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UniversityModfrilms

International300 N. Z eeb Road Ann Arbor. Ml 48106

8300220

Chang, Shih-Chin

EFFECTS OF PLASTIC STRAIN STATE AND HYDROGEN ON THE DEFORMATION AND FRACTURE OF SPHEROIDIZED STEELS

The Ohio State University Ph.D . 1982

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International 300 N. Zeeb Road, Ann Arbor, MI 48106

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UniversityMicrofilms

International

EFFECTS OF PLASTIC STRAIN STATE AND HYDROGEN ON

THE DEFORMATION AND FRACTURE OF SPHEROIDIZED STEELS

D i s s e r t a t i o n

P r e s e n t e d i n P a r t i a l F u l f i l l m e n t o f t h e R e q u ir e m e n ts f o r

t h e D e g re e D o c to r o f P h i lo s o p h y i n th e G rad u ate

S c h o o l o f t h e Ohio S t a t e U n i v e r s i t y

by

S h ih - C h in C hang, B . S c . , M .Sc.

The Ohio S t a t e U n i v e r s i t y

1982

R e a d in g C om m ittee: Approved by

P r o f e s s o r J .P . H ir t h

P r o f e s s o r R.G. H oa g lan d

P r o f e s s o r C .E , M obley

A d v is o r D ep artm en t o f M e t a l l u r g i c a l

E n g in e e r i n g

ACKNOWLEDGMENTS

Sincere gratitude is due to Professor John P. Hirth, the advisor of this auther. This work can't he done without his continuous encouragement and stimulating comments and suggestions. The auther is grateful to his teachers at The Department of Metallurgical Engineering, The Ohio State University their great teaching provides a solid background to his research work. Thanks to Mr. R. Farrar (Bud) for many useful experimental suggestions; to Mr. R.E, Justus for his precise machining of the materials.

The importance of the everlasting love and patience of the auther's family - his parents, wife and daughters - on this work can never be overemphasized. His wife, Shih-Lain, typed the whole manuscript. His 7 and 6 years old daughters, Yu-May and Hsin-May helped on keeping a joyful atomsphere while he was working;

The a u t h e r i s g r a t e f u l to t h e G rad ute S c h o o l o f The Ohio

S t a t e U n i v e r s i t y f o r i t s s u p p o r t d u r in g th e a u t h o r ' s t e n u r e

a s U n i v e r s i t y F e l lo w . The a u th o r a l s o g r a t e f u l l y acknow

l e d g e s t h e su p p o r t o f t h e N a t i o n a l S c i e n c e F o u n d a t io n f o r

s p o n s o r in g t h i s r e s e a r c h u n d er G rant DMR-7815735*

VITA

O c to b e r 2 3 i 19^9 B orn - T a i p e i , Taiw anR e p u b l ic o f C h in a

B .S c . , T s i n g Hua U n i v e r s i t y T a iw a n , R e p u b l ic o f C h in a

M .S c . , T s in g Hua U n i v e r s i t y T aiw an , R e p u b l i c o f C h in a

U n i v e r s i t y F e l l o w , The Ohio S t a t e U n i v e r s i t y , C olum bus, Ohio

R e s e a r c h A s s o c i a t e , D ep artm en t o f M e t a l l u r g i c a l E n g i n e e r i n g ,The Ohio S t a t e U n i v e r s i t y , C olum bus, Ohio

PUBLICATIONS

1. "The D e t e r m in a t io n o f T e n s i l e P r o p e r t i e s from H a r d n e ss M easu rem en ts f o r A l-Zn-M g A l l o y s . " J . M a te r . S c i . , 1976 v o l . 1 1 , p p . 6 2 3 - 6 3 0 .

2 . "Temper E m b r i t t le m e n t o f a Ni-Cr-M o Low A l l o y S t e e l . " M a ter . S c i . ( Q) , 1 9 7 6 , v o l . 8 , p p . 1 1 8 -1 2 6 . ( i n C h in e s e )

3 . "The D e v e lo p m e n t o f 270 Grade PC S t e e l W ire ." T e c h a n i c a l P u b l i c a t i o n s o f The C h in a B ranch o f The I r o n and S t e e l I n s t i t u t e o f S o u t h - E a s t e r n A s i a , 1977* v o l . 5* PP* 1 0 - 1 6 .

if-. " E x p e r im e n ta l M a t e r i a l s S c i e n c e . " W en-Ching Book C o . , T a i p e i , R e p u b l ic o f C h in a , 1 9 7 7 . ( i n C h in e s e )

5 . "A S tu d y o f The S t a t u s o f S m a ll t o M ed iu m -S ized S t e e l P l a n t s i n T a iw a n ." G overnm ent R e p o r t , M i n i s t r y o f E conom ic A f f a i r s , R e p u b l ic o f C h in a , 1 9 7 8 .

1971

1975

1978

1 9 7 9 -1 9 8 2

FIELDS OF STUDY

S t u d i e s i n P h y s i c a l M e t a l lu r g y

P r o f e s s o r s J .P . H i r t h , G. M ey r ick and P .G . Shewmon.

S t u d i e s i n M e c h a n ic a l M e t a l lu r g y and F r a c t u r e M ec h a n ic s

P r o f e s s o r s J .P . H i r t h , J .A . B e g l e y and C.H. P o p e la r .

S t u d i e s i n Therm odynam ics and S t a t i s t i c a l M ech a n ic s

P r o f e s s o r s G.R. S t . P i e r r e and R. S p e i s e r .

S t u d i e s i n M e ta l R e f i n i n g and C a s t in g

P r o f e s s o r s G.R. S t . P i e r r e and C .E . M o b ley .

S t u d i e s i n C o r r o s io n

P r o f e s s o r s F .H . B eck and R .A . R app.

S t u d i e s i n H igh T em perature O x id a t io n , M e t a l l u r g i c a l K i n e t i c s and P o i n t D e f e c t s

P r o f e s s o r R .A . Rapp.

S t u d i e s i n M eta l P h y s i c s

P r o f e s s o r s R. S p e i s e r and D. R ig n e y .

i v

TABLE OF CONTENTS

P ageACKNOWLEDGMENTS ..................................................................................... i i

VITA ................................................................................................................. i i i

LIST OF TABLES .............................. v i i

LIST OF FIGURES ......................... v i i i

C h a p te r

1. INTRODUCTION.............................................................. 1

2 . LITERATURE REVIEW

2 - 1 B a s i c D a ta f o r H ydrogen i n S t e e l s .................. 42 - 2 H ydrogen E f f e c t on M echan ica l. P r o p e r t i e s . 52 - 3 M o d e ls o f H ydrogen D e g r a d a t i o n .......................... 142 - 4 P l a s t i c I n s t a b i l i t y ....................................................... 192 - 5 E f f e c t o f P l a s t i c S t r a i n S t a t e on t h e

H ydrogen In d u c e d C r a c k in g ....................................... 25

3 . EXPERIMENTAL METHODS

3 - 1 M a t e r i a l s S t u d i e d ............................................................ 283 - 2 H ea t T r ea tm en t And S p ec im en P r e p a r a t i o n . . 283 - 3 T e s t s o f U n ch arged S p ec im en s

3 - 3 - 1 U n ch arged Round B ar and P la n eS t r a i n T e n s io n T e s t s ........................ 33

3 - 3 - 2 U n ch arged Bend T e s t ....................................... 333 - 4 T e s t s w i th H ydrogen C h a rg in g

3 - 4 - 1 E l e c t r o l y t e P r e p a r a t i o n andP o l a r i z a t i o n D iagram s .................................. 34

3 - 4 - 2 P r e c h a r g in g T e s t s ............................................ 343 - 4 - 3 Dynamic C h a rg in g T e s t s ................................ 36

3 - 5 M e t a l lo g r a p h ic E x a m in a t io n ..................................... 38

4 . RESULTS

4 - 1 Round Bar T e n s io n T e s t s ........................................... 394 - 1 - 1 L o a d - D e f l e c t i o n C u rves ......................... 394 - 1 - 2 M e c h a n ic a l p r o p e r t i e s .................................. 394 - 1 - 3 S l i p b a n d s on S p ec im en S u r f a c e s . . . . 434 - 1 - 4 F r a c t u r e S u r f a c e s ............................................ 434 - 1 - 5 S u r f a c e C rack s .................................................... 44

v

4 - 2 P la n e S t r a i n T e n s i o n T e s t s ..................................... 574 - 2 - 1 L o a d - D e f l e c t i o n C u r v e s ................................ 574 - 2 - 2 M e c h a n ic a l p r o p e r t i e s ................................... 574 - 2 - 3 S l i p B a n d s on S p e c im e n S u r f a c e s . . . . 614 - 2 - 4 F r a c t u r e S u r f a c e s ............................................. 624 - 2 - 5 C r a c k s and v o i d s ................................................ 63

4 - 3 T h r e e - P o in t B e n d in g T e s t s ........................................ 934 - 3 - 1 L o a d - D e f l e c t i o n C u r v e s and

M e c h a n ic a l P r o p e r t i e s .................................. 934 - 3 - 2 S l i p B ands on N o tc h S u r f a c e s ................ 944 - 3 - 3 F r a c t u r e S u r f a c e s ............................................. 964 - 3 - 4 C r a c k s and V o id s ................................................ 98

5 . DISCUSSION

5 - 1 The E f f e c t o f P l a s t i c S t r a i n S t a t e .................... 1235 - 1 - 1 S l i p B ands on S u r f a c e ................................... 1235 - 1 - 2 D u c t i l i t y and Work H a r d e n in g R a te . . 1265 - 1 - 3 P l a s t i c I n s t a b i l i t y ........................................ 1295 - 1 - 4 C rack I n i t i a t i o n and P r o p a g a t io n . . . 1345 - 1 - 5 The s u p p r e s s i o n o f Y i e l d P o i n t

i n P la n e S t r a i n T e n s i o n T e s t s ............. 1375 - 2 The E f f e c t o f H y d r o g e n ................................................. 139

5 - 2 - 1 L o a d - D e f l e c t i o n C u r v e s andF r a c t u r e S t r a i n s ................................................ 140

5 - 2 - 2 P l a s t i c I n s t a b i l i t y ........................................ 1475 - 2 - 3 C ra ck I n i t i a t i o n and P r o p a g a t io n

i n P r e c h a r g e d S p e c im e n s ............................. 1495 - 2 - 4 C r a c k s i n D y n a m ic a l ly

C h arged S p e c im e n s ............................................. 153

6 . CONCLUSIONS ........................................................................................ 157

LIST OF REFERENCES .................................................................................. 159

v i

LIST OP TABLES

T a b le P age

1 . C o m p o s i t io n o f s t e e l s ......................................................................... 28

2 . Round b a r t e n s i l e p r o p e r t i e s f o rs p h e r o i d i z e d A ISI 1020 s t e e l ....................................................... 42

3 . Round b a r t e n s i l e p r o p e r t i e s f o rs p h e r o i d i z e d A ISI 1090 s t e e l ....................................................... 42

4 . P la n e s t r a i n t e n s i l e p r o p e r t i e s f o rs p h e r o i d i z e d A ISI 1020 s t e e l ....................................................... 60

5- P la n e s t r a i n t e n s i l e p r o p e r t i e s f o rs p h e r o i d i z e d A ISI 1090 s t e e l ....................................................... 60

6 . T h r e e - p o i n t b e n d in g p r o p e r t i e s o fs p h e r o i d i z e d A ISI 1020 s t e e l ....................................................... 93

7- T h r e e - p o in t b e n d in g p r o p e r t i e s o fs p h e r o i d i z e d A ISI 1090 s t e e l ....................................................... 94

8 . The s u r f a c e s t r a i n s f o r c r a c k i n i t i a t i o n ........................ I 33

9» C rack i n i t i a t i o n s i t e s and t h e i r i n i t i a l p r o p a g a t e d i r e c t i o n w i t h r e s p e c t t o th e m ajor t e n s i o n s t r e s s ............................................................................ 134

10 . F r a c t u r e s t r a i n s and d u c t i l i t y l o s s e sf o r h y d r o g e n c h a r g e d s p e c im e n s .................................................. 140

v i i

LIST OF FIGURES

F ig u r e P age

1. S c h e m a t ic r e p r e s e n t a t i o n o f a d i s c o n t i n u t y i n t a n g e n t i a l v e l o c i t y a lo n g a c h a r a c t e r i s t i c s u r f a c e .............. 23

2 . S p h e r o i d i z e d m i c r o s t r u c t u r e o f A ISI 1020 s t e e l ............... 30

3 - S p h e r o i d i z e d m i c r o s t r u c t u r e o f A IS I 1090 s t e e l ............... 30

4 . S p ec im en g e o m e tr y f o r a x is y m m e t r ic (ro u n d b a r )t e n s i o n t e s t s ..................................................................................................... 31

5 . S p ec im en g e o m e tr y f o r p la n e s t r a i n t e n s i o n t e s t s ......... 31

6 . S p ec im en g e o m e tr y f o r t h r e e - p o i n t bend t e s t s .................... 32

7 . F i x t u r e u s e d and t h e d e f i n i t i o n o f m id -sp a n d e f l e c t i o n and p l a s t i c bend a n g le f o r t h r e e -p o i n t bend t e s t ................................................................................................ 32

8 . P o l a r i z a t i o n d ia g ra m f o r s p h e r o i d i z e d A ISI 1020 s t e e l i n 1 N s u l f u r i c a c i d c o n t a i n i n g 1 g / lt h i o u r e a a t room t e m p e r a tu r e ............................................................... 35

9 . P o l a r i z a t i o n d ia g ra m f o r s p h e r o i d i z e d A ISI 1090 s t e e l i n 1 N s u l f u r i c a c i d c o n t a i n i n g 1 g / lt h i o u r e a a t room te m p e r a tu r e ............................................................... 35

1 0 . C e l l f o r dynam ic c h a r g in g o f ro u n d b a r t e n s i o n t e s t s . 37

1 1 . C e l l f o r dynam ic c h a r g in g o f p la n e s t r a i nt e n s i o n t e s t s ...................................................................................................... 37

1 2 . L o a d - d e f l e c t i o n c u r v e s f o r 1020 round b a rt e n s i o n t e s t s ................................................................................................... 40

. . 1 3 • L o a d - d e f l e c t i o n c u r v e s f o r 1090 round b a rt e n s i o n t e s t s ...................................................................................................... 41

1 4 . S l i p b a n d s on t h e s u r f a c e o f u n ch a rg e d 1020 rou n db a r s p e c im e n , t r u e s t r a i n £ = ( a ) 0 . 0 2 5 (U) 0 . 0 6( c ) 0 .1 0 7 ( d ) 0 .2 0 8 ( e ) 0 .5 1 2 ( f ) 1 . 2 1 ..................................... 46

v i i i

15- S l i p b a n d s on t h e s u r f a c e o f u n ch a rg e d 1090 round b a r s p e c im e n , t r u e s t r a i n £ = ( a ) 0 . 0 2 3 (b ) 0 . 0 5 3 ( c ) 0 .0 8 6 (d ) O .I 36 ( e ) O. 2 3 2 ( f ) 0 . 4 9 4 .................................. 47

1 6 . F r a c t u r e s u r f a c e o f u n c h a r g e d 1020 rou n d b a r s p e c im e n ( a ) c u p -a n d -e o n e f r a c t u r e (b ) c e n t r a l r e g i o n ( c ) p e r i p h e r a l r e g i o n ................................................................. 48

17* F r a c t u r e s u r f a c e o f u n c h a r g e d 1090 round b a r sp e c im e n (a ) cu p -a n d c o n e f r a c t u r e (b ) c e n t r a l r e g i o n ( c ) p e r i p h e r a l r e g i o n ............................................................... 49

1 8 . F r a c t u r e s u r f a c e o f p r e c h a r g e d 1020 rou n d b arsp e c im e n ( a ) c u p -a n d -c o n e f r a c t u r e (b ) t h e b o x ed r e g i o n i n (a ) ..................................................................................................... 50

19* F r a c t u r e s u r f a c e o f p r e c h a r g e d 1090 rou n d b a r sp e c im e n (a ) c u p -a n d -c o n e f r a c t u r e (b ) c e n t r a l r e g i o n ( c ) p e r i p h e r a l r e g i o n ............................................................... 51

2 0 . F r a c t u r e s u r f a c e o f d y n a m ic a l ly c h a r g e d 1020 round b a r s p e c im e n (a ) g e n e r a l a p p ea r a n c e(b ) c e n t r a l r e g i o n ( c ) p e r i p h e r a l r e g i o n ............................... 52

2 1 . F r a c tu r e s u r f a c e o f d y n a m ic a l ly c h a r g e d 1090 rou n d b a r s p e c im e n ( a ) g e n e r a l a p p ea ra n ce(b) c e n t r a l r e g i o n ( c ) p e r i p h e r a l r e g i o n ............................... 53

2 2 . S id e s u r f a c e o f u n ch a rg e d round b a r sp e c im e nn e a r f r a c t u r e ( a ) 1020 s t e e l (b ) 1090 s t e e l ........................ 54

23* S id e s u r f a c e o f p r e c h a r g e d ro u n d b a r sp e c im e nn e a r f r a c t u r e (a ) 1020 s t e e l (b ) 1090 s t e e l - ........................ 55

2 4 . S id e s u r f a c e o f d y n a m ic a l ly c h a r g e d round b a r sp e c im e n n e a r f r a c t u r e (a ) 1020 s t e e l (b ) 1090 s t e e l ....................... 56

2 5 . L o a d - d e f l e c t i o n c u r v e s f o r 1020 p la n e s t r a i nt e n s i o n t e s t s ..................................................................................................... 58

2 6 . L o a d - d e f l e c t i o n c u r v e s f o r I 090 p la n e s t r a i nt e n s i o n t e s t s ..................................................................................................... 59

2 7 . S l i p b a n d s on t h e s u r f a c e o f u n ch a rg e d 1020 p la n e s t r a i n t e n s i o n s p e c im e n s , t r u e s t r a i n £ = (a ) 0 . 0 1(b ) 0 . 0 2 8 ( c ) 0 . 1 2 2 (d ) 0 . 1 9 ( e ) 0 . 2 6 ( f ) O.3 9 ................ 67

2 8 . S l i p b a n d s on t h e s u r f a c e o f u n ch a rg e d 1020 p la n e s t r a i n t e n s i o n sp e c im e n a t t r u e s t r a i n £ = 0 . 1 0 5and e t c h e d w i t h 2% n i t a l ........................................................................ 68

i x

2 9 . S l i p b a n d s on t h e s u r f a c e o f u n c h a r g e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n , t r u e s t r a i n £ = ( a ) 0 . 0 3 1 (b ) 0.0*1-9 ( c ) 0.06*1- ( d ) 0 . 0 9 0 ( e ) O .I 37 ( f ) 0 . 2 0 0( g ) 0 . 2 7 7 (h ) 0 .*l-50 ......................................................................................... 69

3 0 . C r a c k s o n MnS p a r t i c l e i n 1090 p l a n e s t r a i n t e n s i o n s p e c im e n w i t h £ = ( a ) 0 . 1 3 7 (b ) 0 . 2 7 7 ........................................ 71

3 1 . S l i p b a n d s o n t h e s u r f a c e o f p r e c h a r g e d 1020 p l a n e s t r a i n t e n s i o n s p e c i m e n s , t r u e s t r a i n £ = ( a ) O .0 3 I ( b ) 0.05*1- ( c ) 0 . 1 1 3 (d ) 0 . 2 3 1 ( e ) 0 . 2 3 1 , h i g h e r m a g n if i c a t i o n t o show t h e v o i d s on s l i p b a n d s ( f ) O.3 9 . . . . 72

3 2 . S l i p b a n d s on t h e s u r f a c e o f p r e c h a r g e d 1090 p l a n e s t r a i n t e n s i o n s p e c i m e n s , t r u e s t r a i n £ = ( a ) 0 . 0 1 6(b ) 0 . 0 *K) ( c ) 0 . 0 6 6 (d ) 0 . 1 5 5 ..............................................................73

33* S l i p b a n d s on t h e s u r f a c e o f d y n a m i c a l ly c h a r g e d1020 p l a n e s t r a i n t e n s i o n s p e c im e n s , t r u e s t r a i n £ =(a ) 0 . 0 1 5 (b ) 0 . 0 2 6 ( c ) 0 . 0 3 9 (d ) 0 . 2 7 0 ................................. 7*1-

3*1-. S l i p b a n d s o n t h e s u r f a c e o f d y n a m i c a l ly c h a r g e d1090 p l a n e s t r a i n t e n s i o n s p e c im e n s , t r u e s t r a i n £ =(a ) 0 . 0 1 5 (b ) 0 . 0 2 0 ........................................................................................ 75

35* F r a c t u r e s u r f a c e o f u n c h a r g e d 1020 p la n e s t r a i n t e n s i o n s p e c im e n ( a ) g e n e r a l a p p e a r a n c e(b ) d im p le d s t r u c t u r e ............................................................................. 76

3 6 . F r a c t u r e s u r f a c e o f u n c h a r g e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n (a ) g e n e r a l a p p e a r a n c e(b ) d im p le d s t r u c t u r e ................................................................................ 77

37* F r a c t u r e s u r f a c e o f p r e c h a r g e d 1020 p la n e s t r a i nt e n s i o n s p e c im e n (a ) g e n e r a l a p p e a r a n c e (b ) d im p le d r e g i o n ( c ) q u a s i - c l e a v a g e r e g i o n ..................................................... 78

3 8 . F r a c t u r e s u r f a c e o f p r e c h a r g e d 1090 p la n e s t r a i nt e n s i o n s p e c im e n ( a ) g e n e r a l a p p e a r a n c e (b ) d im p le d r e g i o n ( c ) q u a s i - c l e a v a g e r e g i o n ..................................................... 79

39* F r a c t u r e s u r f a c e o f d y n a m i c a l ly c h a r g e d 1020 p la n e s t r a i n t e n s i o n s p e c im e n ( a ) g e n e r a l a p p e a r a n c e(b ) s i d e v i e w ( c j d im p le d r e g i o n a t r i d g e (d ) q u a s i c l e a v a g e r e g i o n ................................................................................................. 80

4 0 . F r a c t u r e s u r f a c e o f d y n a m i c a l ly c h a r g e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n ( a ) g e n e r a l a p p a ra n ce( b ) s i d e v i e w ( c ) d im p le d r e g i o n a t r i d g e (d ) q u a s i c l e a v a g e r e g i o n ................................................................................................. 81

x

4 1 . Mid p la n e o f u n ch a rg e d 1020 p la n e s t r a i n t e n s i o nsp e c im e n a t £ = O.3 0 ................................................................................. 82

4-2. Mid p la n e o f p r e c h a r g e d 1020 p la n e s t r a i n t e n s i o nsp e c im e n a t £ = 0 . 3 1 5 ................................................................................. 83

4 3 * Mid p la n e o f u n ch a rg e d 1090 p la n e s t r a i n t e n s i o nsp e c im e n a t £ = 0 . 4 2 ( a ) s u r f a c e c r a c k (b ) i n t e r n a l c r a c k ( c ) i n t e r n a l c r a c k a s s o c i a t e d w i t h MnS ....................... 84

4 4 . Mid p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n a t 6 = £* = 0 . 3 4 2 . A rrow s i n d i c a t e s u r f a c e and i n t e r n a l c r a c k s ................................................................... 85

45 - Mid p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n , s u r f a c e c r a c k form ed a t d eep i n t r u s i o n n e a r a MnS p a r t i c l e ......................................................................... 86

4 6 . Mid p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n ( a ) mode I I c r a c k s (b ) mode I c r a c k .......................... 87

4 7 . Mid p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n sh o w in g i n t e r n a l c r a c k s ( a ) mode I I(b ) mode I ........................................................... 88

4 8 . Mid p la n e o f d y n a m ic a l ly c h a r g e d 1020 p la n e s t r a i n t e n s i o n sp e c im e n a t £ = 0 . 0 8 6 , c r a c k s on o r n e a rs u r f a c e ..................................................................................................................... 89

4 9 . Mid p la n e o f d y n a m ic a l ly c h a r g e d 1020 p la n e s t r a i n t e n s i o n sp e c im e n a t £ = 0 . 0 8 6 , c r a c k s from n e a rs u r f a c e MnS p a r t i c l e s ................................................................................ 90

5 0 . Mid p la n e o f d y n a m ic a l ly c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n a t £ = 0 . 0 2 7 , c r a c k s on s u r f a c e .............. 91

5 1 . Mid p la n e o f d y n a m ic a l ly c h a r g e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n a t £ = 0 . 0 5 2 , c r a c k s on s u r f a c e ........... 92

5 2 . L o a d - d e f l e c t i o n c u r v e s f o r 1020 bend t e s t s ......................... 99

5 3 . L o a d - d e f l e c t i o n c u r v e s f o r 1090 b en d t e s t s ...................... 100

5 4 . S l i p b a n d s on t h e n o t c h s u r f a c e o f u n ch a rg ed 1020bend s p e c im e n , p l a s t i c bend a n g le 6 and n o tc h s u r f a c e s tr a in £ x K = ( a ) 0 . 3 4 ° , 0 .0 1 7 (b ) 0 . 8 4 ° , 0 .0 4 2( c ) 1 . 8 8 ° , 0 . 0 9 4 (d ) 3 - 0 4 ° , 0 . 1 4 ( e ) 5 . 6 ° , 0 . 2 2 4( f ) 11° , 0 .4 0 5 (g ) and (h ) 3 0 ° , 1 .0 3 5 ............................................ 101

55* S l i p b a n d s on th e n o t c h s u r f a c e o f p r e c h a r g e d 1020bend sp e c im e n , p l a s t i c bend a n g le 0 = 25* ............................... 103

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5 6 . S l i p b a n d s on t h e n o tc h s u r f a c e o f d y n a m ic a l ly c h a r g e d 1020 b en d s p e c im e n , p l a s t i c bend a n g le 0 = 3 - 2 5 ° ................................................................................................................ 104-

57* S l i p b a n d s on th e n o t c h s u r f a c e o f d y n a m ic a l ly c h a r g e d 1020 bend s p e c im e n , p l a s t i c bend a n g le0 = 9° ........................................................................................................................ 105

5 8 . S l i p b a n d s on t h e n o tc h s u r f a c e o f u n ch a rg e d 1090b end s p e c im e n , p l a s t i c bend a n g le 0 and n o tc h s u r f a c es t r a i n £** = ( a ) 0 . 1 7 ° , 0 .0 0 7 (b ) 0 . 88° , 0 .0 3 7( c ) 2 . 8 ° , 0 .1 1 7 ( d ) 5 . 1° , 0 . 1 8 ( e ) 1 2 . 1° , 0 .3 8 7( f ) 1 8 . 3 ° i 0 .5 6 7 ( g ) and (h ) 2 3 -4 -° , 7 1 - 5 .................................. 106

59* S l i p b an d s on t h e n o t c h s u r f a c e o f p r e c h a r g e d 1090bend s p e c im e n , p l a s t i c b end a n g le 0 and n o t c h s u r f a c es t r a i n £** = ( a ) 0 . 8 * , 0 .034- (b ) 2 . 2 5 ° , 0 . 0 9 5( c ) 6 .8 3 ° i O.1 7 7 ............................................................................................. 107

6 0 . S l i p b a n d s on t h e n o t c h s u r f a c e o f d y n a m ic a l ly c h a r g e d bend s p e c im e n , p l a s t i c bend a n g le 0 = 1 ,n o t c h s u r f a c e s t r a i n £ * * = 0.04- ....................................................... 108

6 1 . F r a c t u r e s u r f a c e o f u n c h a r g e d 1020 b end sp e c im e n ( a ) g e n e r a l a p p ea r a n c e (b ) f r a c t u r e form ed i n b e n d in g t e s t ( c ) f r a c t u r e form ed b y f a s t b lo wa t l i q u i d n i t r o g e n te m p e r a tu r e ......................................................... 110

6 2 . F r a c t u r e s u r f a c e o f p r e c h a r g e d 1020 bend sp e c im e n (a ) g e n e r a l a p p ea r a n c e (b ) s e c o n d a r y c r a c k s and d im p le d r e g i o n ( c ) d im p le s t r u c t u r e (d ) f r a c t u r e form ed b y f a s t b lo w a t l i q u i d n i t r o g e n t e m p e r a t u r e . . . . m

6 3 . F r a c t u r e s u r f a c e o f d y n a m ic a l ly c h a r g e d 1020 bend sp e c im e n (a ) g e n e r a l a p p ea r a n c e (b ) t r a n s i t i o n r e g i o n ( c ) h y d r o g e n - a s s i s t e d q u a s i - c l e a v a g e r e g i o n(d ) c l e a v a g e r e g i o n ....................................................................................... 112

64-. F r a c t u r e s u r f a c e o f u n c h a r g e d 1090 bend sp e c im e n ( a ) g e n e r a l a p p ea r a n c e (b ) d im p le d r e g i o n( c ) c l e a v a g e r e g i o n ....................................................................................... H 3

6 5 . F r a c t u r e s u r f a c e o f p r e c h a r g e d 1090 bend sp e c im e n( a ) g e n e r a l a p p e a r a n c e (b ) d im p le d r e g i o n( c ) c l e a v a g e r e g i o n ....................................................................................... nZj.

6 6 . F r a c t u r e s u r f a c e o f d y n a m ic a l ly c h a r g e d 1090 bend s p e c im e n (a ) g e n e r a l a p p e a r a n c e (b ) h y d r o g e n - a s s i s t e d q u a s i - c l e a v a g e r e g i o n ( c ) c l e a v a g e r e g i o n ............................. 115

x i i

6 7 . F r a c t u r e s u r f a c e o f u n ch a rg e d 1090 bend sp e c im e nform ed b y f a s t b lo w a t l i q u i d n i t r o g e n te m p e r a tu r e . . . 116

6 8 . N otch mid p la n e o f u n ch a rg e d 1020 bend sp e c im e nw i t h p l a s t i c b end a n g le 0 = 3 0 *2° 117

6 9 * N o tch mid p la n e o f p r e c h a r g e d 1020 bend sp e c im e nw it h p l a s t i c bend a n g le 0 = 2 5 ° ........................................................ 118

7 0 . N o tch mid p la n e o f d y n a m ic a l ly c h a r g e d 1020bend specimen.......................................... 1 1 9

7 1 . N o tch mid p la n e o f u n ch a rg e d 1090 bend s p e c im e nw i t h p l a s t i c b end a n g le 0 = 2 3 ..................................................... 120

7 2 . N o tc h mid p la n e o f p r e c h a r g e d 1090 bend s p e c im e nw it h p l a s t i c b end a n g le 0 = 7 - 2 ° ........................................................ 121

73* N o tch mid p la n e o f d y n a m ic a l ly c h a r g e d 1090b en d sp e c im e n ..................................................................................................... 122

7^ . C h a r a c t e r i s t i c s u r f a c e s and c o r r e s p o n d in g s t r e s s M oh r 's c i r c l e f o r a sp e c im e n un derplane stress condition................................ 1 3 1

7 5 * E q u i - s l i p band d iagram f o r t h e 1090 p la n e s t r a i n sp e c im e n s t e s t e d u n d er d i f f e r e n tc h a r g in g c o n d i t i o n s ...................................................................................... 148

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1. INTRODUCTION

The dam aging e f f e c t s o f h y d ro g en on s t e e l s h a s b e e n r e c o

g n i z e d f o r more th a n a hundred y e a r s . Numerous e f f o r t s have

b e e n d e d i c a t e d t o t h e u n d e r s t a n d in g o f t h i s p ro b lem . I n

much o f t h i s p e r i o d , i t seem s th e o n ly r e s u l t s o f t h e e f f o r t s

was d i s c o v e r i n g more q u e s t i o n s b u t n o t a n s w e r s . T here a re

many a n o m a l ie s and s e e m in g ly c o n t r a d i c t o r y phenomena have

b e e n r e p o r t e d . N e v e r t h e l e s s , i n r e c e n t y e a r s , w i th th e l a r g e

i n c r e a s e i n th e r e s o u r c e s and e f f o r t d e v o te d i n many r e s e a r c h

w o r k s , th e main f e a t u r e s o f t h i s s e t o f phenomena a re b e

com ing more c l e a r .

For s t e e l s , a s p o i n t e d o u t by H ir t h ( l ) , t h e p h en om en ology

o f d u c t i l e f r a c t u r e i n lo w s t r e n g t h s t e e l s and o f c r a c k p ro p a

g a t i o n i n h ig h s t r e n g t h s t e e l s are b e in g d e f i n e d . S im p le me

c h a n ism s a re n o t s u f f i c i e n t t o r a t i o n a l i z e th e g e n e r a l d e g r a

d a t i o n i n p r o p e r t i e s p rod u ced by h y d ro g e n . I n s t e a d a com p lex

p r o c e s s composed o f a number o f c o u p le d m echanism s a c c o u n t s

f o r many o f th e o b s e r v e d d e g r a d a t io n phenomena. T h e r e f o r e ,

t h e n eed to d a y i s t o c o n s t r u c t a s e l f - c o n s i s t e n t u n i f i e d

fram ework to c o v e r th e w hole sp ec tru m o f o b s e r v e d f a c t s .

I n p l a s t i c d e f o r m a t io n , t h e p la n e s t r a i n s t a t e i s one o f

t h e s i m p l e s t p l a s t i c s t r a i n s t a t e s . I t i s a l s o r e c o g in z e d a s

t h e m ost c r u c i a l s t a t e f o r a m a t e r i a l t o m eet i n d e f o r m a t io n

and f r a c t u r e . P la n e s t r a i n d u c t i l i t y o f a m e t a l i s a lw a y s

o b s e r v e d t o be lo w e r t h a n a x is y m m e tr ic t e n s i l e d u c t i l i t y . I n

h y d r o g e n e n v ir o n m e n t s , t h e d u c t i l i t y l o s s o f a m e t a l i n th e

p la n e s t r a i n s t a t e i s r e p o r t e d t o be l a r g e r th a n t h a t i n n o t

c h ed o r sm ooth ro u n d b a r s . The h y d r o g e n e f f e c t on p r o m o t in g

p l a s t i c i n s t a b i l i t y i s m ost p r o n o u n c ed u n d er p la n e s t r a i n

c o n d i t i o n s a s o b s e r v e d i n s lo w b end t e s t s . P r e v i o u s r e s u l t s

s u g g e s t t h a t h y d r o g e n p r o d u c e s s o f t e n i n g f o r i r o n and s t e e l

when d e f o r m a t io n i s by s i n g l e s l i p and h a r d e n in g when deform

a t i o n i s b y m u l t i p l e s l i p . I t i s known t h a t i n g e n e r a l t h e

s u r f a c e s w i t h maximum r e s o l v e d s h e a r s t r e s s a r e d i f f e r e n t

from t h o s e c h a r a c t e r i s t i c s u r f a c e s i n w h ich th e p l a s t i c i n s

t a b i l i t y u s u a l l y t a k e s p l a c e e x c e p t i n th e p la n e s t r a i n s t a t e .

I t i s o f i n t e r e s t t o know w h e th e r t h e s e phenom ena a r e r e l a t e d

t o e a c h o t h e r o r n o t .

Two s p h e r o i d i z e d s t e e l s w i t h 3 p e r c e n t and 13 p e r c e n t v o l

ume f r a c t i o n s o f c a r b id e p a r t i c l e s ; t h r e e h y d ro g e n c h a r g in g

c o n d i t i o n s , u n c h a r g e d , p r e c h a r g e d and d y n a m ic a l ly c h a r g e d ;

and t h r e e sp e c im e n g e o m e t r i e s , a x is y m m e tr ic sm ooth t e n s i o n

b a r , p la n e s t r a i n t e n s i o n b a r and U -n o tc h b en d b a r w ere u s e d

i n t h i s work t o p r o v id e a v a r i e t y o f m i c r o s t r u c t u r e - h y d r o g e n -

p l a s t i c s t r a i n s t a t e c o m b in a t io n s . The m e c h a n ic a l p r o p e r t i e s

w ere d e te r m in e d and th e s l i p b a n d s on s u r f a c e s , f r a c t u r e s u r

f a c e s a s w e l l a s mid p la n e m i c r o s t r u c t u r e s w ere exa m in ed and

com pared f o r t h e d i f f e r e n t c o m b in a t io n s t o g a i n more u n d e r

s t a n d i n g on t h e e f f e c t s o f p l a s t i c s t r a i n s t a t e and h y d ro g e

on m e c h a n ic a l p r o p e r t i e s o f s t e e l s .

2 . LITERATURE REVIEW

2 - 1 B a s i c D a ta f o r H yd rogen i n s t e e l s

F o l l o w i n g i s a l i s t o f some b a s i c d a t a o f h y d r o g e n i n

i r o n and s t e e l ( 1 ) :

(a ) I n t e r s t i t i a l s i t e and s t r a i n f i e l d s I n d i r e c t e v id e n c e

i n d i c a t e t h a t h y d ro g e n i n i r o n o c c u p i e s t e t r a h e d r a l

s i t e s ( 2 , 3 ) - N e v e r t h e l e s s , b y t u n n e l i n g o f H among

n e i g h b o r i n g i n t e r s t i t i a l s i t e s , t h e s t r a i n f i e l d p r o

du ced by H i n i r o n i s a s im p le d i l a t a t i o n (3 * ^ * 5 )*

( b ) . S o l u b i l i t y i The s o l u b i l i t y o f H i n i r o n i n c r e a s e s w i th

i n c r e a s i n g t e m p e r a t u r e . F or t h e room te m p e r a tu r e r e g im e i

t h e Q u ic k -J o n s o n d a t a ( 6 ) g i v e t h e f o l l o w i n g r e l a t i o n

f o r th e atom f r a c t i o n Go o f H i n e q u i l i b r i u m w i t h h yd ro ge

g a s a t p r e s s u r e P ( a t m ) ,

Co = 0 .0 0 1 8 5 P e x p ( - 3 ^ 0 / T ) .

( c ) . P a r t i a l m o la r vo lu m e: The p a r t i a l m o la r vo lum e o f H i n

p u re i r o n i s 0 . 0 0 3 3 nm3/a to r a .

( d ) . D i f f u s i v i t y i I n t h e p e r f e c t l a t t i c e t h e d i f f u s i v i t y o f

H i n i r o n i s D = 2 x 10 7 e x p ( - 8 2 8 / T ) ma/ s . I n s t e e l s

t h e d i f f u s i v i t y i s a f u n c t i o n o f c o m p o s i t i o n , d e g r e e o f

c o l d w o rk , m i c r o s t r u c t u r e , e t c . G a d g ee l and J o h n so n (7 )

fo u n d , a t 200 t o 600°C , t h a t D d e c r e a s e d l i n e a r l y w i t h

i n c r e a s i n g c a rb o n c o n t e n t f o r s p h e r o i d i z e d s t e e l s . X ie

(8 ) d e te r m in e d t h e d i f f u s i v i t i e s o f h y d ro g e n i n t h e s t e e l s

u s e d i n t h i s w ork , a s 8 . 3 ^ x 10 '° m /s f o r s p h e r o i d i z e d

1090 s t e e l and 2 x 10 mx/ s f o r s p h e r o i d i z e d 1020 s t e e l ,

( e ) . T r a p s i I m p u r i t i e s , d i s l o c a t i o n s , i n c l u s i o n - m a t r i x i n t e r

f a c e s , g r a i n b o u n d a r i e s , f r e e s u r f a c e s , e t c , c a n t r a p h y

d r o g e n i n m e t a l s . The c a r b i d e - m a t r i x i n t e r f a c e s and f r e e

s u r f a c e s a r e su ch s t r o n g t r a p s t h a t o n c e s a t u r a t e d w i t h

h y d r o g e n t h e y s h o u ld n o t d e s o r b a t room te m p e r a tu r e i n

m e a s u r a b le t i m e s .

2 - 2 H yd rogen E f f e c t on M e c h a n ic a l P r o p e r t i e s

T h ere a r e a w id e r a n g e o f o b s e r v a t i o n s o f t h e d e g r a d a t io n

o f m a t e r i a l p r o p e r t i e s b y h y d r o g e n . Some o f them a r e c o n f u s

i n g o r e v e n c o n f l i c t i n g . F or e x a m p le , s u g g e s t i o n s h ave b e e n

made t h a t h y d r o g e n s o f t e n s ( 9 , 1 0 , 1 1 , 1 2 ) and h a r d e n s ( 1 3 »1^ * 1 5 )

i r o n . I n t h e i r r e v i e w p a p e r , H ir t h and J o h n so n ( 1 6 ) p r e s e n t e d

n i n e s e p a r a t e p h e n o m e n o lo g ic a l c l a s s i f i c a t i o n s o f h y d ro g e n

d e g r a d a t i o n o f m a t e r i a l s .

F or t h i s work t h e s c o p e w i l l be l i m i t e d t o t h e room tem

p e r a t u r e h y d r o g e n e m b r i t t l e m e n t o f lo w s t r e n g t h s t e e l s . Hy

d r o g e n e m b r i t t l e m e n t may be d e f i n e d a s ( 1 ? ) h y d r o g e n in d u c e d

l o s s e s i n s t r e n g t h a n d /o r d u c t i l i t y and c a n r e s u l t from h y d r o

g e n e f f e c t s on e i t h e r i n i t i a t i o n o r p r o p a g a t io n o f f r a c t u r e .

L ou th a n and M cN itt (1 ? ) sum m arized t e n group o f o b s e r v a t i o n s

6

c h a r a c t e r i z i n g t h e h y d r o g e n e m b r i t t l e m e n t phenom ena and su g

g e s t e d t h a t any v a l i d h y d r o g e n e m b r i t t l e m e n t m echan ism m ust

be c o m p a t ib le w i t h them .

H yd rogen v a r i o u s l y a f f e c t s p l a s t i c f l o w o f lo w « 7 00 M P a)

s t r e n g t h s t e e l s . At room te m p e r a t u r e , a d e c r e a s e i n y i e l d

s t r e n g t h was o b s e r v e d a f t e r e l e c t r o l y t i c c h a r g in g a t l a r g e

h y d r o g e n f u g a c i t y b y R o g e r s (1 8 ) f o r 1020 s t e e l , b y C r a c k n e l l

and P e t c h ( 1 9 ) f o r s t e e l w i t h v a r y i n g c a r b o n c o n t e n t up t o

0 . 6 0 % and b y Lee e t a l (2 0 ) f o r s p h e r o i d i z e d 1095 s t e e l .

O nyew uenyi ( 9 ) n o t i c e d a d e c r e a s e o f t h e m ic r o h a r d n e s s o f

s p h e r o i d i z e d 1090 s t e e l a t i n t e r m e d i a t e and l a r g e i n p u t f u

g a c i t y o f c a t h o d i c a l l y c h a r g e d H. Beachem (2 1 ) o b s e r v e d a

l o w e r f l o w s t r e s s i n t o r s i o n f o r 1020 s t e e l when a c a t h o d i c

c h a r g in g c u r r e n t was a p p l i e d . I n c o n t r a s t , O r ia n i and J o s e p h i c

( 2 2 , 2 3 ) fo u n d t h a t h y d r o g e n i n t h e d i s s o l v e d s t a t e i n v a r i a b l y

h a r d e n s 10*4-5 s t e e l t L u d ers band f o r m a t io n and p r o p a g a t i o n r e

q u i r e d g r e a t e r a p p l i e d s t r e s s ; t h e s l o p e o f t h e s t r e s s - s t r a i n

c u r v e was i n c r e a s e d ; and t h e amount o f p l a s t i c d e f o r m a t io n

o v e r a g i v e n s t r e s s - t i m e h i s t o r y was d e c r e a s e d b y d i s s o l v e d

h y d r o g e n . They a l s o fo u n d m i c r o v o i d s , t h e p o p u l a t i o n o f w h ich

a t a g i v e n d e g r e e o f s t r a i n was i n c r e a s e d b y h y d r o g e n . They

s u g g e s t e d t h a t t h o s e m i c r o v o id s and t h e l a r g e g a s p r e s s u r e

w i t h i n them p r o d u c e t h e a p p a r e n t s o f t e n i n g e f f e c t . H ow ever ,

Kimura e t a l (2*4-,25,2 6 ) u s e d b o th h ig h p u r i t y i r o n w i t h t h e

r e s i d u a l r e s i s t i v i t y r a t i o i n a m a g n e t ic f i e l d (RRRH) o f 3 6 OO

t o 520 0 and J o h n so n -M a tth e y i r o n w i t h RRRH 1500 t o 1 8 0 0 , and

7

fo u n d t h a t t h e s o f t e n i n g t h e y o b s e r v e d was r e v e r s i b l e , t h a t

i s , n o t due t o damage c a u s e d b y h y d r o g e n p r e c i p i t a t i o n . L u n a rsk a

( 2 7 ) t e s t e d w i r e s o f f i v e d i f f e r e n t s t e e l s w i t h a c a t h o d i c

c u r r e n t s w i t c h e d on and o f f d u r in g t h e t e n s i l e t e s t and fo u n d

t h a t a ch an ge o f c a t h o d i c c h a r g in g c a u s e s a r a p i d ch an ge i n

s t r a i n h a r d e n in g r a t e in d e p e n d e n t o f c h e m ic a l c o m p o s i t i o n .

H ow ev er , sh e fo u n d t h a t t h e h a r d e n in g o r s o f t e n i n g e f f e c t o f

h y d r o g e n d e p e n d s on t h e s p e c im e n s i z e , g r a i n s i z e , c o m p o s i t io n

and s t r u c t u r e o f t h e m a t e r i a l and on t h e h y d r o g e n f u g a c i t y .

I t seem s from h e r f i n d i n g s t h a t one c a n n o t c o n c lu d e i n w h ich

d i r e c t i o n t h e h y d r o g e n e f f e c t w i l l b e . The r e s u l t s r e v e a l a

c o m p e t i t i o n b e tw e e n s o f t e n i n g and h a r d e n in g .

H ir t h (1 ) s u g g e s t e d an h y p o t h e s i s f o r t h e r a t i o n a l i z a t i o n

o f t h e h a r d e n i n g - s o f t e n i n g b e h a v io r * When d e f o r m a t io n i s b y

s i n g l e s l i p , h y d r o g e n p r o d u c e s s o f t e n i n g . The s o f t e n i n g i s

a s s o c i a t e d w i t h e n h a n ce d d o u b l e - k i n k n u c l e a t i o n on sc r e w d i s

l o c a t i o n s and t h u s an i n c r e a s e d m o b i l i t y o f th em . I t i s a l s o

p o s s i b l y a s s o c i a t e d w i th e n h a n ce d i n j e c t i o n o f d i s l o c a t i o n s a t

f r e e s u r f a c e s a n d /o r i n t e r p h a s e i n t e r f a c e s . When d e f o r m a t io n

i s by m u l t i p l e s l i p , work h a r d e n in g i s l a r g e r , d i s l o c a t i o n

a r r a y s a r e more c o m p le x , and h a r d e n in g e n s u e s . The e n h a n ced

h a r d e n in g i n t h e l a r g e s t r a i n r e g im e o f d e f o r m a t io n m ig h t be

r e l a t e d t o t h e d i f f i c u l t y o f c r o s s - s l i p i n h y d r o g e n c h a r g e d

i r o n . O th e r p o s s i b i l i t i e s are* t h e p r e s e n c e o f h y d ro g e n i n

d i s l o c a t i o n c o r e s makes d i s l o c a t i o n i n t e r s e c t i o n s more d i f

f i c u l t , v a c a n c i e s s t a b i l i z e d by t r a p p e d h y d ro g e n r e t a r d

r e c o v e r y ; and H a u g m en ts t h e e f f e c t s o f C i n c o r e p i n n i n g and

v i s c o u s c o r e - d r a g .

Lee e t a l ( 2 0 , 2 8 ) fo u n d t h a t p r e c h a r g e d h y d r o g e n p r o m o te s

t h e o n s e t o f p l a s t i c i n s t a b i l i t y o n s p h e r o i d i z e d 1090 s t e e l s .

I n p r e c h a r g e d b en d s p e c i m e n s , c o n c e n t r a t e d s h e a r a l o n g c h a r

a c t e r i s t i c s l i p t r a c e s was f o u n d , i n c o n t r a s t t o u n i fo r m f l o w

i n an u n c h a r g e d s p e c im e n . I n f l a t t e n s i l e s p e c im e n s o f t h e

same m a t e r i a l , an u n c h a r g e d s p e c im e n show ed L iid ers d e fo r m a -

. t i o n b y t h e p r o p a g a t i o n o f a s i n g l e b a n d . I n c h a r g e d s p e c i

m en s , m u l t i p l e L iid er s b a n d s form ed and c o n t i n u e d t o form ah ead

o f p r o p a g a t i n g b a n d s . O r i a n i and J o s e p h i c ( 2 2 , 2 3 ) t e s t e d

s p h e r o i d i z e d 10^5 s t e e l and fo u n d t h a t p r e c h a r g e d h y d r o g e n

l e d t o m u l t i p l e c r i s s - c r o s s L iid ers b a n d s i n c o n t r a s t t o a

s i n g l e ban d f o r t h e u n c h a r g e d c a s e , i n a g r e e m e n t w i t h t h e

r e s u l t s o f Lee e t a l . H o w ev er , t h e y fo u n d t h a t t h e s t r e s s

t o i n i t i a t e and l a t e r a l l y p r o p a g a t e t h e L iid ers b a n d s was i n

c r e a s e d b y h y d r o g e n and t h e r e was no y i e l d p o i n t phenom enon.

So t h e y c o n c lu d e d t h a t b o t h n u c l e a t i o n and p r o p a g a t i o n o f t h e

ban d w ere im peded b y h y d r o g e n .

Lee e t a l fo u n d t h a t b o t h c h a r g e d and u n c h a r g e d s t e e l e v e n

t u a l l y r e v e a l e d p r e f e r e n t i a l v o i d f o r m a t i o n a l o n g c h a r a c t e r i s

t i c s l i p t r a c e s . The v o i d s form ed a t a m u ch a low er s t r a i n i n

t h e p r e c h a r g e d s p e c im e n s and i t w as i n f e r r e d t o b e a c o n s e

q u e n c e o f t h e p l a s t i c i n s t a b i l i t y . C i a l o n e and A sa ro ( 2 9 * 3 0 ,3 1 )

u s e d b o t h i n i t i a l l y c y l i n d r i c a l and c i r c u m f e r e n t i a l l y n o t c h e d

u n i a x i a l l y s t r a i n e d b a r s o f s p h e r o i d i z e d 1 0 1 5 , 1017 and 10^5

s t e e l s , and t e s t e d them u n d er p r e c h a r g in g c o n d i t i o n s . They

fo u n d t h a t v o id i n i t i a t i o n o c c u r r e d w ith lo w e r s t r a i n s and

s t r e s s e s w i t h h y d r o g e n p r e s e n t . An e q u a l l y im p o r ta n t c o n

t r i b u t i o n to th e d u c t i l i t y l o s s w as from h y d r o g e n a c c l e r a t e d

v o id g r o w th and c o a l e s c e n c e . The l a t t e r p r o c e s s t a k e s p l a c e

b y th e p r o p a g a t io n o f v o i d s a lo n g th e g r a i n , and p o s s i b l y

s u b g r a in , b o u n d a r ie s w h ich i n t e r l i n k th e c e m e n t i t e s p h e r o id s .

T hey s u g g e s t e d t h a t h y d r o g e n in d u c e d l o s s e s i n i n t e r f a c i a l

c o h e s io n m ig h t a c c o u n t f o r v o id i n i t i a t i o n , v o i d g r o w th and

v o id c o a le s c e n c e a t lo w e r s t r e s s e s and s t r a i n s . T h ey a l s o

n o t i c e d a p a r t i a l t r a n s i t i o n i n f r a c t u r e mode w h ich w as g e n

e r a l l y fo u n d t o b e a s s o c i a t e d w it h n o n m e t a l l i c i n c l u s i o n s ,

s u g g e s t in g t h a t t h e y a c t a s h y d r o g e n s i n k s d u r in g th e c h a r g

in g p r o c e s s and t h e n , d u r in g d e f o r m a t io n , r e l e a s e a s u f f i c i

e n t l y l a r g e am ount o f h y d r o g e n t o c a u s e q u a s i - c l e a v a g e f r a c

tu r e t o o c c u r .

O r ia n i and J o s e p h ic ( 2 2 ,3 2 ) s t u d i e d s h e e t t e n s i l e sp e c im e n s

o f s p h e r o id iz e d 10^5 s t e e l . T hey fo u n d t h a t h y d r o g e n e n h a n ce d

v o i d n u c l e a t i o n a t c a r b id e p a r t i c l e s b u t had l i t t l e o r no e f

f e c t on v o i d g r o w th , and t h e r e w as no p e r c e p t i b l e r o l e o f e i

t h e r g r a in b o u n d r ie s o r s u b g r a in b o u n d a r ie s .

G arb er e t a l (3 3 » 3 * 0 s t u d ie d rou n d b a r t e n s i o n sp e c im e n s

u n d er p r e c h a r g in g c o n d i t i o n f o r s p h e r o id iz e d 1018 and 1080

s t e e l . I n c o n t r a s t t o o t h e r s t u d i e s t h e y fo u n d t h a t i n b o th

s t e e l s , h y d r o g e n had no s i g n i f i c a n t e f f e c t on e i t h e r th e i n

i t i a t i o n o f v o i d s a t c a r b id e s o r e a r l y g ro w th o f v o i d s p r i o r

to t h e i r l i n k - u p . T h u s, t h e y i n d i c a t e d t h a t i t i s th e e v e n t s

10

o c c u r in g r a t h e r n e a r i n s t r a i n t o f i n a l f r a c t u r e w h ich c o n t r o l

( r e d u c e ) th e t o t a l f r a c t u r e s t r a i n . L in k -u p c a n o c c u r e i t h e r

b y a r a p id a c c e l e r a t i o n i n v o id g r o w th , o r b y an en h an ced

n u c l e a t i o n o f m ic r o v o id s b e tw e e n th e e x i s t i n g v o i d s . They

s u g g e s t e d t h a t t h e s e e f f e c t s may be due l a r g e l y to a p r e s

s u r iz e d v o id m echanism i f h y d ro g en i s t r a n s p o r t e d b y m o b ile

d e s l o c a t i o n s .

The e f f e c t s o f h y d ro g e n on d e g r a d a t io n o f s t e e l s a r e more

p ro n o u n ced w ith i n c r e a s i n g h y d ro g en c o n t e n t or c h a r g in g r a t e

and w ith i n c r e a s i n g s t r e n g t h l e v e l o f th e s t e e l ( 1 7 , 3 5 i 3 6 ) .

T r o ia n o (3 5 ) and h i s a s s o c i a t e s showed t h a t h y d ro g en d e g r a d

ed th e p r o p e r t i e s o f lo w s t r e n g t h s t e e l s , th o s e w ith y i e l d

s t r e n g t h l e s s th a n a b o u t 1 GPa, by d e la y e d c r a c k in g a t s t r e s s

e s b e lo w th e y i e l d s t r e s s and b y th e l o s s o f d u c t i l i t y i n

t e n s i l e t e s t s a s r e f l e c t e d b y a d e c r e a s e d r e d u c t io n o f a r e a .

Toh and B a ld w in ( 3 7 ) d e m o n str a te d t h a t th e d e c r e a s e o f r e d u c

t i o n o f a r e a was g r e a t e s t n e a r room te m p e r a tu r e and d e c r e a s e d

w ith in c r e a s i n g s t r a i n r a t e . T h ese s t e e l s a l s o c a n have a

d e c r e a s e d r e s i s t a n c e t o c r a c k p r o p a g a t io n ( 3 8 ) .

I n g e n e r a l , f i n e r g r a in s i z e r e d u c e s th e d e g r a d a t io n . F or

s p h e r o id iz e d s t e e l s , th e r e s i s t a n c e t o d e g r a d a t io n i n c r e a s e s

w ith d e c r e a s in g c a r b id e s i z e s ( 3 9 > * * 1 ) •

G arber e t a l (3*0 fou n d f o r 1080 s p h e r o id iz e d s t e e l , t h a t

h y d r o g e n d id n o t a f f e c t any p r io r p o r t i o n o f th e f lo w c u r v e

o f a sm ooth round t e n s i o n b a r b u t c a u se d a s m a l le r f r a c t u r e

s t r a i n ( r e d u c t io n o f a r e a ) . F or 1018 s t e e l , h y d ro g en c a u se d

11

a 5# in c r e a s e in f lo w s t r e n g t h and a l s o c a u se d a 25# l o s s i n

r e d u c t io n o f a r e a . C ia lo n e and A saro (3 0 ) o b ta in e d a s im i la r

r e s u l t f o r s p h e r o id iz e d 1045 s t e e l . They fou n d a 22# l o s s

i n d u c t i l i t y f o r h y d ro g en ch a rg ed round b a r s w ith l e s s th a n

1# change i n th e e lo n g a t io n t o f r a c t u r e , no n o t i c e a b le ch an ge

i n th e s t r e s s - s t r a i n c u r v e s , and no change o f th e norm al du c

t i l e r u p tu r e f r a c t u r e m ode.

S e v e r a l a u th o r s have show n, t h a t th e s o - c a l l e d i n t e r n a l

(p r e c h a r g e d ) and e x t e r n a l (d y n a m ic a lly ch a rg ed ) h y d ro g en em

b r i t t l e m e n t are d i s t i n g u i s h e d by h yd rogen b e in g p r e s e n t u n i

fo r m ly or w ith a g r a d ie n t i n c o n c e n t r a t io n d e c r e a s in g from

th e s u r fa c e in w a r d , b u t p r o d u c in g no in h e r e n t d i f f e r e n c e i n

b e h a v io r ( 3 6 ,4 2 ,4 3 ,4 4 ) • H ow ever, h yd rogen s o u r c e s show some

p h e n o n m e n o lo g ic a l e f f e c t s . Page and G e r b e r ic h (4 5 ) foun d

t h a t th e so u r c e o f h y d ro g en a f f e c t s b o th th e c r a c k n u c le a -

t i o n s i t e and th e in c u b a t io n t im e . H ydrogen c r a c k s i n p r e

ch a rg ed n o tc h e d 4340 s t e e l sp ec im en s w ere i n i t i a t e d n e a r th e

e l a s t i c - p l a s t i c boundary w ith an in c u b a t io n t im e w h ich e x h i

b i t e d a l i n e a r d ep en d en ce on n o tc h r a d iu s . H ydrogen c r a c k s

u n der a d y n a m ic a lly ch a rg ed c o n d i t io n w ere i n i t i a t e d n e a r th e

n o tc h s u r fa c e w ith in c u b a t io n t im e s w h ich were r e l a t i v e l y

in d e p e n d e n t o f n o tc h r a d iu s .

S t o l t z (4 6 ) com pared th e d u c t i l i t y l o s s o f sm ooth round

b a r s , n o tc h e d b a r s and p la n e s t r a i n b a r s o f A516 s t e e l (c o n

t a i n i n g 0 .2 1 # C) in 0 .1 MPa h y d ro g en g a s . He fou n d t h a t th e

d u c t i l i t y l o s s e s a re 0 , 1*1# and 32# f o r sm ooth b a r , n o tc h e d

12b a r and p la n e s t r a i n b a r , r e s p e c t i v e l y . T h ere w as no d u c t i

l i t y l o s s f o r sm ooth rou n d b a r sp e c im e n . H ow ever , th e r e w as

a zone o f q u a s i - c l e a v a g e e x te n d in g ~ 1 0 0 um i n t o th e e x t e r i o r

w ith th e r e m a in d e r o f th e sam p le f a i l i n g b y v o id g ro w th and

c o a le s e n c e . I n th e n o tc h e d s a m p le s , a l th o u g h h y d ro g e n

c r a c k s w ere I r e s e n t a t th e r o o t o f th e n o t c h , th e a l l o y s

w ere s u f f i c i e n t l y to u g h t h a t v o id n u c l e a t i o n and g row th a l s o

o c c u r r e d a t th e sam p le c e n t e r - l i n e . The f i n a l d u c t i l i t y was

d e te r m in e d , a s i n th e sm ooth b a r c a s e , b y a c o m p e t i t io n b e

tw e e n th e two f r a c t u r e p r o c e s s e s , one a f f e c t e d and one e s

s e n t i a l l y u n a f f e c t e d b y th e e n v ir o n m e n t . F or p la n e s t r a i n

s p e c im e n s , f r a c t u r e p r o c e e d s b y s h e a r l o c a l i z a t i o n a t

t o th e t e n s i l e a x i s , c o n n e c t in g th e tw o h y d ro g e n in d u c e d

s u r f a c e f la w s one on e a c h s u r f a c e .

Yokogawa e t a l ( 4 7 ,^ 8 ) d id rou n d b a r t e n s i l e t e s t s on i r o n

and ca rb o n s t e e l s w ith ca rb o n c o n t e n t r a n g in g from 0 . 1 8 to

0 .5 7 7 5 1 i n a r g o n and h y d ro g en g a s w ith p r e s s u r e up to 39*3 MP.

T hey fo u n d t h a t y i e l d p o i n t s and u l t im a t e t e n s i l e s t r e n g t h s

w ere a lm o s t in d e p e n d e n t o f th e p r e s s u r e o f e i t h e r h y d ro g e n

o r a r g o n . The e lo n g a t io n and r e d u c t io n o f a r e a i n h y d ro g e n

d e c r e a s e d w ith an in c r e a s e o f th e p r e s s u r e , i n c o n t r a s t w ith

t h o s e i n a r g o n , w h ich w ere in d e p e n d e n t o f p r e s s u r e . The e lo n

g a t i o n and th e r e d u c t io n o f a r e a o f ca rb o n s t e e l s a t t a i n e d

c o n s t a n t v a lu e s a t p r e s s u r e s above a c r i t i c a l v a lu e w h ich

d ep en d ed o n th e ca rb o n c o n t e n t . The c r i t i c a l h y d ro g e n p r e s

su r e and th e c r i t i c a l e lo n g a t io n in c r e a s e d w ith d e c r e a s in g

13

c a r b o n c o n t e n t . T hey a l s o fo u n d , i n a r g o n , t h a t th e f r a c t u r e

s u r f a c e c h a r a c t e r i s t i c s w ere in d e p e n d e n t o f a r g o n p r e s s u r e .

O n ly d im p le d r u p tu r e f r a c t u r e s u r f a c e s w ere o b s e r v e d f o r SUY,

S15C and S 3 5 C , and a m ix ed d im p le and c le a v a g e f r a c t u r e w as

o b s e r v e d f o r S55C . I n h y d r o g e n , th e p r e d o m in a n t o b s e r v a t i o n

w as q u a s i - c l e a v a g e f r a c t u r e p r o p a g a t in g c o n c e n t r i c a l l y from

t h e s u r f a c e t o th e c e n t e r . D im p led r u p tu r e o c c u r r e d i n th e

lo w p r e s s u r e r e g i o n .

I n c o n t r a s t t o Yokogawa e t a l and S t o l t z ' s r e s u l t s , L u n a rsk a

( 1 1 9 ) d y n a m ic a l ly c h a r g e d a n e c k e d t e n s i l e b a r a t a f i x e d

s t r a i n u n t i l i t f a i l e d and fo u n d t h a t th e m id d le r e g i o n o f

th e f r a c t u r e s u r f a c e w as o f a b r i t t l e n a t u r e , w h e r e a s th e

p e r ip h e r a l r e g i o n had a f r a c t u r e o f p l a s t i c c h a r a c t e r .

I n th e t h r e e - p o i n t b en d s t u d i e s o f L ee e t a l ( 2 8 , ^9 ) *

dyn am ic c h a r g in g o f h y d r o g e n w h ile s t r a i n i n g d r a s t i c a l l y

r e d u c e d th e d u c t i l i t y , c h a n g ed th e c r a c k s from a mode I I

s h e a r f r a c t u r e t o a mode I f r a c t u r e , and c h a n g e d th e f r a c

t u r e a p p e a r a n c e from d im p le - r u p tu r e to q u a s i - c l e a v a g e . P r e

c h a r g e d h y d ro g e n r e d u c e d th e d u c t i l i t y b y p r o m o tin g p l a s t i c

i n s t a b i l i t y l e a d in g to a mode I I f r a c t u r e f o r s p h e r o id iz e d

1095 s t e e l . F or p r e c h a r g e d q u en ch ed and tem p ered s t e e l ,

c r a c k s n u c le a t e d i n t e r n a l l y i n a mode I m anner and th e c r a c k

p r o g r e s s e d t o th e s u r f a c e i n mode I I . T h is a u th o r o b s e r v e d

a s i m i l a r r e s u l t ( 5 0 ) on a HSLA NiCop s t e e l and C o s ta and

Thom pson ( 5 1 152) a l s o fo u n d th e . r e s u l t on q u en ch ed and tem

p e r e d and n o m in a l ly p e a r l i t i c 10^5 s t e e l .

14

The i n t e r p r e t a t i o n o f t h e s e r e s u l t s , c o n s i s t e n t w ith th e

e a r l y p r e m ise o f T ro ia n o ( 3 5 ) » i s t h a t th e p r e c h a r g e d h y d ro

g e n a c cu m u la ted t o t h e r e g i o n o f maximum c o n c e n tr a te d i n t e r

n a l s t r e s s a t th e p l a s t i c - e l a s t i c b o u n d a ry , lo w e r e d th e c o h e

s i v e s t r e n g t h and i n i t i a t e d a mode I c r a c k t h e r e . As th e

c r a c k p r o g r e s s e d to th e s u r f a c e , th e p l a s t i c s t r a i n s t a t e

i n f r o n t o f i t a p p ro a ch ed t h a t o f p la n e s t r e s s - p l a n e s t r a i n ,

w h ich w ou ld fa v o r p l a s t i c i n s t a b i l i t y , so t h a t th e f i n a l

s h e a r in g to o k p la c e a lo n g th e s h e a r p la n e w h ich w as i n c l i n e d

a t 4 5 ° t o th e n o tc h s u r f a c e .

2 -3 M od els o f H ydrogen D e g r a d a t io n

A few m echan ism s have b e e n p r o p o se d t o e x p l a in th e h y d ro g e n

e f f e c t s . E ach p r o p o s a l w as b a s e d on some o b s e r v a t i o n s . E ach

i s c o n s i s t e n t w ith some p o r t i o n o f th e r e l e v a n t d a ta . H ow ever,

t h e r e a r e so many v a r i e t i e s o f phenom ena a s s o c i a t e d w ith th e

d e g r a d a t io n o f m a t e r ia l p r o p e r t i e s by h y d ro g e n ( 1 , 1 4 , 1 7 ,

5 3 i 5 4 , 5 5 ) i t h a t none o f th e c u r r e n t ly p r o p o se d m ec h a n ism s,

ta k e n s i n g l y , a p p ea r to be c o m p a t ib le w ith a l l o f th e o b s e r

v a t i o n s . L ou th an and M c n it t ( 5 8 ) p o in t e d o u t " a l l t o o o f t e n

e x p e r im e n ts a r e d e s ig n e d t o i n f e r w e a k n e s s e s i n one o r more

t h e o r i e s , w ith o u t p r o v id in g s o l i d su p p o r t f o r any p r o p o s a l ."

L a ta n is o n e t a l ( 5 6 ) s a id " C o n s id e r in g th e vo lum e o f l i t e r a

tu r e w h ich h a s a p p ea red on t h e s e s u b j e c t s (HE and SCO) , i t

i s no s u p r is e t h a t e v id e n c e ca n be fou n d t o c o n t r a d i c t v i r

t u a l l y e v e r y p o in t o f v ie w ." C o g en t argu m en ts a g a in s t a

s p e c i f i c m echan ism a p p ly in g g e n e r a l l y to a l l d e g r a d a t io n

phenom ena h ave b e e n p r e s e n t e d ( 5 7 * 5 8 ) . T h e r e f o r e , H ir th (1 )

c o n c lu d e d t h a t s im p le m ech an ism s Eire n o t s u f f i c i e n t to r a

t i o n a l i z e th e g e n e r a l d e g r a d a t io n i n p r o p e r t i e s p r o d u c e d b y

h y d ro g e n b u t t h a t th e t o t a l schem e w as a com p lex p r o c e s s

com posed o f a number o f c o u p le d m ech a n ism s.

C o r n e t and l 'a lb o t -B e s n a r d ( 5 9 ) c o n s id e r e d th e m echan ism s

o f h y d ro g e n e m b r it t le m e n t o f i r o n and i t s a l l o y s and su g

g e s t e d t h a t two m ain a v e n u e s t o u n d e r s ta n d in g a re a p p a r e n t:

th e d e c o h e s io n t h e o r y o f T r o ia n o ( 3 5 ) and of O r ia n i ( 3 6 , 7 1 )

and th e i n f l u e n c e o f H on p l a s t i c d e fo r m a t io n . I n a r e c e n t

p a p e r , McMahon ( 6 0 ) t r i e d t o c o n s t r u c t a u n i f i e d v ie w o f

th e e f f e c t s o f h y d ro g e n on th e m e c h a n ic a l b e h a v io r o f i r o n

and s t e e l . He p o in t e d o u t t h a t th e m ost im p o r ta n t a s p e c t s

o f h y d ro g e n b e h a v io r a r e t h a t i t a l t e r s th e b in d in g e n e r g y

o f t h e i r o n l a t t i c e and t h a t i t i s s t r o n g l y tr a p p e d b y d i s

l o c a t i o n c o r e s and i n t e r c r y s t a l l i n e o r p a r t i c l e - m a t r i x i n

t e r f a c e s . E x p e r im e n ts i n d i c a t e t h a t t h e s e e f f e c t s l e a d to

a d e c r e a s e i n l a t t i c e c o h e s io n and an in c r e a s e i n th e m ob i

l i t y o f sc r e w d i s l o c a t i o n s , b u t i n h i b i t c r o s s - s l i p , th e r e b y

p r o m o tin g c o n c e n tr a t e d s h e a r b an d s a n d , u l t i m a t e l y , g l i d e -

p la n e d e c o h s io n i n th e s h e a r b a n d s . The p r e s e n c e o f

im p u r ity -w e a k e n e d g r a in b o u n d a r ie s a l lo w s i n t e r f a c i a l d e c o

h e s i o n and b r i t t l e b e h a v io r a t r e l a t i v e l y lo w s t r e s s l e v e l s .

The p r e s e n c e o f d i s t r i b u t e d p a r t i c l e s p e r m its t r a p p in g o f

h y d ro g e n i n th e p a r t i c l e - m a t r i x i n t e r f a c e s , and u l t i m a t e l y

16 .

p l a s t i c r u p tu r e a t r e d u c e d s t r a i n s .

The a u th o r a g r e e s w ith th e p o in t o f v ie w o f H ir th (1 ) and

McMahon ( 6 0 ) . N e v e r t h e l e s s , i t i s h e l p f u l h e r e to p r e s e n t a

b r i e f r e v ie w o f soihe o f the- m ost w id e ly su p p o r te d m ech an ism s

f o r h y d ro g en d e g r a d a t io n .

( a ) . I n t e r n a l P r e s s u r e T h eory

S u p e r s a tu r a te d h y d ro g e n atom s p r e c i p i t a t e a s m o le c u la r

h y d ro g e n g a s a t i n t e r n a l d e f e c t s . The p r e s s u r e d e v e lo p e d

b y t h i s p r e c i p i t a t i o n i s added t o t h e a p p l ie d s t r e s s and

t h u s lo w e r s th e a p p a r e n t f r a c t u r e s t r e s s . T h is m echanism

was i n i t i a l l y p r o p o s e d b y Z a p ffe ( 6 1 ) and h a s b e e n m o d if ie d

b y s e v e r a l su b s e q u e n t i n v e s t i g a t o r s ( 6 2 - 6 5 )*

In o r d e r to e x p l a in th e h y d ro g en e m b r it t le m e n t o f s t e e l s

i n th e form o f en h a n ced c r a c k gro w th r a t e s a t su b -a tm o s p h e r ic

h y d r o g e n p r e s s u r e s a s r e p o r t e d b y H ancock and J o h n so n ( 6 6 ) ,

th e argum ent h a s b e e n made ( 6 7 - 6 9 ) t h a t d i s l o c a t i o n t r a n s p o r t

c o u ld h ave g e n e r a te d la r g e i n t e r n a l s u p e r s a t u r a t io n and h en ce

l a r g e i n t e r n a l p r e s s u r e s e v e n when th e e x t e r n a l p r e s s u r e was

l o w . W hile a t lo w t e m p e r tu r e s , d i s lo c a t io n - e n h a n c e d , la r g e

s u p e r s a t u r a t io n s i n v o i d s a re p o s s i b l e , a t room te m p e r a tu r e

th e a t t a i n a b l e s u p e r s a t u r a t io n s a r e o n ly a f a c t o r o f 2 o r 3

( 7 0 ) * T h e r e f o r e , th e lo w -p r e s s u r e c r a c k p r o p a g a t io n c o n c lu

s i v e l y e x c lu d e s th e d i s lo c a t io n - e n h a n c e d p r e s s u r e m odel a s

a g e n e r a l one (3 6 , 5 7 *5 8 ) .

17

( b ) . D e c o h e s io n T h eory

D i s s o lv e d h y d ro g en r e d u c e t h e c o h e s iv e s t r e n g t h o f th e

l a t t i c e . T h is m odel w as f i r s t p r o p o s e d b y T r o ia n o ( 3 5 )* a s

a d v a n ced b y O r ia n i i n h i s m o st r e c e n t work ( 3 6 , 7 1 ) » a s i n

h e r e n t i n th e d u c t i l e f r a c t u r e m o d e ls ( 3 0 - 3*0 » and a s m o d e led

f o r m e t a l l o id e f f e c t s (75*76) , and e m e rg e s a s a v a l i d q u a l i

t a t i v e m o d e l. As p o in t e d o u t b y H ir t h ( 1 ) , t h i s i s n o t a

v e r y e x c l u s i v e i d e a , an o v e r - a l l c r a c k in g p r o c e s s i s p o s s i b l e

i n w h ich a l l o f th e o t h e r m ech an ism s c o n t r i b u t e . McMahon ( 6 0 )

h a v e in t e r p p r e t e d t h i s p o in t o f v ie w i n d e t a i l .

( c ) . S l i p S o f t e n in g M od els

Beachem ( 1 0 , 2 1 ) p r o p o s e d t h a t a b so r b e d h y d ro g e n au gm en ts

d i s l o c a t i o n m o t io n . Lynch (5**) s u g g e s t e d t h a t c h e m is o r p t io n

o f h y d r o g e n atom s f a c i l i t a t e s n u c l e a t i o n o f d i s l o c a t i o n s a t

c r a c k t i p s . I f a p p r e c ia b le r e d u c t io n s i n f lo w s t r e s s due t o

d i s s o l v e d h y d ro g en w ere l o c a l i z e d t o r e g i o n s j u s t ah ead o f

c r a c k t i p s , e m b r it t le m e n t due t o s t r a i n l o c a l i z a t i o n w ou ld be

e x p e c t e d . As d i s c u s s e d b e f o r e , en h a n ced d i s l o c a t i o n m o tio n

i s now d e f i n i t e l y e s t a b l i s h e d , w ith s o f t e n i n g b e h a v io r i n

c lu d in g en h a n ced sc re w d i s l o c a t i o n m o b i l i t y , p o s s i b l y en h a n ced

d i s l o c a t i o n i n j e c t i o n a t s u r f a c e s , and t h e p r o m o tio n o f s h e a r

i n s t a b i l i t i e s . H ow ever , h a r d e n in g e f f e c t s a r e a l s o o b s e r v e d ,

so th e id e a o f s o f t e n i n g i s n o t a g e n e r a l o n e . H ir th (1 )

p o i n t s o u t t h a t en h a n ced d i s l o c a t i o n m o tio n ( o r p l a n a r i t y ) i s

n o t a m odel i n i t s e l f , b u t a c o n t r i b u t i n g f a c t o r t o an o v e r a l l

18

d e g r a d a t io n m ech an ism .

( d ) . S u r fa c e E n er g y M odel

P e tc h and S t a b l e s ( 7 7 » 7 8 ) s u g g e s t e d t h a t h y d ro g en a d s o r p

t i o n lo w e r s th e s u r f a c e e n e r g y o f th e s u r f a c e s c r e a t e d in

c r a c k p r o p a g a t io n , lo w e r s th e work o f f r a c t u r e , and h en ce

p r o m o tes c r a c k in g . T h is t h e o r y h a s b e e n m o d if ie d b y o t h e r

i n v e r t i g a t o r s - ( 79» 8 0 ) . A rgum ents a g a i n s t t h i s th e o r y a r eI

t h a t i t g r e a t l y u n d e r e s t im a t e s th e work o f f r a c t u r e , t h a t i t

c a n n o t a c c o u n t f o r d i s c o n t in u o u s c r a c k in g a s i n d ic a t e d by

s o n ic e m i s s i o n s , t h a t i t c a n n o t e x p l a in why t h e te n d e n c y f o r

d e la y e d f a i l u r e ca n be r e v e r s e d on s t r e s s r e m o v a l, and why

o x y g e n , w h ich h a s a g r e a t e r h e a t o f a d s o r p t io n , n o t o n ly f a i l s

t o p rom ote c r a c k in g b u t s t o p s th e h y d ro g e n e f f e c t . M o reo v er ,

O r ia n i ( 3 8 ) p o i n t s o u t t h a t "The id e a t h a t a d s o r p t io n lo w e r s

th e e n e r g y r e q u ir e d t o c r e a t e new s u r f a c e i s n o t n e c e s s a r i l y

l in k e d t o any one m echanism f o r th e g e n e r a t io n o f new c r a c k

s u r f a c e , a s i s t r u e f o r any th erm od yn am ic r e a s o n in g . I t

w ou ld be a s a p p l i c a b l e t o a m echan ism i n v o l v i n g th e b r e a k in g

o f in t e r a t o m ic b on d s a s t o one i n w h ich th e new s u r f a c e i s

p r o d u ce d b y p l a s t i c t e a r i n g ."

( e ) . H y d r id e F o rm a tio n

W e stla k e ( 8 1 ) p r o p o s e d t h a t c r a c k in g c o u ld p r o c e e d b y t h e

fo r m a t io n and c r a c k in g o f a h y d r id e n e a r th e c r a c k t i p . T h is

m od el a p p l ie d t o h y d r id e fo rm in g s y s te m s ( 8 2 ) . But i t met

19

d i f f i c u l t i e s i n e x p l a in i n g th e h y d r o g e n e m b r it t le m e n t o f n on

h y d r id e sy s te m su ch a s i r o n and s t e e l . H ow ever a s d i s c u s s e d

b y H ir th ( 1 ) , t h i s m odel c o u ld a p p ly on an a to m ic s c a l e a t

t h e c r a c k t i p .

2 - 4 P l a s t i c I n s t a b i l i t y

I t i s w e l l e s t a b l i s h e d t h a t m e t a ls a lw a y s u n d erg o some

p l a s t i c d e fo r m a t io n p r i o r t o f r a c t u r e . So a r e q u ir e m e n t f o r

f r a c t u r e o f m e ta l i s th e l o c a l i z a t i o n o f p i a s t i c f lo w ( p l a s t i c

i n s t a b i l i t y ) . The c o n d i t i o n f o r i n i t i a t i n g u n s t a b le f lo w

a f t e r any am ount o f p r i o r s t r a i n i n g i s t h a t som ew here i n th e

m a t e r ia l t h e n e x t in c r e m e n t o f s t r a i n - i n d u c e d h a r d e n in g be

c a n c e le d b y an accom p an y in g s t r a in - in d u c e d s o f t e n i n g . T h en ,

f u r t h e r s t r a i n i n g w ou ld te n d t o c o n c e n tr a t e i n th e p la c e

w here th e r e s i s t a n c e t o c o n t in u e d f lo w i s f i r s t l o s t ( 8 3 )*

The s c a l e o f t h e i n s t a b i l i t y ca n v a r y from t h a t o f n e c k in g

i n a t e n s i o n b a r th r o u g h some in t e r m e d ia t e l e v e l i n v a r io u s

k in d s o f s h e a r -b a n d d e v e lo p m e n t , t o s l i p band fo r m a t io n i n a

g r a i n .

The m ost f a m i l i a r mode o f p l a s t i c i n s t a b i l i t y t o th e m e t a l- '

l u r g i s t i s th e n e c k in g phenom enon i n a t e n s i l e t e s t . D u r in g

t e n s i l e d e fo r m a t io n , n e c k in g b e g in s a t maximum lo a d w here th e

in c r e a s e i n s t r e s s due t o d e c r e a s e i n th e c r o s s - s e c t i o n a r e a

o f t h e sp e c im e n b ecom es g r e a t e r th a n t h e in c r e a s e i n th e lo a d

c a r r y in g a b i l i t y o f th e m e ta l due t o s t r a i n h a r d in in g , t h a t

i s dP =ddA - Add < 0 o r d d /d € <<3 , w here P i s l o a d , A i s

20

c r o s s e c t i o n a l a r e a , 6 i s t r u e s t r e s s and £ i s t r u e s t r a i n .

I n t h i s c a s e t h e i n s t a b i l i t y i s p u r i l y g e o m e t r i c a l .

A more g e n e r a l c o n c e p t o f i n s t a b i l i t y i s th e fo r m a t io n o f

n a rro w s h e a r b an d s i n a s e c t i o n on p la n e s n orm al t o th e d i r

e c t i o n o f z e r o n orm al d is p la c e m e n t v e l o c i t y . I f f lo w i n

t h e s e narrow b a n d s i s n o t b lo c k e d b y th e m ic r o s t r u c t u r e and

t r a n s m i t t e d t o th e r e m a in d e r o f th e s e c t i o n , a d u c t i l e ty p e

f a i l u r e i n t h e band w i l l r e s u l t a t lo w m a g n itu d e s o f n o m in a l

s t r a i n . A c l a s s i c a l ex a m p le o f su ch p l a s t i c i z e d z o n e s i s th e

fo r m a t io n o f L u d ers b an d s i n t e n s i l e t e s t i n g o f lo w ca rb o n

s t e e l .

A n o th e r good exam p le i s th e a d ia b a t ic s h e a r b a n d s o b s e r v e d

i n h ig h s t r a i n r a t e d e fo r m a t io n . I t i s known t h a t d u r in g

p l a s t i c d e fo r m a t io n , m ore th a n 90 p e r c e n t o f t h e work done

i s c o n v e r te d i n t o h e a t a n d , i n m ost c a s e s , th e f lo w s t r e s s

o f a m a t e r ia l d e c r e a s e s w ith in c r e a s i n g te m p e r a tu r e . I f

i n s u f f i c i e n t t im e i s a v a i l a b l e f o r th e h e a t g e n e r a te d t o be

d i s s i p a t e d , th e d eform ed r e g i o n t e n d s t o becom e h o t t e r and

s o f t e r and t h e d e fo r m a t io n t e n d s t o be l o c a l i z e d . T h is ty p e

o f p l a s t i c i n s t a b i l i t y h a s b e e n o b s e r v e d i n a v a r i e t y o f

m e t a ls in c lu d in g s t e e l s , c o p p e r a l l o y s , alum inum a l l o y s and

t i t a n iu m ( 8^ , 8 5 , 8 6 ) . I t h a p p en s i n d i f f e r e n t d e fo r m a t io n

p r o c e s s e s su ch a s s h e a r in g ( 8 ? )» p u n c h in g ( 8 8 ) , h ig h - s p e e d

im p a c t o f s t e e l b a l l s a g a in s t s t e e l t a r g e t p l a t e s ( 8 9 ) and

on r u b b in g and w ear s u r f a c e s ( 9 0 )*

A d ia b a t ic s h e a r o r s l i p i s fu n d a m e n ta lly a h ig h s t r a i n ,

21

h ig h s t r a i n - r a t e p r o c e s s b u t i t c a n o c c u r when th e o v e r a l l

s t r a i n r a t e i s r e l a t i v e l y s m a l l . T h is i s so b e c a u s e o f l o c a l

i z a t i o n o f t h e s t r a i n i n t o a v e r y s m a l l vo lu m e o f a d e fo r m in g

s p e c im e n . The te m p e r a tu r e r i s e i n s h e a r b a n d s may in d u c e

p h a se t r a n s f o r m a t io n s su c h a s t h e r e v e r s a l o f m a r t e n s i t e ( 9 1 )»

r e d i s s o l u t i o n o f h a r d e n in g p r e c e p i t a t e s ( 9 2 ) o r e v e n m e l t in g

( 8 5 ) . Good r e v ie w s on t h i s t o p i c w ere p r e s e n t e d b y B e d fo r d

e t a l ( 9 3 ) and B o g e r s ( 9*0*

The mode o f p l a s t i c i n s t a b i l i t y o f i n t e r e s t i n t h i s w ork i s

a s s o c i a t e d w it h t h o s e d i s c u s s e d b y S p r e tn a k and h i s c o w o r k e r s

( 9 5 - 1 0 3 ) , H u tc h in s o n and T v e rg a a d ( 1 0 ^ , 1 0 5 ) » R i c e and h i s c o

w o r k e r s ( 1 0 6 , 1 0 7 , 1 0 8 ) , Yamamoto ( 1 0 9 ) and M c C lin to c k e t a l

( 110, 111) .

M c C lin to c k e t a l ( 1 1 0 , 1 1 1 ) c o n s id e r e d th e g e o m e t r i c a l s o f t

e n in g c o n t r i b u t i o n a r i s i n g from th e n u c l e a t i o n and g r o w th o f

m ic r o v o id s i n a m a t e r i a l and p r e s e n t e d a s im p le e q u a t io n a s

a n a p p r o x im a t io n o f a f r a c t u r e c r i t e r i o n f o r p r o b le m s o f

p r a c t i c a l i n t e r e s t . R u d n ic k i and R ic e ( 1 0 6 ) d e r iv e d g e n e r a l

c o n d i t i o n s f o r a b i f u r c a t i o n o f p l a s t i c f l o w , c o r r e s p o n d in g

t o t h e l o c a l i z a t i o n o f d e f o r m a t io n i n t o a p la n e r b a n d , w it h

n o p r o v i s i o n f o r i n i t i a l im p e r f e c t i o n s i n th e m a t e r i a l .

Yamamoto ( 1 0 9 ) d e r iv e d c o n d i t i o n s f o r a l o c a l i z a t i o n b i f u r c a

t i o n w ith an i n i t i a l i m p e r f e c t i o n . He fo u n d t h a t th e e x i s

t e n c e o f an i n i t i a l i m p e r f e c t io n g r e a t l y r e d u c e s th e s t r a i n

t o l o c a l i z a t i o n i n b o th a x ia l l y - s y m m e t r i c and p l a n e - s t r a i n

c o n d i t i o n s . He a l s o fo u n d t h a t w ith i n i t i a l i m p e r f e c t i o n s ,

22

t h e c r i t i c a l s t r a i n i s s m a l le r i n th e c a s e o f p l a n e - s t r a i n

e x t e n s i o n th a n i n th e c a s e o f a x ia l ly - s y m m e t r ic e x t e n s i o n .

H u tc h in s o n and T v e rg a a r d ( 1 0 ^ , 1 0 5 ) * s t u d ie d th e g row th

o f a s i n g l e s h e a r hand i n an i n f i n i t e b lo c k o f an in c o m p r e s

s i b l e s o l i d u n d e r g o in g p la n e s t r a i n t e n s i o n o r c o m p r e s s io n .

The s tu d y in c lu d e s a n a ly s e s o f b i f u r c a t i o n and p o s t - b i f u r c a t i o n

b e h a v io u r a s w e l l a s th e s e n s i t i v i t y o f s h e a r band gro w th to

m a t e r ia l i m p e r f e c t io n s . T hey fo u n d t h a t d e p e n d in g on d e t a i l s

o f th e m a t e r ia l m o d e l, s h e a r b an d s may form g r a d u a l ly w ith

r e l a t i v e l y l i m i t e d l o c a l i z a t i o n o f s t r a i n o r t h e y may l o c a l i z e

c a t a s t r o p h i c a l l y w ith an a t t e n d a n t s t r o n g s e n s i t i v i t y t o

i n i t i a l im p e r f e c t io n s i n th e form o f m a t e r ia l in h o m o g e n e ity .

v a n I t e r s o n ( 1 1 2 ) i n d ic a t e d t h a t u n d er a s p e c i a l s t a t e o f

s t r e s s , m a t e r i a l s ca n be c o n v e r te d in t o th e p l a s t i c i z e d s t a t e ,

o r a s t a t e o f m a t te r b e tw e e n th e s o l i d and l i q u i d s t a t e . The

r e s i s t a n c e t o f lo w i s l o s t . A l l th e s t r a i n i s c o n c e n tr a t e d

i n th e s m a ll r e g io n o f m a t e r ia l s i n th e p l a s t i c i z e d s t a t e .

I n a two d im e n s io n a l s e c t i o n norm al to th e s h e a r p la n e , th e

d i r e c t i o n o f th e p l a s t i c i z e d zone i s a lo n g c h a r a c t e r i s t i c

t r a c e s w h ich a r e d i r e c t i o n s o f p u re sh e a r and z e r o e x t e n s i o n a l

s t r a i n . The th e o r y o f c h a r a c t e r i s t i c s i s b a s e d on th e i d e a l

p l a s t i c s t a t e , th e d i r e c t i o n s o f th e c h a r a c t e r i s t i c s , t h u s ,

d ep en d o n ly on th e im p o sed s t r e s s s t a t e . The m ost f a m i l i a r

c h a r a c t e r i s t i c d i r e c t i o n i s t h a t o f L u d ers b a n d s i n p la n e

s t r e s s and s im p le t e n s i o n , n am ely 3 5 - 2 6 ° from th e h o r i z o n t a l .

The c h a r a c t e r i s t i c d i r e c t i o n s a r e fou n d b y s e a r c h in g o u t th e

23

d i r e c t i o n s o f z e r o e x t e n s i o n a l s t r e s s f o r th e d e v i a t o r i c p a r t

o f t h e s t r e s s s t a t e o r th e d i r e c t i o n s o f z e r o n orm al s t r a i n .

O n ly i n th e s t a t e o f s t r e s s o f p la n e s t r a i n a r e th e a l s o

d i r e c t i o n s o f maximum s h e a r s t r e s s .

The s u r f a c e o f c h a r a c t e r i s t i c s i s a s u r f a c e o f d i s c o n t i

n u i t y in t a n g e n t i a l v e l o c i t y ( 1 1 3 ) . a s shown i n f i g u r e 1 .

S in c e n orm al s t r a i n i s n o t a l lo w e d f o r c h a r a c t e r i s t i c t r a c e s ,

d is p la c e m e n t o r v e l o c i t y com p on en ts n orm al t o a s l i p l i n e ( a

s u r f a c e i n t h r e e d im e n s io n s ) m ust be th e same on b o th s i d e s

o f t h e l i n e . A d i s c o n t i n u i t y i n d is p la c e m e n t o r v e l o c i t y

t a n g e n t i a l t o a s l i p l i n e i s a l l o w e d , ' h o w e v e r , so lo n g a s th e

S l i p L in e

Yzn - v m * V2t - Vlt = C o n sta n t( a lo n g th e s l i p l in e ) '

F ig u r e 1 . S c h e m a tic r e p r e s e n t a t io n o f a d i s c o n t i n u i t y i nt a n g e n t i a l v e l o c i t y a lo n g a c h a r a c t e r i s t i c s u r f a c e

24

d i f f e r e n t com p on en ts on e i t h e r s id e o f th e l i n e a r e c o n s t a n t

a lo n g i t s l e n g t h . No f r a c t u r e o c c u r s i f t h i s v e l o c i t y d i s

c o n t i n u i t y i s damped o u t w it h t im e . I f th e d i s c o n t i n u i t y

i n c r e a s e s c o n t in u o u s ly i n m a g n itu d e , f r a c t u r e e n s u e s ( 1 1 4 ) .

C h a k r a b a r t i and S p r e tn a k ( 9 5 ) p o in t e d o u t t h a t a r e a l m at

e r i a l ca n e x h i b i t l o c a l i z a t i o n o f p l a s t i c f lo w i n d i r e c t i o n s

o f p u re s h e a r ; a s so o n a s i t s im u la t e s a dynam ic i d e a l p l a s t i c

s t a t e , and t h a t t h o s e m a t e r i a l s w h ich h ave lo w s t r a i n h a r d e n -I

in g r a t e and lo w s t r a i n r a t e s e n s i t i v i t y may e x h i b i t p l a s t i c

i n s t a b i l i t y i n term o f l o c a l i z a t i o n a lo n g c h a r a c t e r i s t i c s

d u r in g r e l a t i v e l y s lo w d e fo r m a t io n .

S p r e tn a k and h i s c o -w o r k e r s fo u n d t h a t th e t a n g e n t i a l v e l

o c i t y d i s c o n t i n u i t i e s in d e e d c o n s t i t u t e a b a s i c f r a c t u r e i n

i t i a t i o n and p r o p a g a t io n m ode. S p r e tn a k c o n c lu d e d ( 9 7 ) t h a t

th e n e c e s s a r y c o n d i t i o n s f o r su ch f a i l u r e a r e t t h e p r e s e n c e

o f a f r e e s u r f a c e , a s t r e s s / s t r a i n g r a d ie n t and th e m a t e r ia l

m ust s im u la te a p e r f e c t l y p l a s t i c bod y ( lo w work h a r d e n in g

r a t e and lo w s t r a i n r a t e s e n s i v i t y ) .

I n t h e s e c o n d i t i o n s , th e e f f e c t o f a s t r e s s / s t r a i n g r a d ie n t

on p l a s t i c i n s t a b i l i t y r a i s e d c o n s id e r a b le c o n c e r n . I t h a s

b e e n v e r i f i e d i n some c a s e s ( 1 0 0 ) . H ow ever, th e r e a r e a l s o

e v id e n c e s a g a in s t i t . L e e , G o ld er ib erg and H ir th ( 2 8 , 4 9 , 7 3 )

p e r fo r m e d s lo w b en d t e s t s on s p h e r o id iz e d 1095 s t e e l , q u en ch ed

and tem p ered 4^40 s t e e l and DS n i c k e l w ith d i f f e r e n t n o tc h

r o o t r a d i i on t h e sp e c im e n . T hey fo u n d t h a t i n a l l c a s e s ,

th e i n s t a b i l i t y i n i t i a t e d b u t w as in d e p e n d e n t o f th e s t r e s s /

25

strain gradient. Mohamed and Tetelman (115) studied the notch deformation of low carbon steel specimens and also found that the strain at crack initiation was independent of notch root radii.

2-5 Effect of Plastic Strain State on the Hydrogen Induced Cracking

As m en tio n ed i n s e c t i o n 2 - 2 , S t o l t z (4 6 ) fou n d t h a t p la n e

s t r a i n s t a t e c a u se th e h ig h e s t l o s s i n d u c t i l i t y i n a h yd rogen

e n v ir o m e n t . He r a t i o n a l i z e d t h i s phenom enon i n two w ays.

F i r s t , s m a lle r sa m p les sh o u ld be more s e n s i t i v e t o e n v ir o m e n t-

a l l y in d u c e d d u c t i l i t y l o s s e s due to th e h ig h e r f r a c t i o n o f

th e m a t e r ia l a f f e c t e d b y th e e n v ir o m e n t . S im i la r s i z e e f f e c t

h ave b e e n o b se r v e d b y S m ia lo w sk i ( 1 1 6 ) and Asano and O tsuka

( 1 1 7 ) . S e c o n d , b a se d on th e c o n c lu s io n s made by H u tc h in so n

and N e a le ( 1 1 8 ) , p la n e s t r a i n i s a more s u r fa c e im p e r f e c t io n

s e n s i t i v e s t a t e th a n a x isy m m e tr ic t e n s i o n . He argu ed t h a t

f o r th e c a s e o f h y d ro g en in d u c e d s u r fa c e f la w s i n A 516 s t e e l ,

a l l o f th e same d e p th , a lo w e r f r a c t u r e d u c t i l i t y i n th e p la n e

s t r a i n s t a t e w ould be p r e d i c t e d . T h ese two argu m en ts are

c e r t a i n l y r e a s o n a b le . H ow ever, i t s t i l l rem ain ed an open

q u e s t io n w h eth er h yd rogen w ould o r w ou ld n o t prom ote th e i n

i t i a t i o n o f s u r fa c e f la w s more e a s i l y i n th e p la n e s t r a i n

s t a t e th a n i n th e a x isy m m e tr ic s t r a i n s t a t e .

I n lo a d r e l a x a t i o n e x p e r im e n ts , L u narska ( 1 1 9 » 1 2 0 ) fou n d

t h a t f o r u n i fo r m a l ly s t r a in e d s p e c im e n s , th e h yd rogen c h a r g in g

26

c a u se d an a c c e le r a t e d drop i n l o a d . I n c r e a s e d c a th o d ic c u r

r e n t d e n s i t y r e s u l t e d i n a f a s t e r drop i n l o a d . Random ly

d i s t r i b u t e d i n t e r n a l m ic r o c r a c k s w ere o b s e r v e d b u t th e u n i

fo r m ly s t r a i n e d sa m p le s f a i l e d t o b r e a k upon h y d ro g e n c h a r g

i n g , e v e n f o r 600 m in u te s o f c h a r g in g . I n c o n t r a s t , f o r

n e c k e d s a m p le s , a d e c r e a s e i n lo a d r e l a x a t i o n p r o p o t io n a l t o

t h e lo g a r ith m o f tim e was o b s e r v e d u n t i l th e f i n a l d rop i n

lo a d o c c u r r e d a s s o c i a t e d w ith c r a c k fo r m a t io n and gro w th

l e a d in g t o f r a c t u r e .

Nagakawa e t a l ( 1 2 1 , 122) fo u n d t h a t h y d ro g en c h a r g in g

a lw a y s p r o d u ce d an in c r e a s e i n c r e e p r a t e . The m a g n itu d e o f

t h e in c r e a s e go up w ith c h a r g in g c u r r e n t , and becom e z e r o when

c h a r g in g w as s to p p e d .

M a tsu i e t a l (1 2 3 -1 2 5 ) p e r fo r m e d t e n s i l e t e s t s on h ig h pu

r i t y i r o n d u r in g h y d ro g en c h a r g in g . T hey fo u n d t h a t th e f lo w

s t r e s s w as in c r e a s e d b y h y d ro g e n c h a r g in g b e lo w 1.90 ° K and was

d e c r e a s e d r e v e r s i b l y above 190°K . L e s s p u re sp e c im e n s w ere

h a r d e n e d r a t h e r th a n s o f t e n e d b y h y d ro g e n c h a r g in g a t room

te m p e r a tu r e . They a l s o fo u n d t h a t f o r h ig h p u r i t y i r o n s p e

c im e n s d eform ed a t 200°K t o a s t r a i n 6 = 3 t he d i s l o c a t i o n

d e n s i t y was c o n s id e r a b ly in c r e a s e d i n th e h y d r o g e n -c h a r g e d

sp e c im e n . The a v e r a g e d i s l o c a t i o n d e n s i t y i s l a r g e r b y a

f a c t o r o f 5 - 1° i n th e c h a r g e d sp e c im e n s th a n i n th e u n ch a rg ed

s p e c im e n s . The sp e c im e n s d eform ed a t room te m p e r a tu r e w ith

and w ith o u t c h a r g in g show no a p p r e c ia b le d i f f e r e n c e i n d i s

l o c a t i o n d e n s i t y . G ourm elon ( 1 2 6 ) fo u n d f o r z o n e - r e f in e d

i r o n t h a t h y d ro g e n p r o d u ce d s o f t e n i n g f o r s t r a i n s l e s s th a n

2 p e t b u t h a r d e n in g f o r l a r g e r s t r a i n s . C orn et e t a l ( 1 2 7 )

show ed b y m eans o f t r a n s m is s io n e l e c t r o n m ic r o sc o p y t h a t t a n

g l i n g o f d i s l o c a t i o n s o c c u r e d a t 2 p e t o r g r e a t e r s t r a i n .

B a sed on t h e s e t e n s i l e , c r e e p and lo a d r e l a x a t i o n r e s u l t s

and many o t h e r o b s e r v a t i o n s , H ir th (1 ) s u g g e s t e d an h y p o t h e s i s

f o r th e r a t i o n a l i z a t i o n o f th e h a r d e n in g - s o f t e n in g b e h a v io r .

He p r o p o s e d t h a t when d e fo r m a t io n i s b y s i n g l e s l i p , h y d r o g e n

p r o d u c e s s o f t e n i n g , a s s o c i a t e d w ith th e en han cem en t o f d o u b le

k in k n u c l e a t io n on sc r e w d i s l o c a t i o n s and th u s an enh an cem en t

o f t h e i r m o tio n and p o s s i b l y w it h en h a n ced i n j e c t i o n o f d i s

l o c a t i o n s a t f r e e s u r f a c e s . When d e fo r m a t io n i s b y m u l t ip le

s l i p , work h a r d e n in g i s l a r g e r , and h a r d e n in g e n s u e s .

3 - EXPERIMENTAL METHODS

3 - 1 M a t e r ia l s S tu d ie d

The m a t e r i a l s u s e d i n t h i s s tu d y w ere A ISI 1020 and A ISI

1090 s t e e l s . C o m p o s it io n s o f t h e s t e e l s a r e shown i n t a b l e 1 .

The 1020 s t e e l was r e c e i v e d i n th e form o f 25 mm sq u a r e h o t -

r o l l e d b a r , w h ile th e 1090 s t e e l was r e c e i v e d i n t h e form o f

38 mm x 13 mm h o t - r o l l e d b a r .

T a b le 1 . C o m p o s it io n o f s t e e l s (wt?S) •

s t e e l C Mn S i P S

A ISI 1020 0 . 2 0 0 . 5 9 0 . 1 8 0 . 0 0 9 0 . 0 3 0

A ISI 1090 0 . 8 8 0 . 7 5 0 . 1 9 0 . 0 1 7 0 . 0 3 6

3 - 2 H ea t T rea tm en t and S p ec im en P r e p a r a t io n

The s p h e r o id iz e d m ic r o s t r u c t u r e o f 1020 s t e e l w as o b t a in e d

b y a u s t e n i t i z i n g a t 1000°C f o r one h o u r , q u e n c h in g i n q a t e r

and te m p e r in g a t 690°C f o r 7 d a y s .

The s p h e r o i d iz a t io n o f t h e 1090 s t e e l w as done a t E l e c t r i c

28

29

Heat Treating Company in Columbus, Ohio. The as-received bars were held for two hours at 7 5 0 °C, then furnace cooled to 704°C, held for twenty hours and then air cooled to room temperature.

F ig u r e s 2 and 3 show th e t y p i c a l m ic r o s t r u c t u r e s o f sp h e r o

i d i z e d 1020 and 1090 s t e e l . The g r a in s i z e s i n t h e 1020 s t e e l

r a n g e d from 5 to 40 um w ith an a v e r a g e o f 10 um. The a v e r a g e

c a r b id e s i z e i n th e 1020 s t e e l w as 0 . 4 um w ith some la r g e

c a r b id e s ( 1 . 5 um) d i s t r i b u t e d a lo n g th e g r a in b o u n d a r ie s .

In the 1090 steel the average grain size was 15 um, the carbide size ranged from 0.5 to 4 um with an average of 1.2 um.

Figures 4, 5 and 6 show the dimensions of axisymmetric (round bar) tension specimens, plain strain tension specimens and bend specimens. Some of the important directions and planes are also shown in these figures. The notch root radius of the 1020 bend specimens is 1 mm while that of the 1090

specimen is 1.19 mm.The g a u g e l e n g t h s u r f a c e s o f b o th rou n d b a r and p la n e s t r a i n

t e n s i o n sp e c im e n s a s w e l l a s th e n o tc h s u r f a c e o f b en d s p e c i

men w ere p o l i s h e d s u c c e s s i v e l y th r o u g h 2 4 0 ,3 2 0 ,4 0 0 and 600

g r i t s i l i c o n c a r b id e a b r a s iv e p a p e r s and w ith diam ond p a s t e

from 6 um th r o u g h 1 /4 um. The p o l i s h i n g d i r e c t i o n o f th e l a s t

s t e p w as p a r a l l e l t o t h e d i r e c t i o n o f a c t i o n o f th e maximum

t e n s i o n s t r e s s . The r e s u l t i n g p o l i s h e d s u r f a c e s w ere good

en ou gh f o r m e t a l lo g r a p h ic e x a m in a t io n s . S p ec im en s w ere p o

l i s h e d , u l t r a s o n i c a l l y c le a n e d i n a c e to n e and a i r b lo w n d ry

F ig u r e 2 . S p h e r o id iz e d m ic r o s t r u c t u r e o f A ISI 1020 s t e e l

F ig u r e 3* S p h e r o id iz e d m ic r o s t r u c t u r e o f A IS I 1090 s t e e l

31

bo

i.35mnn Radiusll.lm m No.14 T h r e a d s

IDIO

8.7 mmJ l t -

9.5mml 12.7 mm

76.2 mm

F ig u r e k* S p ecim en g e o m e tr y f o r a x isy m m e tr ic (ro u n d b a r ) t e n s i o n t e s t s

t

I

i

i

I22mm-

2mm f\ IRadius'J. ,1

EE

IDCMCD

2 mm-

x(l)I * ,z (3 )

y(2)

<-6 mm

F ig u r e 5* S p ecim en g e o m e try f o r p la n e s t r a i n t e n s i o n t e s t s

Notch Mid Plane

y(2)

z(3)

—>|5.5mm[< 22mm Rolling Direction2F

F ig u r e 6 . S p ec im en g e o m e try f o r t h r e e - p o i n t bend t e s t s

Specimen

12.7mm Hardened S tee l Pins-.

Plunger1.93 mm Radius

~ “ T Mid Span Deflection

Plastic Bend Angle

SteelTestAnvil

in4 4 mm

75mm

F ig u r e 7- F ix t u r e u s e d and th e d e f i n i t i o n o f m id -sp a n d e f l e c t i o n and p l a s t i c bend a n g le f o r t h r e e - p o i n t bend t e s t

33

im m e d ia te ly "before t e s t i n g o r h y d ro g en c h a r g in g .

3 - 3 T e s t s o f U n ch arged S p ec im en s

3 - 3 - 1 U n ch arged Round B ar and P la n e S t r a i n T e n s io n T e s t s

The gau ge l e n g t h s o f th e t e n s i o n sp e c im e n s w ere m arked

w ith a knoop m ic r o h a r d n e s s t e s t e r w ith a lo a d o f 10 g , m aking

in d e n t a t i o n s o f 1 um d e p th on th e s p e c im e n s . A l l t e n s i o n

t e s t s w ere p e r fo r m ed a t room 'tem p era tu re w ith a 5000 k g f c a

p a c i t y I n s t r o n m ach in e t h a t p r o v id e d th e lo a d (P ) v e r s u s

d e f l e c t i o n r e c o r d . H ow ever , i n o r d e r to g e t a more a c c u r a te

d e f l e c t i o n , th e d i s t a n c e s "between gau ge l e n g t h m arks w ere

m easu red u n d er a 100 x o p t i c a l m ic r o sc o p e w ith t r a v e l i n g s t a g e

a f t e r th e t e n s i o n t e s t . The s m a l l e s t d ia m e te r on a round b a r

w as m easu red w ith a s l i d e c a l i p e r and t h e s m a l l e s t t h i c k n e s s

on th e p la n e s t r a i n sp e c im e n s was m easu red w ith a p o in t e d

m ic r o m e te r . The t r u e s t r a i n s w ere c a l c u l a t e d from gauge

l e n g t h d a ta b e f o r e u n ifo r m s t r a i n and from c r o s s - s e c t i o n

a r e a o r t h i c k n e s s d a t a , f o r round b a r and p la n e s t r a i n s p e c i

m en s, r e s p e c t i v e l y , a f t e r u n ifo rm s t r a i n . The m o st f r e q u e n t ly

u s e d c r o s s h e a d sp e e d u s e d i n t h i s work w as 0 . 0 5 c n /m in w h ich

c o r r e s p o n d s t o a s t r a i n r a t e o f 2 .6 x 10 / s e c f o r rou n d b a r -3

and 1 .4 x 10 / s e c f o r p la n e s t r a i n s p e c im e n s .

3 - 3 - 2 U n ch arged Bend T e s t

B end t e s t s w ere c a r r ie d o u t on a I n s t r o n m ach in e w it h a

5000 k g f c a p a c i t y GRM c o m p r e s s io n / t e n s io n lo a d c e l l . The

34

c r o s s h e a d sp e e d w as 0 . 0 5 cm /m in . The 3 - p o i n t b e n d in g f i x t u r e

i s shown i n F ig u r e 7* The m id -sp a n d e f l e c t i o n £>w as m eas

u r e d u n d er a 100 x o p t i c a l m ic r o s c o p e w ith a t r a v e l i n g s t a g e .

The p l a s t i c b en d a n g le 0 w as c a l c u l a t e d from th e r e l a t i o n

0 = 2 S in ( ^ / 2 ? . 5 ) -

3 - 4 T e s t s w ith H ydrogen C h a rg in g

3 - 4 - 1 E l e c t r o l y t e P r e p a r a t io n and P o l a r i z a t i o n D iagram s

The e l e c t r o l y t e u s e d f o r h y d r o g e n c h a r g in g e x p e r im e n ts was

1 N s u l f u r i c a c id w i t h 1 g / l o f t h io u r e a a s a h y d ro g e n recom

b i n a t i o n p o i s o n . The s o l u t i o n was b u b b led w ith h ig h p u r i t y

n i t r o g e n f o r a t l e a s t 8 h o u r s p r i o r t o c h a r g in g t o r e d u c e th e

amount o f o x y g e n i n s o l u t i o n . B e fo r e b u b b l in g , th e h ig h pu

r i t y n i t r o g e n w as p a s s e d th r o u g h t h r e e g a s -w a s h in g b o t t l e s

o f chrom ous s o l u t i o n s and one s a s - w a s h in g b o t t l e o f e l e c t r o

l y t e t o rem ove o x y g e n ( 1 2 8 ) .

The p o l a r i z a t i o n d ia g ra m s w h ich d e f in e th e c h a r g in g c o n d i

t i o n s o f th e s t e e l s w ere o b ta in e d w ith a M odel 350 C o r r o s io n

M easurem ent C o n so le made b y EG&G P r in c e t o n A p p lie d R e se a r c h

w ith a p o t e n t i a l in c r e a s e sp e e d o f 0 . 1 m v /se c and a r e shown i n

f i g u r e s 8 and 9 » f o r th e 1020 and 1090 s t e e l s , r e s p e c t i v e l y .

3 - 4 - 2 P r e c h a r g in g T e s t s

I n h y d ro g e n c h a r g ed t e s t s , s t a i n l e s s s t e e l w ir e l e a d s w ere

s p o t w e ld e d t o th e s p e c im e n s . The sa m p le s w ere th e n c o a te d

w ith n o n c o n d u c t in g a c id r e s i s t a n t p a in t (m ic r o s to p ) e x c e p t on

10'« - 2 10 I I0C 10*

Current Density, A /m 210' 10'

F ig u r e 8 . P o l a r i z a t i o n d iagram f o r s p h e r o id iz e d A ISI 1020 s t e e l i n 1 N s u l f u r i c a c id c o n t a in in g 1 g / l t h io u r e a a t room te m p e r a tu r e

toCM

X -100

■E° -200

<2 -3 0 0

— -4 0 0

-500

Current D ensity, A /m 2F ig u r e P o l a r i z a t i o n d iagram f o r s p h e r o id iz e d A ISI 1090

s t e e l i n 1 N s u l f u r i c a c id c o n t a in in g 1 g / l t h io u r e a a t room te m p e r a tu r e

36

t h e r e g io n t o be c h a r g e d .

The anode w as made o f P t f o i l f o r t e n s i o n sp e c im e n s and o f

P t w ir e f o r b en d b a r n o t c h e s . An A ard vark P o t e n t i o s t a t -

G a lv a n o s ta t m od el V-2LR w as u s e d t o p r o v id e t h e c h a r g in g

c u r r e n t . I n o r d e r to a v o id o x i d a t i o n o f t h e sam p le s u r f a c e

b e f o r e c h a r g in g , th e sp e c im e n w as p u t on s i t e and th e g a lv a -

n o s t a t was tu r n e d on b e f o r e th e e l e c t r o l y t e w as added i n t o

th e c h a r g in g c e l l .

A f t e r two h o u r s o f c h a r g in g a t a c u r r e n t d e n s i t y o f 100 A/m3, ,

t h e sp ec im en w as rem oved from th e e l e c t r o l y t e , r in s e d i n f lo w

in g w a t e r , c le a n e d o f f th e m ic r o s to p c o a t i n g , r in s e d i n a c e t o

n e , b lo w d r ie d w ith p r e s s u r iz e d a i r and p u t in t o th e t e s t

a p p a r a tu s . The lo a d w as a p p l ie d t o t h e sp e c im e n w i t h in two

m in u te s a f t e r c h a r g in g s to p p e d . The q u a s i-d y n a m ic mode o f

t e s t i n g (1 2 9 ) w as n o t a d o p te d i n t h i s work b e c a u s e any r e s i d

u a l e l e c t r o l y t e on sp e c im e n s u r f a c e i s l i k e l y t o a f f e c t th e

t e s t r e s u l t . The r e m a in in g p a r t o f t h e p r e c h a r g e t e s t s was

e s s e n t i a l l y th e same a s t h a t on u n ch a rg e d s p e c im e n s .

3 - 4 - 3 Dynam ic C h a rg in g T e s t

Dynam ic c h a r g in g t e s t s w ere e s s e n t i a l l y th e same a s th e

p r e c h a r g in g t e s t s e x c e p t t h a t l o a d s w ere a p p l ie d t o - t h e s p e

c im e n s im u lt a n e o u s ly w ith t h e c h a r g in g c u r r e n t . The dynam ic

c h a r g in g c e l l s f o r rou n d b a r and p la n e s t r a i n sp e c im e n s a r e

shown i n f i g u r e 10 and 11 r e s p e c t i v e l y . The dynam ic c h a r g in g

c e l l u s e d f o r th e b en d t e s t w as d e s c r ib e d i n o n u e w u e n y i’ s

37

■TeflonSpecimen (Cathode)

Glass CylinderD= 140 mm H= 90 mm

Pt Cylinder (Anode)D=25 mm H=25 mm S e a le d with Silicone

R ubber

Teflon

F ig u r e 1 0 . C e l l f o r dynam ic c h a r g in g o f rou n d b a r t e n s i o n t e s t s

P t Foil (Anode)

Glass Cylinder D= 140 mm H=20 mmTeflon

Sealed with Teflon Tape

Specim en (C athode) \ ___

F ig u r e 1 1 . C e l l f o r dynam ic c h a r g in g o f p la n e s t r a i n t e n s i o n t e s t s

38

work (129)- At the end of the test, specimens were cleaned quickly as described in the preceding section to avoid excess surface corrosion.

3-5 Metallographic ExaminationA f t e r m e c h a n ic a l t e s t i n g w as c o m p le te d , sp ec im en s u r f a c e s

w ere s t u d ie d b y b o th o p t i c a l and s c a n n in g e l e c t r o n m ic r o sc o p y

(SEM ). The s h o r t w o rk in g d i s t a n c e w as u s e d f o r SEM w h en ever

p o s s i b l e t o g iv e a b e t t e r c o n t r a s t on th e im age o f t h e n e a r ly

f l a t s u r f a c e s . E n erg y d i s p e r s i v e a n a l y s i s o f X -r a y s (EDAX)

w as u se d t o i d e n t i f y i n c l u s i o n s .

A f t e r s u r f a c e s ( i n c lu d i n g f r a c t u r e s u r f a c e s ) w ere s t u d i e d ,

sp e c im e n s w ere s e c t i o n e d l o n g i t u d i n a l l y th r o u g h th e m id p la n e

t o p e r m it e x a m in a t io n o f th e c r a c k and v o id f o r m a t io n . When

p o l i s h i n g th e m id p la n e s p e c im e n s , e s s e n t i a l s t e p s w ere ta k e n

t o p r e v e n t ed ge r o u n d in g and i n c l u s i o n t e a r o f f .

RESULT

4 - 1 Round B elt T e n s io n T e s t s

4 - 1 - 1 L o a d - d e f l e c t i o n C u rves

F ig u r e s 12 and 13 show th e g e n e r a l sh ap e o f l o a d - d e f l e c t i o n

c u r v e s f o r rou n d b a r t e n s i o n t e s t s o f A ISI 1020 and A ISI 1090

s p e c im e n s . A l l c u r v e s show v e r y c l e a r y i e l d p o i n t s and y i e l d

p o in t e l o n g a t i o n s . T here i s l i t t l e d i f f e r e n c e b e tw e e n th e

y i e l d p o i n t s o b s e r v e d on c h a r g e d and on u n ch a rg ed s p e c im e n s .

F or A ISI th e 1020 s p e c im e n s , th e f r a c t u r e lo a d s o f sp e c im e n s

u n d er d i f f e r e n t c h a r g in g c o n d i t i o n s a r e a b o u t th e sam e. In

c o n t r a s t , f o r th e A ISI 1090 sp e c im e n s th e f r a c t u r e lo a d i s

lo w e s t f o r u n ch a rg ed and h i g h e s t f o r d y n a m ic a lly c h a r g e d

sp e c im e n s .

4 - 1 - 2 M e c h a n ic a l P r o p e r t i e s

The m e c h a n ic a l p r o p e r t i e s o b t a in e d from rou n d b a r t e n s i o n

t e s t s a re shown i n t a b l e s 2 and 3 f o r 1020 and 1090 s p e c im e n s ,

r e s p e c t i v e l y . I n th e t a b l e s , Y i s th e lo w e r y i e l d s t r e n g t h ,

UTS i s th e e n g in e e r in g u l t im a t e t e n s i l e s t r e n g t h , €u i s th e

t r u e s t r a i n a t UTS, RA i s th e r e d u c t io n o f a r e a and i s th e

t r u e f r a c t u r e s t r a i n c a l c u l a t e d from t h e r e d u c t io n o f a r e a .

39

20

18

16

14

12

■g 10

5

8

6

4

2

° c

cure :

Uncharged

Precharged

DynamicallyCharged

\

6 8 10 Deflection (mm)

2.. L o a d - d e f l e c t i o n c u r v e s f o r 1020 rou n d b a r t e n s i o n t e s t s

20

18

16

■14

12

10

8

6

4

2

0

I j r

Uncharged — — —

P recharged --------------

Dynamically C h a rg e d ,

1 ■ t : I__________ J _______\___L2 4 6 8 10

Deflection (m m )

. L o a d - d e f l e c t i o n c u r v e s f o r 1090 rou n d b ar t e n s i o n t e s t s

T a b le 2 . Round b a r t e n s i l e p r o p e r t i e s f o r

s p h e r o id iz e d A IS I 1020 s t e e l .

Y(MPa) UTS (MPa) £ u RA £ f

U n charged 286 432 0 .2 0 7 0 .7 7 1 . 4 7

P r e c h a r g e d 297 0 . 1 9 0 . 7 2 1 . 2 8

D y n a m ic a llyC harged 289 4 3 4 0 .1 6 0 .3 ^ O .in

T a b le 3* Round b a r t e n s i l e p r o p e r t i e s f o r

s p h e r o id iz e d A IS I 1090 s t e e l

Y(MPa) UTS(MPa) £ u RA £ f

U n ch arged 315 530 0 . 2 1 0 .6 5 1.04-

P r e c h a r g e d 350 558 0 . 2 0 0 .^ 7 0 . 6 3

D y n a m ic a llyc h a r g ed 32^ 522 0 .0 9 0 . 1 0 5 0 . 11

4 3

k - 1 -3 • S l i p B ands on S p ec im en S u r f a c e s

As th e t e n s i o n sp e c im e n s d e fo rm e d , s l i p b an d s s t a r t e d t o

form a s so o n a s th e y i e l d p o i n t s w ere r e a c h e d . F ig u r e 14

sh ow s t h e s l i p band d e v e lo p m en t from S = 0 .0 2 5 "to 1 .2 1 f o r

u n ch a rg e d 1020 s p e c im e n s .

I n t h i s f i g u r e and a l l th e f o l lo w in g f i g u r e s o f t e s t e d s p e c

im e n s , th e d i r e c t i o n o f maximum a p p l ie d t e n s i l e s t r e s s i s th e

t o p -b o t to m d i r e c t i o n i f n o t m e n tio n e d o t h e r w is e .

F ig u r e 15 sh ow s th e s l i p band d ev e lo p m en t f o r u n ch a rg ed 1090

s p e c im e n s from £ = 0 .0 2 3 t o 0 .4 9 4 . I t i s n o te w o r th y t h a t th e

s l i p b an d s a r e more i n t e n s e on 1020 sp e c im e n s th a n on 1090

s p e c im e n s . F or b o th m a t e r ia l s th e i n t e r s e c t i o n o f s l i p b a n d s

b e g in s a t a s t r a i n s m a l le r th a n 0 . 0 6 .

4 - 1 - 4 F r a c tu r e S u r f a c e s

The f r a c t u r e s u r f a c e s o f 1020 and 1090 u n ch a rg ed round b ar '

t e n s i o n sp e c im e n s a r e shown i n f i g u r e s 16 and 17* F or b o th

m a t e r i a l s , f r a c t u r e o c c u r s i n a t y p i c a l c u p -a n d -c o n e m ode.

B a s i c a l l y t h e d im p le s t r u c t u r e i s th e same a t th e c e n t e r and

p e r ip h e r y , e x c e p t t h a t , f i r s t , th e c e n t r a l r e g io n lo o k s f i b r

o u s u n d er lo w m a g n i f i c a t io n a n d , s e c o n d , th e d im p le s a t th e

p e r ip h e r y a r e t i l t e d .

F ig u r e s 18 and 19 a r e f r a c t u r e s u r f a c e s o f p r e c h a r g e d 1020

and 1090 rou n d b a r t e n s i o n s p e c im e n s . T h ese sp e c im e n s f a i l e d

i n th e c u p -a n d -c o n e mode a s f o r t h e u n ch a rg ed s p e c im e n s .

44

H ow ever , o n th e 1020 sp e c im e n s t h e r e w ere more l a r g e r d im p le s

i n t h e c e n t r a l r e g i o n th a n a t t h e p e r ip h e r y . F or p r e c h a r g e d

1090 s p e c im e n s , t h e r e w ere many s e c o n d a r y c r a c k s and b r i t t l e

r e g i o n s on th e c e n t r a l p a r t o f th e f r a c t u r e s u r f a c e . The

se c o n d a r y c r a c k s w ere u s u a l l y a t g r a in b o u n d a r ie s w here c a r

b id e p a r t i c l e s a r e l a r g e r and c l o s e r t o one a n o th e r . No r e

l a t i o n was fo u n d b e tw e e n MnS i n c l u s i o n s and se c o n d a r y c r a c k s .

F ig u r e 20 sh ow s th e f r a c t u r e s u r f a c e o f a d y n a m ic a lly c h a r g

e d 1020 rou n d b a r sp e c im e n . The f r a c t u r e s u r f a c e show s q u a s i

c le a v a g e a t th e p e r ip h e r a l r e g i o n w here th e c r a c k p r o p a g a te d

n orm al t o t h e t e n s i l e a x i s and i t r e v e a l s d im p le r u p tu r e i n

th e c e n t r a l r e g io n w here i t was i n c l i n e d a b o u t 45 ° t o th e

t e n s i l e a x i s .

The f r a c t u r e s u r f a c e s o f d y n a m ic a l ly c h a r g e d 1090 ro u n d

t e n s i o n sp e c im e n s w ere a l l o f a b r i t t l e n a tu r e and p e r p e n d ic

u l a r t o th e t e n s i l e a x i s . H ow ever , a s shown i n f i g u r e 2 1 ,

t h e r e a r e two d i f f e r e n t t y p e s o f b r i t t l e f r a c t u r e s u r f a c e s .

C lo s e to t h e p e r ip h e r y , th e f r a c t u r e s u r f a c e was r e l a t i v e l y

f l a t w ith Q u a s i- c le a v a g e i n c h a r a c t e r w h ile i n th e c e n t r a l

r e g io n a t y p i c a l t r a n s g r a n u la r c le a v a g e s u r f a c e d o m in a te d .

4-1-5 Surface CracksF ig u r e 22 show s th e s id e s u r f a c e s o f 1020 and 1090 u n ch a rg ed

rou n d b a r s n e a r t h e f r a c t u r e z o n e . T h ere a r e some s u r f a c e

v o i d s p r e s e n t b u t no s u r f a c e c r a c k s . A l s o , th e s l i p b a n d s

o n t h e 1090 sp e c im e n s a r e much f i n e r th a n t h o s e o n 1020

45

s p e c im e n s .

The s i d e s u r f a c e o f a p r e c h a r g e d 1020 rou n d b a r n e a r th e

f r a c t u r e r e g i o n i s s i m i l a r t o t h a t o f t h e u n ch a rg e d s p e c im e n s .

On t h e o t h e r h a n d , a s shown i n f i g u r e 2 3 - b , th e s i d e s u r f a c e

o f a p r e c h a r g e d 1090 rou n d b a r show s a number o f s u r f a c e

c r a c k s .

The s u r f a c e c r a c k s a r e w e l l d e v e lo p e d on d y n a m ic a lly c h a r g e d

sp e c im e n s a s shown i n f i g u r e 2 4 . The number and s i z e s o f

s u r f a c e c r a c k s on th e 1020 rou n d b a r a r e b o th much la r g e r

th a n t h o s e on t h e 1090 rou n d b a r .

(c)

SR S-iist■ J**> «W V- *

‘il*;:■ wr

lOum

lO li m

10pm

«)

lOum

CO <f>F ig u re 14-. S l i p bands on th e su r fa c e o f uncharged 1020 round

b ar sp ec im en , tr u e s t r a i n £ = (a ) 0 . 0 2 5 (b) 0 . 0 6 (c ) 0 .1 0 7 (d ) 0 . 2 0 8 ( e ) 0 .5 1 2 ( f ) 1 .2 1

(O ) ( f )

F ig u r e 15* S l i p b a n d s on th e s u r f a c e o f u n c h a rg e d 1090 rou n d b a r sp e c im e n , t r u e s t r a i n £ = ( a ) 0 . 0 2 3 ("b) 0 . 0 5 3 ( c ) 0 .0 8 6 (d ) O .I 36 ( e ) 0 .2 3 2 ( f ) 0 .4 9 4

48

10um

(b ) ( c )

F ig u r e 1 6 . F r a c t u r e s u r f a c e o f u n c h a r g e d 1020 ro u n d b a rs p e c im e n ( a ) c u p -a n d -c o n e f r a c t u r e ( b ) c e n t r a lr e g i o n ( c ) p e r i p h e r a l r e g i o n

(To) (c)

F ig u r e 1? . F r a c tu r e s u r fa c e o f u n ch arged 1090 round "barsp ec im en (a ) c u p -a n d -c o n e f r a c t u r e (b ) c e n t r a lr e g io n ( c ) p e r ip h e r a l r e g io n

50

(b )

F ig u r e 18 . F r a c tu r e s u r fa c e o f p r e c h a r g e d 1020 round "barsp ec im en (a ) c u p -a n d -c o n e f r a c t u r e (h ) th e "boxedr e g io n i n (a )

51

(b) (c)

F ig u r e 1 9 . F r a c t u r e s u r f a c e o f p r e c h a r g e d 1090 ro u n d b a rs p e c im e n (a ) c u p -a n d -c o n e f r a c t u r e (b ) c e n t r a lr e g i o n ( c ) p e r i p h e r a l r e g i o n

1 0 |im

F ig u r e 2 0 . F r a c tu r e s u r fa c e o f d y n a m ic a lly ch arged 1020round b a r sp ec im en (a ) cu p -a n d -co n e f r a c tu r e(t>) c e n t r a l r e g io n ( c ) p e r ip h e r a l r e g io n

53

(a)

1 0 p m

(b) (c)

F igu re 21 . F ractu re su r fa c e o f d y n a m ica lly charged 1090round bar specim en (a ) cup-an d-cone fr a c tu r e(h) c e n tr a l r e g io n (c ) p e r ip h e r a l r e g io n

5 4

(13)

F igu re 22 . S id e su r fa c e fo r uncharged round bar specim en n ear fr a c tu r e (a ) 1020 s t e e l (b ) 1090 s t e e l

F ig u r e 23*

F ig u r e

57

4 - 2 P la n e S t r a i n T e n s io n T e s t s

4 - 2 - 1 L o a d - D e f le c t io n C u rves

F ig u r e s 25 and 26 show th e g e n e r a l sh ape o f l o a d - d e f l e c t i o n

c u r v e s i n p la n e s t r a i n t e n s i o n t e s t s f o r 1020 and 1090 s p e c i

m en s. T e s t e d u n d er d i f f e r e n t c h a r g in g c o n d i t i o n s , a l l 1020

sp e c im e n s show ed a r e g io n o f s t a b l e c r a c k p r o p a g a t io n a f t e r

th e u n s t a b le c r a c k p r o p a g a t io n r e g io n . I n c o n t r a s t , a l l 1090

sp e c im e n s f r a c t u r e d im m e d ia te ly a f t e r ■unstable c r a c k p r o p a g a

t i o n . A n o th er n o te w o r th y phenom enon i s t h a t m ost o f th e lo a d -

d e f l e c t i o n c u r v e s d id n o t h ave a y i e l d p o i n t , w h ile o n ly a

s m a ll f r a c t i o n o f them show ed a f a i n t " k n ee” on th e c u r v e s .

4 - 2 - 2 M e c h a n ic a l P r o p e r t i e s

The mechanical properties obtained from plane-strain tension tests are shown in tables 4 and table 5* The yield strengths shown were obtained from those load-deflection curves with a faint "knee". The numbers in parentheses are the fraction of specimens with a knee on their load-deflection curves. The true fracture strains £f were calculated from the change of thickness Rt by the equation Sf = -ln(l-Rt).

Load

(k

N)

28UnchargedPrechargedDynamicallyCharged

24

20

Deflection ( m m )

F ig u r e 25 • L o a d - d e f l e c t i o n c u r v e s f o r 1020 p la n e s t r a i n t e n s i o n t e s t s * A rrow s i n d ic a t e d e f l e c t i o n s w here s u r f a c e c r a c k s s t a r t to form .

Load

(k

N)

28

24

20

8 UnchargedPrechargedDynamicallyCharged

4

0 6: 4Deflection (m m )20

F ig u r e 2 6 . L o a d - d e f l e c t i o n c u r v e s f o r 1090 p la n e s t r a i n t e n s i o n t e s t s . A rrow s i n d i c a t e d e f l e c t i o n s w here s u r f a c e c r a c k s s t a r t t o form .

60

T a b le 4 , P la n e s t r a i n t e n s i l e p r o p e r t i e s

f o r s p h e r o id iz e d A ISI 1020 s t e e l .

Y(MPa) UTS(MPa) E sc £ u Rt £ f

U n ch arged 2 8 9 ( 2 /5 ) 4-80 0 .3 4 0 . 2 3 0 . 6 2 0 . 9 7

1P r e c h a r g e d N o( O / l l ) 468 0 . 3 0 0 . 2 0 7 0 .4 0 0 . 5 1

D y n a m ic a llyC harged

N o (0 /9 ) 460 0 . 0 1 5 0 .0 8 6 0 .2 2 0 . 2 5

T a b le 5* P la n e s t r a i n t e n s i l e p r o p e r t i e s

f o r s p h e r o id iz e d A ISI 1090 s t e e l *

Y(MPa) UTS(MPa) E s c £ u R t £ f

U n ch arged 3 4 3 ( 3 /9 ) 602 0 .2 8 0 . 2 4 3 0 . 5 1 0 . 7 1

P re c h a r g ed 3 5 7 ( 4 /1 1 ) 590 0 .1 0 0 . 1 7 0 .2 9 0 . 3 4


3 5 7 ( 4 /7 ) 557 0 . 0 1 5 0 . 0 6 0 .0 6 0 . 0 6

61

if—2 - 3 S l i p B ands on S p ec im en S u r fa c e s

S l i p b a n d s on sp e c im e n s u r f a c e s form a s p l a s t i c d e fo r m a t io n

b e g i n s . F ig u r e 2? show s th e s l i p band d e v e lo p m en t on an un

c h a r g e d 1020 p la n e s t r a i n t e n s i o n sp e c im e n from £ = 0 . 0 1 to

0*39* The s u r f a c e o f a sp e c im e n a t £ = 0 .1 0 5 was e tc h e d w ith

2?S n i t a l t o r e v e a l th e m ic r o s t r u c t u r e . As shown i n f i g u r e 2 8 ,

s l i p b an d s w ere gro u p ed b y th e o r i g i n a l a u s t e n i t e g r a in

b o u n d a r ie s .

F ig u r e 29 show s th e d e v e lo p m en t o f s l i p b a n d s on u n ch a rg ed

1090 p la n e s t r a i n t e n s i o n sp e c im e n s . MnS i n c l u s i o n s w ere

fo u n d t o be c r a c k e d a t s m a l l s t r a i n s « 0 . 0 5 ) * C ra ck s i n MnS

p a r t i c l e s in c r e a s e i n number and w id th a s s t r a i n i n c r e a s e s .

H o w ev er , a s shown i n f i g u r e 30» t h e y do n o t grow i n t o th e

m a t r ix . The f i n a l f u r f a c e c r a c k s w ere i n i t i a t e d from v o id s

w h ich grew and c o a le s c e n c e d a t c e m e n t i t e - f e r r i t e i n t e r f a c e s

a lo n g s l i p band t r a c e s .

The s l i p b an d s on a p r e c h a r g e d 1020 p la n e s t r a i n t e n s i o n

sp e c im e n a r e shown i n f i g u r e 31 f o r s t r a i n s from £ = 0 . 0 3

t o O .3 5 . V o id s b agan to form a lo n g s l i p b an d s a t a s t r a i n

s m a l le r th a n 0 .1 1 . F ig u r e 3 1 - e i s an exam ple o f v o i d s w i t h in

s l i p b a n d s .

F ig u r e 32 show s th e s l i p b a n d s on p r e c h a r g e d 1090 p la n e

s t r a i n t e n s i o n s p e c im e n s . C om paring t h i s r e s u l t w ith f i g u r e

2 9 , one f i n d s t h a t th e d ev e lo p m en t o f s h e a r b a n d s w as p r o m o te -

e d b y h y d ro g en c h a r g in g . F or exam p le ( f i g u r e 3 2 - c and f ig u r e

2 9 - e ) , th e s l i p b an d s on a p r e c h a r g e d sp ec im en a t a s t r a i n

62

o f 0 . 0 6 6 r e s e m b le th o s e on u n ch a rg ed sp e c im e n s a t a s t r a i n

of* 0 . 1 3 7 w ith e v e n la r g e r s u r f a c e v o i d s on s l i p b an d s i n th e

l a t t e r c a s e .

The c h a n g e s i n s u r f a c e to p o g r a p h y o f d y n a m ic a lly c h a r g e d

1020 p la n e s t r a i n sp e c im e n s a re shown i n f i g u r e 3 3 . I n t h i s

c a s e , v o i d s w i t h in s l i p b a n d s form ed a t a s t r a i n s m a l le r th a n

0 . 0 2 6 , and th e y s u b s e q u e n t ly c o n n e c te d to form s u r f a c e c r a c k s .

H ow ever , s u r f a c e c r a c k s n eed n o t b e n e c e s s a r i l y i n i t i a t e d

from s l i p b a n d s . I n f i g u r e 3 3 - b , t h e r e a re e x a m p les o f c r a c k s

form ed b o th w i t h in and o u t s id e th e s l i p b a n d s . The f r a c t u r e

s t r a i n o f d y n a m ic a lly c h a r g e d 1020 p la n e s t r a i n sp e c im e n s was

fo u n d t o be O .2 7 . F ig u r e 3 3 -d i s a v ie w o f th e s id e s u r f a c e

o f a f r a c t u r e d sp e c im e n . I t i s i n t e r e s t i n g to n o t i c e t h a t

t h e s l i p b an d s on i t r e se m b le t h o s e i n f i g u r e 3 1- a ( £ = 0 . 0 3 1 )

o r 3 1 -b ( £ = 0 . 0 5 *0 , w h ile th e ru m p lin g c o n f i g u r a t io n s on i t

r e s e m b le s t h o s e i n f i g u r e 31 - c ( £ = 0 . 1 1 3 ) o r 31 - d ( £ = 0 . 2 3 ) .

The s u r f a c e c r a c k s on d y n a m ic a l ly c h a r g ed 1090 sp e c im e n

form ed a t a s t r a i n a s lo w a s 0 .0 1 5 * S im i la r to th e r e s u l t

f o r 1020 s p e c im e n s , c r a c k s can form e i t h e r w i t h in o r o u t s id e

th e s l i p b a n d s a s shown i n f i g u r e 3k.

k -2 -k F r a c tu r e S u r f a c e s

The f r a c t u r e o f b o th 1020 and 1090 u n ch a rg ed p la n e s t r a i n

sp e c im e n s o c c u r e d a lo n g sp a n te d t r a j e c t o r i e s i n a V -sh a p e and

t h e r e s u l t i n g f r a c t u r e s u r f a c e s w ere com posed o f d im p le s w ith

a s i z e ab ou t e q u a l t o th e c a r b id e p a r t i c l e s p a c in g a s shown

63

i n f i g u r e s 35 and 3 6 .

F ig u r e 37 show s a f r a c t u r e s u r f a c e c h a r a c t e r i s t i c o f th e

p r e c h a r g e d 1020 s p e c im e n s . The f r a c t u r e i s V - s la n t i n sh ap e

and th e s u r f a c e i s com posed o f r e g io n s o f a d im p led a p p ea r e n c e

s i m i l a r to th e s u r f a c e o f u n ch a rg ed s p e c im e n s . H ow ever , th e r e

a r e a l s o r e g i o n s o f b r i t t l e a p p ea ra n ce w ith a MnS p a r t i c l e a t

t h e i r c e n t e r a s i n d ic a t e d by th e a rro w s i n f i g u r e 37* T hese

b r i t t l e r e g io n s r e se m b le th e n e a r - s u r f a c e f l a t r e g i o n on dy

n a m ic a l ly c h a r g ed sp e c im e n s ( f i g u r e 20- c o r f i g u r e 3 9 - c ) and

a r e d i f f e r e n t from th e c le a v a g e f r a c t u r e s u r f a c e s form ed a t

lo w te m p e r a tu r e s ( f i g u r e 6 l - c ) o r a t h ig h s t r e s s i n t e n s i t i e s

( f i g u r e 63- c ) . A s i m i la r r e s u l t was fo u n d on th e f r a c t u r e

s u r f a c e o f p r e c h a r g e d 1090 p la n e s t r a i n sp e c im e n s a s shown

i n f i g u r e 3 8 -

F ig u r e s 39 and 40 a r e f r a c t u r e s u r f a c e s o f d y n a m ic a lly

c h a r g e d 1020 and 1090 p la n e s t r a i n t e n s i o n s p e c im e n s . The

f r a c t u r e o c c u r s a s two f l a t c r a c k s c o n n e c te d by an a lm o s t

v e r t i c a l s t e p , w ith th e f l a t r e g io n s h a v in g a q u a s i - c l e a v a g e

a p p e a r a n c e . T h ere a r e d u c t i l e d im p le s on th e s t e p .

4 - 2 - 5 C ra ck s and V o id s

The c r a c k s and v o i d s on p la n e s t r a i n t e n s i o n sp e c im e n s w ere

ex a m in ed on t h e m id -p la n e s e c t i o n . A l l o f th e c r a c k s on th e

p la n e s t r a i n t e n s i o n sp e c im e n s i n i t i a t e d a t th e m id d le o f th e

gau ge l e n g t h w here th e p la n e s t r a i n c o n d i t io n w as s a t i s f i e d .

F ig u r e 41 show s a s u r f a c e c r a c k s t a r t e d a t a s t r a i n o f 0 . 3 0

64

f o r an u n ch a rg e d 1020 s p e c im e n . The c r a c k w as l o c a t e d a t a

d eep s u r f a c e i n t r u s i o n and f o l lo w e d th e d i r e c t i o n o f t h e

c h a r a c t e r i s t i c t r a c e w h ich w as 4 5 ° t o th e t e n s i o n d i r e c t i o n .

N ear th e c r a c k t i p m ost v o i d s a r e form ed a lo n g th e c h a r a c t e r -

i s t i c t r a c e s . H ow ever, t h e r e i s n o t much d i f f e r e n c e i n th e

num bers and s i z e s o f v o id s form ed n e a r th e s u r f a c e and a t th e

c e n t e r . The s u l f i d e p a r t i c l e s c r a c k e d r e a d i l y , b u t t h o s e

c r a c k s u s u a l l y d id n o t grow i n t o th e m a tr ix .

F ig u r e 4 2 show s th e m id p la n e o f a p r e c h a r g e d 1020 sp e c im e n

a f t e r a s t r a i n o f 0 . 3 I . The a p p ea r a n c e i s s i m i la r to t h a t

f o r an u n ch a rg ed sp e c im e n a t a b o u t th e same s t r a i n e x c e p t

t h a t th e s u r f a c e r u m p lin g i s much more p r o n o u ce d i n th e p r e

c h a r g e d c a s e .

The s u r f a c e c r a c k s on th e 1090 u n ch a rg ed sp e c im e n s t y p i c a l l y

form ed a t th e d e e p e s t s u r f a c e i n t r u s i o n and f o l lo w e d th e 4 5 °

c h a r a c t e r i s t i c t r a c e s a s shown i n f i g u r e 4 3 - a . T h ere a r e

a l s o c r a c k s form ed i n t e r n a l l y b y v o i d s c o n n e c t in g a lo n g th e

c h a r a c t e r i s t i c t r a c e s a t 4 5 ° t o th e t e n s i o n d i r e c t i o n .

F or p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n s , c r a c k s

o f com p arab le s i z e s form ed a t th e s u r f a c e and i n th e i n t e r i o r

a s i n d ic a t e d b y th e arrow s i n f i g u r e 4 4 . S u r fa c e c r a c k s i n

g e n e r a l a r e a s s o c i a t e d w ith MnS p a r t i c l e s n e a r th e s u r f a c e a s

shown i n f i g u r e s 45 and 4 6 . F ig u r e 45 show s two c l o s e l y l o

c a te d s u r f a c e i n t r u s i o n s . The one n e a r a MnS p a r t i c l e form ed

a c r a c k w h ile th e o th e r d id n o t , e v e n th o u g h i t was th e d e e p e r

o n e . C ra ck s form ed i n t e r n a l l y may o r may n o t be a s s o c i a t e d

65

w ith MnS p a x t i c l e s a s shown i n f i g u r e 47* The p la n e s o f m o st

c r a c k s a r e i n c l i n e d a t 4 5 ° t o th e t e n s i l e d i r e c t i o n . H ow ever,

t h e r e a r e a l s o a few c r a c k s "both a t th e s u r f a c e and i n th e

i n t e r i o r w h ich a r e p e r p e n d ic u la r to th e t e n s i l e s t r e s s .

I n u n ch a rg e d and p r e c h a r g e d p la n e s t r a i n t e n s i o n sp e c im e n s

f o r b o th 1020 and 1090 s t e e l , t h e r e i s l i t t l e d i f f e r e n c e i n

t h e num bers and s i z e s o f th e v o i d s form ed n e a r th e s u r f a c e o r

a t th e c e n t e r e x c e p t a t th e t i p o f a c r a c k . The p red o m in a n t

mode o f v o id fo r m a t io n w as b y i n c l u s i o n - f e r r i t e i n t e r f a c e

d e c o h e s io n . C r a c k in g o f c e m e n t i t e i s a l s o o b s e r v e d b u t w ith

a much lo w e r f r e q u e n c y . MnS p a r t i c l e s c r a c k e d and th e i n t e r

f a c e b e tw e e n MnS and Fe s e p a r a t e d r e a d i l y . H ow ever , i n un

c h a r g e d s p e c im e n s , th e c r a c k s on MnS p a r t i c l e s d id n o t p r o

p a g a te in t o t h e m a tr ix .

S u r fa c e c r a c k s form ed a t v e r y s m a l l s t r a i n s and i n la r g e

num bers on d y n a m ic a l ly c h a r g e d s p e c im e n s . F ig u r e s A8 and -9

d e p ic t th e c r a c k s on d y n a m ic a lly c h a r g e d 1020 p la n e s t r a i n

t e n s i o n s p e c im e n s . The c r a c k s form ed a t th e s u r f a c e o r i n a

r e g i o n w i t h in 100 um o f th e s u r f a c e , and a l l o f them w ere i n

a p la n e n orm al t o th e t e n s i l e s t r e s s . F ig u r e ^9 show s c r a c k s

form ed on a n e a r s u r f a c e MnS p a r t i c l e and p r o p a g a te d i n t o th e

m a t r ix .

The c r a c k s on d y n a m ic a l ly c h a r g e d 1090 sp e c im e n s a r e form ed

a t th e s u r f a c e . T hey i n i t i a t e a t any p r e f e r e n t i a l d e f e c t s i t e ,

su ch a s a c o m e n t i t e - f e r r i t e i n t e r f a c e , a g r a in b o u n d a ry , o r

an i n c l u s i o n . A t i n i t i a t i o n , th e c r a c k s f o l lo w e d th e

i n t e r f a c e o r i e n t a t i o n s o f t h e s e p r e f e r e n t i a l d e f e c t s a s shown

i n f i g u r e 5°* T h en , w i t h i n a d i s t a n c e o f one o r two Fe3C p a r

t i c l e s i z e s (<5 u m ), th e y ch an ge t h e i r d i r e c t i o n so t h a t th e

c r a c k p la n e b ecom es n orm al t o t h e t e n s i l e a x i s . The c r a c k s

w ou ld e v e n p a s s th r o u g h c e m e n t i t e p a r t i c l e s r a t h e r th a n d e

c o h e r e and f o l l o w th e p a r t i c l e - m a t r i x i n t e r f a c e t o k eep t h e i r

n orm al o r i e n t a t i o n . E xam p les ca n be s e e n i n f i g u r e s 5 0 -b

and 51-a * «1

I n c o n t r a s t t o sp e c im e n s t e s t e d u n d er u n ch a rg e d o r p r e c h a r g

ed c o n d i t i o n s , t h e r e i s no e v id e n c e o f v o i d s form ed i n t e r n a l l y

o r i n f r o n t o f c r a c k s f o r th e d y n a m ic a l ly c h a r g e d s p e c im e n s .

6?

F ig u r e 2 7 . S l i p bands on th e su r fa c e o f un ch arged 1020 p la n e s t r a i n t e n s io n sp ec im en , tr u e s t r a i n £ = (a ) 0 . 0 1 (b ) 0 . 0 2 8 ( c ) 0 . 1 2 2 (d ) 0 . 1 9 ( e ) 0 . 2 6 ( f ) 0 . 3 9

68

1 0 p m

F ig u r e 2 8 . S l i p ban ds on th e s u r fa c e o f un ch arged 1020 p la n e s t r a i n t e n s io n sp ec im en , tr u e s t r a i n £ = 0 . 1 0 5 , e tc h e d w ith 2fo n i t a l

69

1 0 |im

(<D10>im

F ig u r e 29* S l i p "bands o n th e s u r f a c e o f u n ch a rg e d 1090 p la n e s t r a i n t e n s i o n s p e c im e n , t r u e s t r a i n £ = (a ) 0 . 0 3 1 (b ) 0 .0 4 9 ( c ) 0 .0 6 4 (d ) 0 . 0 9 0 ( e ) 0 . 1 3 7 ( f ) 0 . 2 0 0 (g ) 0 .2 7 7 (h ) 0 .4 5 0

F ig u r e 29* ( c o n t in u e d )

70

l O p m

(e) <h)

71

(a)

(b)

F ig u re 30 . Cracks on MnS p a r t i c l e in 1090 p lan e s t r a in t e n s io n specim en w ith £ = (a ) 0 . 13 7 (b ) 0 . 277

72

10|im

( o ) ( f )

F ig u r e 3 1 . S l i p ban ds on th e s u r fa c e o f p r e c h a r g e d 1020 p la n e s t r a i n t e n s i o n sp e c im e n , t r u e s t r a i n £ = (a ) 0 . 0 3 1 (b ) 0 .0 5 ^ ( c ) 0 .1 1 3 (d ) O .2 3 I ( e ) 0 .2 3 1 , h ig h e r magn i f i c a t i o n to show th e v o id s on s l i p b an d s ( f ) 0 .3 9

73

10|>m

10j»m

( C ) ( J )

F ig u r e 3 2 . S l i p b a n d s on t h e s u r f a c e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n , t r u e s t r a i n £ = (a ) 0 . 0 1 6 (b ) 0 . 0^0 ( c ) 0 . 0 6 6 (d ) 0 . 1 5 5

7^

».y>r >;‘r'

10|im

(c) (d)

F ig u r e 33* S l i p b a n d s on t h e s u r f a c e o f d y n a m ic a l ly c h a r g e d 1020 p la n e s t r a i n t e n s i o n s p e c im e n , t r u e s t r a i n £ = ( a ) 0 . 0 1 5 (*>) 0 .0 2 6 ( c ) 0 . 0 3 9 (d ) 0 .2 7 0

75

(b )

F ig u re 3^* S l ip bands on th e su r fa c e o f d y n a m ica lly charged1090 p la n e s t r a i n t e n s io n sp ec im en , tr u e s t r a i n£ = (a ) 0.015 (b ) 0.020

76

(b)

F ig u re 3 5 * F ra ctu re s u r fa c e o f uncharged 1020 p la n e s t r a int e n s io n sp ecim en (a ) g e n e r a l appearance(b ) d im ple s tr u c tu r e

7 7

(b )

F ig u r e 3 6 . F r a c tu r e s u r fa c e o f u n ch arged 1090 p la n e s t r a i nt e n s io n sp ec im en (a ) g e n e r a l app earance(b ) d im p le s t r u c t u r e

7 8

(b ) ( c )

F ig u r e 37* F r a c tu r e s u r fa c e o f p r e c h a r g e d 1020 p la n e s t r a i nt e n s io n sp ec im en (a ) g e n e r a l ap p earan ce(b ) d im p le r e g io n ( c ) q u a s i - c le a v a g e r e g io n

(b) (c)

F ig u r e 38* F r a c tu r e s u r fa c e o f p r e ch a rg e d 1090 p la n e s t r a i nt e n s io n sp ec im en (a ) g e n e r a l ap p earan ce(b ) d im ple r e g io n ( c ) q u a s i - c le a v a g e r e g io n

80

tOOpm

(a)

;.% y‘' A .\~yt ■

CO (d>

F ig u r e 39* F r a c tu r e s u r f a c e o f d y n a m ic a lly c h a r g ed 1020 p la n e s t r a i n t e n s i o n sp e c im e n (a ) g e n e r a l a p p ea ra n ce (b ) s id e v ie w ( c ) d im p le r e g io n a t r id g e (d ) q u a s i - c l e a v a g e r e g io n

( e ) ( d )

F ig u r e 4 0 . F r a c tu r e s u r f a c e o f d y n a m ic a lly c h a r g ed 1090S t r a i n t e n s i o n sp e c im e n (a ) g e n e r a l ap p ea ra n ce (b ) s id e v ie w ( c ) d im p le r e g io n a t r id g e (d ) q u a s i - c l e a v a g e r e g io n

0>)

F ig u re k l . Mid p la n e o f uncharged 1020 p la n e s t r a in t e n s io n specim en a t £ = O.30

83

■Yl-v' vIH'*' fv * * ' ' ' 51 i “. *3 'I i

( b )

F ig u re ^ 2 . Mid p la n e o f p rech a rg ed 1020 p la n e s t r a i n t e n s io n sp ecim en a t £ = 0 . 3 1 5

(b ) (c )

F ig u r e ^ 3 . Mid p la n e o f u n ch arged 1090 p la n e s t r a i n t e n s io n sp ec im en a t £ = O.k-2 ( a ) s u r fa c e cra ck (b ) i n t e r n a l c r a c k ( c ) i n t e r n a l c ra c k a s s o c ia t e d w ith MnS

F ig u r e V k M id p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n a t £ = £ f = 0 .3 ^ 2 . A rrow s i n d i c a t e s u r f a c e and i n t e r n a l c r a c k s

F ig u r e ^5* Mid p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n , s u r f a c e c r a c k form ed a t d eep i n t r u s i o n n e a r a MnS p a r t i c l e

8 7

(Id)

F ig u re 4 6 . Mid p la n e o f p rech a rg ed 1090 p la n e s t r a in t e n s io n sp ecim en (a ) mode I I c ra c k s (t>) mode I crack

88

(b )

F ig u r e k7 . M id p la n e o f p r e c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n sh o w in g i n t e r n a l c r a c k s ( a ) mode I I (b ) mode I

89

1 0 p m

Cb) ( c )

F ig u r e ^ 8 . M id p la n e o f d y n a m ic a l ly c h a r g e d 1 0 2 0 p la n es t r a i n t e n s i o n s p e c im e n a t £ = 0 . 0 8 6 ,c r a c k s o n o r n e a r s u r f a c e

( b )

F ig u r e ^ 9 . Mid p la n e o f d y n a m ic a lly c h a r g ed 1020 p la n es t r a i n t e n s i o n sp ec im en a t £ = 0 . 0 8 6 , c r a c k sfrom n ea r s u r fa c e MnS p a r t i c l e s

(b )

F ig u r e 5 0 . M id p la n e o f d y n a m ic a l ly c h a r g e d 1090 p la n e s t r a i n t e n s i o n sp e c im e n a t £ = 0 . 0 2 7 * c r a c k s on s u r f a c e

92

1 Of i m

F ig u r e 5 1 . Mid p la n e o f d y n a m ic a lly c h a rg ed 1090 p la n es t r a i n t e n s i o n sp ec im en a t £ - 0 . 05*2,c r a c k s on s u r fa c e

9 3

4-3 Three-Point Bending Tests4 - 3 - 1 L o a d - D e f le c t io n C u rves and M e c h a n ic a l P r o p e r t i e s

F ig u r e s 52 and 53 show t y p i c a l l o a d - d e f l e c t i o n c u r v e s f o r

1020 and 1090 t h r e e - p o i n t b e n d in g t e s t s . S u r fa c e c r a c k s w ere

form ed b e f o r e t h e maximum lo a d w as r e a c h e d f o r a l l s p e c im e n s .

F or 1020 u n ch a rg e d and p r e c h a r g e d s p e c im e n s , th e d e f l e c t i o n s

a t f r a c t u r e w ere so l a r g e t h a t th e t e s t s t e r m in a te d when th e

b e n t sp e c im e n r e a c h e d t h e l i m i t o f th e a n v i l . N e v e r t h e l e s s ,

t h e r e w ere a lr e a d y w e l l d e v e lo p e d c r a c k s on them . The g e n e r

a l y i e l d i n g lo a d (GY) , maximum lo a d (UL) and th e m id sp an

d e f l e c t i o n a t f r a c t i i r e (A f) w ere l i s t e d i n t a b l e s 6 and 7 .

T a b le 6 . T h r e e - p o in t b e n d in g p r o p e r t i e s

o f s p h e r o id iz e d A ISI 1020 s t e e l .

GY(KN) UL (KN) A f (mm)

U n ch arged 6 > 1 7 > 1 9

P r e c h a r g e d 6 13 > 1 9

D y n a m ic a llyC h arged

5*7 8 .3 8

Table 7 . Three-point bending propertiesof spheroidized Alfl 1090 steel.

GY (KN) UL (KN) ^ f (mm)

U n ch arged 6 . 0 7 16.14 9*5

P r e c h a r g e d 6.67 * 1 1 .7 6 5


6.07 10.29 2 . 5

3 - 2 Slip Bands on Notch SurfacesThe slip bands on notch surfaces of bend specimens develop

ed when the local plastic yielding load (about one-third of the general yielding load) was-reached. The slip band development for uncharged 1020 bending specimen is shown in figure 5^. The arrow in figure 54-h denotes the region where figure 54-a to 54-g were located. The strains shown for bend specimens were calculated from equations 1 to 3 as discussed in section 5-1-1*

Figure 5^-b reveals that the MnS inclusions cracked at a small strain, but these cracks did not propagate into the matrix. In figure 5^-ff a surface crack developed from a surface void at a cementite-ferrite interface.

95

F ig u r e 55 show s th e n o tc h s u r f a c e o f a p r e c h a r g e d sp e c im e n

"bent to 9 = 2 5 Compari ng t h i s r e s u l t w ith t h o s e i n f i g u r e s

5 ^ -g and 5 ^ -h one s e e s t h a t th e s l i p b a n d s and c r a c k s a r e

e v e n more d e v e lo p e d th a n f o r th e u n ch a rg ed sp e c im e n b e n t to

0 = 30c.F ig u r e s $6 and 57 r e p r e s e n t th e n o tc h s u r f a c e o f d y n a m ic a l

l y c h a r g ed 1020 sp e c im e n s b e n t t o © = 3 *2 5 °an d 9°» r e s p e c

t i v e l y . I n th e d y n a m ic a l ly c h a r g e d sp e c im e n t h e r e a r e many

s u r f a c e c r a c k s sp r e a d w id e ly on th e n o tc h s u r f a c e . The s l i p

b a n d s on a sp e c im e n b e n t to a l a r g e r a n g le ( f i g u r e 5 7 ) a r e

l i t t l e d i f f e r e n t from th o s e on sp e c im e n s b e n t to a s m a l l a n g le

( f i g u r e 5 6 ) • w h ich m eans t h a t th e fo r m a t io n o f s u r f a c e c r a c k s

r e l e a s e s s t r e s s e s on th e n o tc h s u r f a c e so t h a t i t d o e s n o t

p l a s t i c a l l y s t r a i n f u r t h e r on ce th e c r a c k s fo rm ed .

F ig u r e 58 sh ow s th e s l i p band d e v e lo p m en t f o r u n ch a rg ed

1090 bend s p e c im e n s . F ig u r e 5 8 - a show s t h a t th e s l i p b an d s

w ere form ed b e f o r e th e g e n e r a l y i e l d lo a d was r e a c h e d . F ig u r e

5 8 - c show s t h a t s u r f a c e v o i d s s t a r t to form a t £xx= 0 . 1 2 .

The v o id s c o n n e c te d t o form a s u r f a c e c r a c k a s shown i n f i g u r e

5 8 - d . At a s u r f a c e s t r a i n e q u a l t o th e f r a c t u r e s t r a i n i n

th e p la n e s t r a i n t e n s i o n t e s t , many s u r f a c e c r a c k s fo rm ed .

The s l i p band d ev e lo p m en t c h a r a c t e r i s t i c o f p r e c h a r g e d

1090 sp e c im e n s i s shown i n f i g u r e 59* F ig u r e 60 d e p i c t s th e

s l i p b a n d s on a d y n a m ic a lly c h a r g e d sp e c im e n b e n t t o a s u r f a c e

s t r a i n o f £xx= 0 . 0 4 . The s u r f a c e c r a c k i s j u s t a b o u t to form .

9 6

T h is o b s e r v a t io n i s c o n s i s t e n t w ith th e work o f L ee e t a l

( 28) - , who fo u n d th e s u r f a c e s t r a i n f o r c r a c k i n i t i a t i o n on

d y n a m ic a l ly c h a r g e d sp ec im en t o b e O .0 5 .

3 F r a c tu r e S u r f a c e s

S in c e u n ch a rg ed and p r e c h a r g e d 1020 bend sp e c im e n s can

n o t bend t o f r a c t u r e i n th e t e s t s , th e y w ere b e n t t o th e

l i m i t o f th e c o m p r e s s io n a n v i l , c o o le d to l i q u i d n i t r o g e n

t e m p e r a tu r e , and f r a c t u r e d c o m p le t e ly b y a b lo w w ith a hammer

t o r e v e a l th e r o o m -te m p e r a tu r e f r a c t u r e s u r f a c e . S u r fa c e

c r a c k s on u n ch a rg ed 1020 bend sp ec im en w ere q u i t e s h a l lo w a s

shown i n f i g u r e 6 1 . The c r a c k e d s u r f a c e was com posed o f dim

p l e s and se c o n d a r y c r a c k s . The f a s t f r a c t u r e s u r f a c e form ed

a t l i q u i d n i t r o g e n te m p e r a tu r e i s t r a n s g r a n u la r c le a v a g e i n

n a t u r e .

F ig u r e 62 r e p r e s e n t s th e f r a c t u r e s u r f a c e o f p r e c h a r g e d

1020 bend s p e c im e n s . The f r a c t u r e s u r f a c e c o n t a in s d im p le s

and many se c o n d a r y c r a c k s . As c a n b e m easu red on f i g u r e 6 2 - a ,

th e c r a c k had p r o p a g a te d a d i s t a n c e o f a b o u t ^ .8 mm when th e

t e s t w as s to p p e d . T h ere a r e many la r g e r d im p le s on t h i s

s u r f a c e com pared w ith th e f r a c t u r e s u r f a c e o f an u n ch a rg e d

sp e c im e n . L ik e th e u n ch a rg ed sp e c im e n , th e f r a c t u r e s u r f a c e

p r o d u ce d a t l i q u i d n i t r o g e n te m p e r a tu r e i s c le a v a g e i n n a t u r e .

F ig u r e 63 show s th e f r a c t u r e s u r f a c e o f a d y n a m ic a l ly c h a r g

e d 1020 bend sp e c im e n . The f r a c t u r e s u r f a c e c o n s i s t s o f th r e e

r e g i o n s . I n a r e g io n from th e n o tc h r o o t to a b o u t 5 - 2 mm

97

b e lo w t h e n o tc h th e f r a c t u r e s u r f a c e i s f l a t h y d ro g e n a s s i s t e d

q u a s i - c l e a v a g e i n form . N e x t , t h e r e i s a c le a v a g e r e g i o n ,

f o l lo w e d b y th e d u c t i l e s h e a r l i p . A lth o u g h i t i s form ed i n

a room te m p e r a tu r e t e s t , t h e c le a v a g e r e g i o n ( f i g u r e 6 3 - c )

r e s e m b le s t h a t on a s u r f a c e form ed b y f a s t f r a c t u r e a t l i q u i d

n i t r o g e n te m p e r a tu r e ( f i g u r e 6 l - c and 6 2- d ) .

The f r a c t u r e s u r f a c e o f an u n ch a rg ed 1090 bend sp e c im e n i s

shown i n f i g u r e 6 4 . N ear th e n o tc h r o o t , t h e r e i s a c r e s c e n t

sh a p e d , d u c t i l e , s h e a r - c r a c k r e g i o n . The d e p th o f th e s h e a r

c r a c k r e g io n i s a b o u t 1 mm. The m ajor a r e a on th e f r a c t u r e

s u r f a c e h a s a t r a n s g r a n u la r c le a v a g e a p p ea ra n ce a s t h a t

form ed b y f a s t f r a c t u r e a t l i q u i d n i t r o g e n te m p e r a tu r e ( f i g u r e

6 7 ) . T h ere i s a s h e a r o f f zon e a b o u t 0 .1 mm w id e w h ich i s

d im p le d on th e p e r ip h e r y o f t h e f r a c t u r e s u r f a c e .

The bend f r a c t u r e s u r f a c e s o f th e p r e c h a r g e d 1090 sp e c im e n s

w ere s i m i l a r t o t h o s e f o r u n ch a rg ed s p e c im e n s , e x c e p t t h a t

th e d e p th o f th e s h e a r c r a c k r e g i o n on th e p r e c h a r g e d s p e c i

men i s o n ly a b o u t 0 . 5 mm ( f i g u r e 6 5 ) .

The f r a c t u r e s u r f a c e o f a d y n a m ic a lly c h a r g ed 1090 bend

sp e c im e n i s shown i n f i g u r e 6 6 . N ear th e n o tc h r o o t i s a

h y d r o g e n - a s s i s t e d , q u a s i - c l e a v a g e r e g io n a b o u t 0.2mm w id e .

The o t h e r p a r t o f th e f r a c t u r e s u r f a c e i s t r a n s g r a n u la r

c le a v a g e i n a p p e a r a n c e .

98

4-3-4 Cracks and VoidsF ig u r e s 68 t o 73 show th e n o tc h raid p la n e o f 1020 and 1090

sp e c im e n s "bent u n d er d i f f e r e n t c h a r g in g c o n d i t i o n s u n t i l w e l l

d e v e lo p e d s u r f a c e c r a c k s w ere o b s e r v e d . The o v e r a l l mode o f

f r a c t u r e o f b o th 1020 and 1090 u n ch a rg ed and p r e c h a r g e d s p e c

im en s w ere t h a t i p l a s t i c f lo w l o c a l i z e d a lo n g c h a r a c t e r i s t i c

s l i p l i n e t r a c e s o f p l a s t i c s h e a r i n s t a b i l i t y ; f o l lo w e d by

v o id f o r m a t io n , g r o w th , c o a le s c e n c e ; a n d , s u b s e q u e n t ly , th e/

i n i t i a t i o n o f c r a c k s from th e s u r f a c e a lo n g t h e s e t r a c e s .

S im i la r t o th e r e s u l t o f Lee e t a l ( 2 8 ) th e v o id d e n s i t y i n

t h e s e sp e c im e n s i s r e l a t i v e l y i n t e n s e n e a r th e n o tc h s u r f a c e

and a lo n g th e c h a r a c t e r i s t i c s l i p l i n e s .

I n c o n t r a s t , c r a c k s o n d y n a m ic a lly c h a r g e d sp e c im e n s o c c u r -

e d i n th e c r a c k o p e n in g mode (mode I ) , w ith t h e c r a c k p la n e

p e r p e n d ic u la r to th e n o tc h s u r f a c e and p r o p a g a t in g a lo n g th e

p la n e w ith maximum t e n s i l e s t r e s s a c t i n g a c r o s s i t . O n ly a

few i s o l a t e d v o i d s w ere o b s e r v e d on th e m id p la n e o f t h e s e

s p e c im e n s . As w as a l s o tr u e f o r p la n e s t r a i n t e n s i o n s p e c i

m en s, c r a c k s w ere o c c a s i o n a l l y o b s e r v e d to p a s s th r o u g h a

c a r b id e r a t h e r th a n c a u s in g d e c o h e s io n o f th e f e r r i t e - c a r b i d e

i n t e r f a c e and f o l lo w in g t h i s p a th . As a c r a c k grew i t som e

t im e s b r a n c h e d . P l a s t i c f lo w and v o i d s w ere o b s e r v e d i n th e

b r a n c h in g r e g i o n a s shown i n f i g u r e 7 0 - c .

Lo

ad

(kN

)

18

16

14

12

10

8

6

4

2

00 2 4 6 8 10 12 14 16 18. 20

Mid-Span Deflection (mm)

Uncharged

Precharged

DynamicallyCharged

F ig u re 5 2 . L o a d -d e f le c t io n cu rv es fo r 1020 bend t e s t s

18

16

14

12

2 io

1 8o_J

6

4

2

0

53. ]

Uncharged

Precharged

Dynamically Charged

6 8 10 12 14Mid-Span Deflection (mm)

- d e f l e c t i o n cu rv es fo r 1090 bend t e s t s

101

P H

10m™ lopm

(■:) ( <J)

F ig u r e 54*. S l i p "bands on th e n o tc h s u r f a c e o f u n ch a rg ed 1020 bend sp e c im e n , p l a s t i c b en d a n g le 0 and n o tc h s u r f a c e s t r a i n £ x x = (a ) 0.34*° » 0 . 0 1 ?( b ) 0 . 8 V , 0 . 0 4 2 ( c ) 1 . 8 8 ° , 0 . 0 9 4 ( d) 3 - 0 4 ° , 0 . 1 4 ( e ) 5 * 6 ° , 0.224* ( f ) 1 1 ° , 0 .4-05 (g ) and (h ) 3 0 ° , 1 . 0 3 5

F ig u r e 5^* ( c o n t in u e d )102

100/jm

(a )

(b )

F ig u r e 55* S l i p bands on th e n o tc h s u r fa c e o f p r e ch a rg e d1020 bend sp ec im en , p l a s t i c bend a n g le Q = 25°

lOJj-

(b )

F ig u r e 5 6 . S l i p bands on th e n o tc h s u r fa c e o f d y n a m ic a llych arged 1020 bend sp ec im en , p l a s t i c bend a n g lee = 3 . 2 5 0

F ig u re 57-

106

- K '

lOjiro

F ig u r e 5 8 . S l i p b a n d s on th e n o tc h s u r f a c e o f u n ch a rg e d 1090 b en d sp e c im e n , p l a s t i c bend a n g le 0 and n o tc h s u r f a c e s t r a i n £ x x - ( a) 0 . 1 7 ° , 0 . 0 0 7 (b) 0 . 8 8 ° , 0 . 0 3 7 ( c ) 2 . 8 ° , 0 . 1 1 7 ( d) 5 . 1 ° , 0 . 1 8 ( e ) 1 2 . 1 ° , 0 . 3 8 7 ( f ) 1 8 . 3 ° , 0 . 5 6 7 ( g ) and (h) 2 3 . ^ , 7 1 . 5

F ig u r e 58* ( c o n t in u e d )

107

1 0 p m

(b ) (c )

F ig u re 59- S l i p bands on th e n o tch s u r fa c e o f p rech arged 1090 bend sp ec im en , p l a s t i c bend a n g le 0 and n o tch su r fa c e s t r a i n £ x x = (a ) 0 . 8 ° , 0 . 03^(b ) 2 . 2 5 ° , O.095 ( c ) 6 . 8 3 ° , 0 . 1 7 7

109

jtfiSS®!**.

1 Opm

JW ,

1 0 p m

F ig u r e 6 0 . S l i p b an d s on th e n o tc h s u r fa c e o f d y n a m ic a llyc h a r g e d bend sp e c im e n , p l a s t i c b en d a n g le 0 = 1°,n o tc h s u r fa c e s t r a i n £ x x = 0 .0 ^

(b ) ( c )

F ig u r e 6 1 . F r a c tu r e s u r f a c e o f u n c h a r g e d 1020 b en d sp e c im e n ( a ) g e n e r a l a p p e a r a n c e (b ) f r a c t u r e form ed i n b e n d in g t e s t ( c ) f r a c t u r e form ed b y f a s t b lo w a t l i q u i d n i t r o g e n te m p e r a tu r e

I l l

F ig u r e 6 2 . F r a c tu r e s u r f a c e o f p r e c h a r g e d 1020 "bend sp e c im e n ( a ) g e n e r a l a p p e a r a n c e (h ) s e c o n d a r y c r a c k s and d im p le d r e g i o n ( c ) d im p le s (d ) f r a c t u r e form ed b y f a s t b lo w a t l i q u i d n i t r o g e n te m p e r a tu r e

112

HI

■ ■ lOOjjm

(c) (d)

F ig u r e 6 3 . F r a c tu r e s u r f a c e o f d y n a m ic a lly c h a r g ed1020 bend sp e c im e n (a ) g e n e r a l a p p ea ra n ce (b ) t r a n s i t i o n r e g io n ( c ) h i d r o g e n - a s s i s t e d q u a s i - c l e a v a g e r e g io n (d ) c le a v a g e r e g io n

113

1 0 0 /70,

(a )

10pm

(b) ( c )

F igu re 6*K F ractu re su r fa c e o f uncharged 1090 bend specim en(a ) g e n e r a l appearance (b ) dim pled r e g io n(c ) c le a v a g e r e g io n

114

(b ) ( c )

F ig u r e 6 5 • F r a c tu r e s u r fa c e o f p r e c h a r g e d 1090 bend sp ec im en(a ) g e n e r a l ap p earan ce (b ) d im p led r e g io n(c ) c le a v a g e r e g io n

115

10pmkS

(b ) ( c )

F ig u r e 6 6 . F r a c tu r e s u r f a c e o f d y n a m ic a lly c h a r g ed 1090 bendsp e c im e n (a ) g e n e r a l a p p ea ra n ce (b ) h y d r o g e n -a s s i s t e d q u a s i - c l e a v a g e r e g i o n ( c ) c le a v a g e r e g io n

116

F ig u r e 6 7 . F r a c tu r e s u r f a c e o f u n ch a rg ed 1090 b en d sp e c im e nform ed b y f a s t b lo w a t l i q u i d n i t r o g e n te m p e r a tu r e

(b )

F ig u re 6 8 . N otch mid p la n e o f u n ch arged 1020 bend sp ec im en w ith p l a s t i c bend a n g le 6 = 3 0 .2°

118

00m *i.

(b) ( c )

F igu re 69* N otch mid p la n e o f p rech arged 1020 bend specim enw ith p l a s t i c bend a n g le Q = 25 °

(b ) (c )

F ig u r e 7 0 . N otch mid p la n e o f d y n a m ic a lly ch a rg ed 1020 bend sp ec im en

120

(a)

tS r *i<£'

wF ig u r e 7 1 . N otch mid p la n e o f un ch arged 1090 bend sp ec im en

w ith p l a s t i c bend a n g le Q = 23

121

(b )

F ig u re 7 2 . Notch mid p la n e o f p rech arged 1090 bend specim enw ith p l a s t i c bend a n g le 0 = 7 .2°

( b ) ( c)

F ig u r e 73* N otch m id p la n e o f d y n a m ic a lly ch a rg ed 1090 hend sp ec im en

5 . DISCUSSION

5 - 1 The E f f e c t o f P l a s t i c S t r a i n S t a t e

5 - 1 - 1 S l i p Bands on S u r fa c e

Pond (1 2 9 ) s t u d ie d c r y s t a l s o f a number o f f a c e - c e n t e r e d

c u b ic m e t a ls and one b o d y -c e n t e r e d c u b ic m e ta l and fo u n d t h a ti

s l i p b an d s a r e in h o m o g e n e o u s ly d i s t r i b u t e d th r o u g h o u t a d e

form ed c r y s t a l and t h a t th e s l i p p la n e s w i t h i n a s l i p band

a r e n o t u n ifo rm i n num ber, d i s p e r s i o n and e x t e n t o f g l i d i n g .

So when m aking c o m p a r iso n o f th e s l i p b an d s on d i f f e r e n t s p e c

im e n s , one h a s to c o n s id e r b o th g e n e r a l a p p ea ra n ce and s p e c i a l

r e g i o n s su ch a s th e r e g io n w ith th e h i g h e s t e x t e n t o f g l i d i n g .

The s t r a i n o f a round b a r o r p la n e s t r a i n t e n s i o n sp e c im e n

c a n be d e te r m in e d b y th e d i r e c t m easu rem en t o f e lo n g a t io n o r

r e c u c t i o n o f a r e a o r t h i c k n e s s . F o r a bend s p e c im e n , th e

p r i n c i p a l s u r f a c e s t r a i n £ x x h a s to be c a l c u l a t e d i n d i r e c t l y

from th e m easu red p l a s t i c bend a n g le . By u s e o f th e s u r f a c e

g r o o v e m eth od , G o ld en b e rg e t a l (7 3 ) o b ta in e d th e n o tc h r o o t

p r i n c i p a l s t r a i n £ x x a s a f u n c t i o n o f p l a s t i c d e f l e c t i o n a n g le

0 . F or a sp e c im e n w ith n o tc h r a d iu s r = 1 .1 9 th e r e l a t i o n

i s £ xx = O.Oij-20 f o r 0 < 2 . 6 °

£ x x = 0 .0 2 9 0 + 0 * °3 6 f o r 0 > 2 . 6 ° E q u a t io n (1 )

123

124

B a se d on th e r e s u l t s o f G o ld en b erg e t a l and G r i f f i s e t a l

( 1 0 1 ) , th e a u th e r d e r iv e d a f u n c t i o n f o r r = 1 . 0 0 mm

£ x x = 0 . 0 5 0 0 f o r © < 2 . 3 5 °

£ x x = 0 .0 3 3 0 + 0 .0 4 0 f o r 0 > 2 . 3 5 ° E q u a t io n (2 )

By th e m easu rem en t o f th e maximum n o tc h o p e n in g d is p la c e m e n t ,

O nyew uenyi ( 1 3 0 ) , on th e b a s i s o f th e r e s u l t o f G o ld en b erg e t

a l , d e r iv e d a r e l a t i o n b e tw e e n 0 and£Hkx f o r th e h y d ro g en

c h a r g e d c a s e w ith r = 1 . 1 9 mm

£H xx = 0 .0 1 9 0 * 0 .0 4 7 f o r 0 > 2 . 6 ° E q u a t io n ( 3 )

The n o tc h s u r f a c e s t r a i n s i n t h i s work w ere c a l c u l a t e d from

e q u a t io n s ( 1 ) —( 3 ) • By co m p a rin g th e s l i p b a n d s on bend n o tc h

s u r f a c e s and on p la n e s t r a i n t e n s i o n sp e c im e n s ( f o r e x a m p le ,

f i g u r e 5^ and f i g u r e 5 7 )> one f i n d s t h a t th e a g r ee m e n t i s

e x c e l l e n t .

H ow ever, s in c e t h e r e i s a s t r a i n g r a d ie n t i n th e bend s p e c

im en , i t i s r e a s o n a b le to f i n d t h a t th e s u r f a c e v o i d s and s u r

f a c e c r a c k s a re c o n s t r a in e d b y th e l e s s s t r a i n e d m a t e r ia l

u n d er th em . A lso th e c u r v ed c h a r a c t e r i s t i c s l i p t r a j e c t o r y

i n th e b en d b a r s com pared t o th e p la n a r t r a j e c t o r y i n p la n e -

s t r a i n t e n s i o n may be a f a c t o r f a v o r in g v o i d l i n k i n g i n th e

l a t t e r c a s e . F or e x a m p le , f i g u r e 2 9 -g show s t h a t th e a c r a c k

w as fou n d on 1090 u n ch a rg ed p la n e s t r a i n sp e c im e n s a t £ =

0 . 2 8 , w h ile on a bend sp ec im en a t a s u r f a c e s t r a i n £ x x = 0 . 3 9

( f i g u r e 5 8 - e ) , e v e n th o u g h t h e r e i s e v id e n c e o f v o id c o n n e c

t i o n , th e c o n n e c t io n h a s n o t p ro d u ced a c r a c k . The f r a c t u r e

s t r a i n o f p la n e s t r a i n sp e c im e n s c o r r e s p o n d s t o th e c o n d i t i o n

125

f o r b en d sp e c im e n s when t h e r e a r e many s u r f a c e c r a c k s on th e

U -n o tc h s u r f a c e ( f i g u r e s 5 8 - g and 5 8 - h ) .

F or s t r a i n s s m a l le r th a n 0 . 1 , th e a g reem en t f o r th e d e g r e e

o f s l i p and s l i p c o n f ig u r a t io n i n s l i p b a n d s b e tw e e n rou n d

b a r s and p la n e s t r a i n t e n s i o n o r bend sp e c im e n s i s a l s o g o o d .

H o w ev er , on rou n d b a r s u r f a c e s t h e r e a r e num erous s l i p band

i n t e r s e c t i o n s a s d i s c u s s e d i n t h e n e x t s e c t i o n .

The s l i p b a n d s on t h e 1020 sp e c im e n s a r e i n g e n e r a l more

i n t e n s e th a n t h o s e on th e 1090 sp e c im e n s t e s t e d u n d er t h e same

c o n d i t i o n s . T h is m eans t h a t th e am ount o f s l i d e i n e a c h band

i n th e 1020 sp e c im e n s i s more th a n t h a t i n th e 1090 s p e c im e n s .

T h is f a c t i s c o n s i s t e n t w ith A s h b y 's (1 3 1 ) o b s e r v a t io n on

c o p p e r s i n g l e c r y s t a l s c o n t a in in g SiO p a r t i c l e s . He fo u n d

t h a t th e d e fo r m a t io n becam e more hom ogeneous a s th e vo lum e

f r a c t i o n o f p a r t i c l e s w as in c r e a s e d so t h a t s l i p l i n e s becam e

f i n e r and f i n a l l y u n o b s e r v a b le .

The vo lum e f r a c t i o n o f c a r b id e i n t h e 1090 s t e e l i s 1 3 -2 ^ ,

w h ile i t i s 3% i n th e 1020 s t e e l . I n d e fo r m a t io n , th e c a r b id e

p a r t i c l e s a c t a s th e b a r r i e r s t o d i s l o c a t i o n m ovem ent. T hey <

f o r c e d i s l o c a t i o n s t o c r o s s - s l i p a s s t r a i n i s i n c r e a s e d , r e

s u l t i n g i n in c r e a s e d a p p l ie d s t r e s s and th e a p p ea r a n c e o f new

d i s l o c a t i o n s o u r c e s . T h e r e f o r e , a s th e number and vo lum e

f r a c t i o n o f c a r b id e p a r t i c l e s i s in c r e a s e d , th e s l i p b a n d s

w i l l i n c r e a s e i n number and d e c r e a s e i n e x t e n t o f s h e a r .

126

5 - 1 - 2 D u c t i l i t y and Work H a rd en in g R a te

I t i s w e l l knwon t h a t f o r a g i v e n m a t e r i a l , th e p la n e s t r a i n

d u c t i l i t y i s u s u a l l y lo w e r th a n t h a t m easu red from an axisym m e-

t r i c t e n s i l e t e s t . I n f a c t , th e d u c t i l i t y c o n t in u o u s ly

d e c r e a s e s a s one p r o c e e d s from a m in or t o m ajor s t r a i n r a t i o

6 a / £ i o f 0 . 5 ( a x isy m m e tr ic t e n s io n ) t o 0 ( t h e v a lu e f o r p la n e

s t r a i n ) ( 1 3 2 ) . T h is t r e n d i s e v id e n t i n a co m p a r iso n o f th e

rou n d b a r and p la n e s t r a i n t e n s i o n t e s t r e s u l t s i n t h i s w ork .

W agoner (133»3-3*0 fo u n d t h a t 2 0 3 6 -T 4 alum inum work h a r d e n s

s i g n i f i c a n t l y l e s s i n p la n e s t r a i n th a n i n u n i a x i a l t e n s i o n

and th e work h a r d e n in g f o r two co m m erc ia l s h e e t s t e e l s i s a l s o

lo w e r i n p la n e s t r a i n t e n s i o n b eyon d th e u n ifo r m s t r a i n .

The o b s e r v a t io n s m en tio n e d ab ove a re r e l a t e d t o one a n o th e r

i n p l a s t i c d e fo r m a t io n . S in c e a lo w e r work h a r d e n in g r a t e

p r o m o te s p l a s t i c i n s t a b i l i t y ( e i t h e r n e c k in g o r th e l o c a l i z e d

f lo w a lo n g c h a r a c t e r i s t i c t r a c e s ) , i t c a u s e s e a r l i e r f a i l u r e

and lo w e r d u c t i l i t y . I r i c i b a r e t a l ( 1 3 5 ) fo u n d t h a t th e

a c c u m u la t io n o f m ic r o s t r u c t u r a l damage d ep en d s n o t o n ly on

th e l o c a l l e v e l o f th e f lo w s t r e s s and th e im p osed s t r a i n r a t e ,

b u t on th e work h a r d e n in g r a t e o f th e m a tr ix . I f th e work

h a r d e n in g c a p a c i t y ca n be r e s t o r e d i t i s p o s s i b l e t o c o a x th e

m a t e r ia l t o much la r g e r s t r a i n s p r io r to f a i l u r e .

C h a k r a b a r ti and S p r e tn a k ( 9 6 ) p o in t e d o u t t h a t i n t h i n s h e e t

s p e c im e n s , th e s t r e s s i n th e t h i c k n e s s d i r e c t i o n ( (S3) i s a s

sumed t o b e z e r o , y i e l d i n g a p la n e s t r e s s c o n d i t i o n . As th e

g e o m e tr ic c o n s t r a i n t t o c o n t r a c t i o n i n th e t r a n s v e r s e

127

d i r e c t i o n i s in c r e a s e d , a t e n s i l e s t r e s s ^ 2 i s g e n e r a te d i n

in c r e a s i n g m a g n itu d e . As th e r a t i o c5 2 /c jl r e a c h e s a v a lu e o f

0 .5 »‘ t h e c o n d i t i o n f o r p la n e s t r a i n £ 2 =0 i s a c h ie v e d . T h u s ,

th e s t a t e o f s t r e s s (^1, ( 3 l / 2 , 0 r e p r e n t s a c o m b in a t io n o f

p la n e s t r e s s and p la n e s t r a i n . The C la u s in g p la n e s t r a i n

sp e c im e n u s e d i n t h i s w ork and m ost s h e e t p la n e s t r a i n s p e c i

m ens u s e d i n o t h e r r e s e a r c h e s a r e " p la n e s t r e s s - p l a n e s t r a in "

t e n s i l e s p e c im e n s .

I n t e n s i o n t e s t s , th e maximum r e s o lv e d s h e a r s t r e s s o c c u r s

o n t h o s e p la n e s t h a t a re i n c l i n e d a t ^ 5 ° to th e t e n s i l e a x i s .

I n u n i a x i a l t e n s i o n th e p la n e s w ith t h e maximum r e s o l v e d s h e a r

s t r e s s form c o n e s w ith th e t e n s i l e d i r e c t i o n a s th e sym m etry

a x i s an d , b e c a u s e o f th e ch an ge i n Schm id f a c t o r ( 1 3 6 ) th r o u g h

a g r a in , s e v e r a l s y s te m s c o u ld be th e p r im a ry s l i p sy s te m i n

a g r a in , e a c h one a c t i v e i n a d i f f e r e n t p o r t io n . I n c o n t r a s t ,

i n a p la n e s t r a i n sp e c im e n , b e c a u s e o f th e g e o m e t r ic a l co n

s t r a i n t , o n ly p la n e s n e a r l l y p a r a l l e l to th e two s e t s o f

m u tu a lly p e r p e n d ic u la r p la n e s o f maximum r e s o l v e d s h e a r s t r e s s

can c o n t r ib u t e to d e fo r m a t io n , and o n ly one s l i p sy s te m w i l l ,

i n g e n e r a l , be th e p r im a ry s l i p sy s te m w ith maximum Schm id

f a c t o r .

T h e r e f o r e , u n d er th e s t a t e o f u n i a x i a l t e n s i o n , c h a n c e s a re

t h a t t h e r e axe s e v e r a l s l i p s y s te m s i n a g r a in w h ich a re a c t

i v e a t th e o n s e t o f d e fo r m a t io n . D i s l o c a t i o n s on s e v e r a l

s l i p s y s te m s s t a r t to move w i t h i n a r e l a t i v e l y s m a ll in c r e m e n t

o f a p p l ie d s t r a i n . B e ca u se o f th e i n t e r a c t i o n b e tw e e n

128

d i s l o c a t i o n s on i n t e r s e c t i n g s l i p s y s t e m s , d e fo r m a t io n b y du

p l e x s l i p r e s u l t s i n a h ig h d e g r e e o f s t r a i n h a r d e n in g . On th e

o t h e r h a n d , u n d er th e p la n e s t r e s s - p l a n e s t r a i n s t a t e , i t i s

v e r y l i k e l y t h a t i n a g iv e n g r a in t h e r e i s a l a r g e d i f f e r e n c e

b e tw e e n th e Schm id f a c t o r s on th e p r im a ry s l i p sy s te m and on

o t h e r s l i p s y s t e m s . The d i s l o c a t i o n s on th e o t h e r s l i p s y s

tem s th e n w ou ld be a c t i v a t e d o n ly a f t e r th e p r im a ry sy s te m

had b e e n r o t a t e d and work h a rd en ed t o an amount l a r g e enough

t o o f f s e t th e d i f f e r e n c e s o f Schm id f a c t o r b e tw e e n them .

A l s o , i n d i f f e r e n t g r a i n s , t h e s e "m ost a c t iv e " s l i p p la n e s

a r e i n c l i n e d o n ly b y r e l a t i v e l y s m a ll a n g le s to and h e n c e

r o u g h ly l y i n g a lo n g one o f th e two s e t s o f p la n e s w ith m a x i

mum r e s o l v e d sh e a r s t r e s s . S im i la r id e a s have b e e n d i s c u s s e d

b y Hahn e t a l ( 1 3 7 , 1 3 8 ) .

T h is id e a s ca n b e v e r i f i e d by com p arin g th e s u r f a c e s l i p

b a n d s form ed on round b a r and p la n e s t r a i n t e n s i o n s p e c im e n s .

F or e x a m p le , i n f i g u r e 14- t h e r e a r e a l o t o f i n t e r s e c t i n g

s l i p b a n d s , w h ile i n f i g u r e 2 7 , a much s m a l le r number o f s l i p

band i n t e r s e c t i o n s w ere o b s e r v e d a t h ig h s t r a i n s . The above

r e a s o n in g e x p l a in s th e g e n e r a l t r e n d o f lo w e r work h a r d e n in g

r a t e and d u c t i l i t y o f m a t e r i a l s u n d er th e p la n e s t r a i n s t a t e .

W agoner ( 1 3 9 ) n o t i c e d t h a t , i n g e n e r a l , th e lo w e r in g o f work

h a r d e n in g r a t e u n d er th e p la n e s t r a i n s t a t e was m ore p ro n o u n c

ed i n f . c . c . m e t a ls th a n i n b . c . c . m e t a l s . T h is o b s e r v a t io n

c a n be r a t i o n a l i z e d b y th e id e a d i s c u s s e d a b o v e , t o o . The

s t a c k in g f a u l t e n e r g i e s o f f . c . c . m e t a ls a r e known to be lo w e r

129

th a n t h o s e o f h . c . c . m e t a l s . I n h ig h s t a c k in g f a u l t e n e r g y

m a t e r i a l s , th e d i s l o c a t i o n s ca n e a s i l y c r o s s - s l i p w h ile th e

d i s l o c a t i o n m ovem ents i n lo w s t a c k in g f a u l t e n e r g y m a t e r i a l s

t e n d t o be p la n a r . As a lr e a d y d i s c u s s e d , d u p le x s l i p t a k e s

p la c e e a s i l y i n th e c a s e o f u n i a x i a l t e n s i o n w h ile s i n g l e s l i p

i s fa v o r e d i n th e p la n e s t r a i n s t a t e . P la n a r s l i p t e n d s to

i n c r e a s e t h e p o te n c y o f d i s l o c a t i o n p i l e u p s and h e n ce make

i t e a s i e r t o t r a n s m it s l i p a c r o s s b o u n d a r ie s . T h u s, worki

h a r d e n in g s h o u ld be r e l a t i v e l y lo w f o r p l a n e - s t r a i n f . c . c .

m e t a l s . On th e o t h e r h an d , d i s l o c a t i o n b a r r i e r s a r e s t r o n g e r

f o r i n t e r s e c t i n g s l i p s y s te m s i n f . c . c . m e t a l s , so work h a r d

e n in g s h o u ld be r e l a t i v e l y h ig h f o r u n i a x i a l t e n s i o n s p e c im e n s .

Thus th e d i f f e r e n c e i n work h a r d e n in g f o r p la n e s t r a i n v e r s u s

u n i a x i a l sp e c im e n s s h o o ld be la r g e i n f . c . c . m e t a ls a s o b s e r v e d .

5 - 1 - 3 P l a s t i c I n s t a b i l i t y

P l a s t i c i n s t a b i l i t y i n th e form o f n e c k in g i s w id e ly r e c

o g n iz e d . D u r in g t e n s i l e d e fo r m a t io n , n e c k in g b e g in s a t a

maximum lo a d w here th e in c r e a s e i n s t r e s s c a u se d by th e d e

c r e a s e i n th e c r o s s - s e c t i o n a l a r e a o f a sp e c im e n b ecom es

g r e a t e r th a n th e in c r e a s e i n th e lo a d c a r r y in g a b i l i t y o f th e

m e ta l due to s t r a i n h a r d e n in g ; t h a t i s t dP = d dA + Ad<5 < 0

o r d<5/d£,<($» I n s t e a d , th e mode o f p l a s t i c i n s t a b i l i t y o f

i n t e r e s t i n t h i s work i s t h e l o c a l i z a t i o n o f p l a s t i c f lo w

a lo n g c h a r a c t e r i s t i c t r a c e s a s d e s c r ib e d i n s e c t i o n 2 - 4 .

130

The c h a r a c t e r i s t i c s a r e t r a c e s o f p la n e s o f p i r e s h e a r

d e fo r m a t io n . They ca n he fo u n d h y d e te r m in in g t h e p la n e s

o f p u re s h e a r s t r e s s from th e d e v i a t o r i c p a r t o f th e s t r e s s

s t a t e . F ig u r e 74 show s th e c h a r a c t e r i s t i c s and th e s t r e s s

M oh r's c i r c l e f o r a sp e c im e n u n d er a p la n e s t r e s s (<^i, da ,

d'3 = 0) c o n d i t i o n . The a n g le s b e tw e e n th e c h a r a c t e r i s t i c

t r a c e s and th e t e n s i l e a x i s ch an ge w ith th e s t r e s s s t a t e a s

th e f o l lo w in g r e l a t i o n s *

<f>3 = 1 /2 COS on 6 3 p la n eT3 3 ( d a - d , )

E q u a t io n (4 )

& = l /2 COS 1 —^~^3 - on 6* p la n e3 ( 6a - 6, )

I n th e c a s e o f u n i a x i a l t e n s i o n , d a = 63 = 0 , 4* ~ $3 =

5 4 . 7 4 0 . i n c r e a s e s w h ile ^ d e c r e a s e s a s da, i n t e n s i o n

i n c r e a s e s from z e r o . At = 0 . 5 * th e s t r e s s s t a t e b ecom es

one o f p la n e s t r e s s - p l a n e s t r a i n , and e a c h two s e t s o f c h a r

a c t e r i s t i c s c o l l a p e s in t o one w ith A - 90° and = 4 5 ° . I n

t h i s c a s e th e c h a r a c t e r i s t i c p la n e s c o in c id e th e p la n e s o f

maximum r e s o l v e d sh e a r s t r e s s . At th e s t a g e w here c h a r a c t e r

i s t i c b e h a v io r o c c u r s , th e f lo w i s e s s e n t i a l l y p l a s t i c i n

n a tu r e and c r y s t a l s l i p h a s no m ea n in g . N o n e t h e le s s , f o r u n i

a x i a l s p e c im e n s , th e ’’l o c a l " c h a r a c t e r i s t i c s make a con e w ith

th e t e n s i l e a x i s a s th e sy m m etr ic a x i s and i n t e r s e c t i o n i s

e n h a n ce d . E ven n e a r th e i d e a l p l a s t i c l i m i t , th o u g h , i n t e r

s e c t i o n s h o u ld d i s r u p t i n s t a b i l i t y and in c r e a s e th e l o c a l work

h a r d e n in g r a t e . On th e o t h e r h a n d , no su ch h a r d e n in g or

F ig u re 7^i C h a r a c t e r is t ic s u r fa c e s and co rresp o n d in g s t r e s s M ohr's c i r c l e fo r a specim en under p la n e s t r e s s c o n d it io n

132

i n t e r r u p t i o n o c c u r s i n p la n e s t r a i n . T h e r e f o r e , th e m a t e r ia l

u n d er th e p la n e s t r e s s - p l a n e s t r a i n s t r e s s s t a t e i s i n t r i n s

i c a l l y more s u s c e p t i b l e t o c o n c e n tr a t e s h e a r o r p l a s t i c

i n s t a b i l i t y .

On th e b a s i s o f a c o m p a r iso n o f th e s h e a r s t r a i n a t w h ich

th e f o lw s t a r t s t o l o c a l i z e on t o r s i o n b a r s w ith d i f f e r e n t

d ia m e te r s and on U and V -n o tc h e d bend b a r s , S p r e tn a k e t a l

( 9 7 ) had p o s t u l a t e d t h a t p l a s t i c i n s t a b i l i t y i s fa v o r e d b y a

s t r e s s / s t r a i n g r a d i e n t . T hey fo u n d th e s t r a i n f o r p l a s t i c

i n s t a b i l i t y i n i t i a t i o n d e c r e a s e s a s th e s t r e s s / s t r a i n g r a d ie n t

i n c r e a s e s . H ow ever , Mohamed and T e te lm a n ( 1 1 5 ) s t u d ie d th e

d e fo r m a t io n o f lo w ca rb o n s t e e l sp e c im e n s w ith d i f f e r e n t n o tc h

r o o t r a d i i and o b s e r v e d t h a t th e s t r a i n a t c r a c k i n i t i a t i o n

w as in d e p e n d e n t o f n o tc h r o o t r a d i i . Lee e t a l ( 2 8 , 4 - 9 »73)

s t u d ie d s p h e r o id iz e d A ISI 1095 s t e e l , q u en ch ed and tem p ered

4-340 s t e e l and d i s p e r s io n - s t r e n g t h e n e d n i c k e l (14-0) and fo u n d

t h a t , i n a l l c a s e s , th e c r a c k i n i t i a t i o n a s s o c i a t e d w ith p l a s

t i c i n s t a b i l i t i e s o c c u r r e d a t s t r a i n s w h ich w ere in d e p e n d e n t

o f r o o t r a d i i . S in c e a s m a ll n o tc h r o o t r a d iu s m eans a h ig h e r

s t r e s s / s t r a i n g r a d i e n t , th e in d e p e n d e n c e o f c r i t i c a l s t r a i n

on n o tc h r o o t r a d iu s im p l i e s an in d e p e n d e n c e o f p l a s t i c i n

s t a b i l i t y o n s t r e s s / s t r a i n g r a d i e n t .

I n t h i s w o rk , a s d i s c u s s e d p r e v i o u s l y , th e s i m i l a r i t y o f

s u r f a c e s l i p b a n d s on b en d sp e c im e n s and p la n e s t r a i n t e n s i o n

sp e c im e n s a t th e same s u r f a c e s t r a i n im p l ie s t h a t th e p l a s t i c

133

s t r a i n g r a d ie n t h a s l i t t l e e f f e c t on th e o n s e t o f p l a s t i c


M breoverp th e a u th e r fo u n d t h a t th e s u r f a c e c r a c k s on p la n e

s t r a i n sp e c im e n s i n i t i a t e d a t a s t r a i n s m a l le r th a n th e s u r

f a c e s t r a i n n e e d e d f o r c r a c k i n i t i a t i o n on U -n o tc h bend s p e c

im en 's, a s shown i n t a b le 8 . T h is r e s u l t r e v e a l s a t r e n d

o p p o s i t e t o t h a t p r o p o s e d b y S p r e tn a k e t a l . H ow ever, a s

d i s c u s s e d i n th e p r e v io u s s e c t i o n , t h i s r e s u l t i s p l a u s i b l e ,

s i n c e th e c o n s t r a i n t o f th e in n e r m a t e r ia l on th e o u t e r r e

g io n o r th e c u r v e d c h a r a c t e r i s t i c t r a c e f o r th e b en d b a r ca n

lo w e r th e te n d e n c y o f th e v o i d s o r m ic r o c r a c k s on th e n o tc h

s u r f a c e to o p e n . T h u s, th e C rack s can o n ly be o b s e r v e d a t a

l a r g e r s t r a i n .

T a b le 8 . The s u r f a c e s t r a i n f o r c r a c k i n i t i a t i o n

1090u n ch a rg ed

1090 p r e c h a r g e d

1090d y n a m ic a lly

ch a rg ed

1020u n ch a rg ed

p la n e s t r a i n 0 . 2 8 0 . 1 0 0 . 0 1 5 0 . 3 ^

b en d 0.*K) 0 . 1 7 0 . 0 5 0 . 7 0

134

5 - 1 - 4 C rack I n i t i a t i o n and P r o p o g a t io n

T a b le 9 su m m arizes d a ta f o r th e c r a c k i n i t i a t i o n s i t e and

t h e i n i t i a l d i r e c t i o n o f c r a c k p r o p a g a t io n w ith r e s p e c t to

t h e p r i n c i p a l t e n s i l e s t r e s s a x i s f o r sp e c im e n s u n d er d i f f e r

e n t s t r e s s s t a t e s and c h a r g in g c o n d i t i o n s . The c r a c k i n i t i a

t i o n and p r o p o g a t io n i n d y n a m ic a lly c h a r g ed sp e c im e n s a re

s t r o n g l y a f f e c t e d b y h y d ro g e n c h a r g in g a s d ic u s s e d i n a l a t e r

s e c t i o n . I n t h i s s e c t i o n th e e f f e c t o f s t r e s s s t a t e o n ly i s

c o n s id e r e d .

T a b le 9 . C rack i n i t i a t i o n s i t e s and t h e i r i n i t i a l p r o p a g a te d i r e c t i o n w ith r e s p e c t to th e m a jo r t e n s i o n s t r e s s

U n ch arged P r e c h a r g e d D y n a m ic a llyC harged

Round B ar I n t e r n a l I n t e r n a l o r S u r fa c eS u r fa c e

O90 90° o r 4 5 °* 90°

P la n e S t r a in S u r fa c e or S u r fa c e o r S u r fa c eI n t e r n a l I n t e r n a l

I , , , 0 _ _ o45 o r 90 4 5 ° o r 90° 90°

Bend S u r fa c e S u r fa c e S u r fa c e

4 5 ° 45° 90°

* f o r i n t e r n a l c r a c k 9 0 ° o n ly .

135

The u s u a l way o f d e s c r i b i n g th e cup and co n e f r a c t u r e p r o

c e s s o f a r o u n d t e n s i o n b a r i s t h a t ( 1 4 1 ) a t some c r i t i c a l

p o i n t , th e t r i a x i a l t e n s i l e s t r e s s c o n d i t i o n w i t h i n t h e n e c k e d

r e g i o n c a u s e s s m a l l p a r t i c l e s w i t h i n th e m a t e r ia l t o e i t h e r

f r a c t u r e o r s e p a r a t e from t h e m a t r ix . The r e s u l t i n g m ic r o

v o i d s t h e n u n d erg o a p e r io d o f g r o w th and e v e n t u a l c o a l e s c e n c e ,

p r o d u c in g an i n t e r n a l , d i s c - s h a p e d c r a c k o r i e n t e d n o rm a l t o

t h e t e n s i l e s t r e s s a x i s . F i n a l f r a c t u r e th e n o c c u r s b y ai

s h e a r in g o f f p r o c e s s a lo n g a c o n i c a l s u r f a c e o r i e n t e d a t 45

d e g r e e s t o th e s t r e s s a x i s .

As d i s c u s s e d b e f o r e , i n a ro u n d b a r t e n s i o n s p e c im e n , b o th

t h e 45 d e g r e e maximum r e s o l v e d s h e a r s t r e s s p la n e s and th e

5 4 . 7 4 d e g r e e p u r e s h e a r s u r f a c e s a r e c o n i c a l l y d i s t r i b u t e d

a ro u n d t h e t e n s i l e s t r e s s a x i s . U nder t h i s c o n d i t i o n , th e

s h e a r i n s t a b i l i t y i s s u p p r e s s e d b u t p l a s t i c d e fo r m a t io n o r

s h e a r in g i s s t i l l o c c u r r in g . The p l a s t i c i n c o m p a t i b i l i t y

s t r e s s e s g e n e r a t e d a t th e i n t e r f a c e s b e tw e e n t h e h a r d p a r t

i c l e s and th e m a tr ix ( 7 3 ) c a u s e v o i d s to i n i t i a t e and g ro w .

Once th e v o i d s h a v e form ed and c o a le s c e n c e d t o becom e a s m a ll

d i s c - s h a p e d c r a c k , t h e s t r e s s s t a t e i n f r o n t o f t h e c r a c k

w i l l becom e m ore and m ore f a v o r a b le f o r p l a s t i c i n s t a b i l i t y

t o ta k e p l a c e . The c r a c k w i l l t h e n t e n d t o f o l l o w th e w eak

e n e d m a t e r i a l l y i n g a lo n g th e p l a s t i c i n s t a b i l i t y s u r f a c e s .

H o w ev er , t h e p a th o f th e c r a c k s h o u ld be c o n f in e d t o t h e n e c k

e d r e g i o n , and t h e r e th e s t r e s s s t a t e i s f a v o r a b le f o r b o th

136

u n s t a b le ( b r i t t l e ) and s t a b l e ( d u c t i l e ) c r a c k g r o w th . D epend

in g on th e r e s i s t a n c e o f a m a t e r ia l t o t h e d i f f e r e n t m odes o f

f r a c t u r e , t h e r e may o r may n o t b e an u n s t a b le c r a c k p r o p o g a

t i o n r e g i o n ( r a d i a l r e g i o n ) . I n e i t h e r c a s e , when th e r e m a in

in g p a r t o f th e sp e c im e n i s s m a l l e n o u g h , s i m i la r t o a t h i n -

w a l le d tu b e s u b j e c t t o t e n s i l e s t r e s s and th e c o n s t r a i n t o f

c o n s t a n t d ia m e t e r , t h e s t r e s s s t a t e i s p la n e s t r e s s - p l a n e

s t r a i n i n f r o n t o f th e i n t e r n a l c r a c k . The s u r f a c e o f p u re

s h e a r th e n c o i n c i d e s w ith th e maximum r e s o l v e d s h e a r s t r e s s

s u r f a c e , and th e s h e a r i n s t a b i l i t y in d u c e d b y t h i s s t r e s s

s t a t e l e a d s to th e f i n a l s e p a r a t io n o f th e sp e c im e n .

T h is form o f c r a c k fo r m a t io n w as o b s e r v e d on u n ch a rg e d and

p r e c h a r g e d round b a r sp e c im e n s i n t h e p r e s e n t work f o r b o th

1020 and 1090 s t e e l s . I t was a l s o r e p o r t e d b y C ia lo n e and

A saro ( 3 0 , 3 1 ) f o r p r e c h a r g e d s p h e r o id iz e d 10^5 s t e e l .

I n u n ch a rg ed and p r e c h a r g e d p la n e s t r a i n t e n s i l e sp e c im e n s

c r a c k s form ed p r e d o m in a n tly a t th e s u r f a c e a s shown i n f i g u r e

4 l t o 4-7• H ow ever, t h e r e a r e a l s o i n t e r i o r c r a c k s fo rm ed .

T h ere i s some e v id e n c e o f i n t e r i o r c r a c k s i n u n ch a rg e d s p e c

im e n s , b u t th e y a r e more f r e q u e n t l y fou n d i n th e p r e c h a r g e d

sp e c im e n s a s shown i n f i g u r e ^7•

A l l c r a c k s i n u n ch a rg ed p la n e s t r a i n sp e c im e n s and m ost

c r a c k s i n p r e c h a r g e d sp e c im e n s p r o p o g a te f o l l o w in g th e d i r e c

t i o n o f th e c h a r a c t e r i s t i c p la n e s . T h is i s n o t d i f f i c u l t to

u n d e r s ta n d s in c e in p la n e s t r a i n t e n s i o n s p e c im e n s , th e s t r e s s

s t a t e i s a b o u t th e same th r o u g h o u t th e sp e c im e n e x c e p t t h a t

137

t h e s t r e s s (53 i n th e t h i c k n e s s d i r e c t i o n i s s l i g h t l y l a r g e r

th an , z e r o i n th e i n t e r i o r . T h e r e f o r e , one can e x p e c t t h a t

t h e c r a c k s w i l l form p r e d o m in a n t ly a t t h e s u r f a c e a lo n g th e

c h a r a c t e r i s t i c p la n e . The fo r m a t io n o f c r a c k s a lo n g c h a r a c

t e r i s t i c p la n e s i n th e i n t e r i o r i s a l s o p o s s i b l e a t h ig h e r

a p p l ie d s t r e s s , b u t t o a l e s s e r e x t e n t .

Some o f th e s u r f a c e and i n t e r n a l c r a c k s i n p r e c h a r g e d p la n e

s t r a i n sp e c im e n s a r e fo u n d t o s t a r t i n a d i r e c t i o n norm al to

t h e a p p l ie d t e n s i o n a x i s a s shown i n f i g u r e s 4 6 -b and 4 7 - b .

T h is phenom enon i s c a u s e d b y th e c h a r g e d h y d ro g e n a s d i s c u s s e d

l a t e r .

I n b en d s p e c im e n s , a l l c r a c k s a r e form ed a t th e n o tc h s u r

f a c e and f o l l o w th e c h a r a c t e r i s t i c t r a c e s . F r e q u e n t ly , a s i n

f i g u r e 71 and 7 2 , th e c r a c k p a th a b r u p t ly c h a n g e s from one

c h a r a c t e r i s t i c t r a c e to one p e r p e n d ic u la r t o i t . S im i la r

o b s e r v a t io n s h a s b e e n made by G r i f f i s and S p r e tn a k ( 1 0 1 ) on

A ISI 4340 s t e e l and b y L e e , G o ld en b erg and H ir th ( 4 9 , 7 3 ) on

q u en ch ed and tem p ered A ISI 43^0 and s p h e r o id iz e d A ISI IO95

s t e e l s . H ow ever, no m a t te r how th e c r a c k z ig z a g s from one t o

th e o t h e r c h a r a c t e r i s t i c p la n e d u r in g i t s p r o p o g a t io n , th e

m ain f r a c t u r e a lw a y s r o u g h ly f o l l o w s th e m id -sp a n p la n e ( t h e

r e g i o n o f minimum c r o s s s e c t i o n ) .

5 - 1 - 5 The S u p p r e s s io n o f th e Y ie ld P o in t i n P la n e S t r a i n T e s t s

As shown i n f i g u r e s 25 and 2 6 , an i n t e r e s t i n g o b s e r v a t io n

made i n p la n e s t r a i n t e n s i o n t e s t s i s th e s u p p r e s s io n o f y i e l d

138

p o i n t phenom enon. O n ly a r e l a t i v e l y s m a ll f r a c t i o n o f th e

l o a d - d e f l e c t i o n c u r v e s show ed a f a i n t "knee" a t th e s t r e s s

c l o s e to th e lo w e r y i e l d s t r e n g t h o b ta in e d i n round b a r t e n

s i o n t e s t s . The s u p p r e s s io n o f th e y i e l d p o i n t i s more p r o

n ou n ced f o r 1020 sp e c im e n s th a n f o r 1090 s p e c im e n s . F or 1020

s p e c im e n s , th e s u p p r e s s io n i s more p ro n o u n ced i n h y d ro g e n

c h a r g e d sp e c im e n s th a n i n u n ch a rg e d sp e c im e n s a s shown i n

t a b l e s 4 and 5*

The s l i p b a n d s on 1020 sp e c im e n s a r e i n g e n e r a l more i n t e n s e

th a n t h o s e on 1090 sp e c im e n s t e s t e d u n d er th e same c o n d i t i o n s .

A l s o , c h a r g e d h y d ro g e n p r o m o tes th e in h o m o g e n ity o f th e s l i p

b a n d s ( s e c t i o n 5 - 2 - 2 ) on d eform ed s p e c im e n s . T here i s e v i

d en c e (1 ) s u g g e s t in g t h a t c r o s s - s l i p i s more d i f f i c u l t i n

h y d r o g e n -c h a r g e d i r o n .

B a sed on t h e s e f a c t s and th e id e a d i s c u s s e d i n p r e v io u s

s e c t i o n s , t h a t s l i p i s more p la n a r f o r th e p la n e s t r a i n s t a t e ,

one c a n e x p e c t f o r t h i s s t a t e t h a t : th e p la n e s w ith maximum

r e s o l v e d s h e a r s t r e s s c o in c id e w ith th e p u re s h e a r p la n e s ;

t h e s l i p p la n e s i n d i f f e r e n t g r a in s a r e r o u g h ly a l i g n e d , so

t h a t th e d i s l o c a t i o n s c a n b e r e a d i l y t r a n s m it t e d a c r o s s th e

b o u n d a ry b y so u r c e a c t i v a t i o n ; th e s t r e s s n e c e s s a r y f o r th e

d e fo r m a t io n c o n t in u in g i n t o n e ig h b o r g r a in s i s s m a l le r ; a s a

c o n se q u e n c e th e g e n e r a t io n o f a l a r g e d i s l o c a t i o n d e n s i t y b y

p r o p a g a t io n o f s l i p a c r o s s g r a in s i s e a s y ; and t h e r e f o r e th e

u p p e r y i e l d p o in t phenom enon, a s s o c i a t e d w ith lo w d i s l o c a t i o n

d e n s i t i e s a t t h e o n s e t o f y i e l d i n g , i s s u p p r e s s e d .

139

5 - 2 The E f f e c t o f H ydrogen

On th e h a s i s o f p r e v io u s work done on th e same m a t e r i a l s

u s e d i n t h i s r e s e a r c h , th e f o l lo w in g f a c t s c a n be em p lo y ed f o r

th e d i s c u s s i o n on t h e e f f e c t o f h y d ro g en i n t h i s work ( 8 , 9 ) •

1. The c r i t i c a l c h a r g in g c u r r e n t d e n s i t y f o r i r r e v e r s i b l e

damage i n t h e form o f v o i d s f o r s p h e r o id iz e d 1090 s t e e l i n

IN HaSO j. p o is o n e d w ith l g / l t h io u r e a i s 135 A/nf1 . H yd rogen

c h a r g in g s o f t e n s th e m a t e r ia l i n sa m p le s c h a r g e d a t 100 A/m3*

f o r 0 . 5 to 10 h o u r s b u t t h e s e e s s e n t i a l l y r e c o v e r e d t h e i r l o s s

i n m ic r o h a r d n e s s a f t e r 100 h o u r s o f room te m p e r a tu r e a g in g .

2 . W ith a c h a r g in g c u r r e n t d e n s i t y o f 100 A/m1, , th e s u r f a c e

f u g a c i t y o f h y d r o g e n on 1090 s t e e l i s 1 . 0 2 GPa. The e f f e c t i v e

d i f f u s i v i t y o f h y d ro g en i n 1090 s t e e l i s 8 . 3^ x 10 10 ma/ s .

3 . W h ile t h e r e i s no d a ta a v a i l a b l e f o r th e 1020 s t e e l s u se d

i n t h i s work e x p o se d t o a c h a r g in g c u r r e n t d e n s i t y o f 100 A/mx ,

th e r e s u l t s o b t a in e d w ith a c h a r g in g c u r r e n t o f 50 A/ma c o u ld

p r o v id e a good r e f e r e n c e f o r d i s c u s s i o n . W ith a c h a r g in g

c u r r e n t o f 50 A/nf* th e e f f e c t i v e d i f f u s i v i t y o f h y d ro g en i n

s p h e r o id iz e d 1020 s t e e l i s 2 x 10~* ma/ s w h ile t h a t i n 1090

s t e e l i s 5 x 10_/0 mx/ s . W ith t h i s c h a r g in g c u r r e n t d e n s i t y ,

th e s u r f a c e f u g a c i t y o f h y d ro g en on 1020 s t e e l i s 1 .7 GPa

w h ile on 1090 s t e e l i s 0 .8 GPa.

The f a c t s m e n tio n e d above a r e c o n s i s t e n t w ith th e r e s u l t s

o f G a d g e e l and J o h n so n ( 7 ) * who u s e d a g a s p e r m e a t io n t e c h

n iq u e . T hey fo u n d t h a t a t 200 t o 6 0 0 ° C i n s p h e r o id iz e d s t e e l s

d i f f u s i v i t y d e c r e a s e d l i n e a r l y w ith in c r e a s i n g c a rb o n c o n t e n t

u n t i l f o r 1090 s t e e l i t w as r e d u c e d b y a f a c t o r o f 3-

140

5 - 2 - 1 L o a d - D e f le c t io n C u rves and F r a c tu r e S t r a i n s

I n f i g u r e 13 and f i g u r e 1 4 , one can f i n d no d i f f e r e n c e on

t h e l o a d - d e f l e c t i o n c u r v e s b e f o r e u n ifo rm s t r a i n f o r u n ch a rg ed

and p r e c h a r g e d round b a r sp e c im e n s f o r b o th A ISI 1020 and 1090

s t e e l s . H ow ever, t h e r e a re s o m e d u c t i l i t y l o s s e s a s e x p r e s s e d

b y th e f r a c t u r e s t r a i n d e r iv e d from th e r e d u c t io n o f a r e a a s

shown i n t a b l e 8 . S i m i l i a r r e s u l t s have a l s o b e e n r e p o r t e d

b y C ia lo n e e t a l ( 3 9 * 3 0 , 3 1 ) and Tompson e t a l ( 3 4 ) .

T ab le 1 0 . F r a c tu r e s t r a i n s and d u c t i l i t y l o s s e s

f o r h y d ro g e n c h a r g ed sp e c im e n s

A ISI 1020 S t e e l A IS I 1090 S t e e l

Round B ar P la n e S t r a i n Round B ar P la n e S t r a i n

U n ch arged 1 . 4 ? 0 . 9 7 1 . 0 4 0 . 7 1 3

P r e c h a r g e d 1 . 2 8 0 . 5 1 O. 6 3 0 .3 4 2

D u c t i l i t yL o ss 1395 4796 3995 5295

D y n a m ic a llyC harged 0 .4 1 2 0 . 2 5 0 . 1 1 0 . 0 6

D u c t i l i t yL o ss 72% 74% 91.596

141

From th e e f f e c t i v e d i f f u s i v i t i e s o f h y d ro g en i n s p h e r o id iz e d

1090 and 1020 s t e e l s u s e d i n t h i s w ork , one ca n c a l c u l a t e b y

u s in g s ta n d a r d m a th e m a tic a l s o l u t i o n s (1 4 2 ) t h a t th e h y d ro g en

d i s t r i b u t i o n s i n b o th s t e e l s s h o u ld h ave b e e n v e r y u n ifo rm i n

b o th p la n e s t r a i n and rou n d b a r t e n s i l e sp e c im e n s a t th e end

o f th e p r e c h a r g in g t r e a t m e n t . W ith th e g e o m e try and th e t e n

s i o n t e s t t im e u s e d ('v'10 m in u te s f o r p la n e s t r a i n a n d ^ 2 0

m in u te s f o r rou n d b a r t e n s i l e sp e c im e n s ) th e h y d ro g en l o s s a tI

t h e c e n t e r o f p la n e s t r a i n sp e c im e n s i s a b o u t th e same a s o r

e v e n s l i g h t l y h ig h e r th a n t h a t a t th e c e n t e r o f ro u n d b a r s .

T h e r e f o r e , one m ig h t e x p e c t t h a t th e d u c t i l i t y l o s s c a u se d by

p r e c h a r g e d h y d ro g en sh o u ld be a b o u t th e same o r e v e n h ig h e r

i n rou n d b a r s p e c im e n s . N e v e r t h e l e s s , a s shown i n f i g u r e s

25 and 2 6 , f o r b o th A ISI 1020 and 1090 m a t e r i a l s , th e lo a d -

d e f l e c t i o n c u r v e s f o r p r e c h a r g e d p la n e s t r a i n sp e c im e n s a re

som ewhat lo w e r th a n th o s e f o r u n ch a rg ed sp e c im e n s . A l s o ,

one can f i n d from t a b l e 8 t h a t th e d u c t i l i t y l o s s e s u n der th e

p r e c h a r g e d c o n d i t i o n a r e much h ig h e r i n p la n e s t r a i n th a n i n

round b a r s p e c im e n s .

As d i s c u s s e d b e f o r e , u n d er th e p la n e s t r e s s - p l a n e s t r a i n

s t a t e th e c h a r a c t e r i s t i c p la n e s c o in c id e w ith th e p la n e s o f

maximum r e s o lv e d sh e a r s t r e s s . The n e a r ly p la n a r s h e a r

g r e a t l y d e c r e a s e s th e ch a n ce o f s h e a r band i n t e r s e c t i o n and

f lo w l o c a l i z a t i o n o r p l a s t i c i n s t a b i l i t y i s fa v o r e d u n d er th e

p la n e s t r a i n s t a t e .

U nder th e c h a r g in g c o n d i t io n s u se d i n t h i s w ork , th e r e i s

e v id e n c e t h a t d i s l o c a t i o n i n j e c t i o n from s e c o n d p h a se p a r t i c l e

m a tr ix i n t e r f a c e s i s en h a n ced and t h a t th e m o b i l i t y o f d i s

l o c a t i o n s i s in c r e a s e d b y th e c h a r g e d h y d ro g en ( 9 ) » H e n c e ,

th e lo w e r in g o f th e l o a d - d e f l e c t i o n c u r v e and th e g r e a t e r

d e c r e a s e i n d u c t i l i t y o f th e p r e c h a r g e d p la n e s t r a i n sp e c im e n s

ca n be i n t e r p r e t e d a s f o l l o w s i A t th e end o f th e c h a r g in g

t r e a t m e n t , th e c h a r g e d h y d ro g e n s t a r t s t o d i f f u s e o u t o f th e

sp e c im e n . U s in g th e e f f e c t i v e d i f f u s i v i t y , one can f in d th e

d i f f u s i o n d i s t a n c e s d u r in g th e t e s t t o be 0 .7 ram f o r 1090

sp e c im e n s and 1 .1 mm f o r 1020 s p e c im e n s . T h e r e f o r e , u n tra p p e d

h y d r o g e n w ou ld be d r a in e d o v e r a lm o s t th e w h o le t h i c k n e s s o f

p la n e s t r a i n sp e c im e n s f o r b o th m a t e r i a l s . D eep h y d r o g e n

t r a p s su ch a s p r i o r a u s t e n i t e g r a in b o u n d a r ie s and f e r r i t e -

i n c l u s i o n i n t e r f a c e s f u n t io n a s a h y d ro g e n r e s e r v o i r i n p r e

c h a r g in g t e s t s . They a c c u m u la te h y d r o g e n d u r in g th e c h a r g in g

t r e a tm e n t and ca n r e l e a s e i t d u r in g s t r a i n i n g b y i n t e r a c t i o n

o f th e t r a p s w ith d i s l o c a t i o n s o r p i l e u p s . The tr a p p e d h y

d r o g e n i s p o s t u l a t e d t o en h en ce d i s l o c a t i o n so u r c e o p e r a t io n

a t i n c l u s i o n - f e r r i t e i n t e r f a c e s r e s u l t i n g i n th e i n j e c t i o n

o f d i s l o c a t i o n s . T hose d i s l o c a t i o n s i n j e c t e d from th e 1 h y d ro

g e n r e s e r v o i r have a h ig h e r m o b i l i t y due to th e h y d ro g e n a s

s o c i a t e d w ith them . T h u s , th e a p p l ie d s t r e s s n e e d e d f o r t h e i r

m ovem ent i s s m a l l e r , and th e lo a d i s s m a l le r a t a g iv e n d e

f l e c t i o n . F u r t h e r , th e h y d ro g e n t r a n s p o r t e d by d i s l o c a t i o n s

m ig h t b e r e l e a s e d when d i s l o c a t i o n i n t e r e c t w ith some b a r r ie r

w ith a c o n se q u e n t lo c a l , h y d ro g e n c o n c e n t r a t io n in c r e a s e a lo n g

1^3

th e B l ip p la n e o f th e d i s l o c a t i o n s o u r c e . I n t u r n , t h i s h y

d r o g e n c o u ld make th e m ovem ent o f d i s l o c a t i o n s a lo n g th e s l i p

p la n e e a s i e r , th u s p r o m o tin g p l a s t i c i n s t a b i l i t y and r e s u l t i n g

i n a d u c t i l i t y l o s s .

On th e o t h e r h a n d , th e d u p le x s l i p i n round b a r s m akes

d i s l o c a t i o n s i n t e r s e c t i o n more p r e v a le n t . H e n c e , th e s t r e s s

n e c e s s a r y f o r d i s l o c a t i o n m ovem ent to a c c o m p lis h th e p l a s t i c

d e fo r m a t io n i s p r e d o m in a n tly th e s t r e s s n e e d e d f o r d i s l o c a t i o n

t o p e n e t r a t e th e f o r e s t form ed o f o t h e r d i s l o c a t i o n s . T h is

s t r e s s i s a f u n c t io n o f th e d i s l o c a t i o n d e n s i t y and th e l i n e

t e n s i o n o f d i s l o c a t i o n s and h a s l i t t l e t o do w ith th e c h a r g e d

h y d r o g e n . S o , th e r e i s l i t t l e d i f f e r e n c e b e tw e e n th e s t r e s s -

s t r a i n c u r v e s o f p r e c h a r g e d and u n ch a rg ed round b a r sp e c im e n s

b e f o r e u n ifo r m s t r a i n i s a c h ie v e d . A f t e r a s m a l l d i s c - s h a p e d

c r a c k i s form ed a t th e c e n t e r o f th e n e c k e d r e g io n a s d i s c u s s

e d i n s e c t i o n 5 -1 -^ * th e s t r e s s s t a t e i n f r o n t o f th e c r a c k

g r a d u a l ly a p p r o a c h e s th e p la n e s t r a i n c o n d i t i o n , and th e e f

f e c t o f h y d ro g en b ecom es more e v id e n t a s i n p la n e s t r a i n t e n

s i o n s p e c im e n s . T h is e x p l a in s th e o b s e r v a t io n made by many

g r o u p s ( 3 1 i 3^) t h a t f o r p r e c h a r g e d , u n n o tc h e d rou n d b a r t e n

s i o n s p e c im e n s , t h e r e i s l i t t l e u n ifo r m s t r a i n and e lo n g a t i o n

ch an ge b u t th e r e d u c t io n o f a r e a o r f r a c t u r e s t r a i n i s s m a ll

com pared t o th e u n ch a rg ed c a s e .

The c u c t i l i t y l o s s e s i n p r e c h a r g e d p la n e s t r a i n t e n s i o n

sp e c im e n s f o r 1020 and f o r 1090 sp e c im e n s a r e q u i t e c l o s e ,

w ith 1020 s t e e l a b o u t l e s s th a n 1090 s t e e l . I n c o n t r a s t ,

14-4-

i n th e rou n d b a r c a s e , t h e d u c t i l i t y l o s s o f t h e p r e c h a r g e d

1020 s p e c im e n i s o n l y one t h i r d o f t h a t o f t h e p r e c h a r g e d

1090 s p e c im e n . T h is a l s o c o u ld b e e x p la n e d b y th e d i s c u s s i o n

a b o v e . S i n c e t h e c a r b id e volum e f r a c t i o n i s much h i g h e r i n

1090 s t e e l , t h e r e i s more tr a p p e d h y d ro g e n i n th e 1090 round

b a r t h e n i n t h e 1020 s p e c im e n a s t h e y r e a c h th e n e c k in g p o i n t .

T h e r e f o r e , t h e p l a s t i c i n s t a b i l i t y i s l e s s en h a n ced i n 1020

b a r b e c a u s e a l e s s e r amount o f h y d r o g e n i s a v a i l a b l e . T h is

p r o p o s a l i s a l s o s u p p o r te d b y t h e f a c t t h a t f o r t h e same

s p e c im e n g e o m e tr y , th e d u c t i l i t y l o s s c a u se d by p r e c h a r g e d

h y d r o g e n i s h i g h e r f o r 1090 s t e e l th a n f o r 1020 s t e e l .

F or d y n a m ic a l ly c h a r g e d s p e c im e n s , th e f r a c t u r e s t r a i n s are

d r a s t i c a l l y d e c r e a s e d . I n t e r e s t i n g l y , u n l i k e t h e p r e c h a r g e d

c a s e s , t h e d u c t i l i t y l o s s e s a r e a b o u t t h e same f o r round b a r

and f o r p la n e s t r a i n t e n s i o n s p e c im e n s . N am ely , 72 and 74-

p e r c e n t f o r 1020 s p e c im e n s and 89 and 91*5 p e r c e n t f o r 1090

s p e c im e n s . From t h e o b s e r v a t i o n s o f th e m id p la n e and o f th e

f r a c t u r e s u r f a c e , one r e c a l l e s t h a t i n t h e d y n a m ic a l ly c h a r g e d

s p e c im e n s t h e s u r f a c e c r a c k s form ed a t s m a l l s t r a i n and grew

i n a mode I o p e n in g mode w i t h t h e p r o p a g a t io n d i r e c t i o n norm al

to th e a p p l i e d t e n s i l e a x i s . S u r f a c e c r a c k s r e l e a s e d th e

s t r a i n o f o t h e r s u r f a c e m a t e r i a l . T h e r e f o r e , a f t e r th e form

a t i o n o f s u r f a c e c r a c k s th e a p p l i e d s t r a i n was a c o m p l is h e d by

t h e d e f o r m a t io n o f th e i n t e r i o r m a t e r i a l u n t i l t h e s u r f a c e

c r a c k s grew to a c e r t a i n s i z e , w hereupon th e f i n a l f r a c t u r e

h a p p en ed . F i n a l f r a c t u r e t o o k one o f two fo r m s , mode I f a s t

145f r a c t u r e o r mode I I s h e a r i n g o f f , d e p e n d in g on th e r e s i s t e n c e

o f t h e s t e e l to th e d i f f e r e n t m odes o f f r a c t u r e » t h e s i z e o f

s u r f a c e c r a c k s ; and t h e s i z e ( t h i c k n e s s ) o f t h e r e m a in in g l i

g a m en t. No m a t t e r w h ich form t h e f i n a l f r a c t u r e t o o k , i t

c o n t r i b u t e d l i t t l e t o t h e f r a c t u r e s t r a i n m easured from th e

r e d u c t i o n o f a r e a o r t h e r e d u c t i o n o f t h i c k n e s s . T h e r e f o r e ,

t h e f r a c t u r e s t r a i n i n d y n a m ic a l ly c h a r g e d sp e c im e n s i s m a in ly

d e t e r m in e d by th e m a t e r i a l d e f o r m a t io n d u r in g th e p e r i o d o f

s t a b l e s u r f a c e c r a c k p r o p a g a t i o n . The p r o p a g a t io n o f a mode

I s u r f a c e c r a c k d ep en d s on t h e a p p l i e d t e n s i l e s t r e s s and th e

amount o f h y d ro g e n a v a i l a b l e . S i n c e t h e s e f a c t o r s had s i m i l a r

v a l u e s f o r t h e rou n d b a r and t h e p la n e s t r a i n t e n s i o n s p e c i

m e n s , t h e y t h e r e f o r e had s i m i l a r f r a c t u r e s t r a i n v a l u e s .

The r e s u l t s f o r d y n a m ic a l ly c h a r g e d s p e c im e n s seem n o t to

a g r e e w i th t h a t o b s e r v e d by S t o l t z ( 4 6 ) , who t e s t e d t h e s p e c

im en s i n 0 .1 MPa f l o w i n g h y d r o g e n and i n a i r . He fo u n d t h a t

t h e r e i s no d u c t i l i t y l o s s f o r a sm ooth u n i a x i a l t e n s i o n bar

b u t a 32 p e r c e n t d u c t i l i t y l o s s f o r a p la n e s t r a i n b a r . T h is

d i s c r e p e n c y may be c a u se d by th e d i f f e r e n t h y d r o g e n f u g a c i t y

u s e d . I n t h i s w ork , t h e f u g a c i t y o f h y d ro g e n u s e d w as more

t h e n 1 GPa, much l a r g e r th a n t h a t u s e d b y S t o l t z . I f th e

h y d r o g e n f u g a c i t y i s l o w , i t may n o t be enough t o lo w e r th e

c o h e s i v e s t r e n g t h t o a d e g r e e t o form mode I s u r f a c e c r a c k s .

The e f f e c t o f h y d ro g en t h e n w ou ld b e m a in ly t o prom ote th e

d i s l o c a t i o n movement and t h u s p l a s t i c i n s t a b i l i t y . As d i s

c u s s e d p r e v i o u s l y , i n rou n d b a r s p e c im e n s th e d i s l o c a t i o n s

146a r e r e a d i l y t o t a n g l e and c a u s e work h a r d e n in g o f t h e s p e c im e n ,

so t h a t p l a s t i c i n s t a b i l i t y becom es im p o r ta n t o n l y a f t e r t h e

d i s c - s h a p e d c r a c k h a s form ed a t th e c e n t e r o f t h e n e c k e d r e

g i o n . T h e r e f o r e , t h e e f f e c t o f e x t e r n a l l y s u p p l i e d h y d r o g e n

i s n o t o b v i o u s . On t h e o t h e r h a n d , f o r p la n e s t r a i n t e n s i o n

s p e c im e n s , s i n c e s i n g l e s l i p and l a t e r , p la n a r s h e a r o c c u r ,

t h e e f f e c t o f h y d ro g e n on p r o m o t in g s h e a r i n s t a b i l i t y i s more

e v i d e n t .

The work o f Yokogawa e t a l ( 4 7 , 4 8 ) p r o v id e a l i n k b e tw e e n

t h i s work and t h e r e s u l t o f S t o l t z and g i v e s t r o n g s u p p o r t

t o t h e above r e a s o n i n g . They t e s t e d round b a r s p e c im e n s o f

f o u r c a r b o n s t e e l s i n a r g o n and i n h y d ro g e n g a s from 1 t o 3 9 -9

MPa, and fo u n d t h a t : ( 1 ) . E l o n g a t i o n and r e d u c t i o n o f a r e a i n

h y d r o g e n d e c r e a s e w i t h an i n c r e a s e i n th e p r e s s u r e i n c o n t r a s t

w i t h t h o s e i n a r g o n w h ich a r e u n a f f e c t e d . The e l o n g a t i o n and

t h e r e d u c t i o n o f a r e a o f c a rb o n s t e e l s a t t a i n c o n s t a n t v a l u e s

a t a p r e s s u r e above a c r i t i c a l v a l u e w h ich d ep en d s on th e

c a r b o n c o n t e n t . The c r i t i c a l h y d ro g e n p r e s s u r e and t h e c r i t

i c a l e l o n g a t i o n i n c r e a s e w i t h d e c r e a s i n g c a rb o n c o n t e n t .

( 2 ) . I n h y d r o g e n , p r e d o m im a n tly o b s e r v e d i s a q u a s i - c l e a v a g e

f r a c t u r e p r o p a g a t in g c o n c e n t r i c a l l y from th e s u r f a c e t o th e

c e n t e r . D im ple r u p tu r e s u r f a c e s a r e a l s o o b s e r v e d i n th e lo w

p r e s s u r e r e g i o n .

T h e r e f o r e , i t i s c o n c lu d e d t h a t h y d ro g e n can b o th prom ote

th e d i s l o c a t i o n movement and r e d u c e t h e c o h e s i v e s t r e n g t h o f

s t e e l s . Which e f f e c t i s m a n i f e s t e d i n a t e s t d ep en d s on th e

a p p l i e d s t r e s s s t a t e and t h e amount o f h y d ro g e n a v a i l a b l e .

147

5 - 2 - 2 P l a s t i c I n s t a b i l i t y

F ig u r e 75 i s a q u a l i t a t i v e d iagram sh o w in g t h e e q u i - s l i p

band s t r a i n o f 1090 p la n e s t r a i n t e n s i o n sp e c im e n s b a s e d on

th e c o m p a r iso n o f s l i p ban ds d e v e lo p e d on them u n d er d i f f e r

e n t h y d ro g e n c h a r g in g c o n d i t i o n s ( f i g u r e s 3 0 , 3 2 , 3^ and

t a b l e 5)■ I t show s t h a t f o r a g i v e n c o n f i g u r a t i o n o f s l i p

b a n d s , th e c h a r g e d sp e c im e n r e q u i r e s a s m a l l e r s t r a i n . That

i s t h e s h e a r i s more in h om o gen eo u s and l o c a l i z e d u n d er th e

h y d r o g e n c h a r g e d c o n d i t i o n . T h is r e s u l t f u r t h e r c o n f ir m s t h e

o b s e r v a t i o n s o f Lee ( 2 0 ,2 8 ) and O nyewuenyi ( 9 , 1 4 3 ) t h a t h y

d r o g e n c h a r g in g s o f t e n s t h e m a t e r i a l u s e d i n t h i s work and

p r o m o te s s h e a r i n s t a b i l i t y . I n f i g u r e 75» t h e s t r a i n s f o r

s u r f a c e c r a c k i n i t i a t i o n a r e a r b i t r a r i l y d e f i n e d a s t h e s t r a i n s

a t w h ich s u r f a c e v o i d s s t a r t t o c o n n e c t w i t h e a c h o t h e r .

H u tc h in s o n and T v e rg a a r d (1 0 4 ) d e r iv e d th e c r i t i c a l s t r a i n s

f o r s u r f a c e b i f u r c a t i o n i n p la n e s t r a i n and u n i a x i a l t e n s i o n

o r c o m p r e s s io n c a s e s ( e q u a t i o n 2 . 2 4 and f i g u r e 4 i n r e f e r e n c e

1 0 4 ) . F or th e 1090 s p h e r o i d i z e d s t e e l u s e d i n t h i s work un der

u n ch a rg e d c o n d i t i o n , th e work h a r d e n in g c o e f f i c i e n t s a re 0 . 2 4

i n p la n e s t r a i n t e n s i l e t e s t s and 0 .2 5 5 i n u n i a x i a l t e n s i l e

t e s t s . The c o r r e s p o n d c r i t i c a l s t r a i n s c a l c u l a t e d from

H u t c h in s o n and T v e r g a a r d * s r e s u l t s a r e 0 . 4 2 f o r p la n e s t r a i n

t e s t s and O.6 0 6 u n i a x i a l t e n s i o n t e s t s .

As r e p o r t e d i n s e c t i o n 4 - 1 - 5 and d i s c u s s e d i n s e c t i o n 5 - 1 - 4 ,

f o r u n ch a rg e d round b a r s , t h e c r a c k i s i n i t i a t e d from t h e

i n t e r i o r o f t h e n e c k and t h e r e i s no s u r f a c e c r a c k e v e n a t a

DynamicallyCharged

Fracture

Precharged■Surface Cracks Initiation - -----------------------------

Fracture Strain of Uncharged Specimens is 0.71

SurfaceVoids

Uncharged

0.40.2 0.30 0.1

True Strain

F igu re 75* E q u i - s l i p band diagram f o r the 1090 p lan e s t r a i n specim ens t e s t e d under d i f f e r e n t ch arg in g c o n d i t io n s

149

s t r a i n n e a r t h e f r a c t u r e s t r a i n . A l s o , t h e d e v e lo p m e n t o f

s u r f a c e m o rp h o lo g y w i t h s t r a i n i s fo u n d t o be a c o n t in u o u s

e v e n t . T h e r e f o r e , t h e o n l y w e l l - d e f i n e d e x p e r im e n t a l r e s u l t s

s u i t a b l e t o compare w i t h t h e t h e o r e t i c a l v a l u e s are t h e f r a c

t u r e s t r a i n s . S u p p o se t h e c r i t i c a l s t r a i n f o r s u r f a c e b i f u r

c a t i o n i s p r o p o t i o n a l t o t h e f r a c t u r e s t r a i n , ( T h i s i s p r o b a b ly

a r e s o n a b l e a p p r o x im a t io n f o r t h e p la n e s t r a i n sp e c im e n w here

i n s t a b i l i t y and f r a c t u r e i n i t i a t e a t t h e s u r f a c e , b u t i s p r o

b a b l y a lo w e r bound e s t i m a t e f o r t h e u n i a x i a l c a s e w here

i n s t a b i l i t y i n i t i a t e s a t th e s u r f a c e b u t f r a c t u r e i n th e i n

t e r i o r . ) t h e n t h e r a t i o o f t h e c r i t i c a l b i f u r c a t i o n s t r a i n o f

a p la n e s t r a i n t e n s i l e sp e c im e n to t h a t o f a u n i a x i a l t e n s i l e

sp e c im e n sh o u ld e q u a l th e r a t i o o f t h e i r f r a c t u r e s t r a i n s .

The e x p e r im e n t a l r e s u l t i n t h i s work i s 0 . 7 1 / 1 * 0 4 = 0 . 6 8 and

t h e t h e o r e t i c a l v a l u e i s 0 . 4 2 / 0 . 6 0 6 = 0 .6 9 * The a g reem en t i s

v e r y good b u t s h o u ld n o t be o v e r e m p h a s iz e d s i n c e t h e p o s t u l a t

ed p r o p o r t i o n a l i t y b e tw e e n t h e c r i t i c a l b i f u r c a t i o n s t r a i n

and t h e f r a c t u r e s t r a i n i s o n l y a ro u g h a p p ro x m a tio n . The

c o r r e l a t i o n w i t h t h e m e c h a n ic s p r e d i c t i o n s are i n t e r e s t i n g ,

h o w e v e r , and s u g g e s t f u r t h e r work t o t e s t th e c o r r e l a t i o n i n

more d e t a i l .

5 - 2 - 3 C rack I n i t i a t i o n and P r o p a g a t io n i n P r e c h a r g e d S p ec im en s

The c r a c k i n i t i a t i o n and p r o p a g a t i o n i n p r e c h a r g e d rou n d b ar

t e n s i l e s p e c im e n s a r e s i m i l a r t o t h o s e i n u n ch a rg e d s p e c im e n s .

The d i s c - s h a p e d c r a c k form ed a t th e c e n t e r o f a n e c k e d r e g i o n

150and p r o p a g a te d i n a r a d i a l d i r e c t i o n norm al t o t h a t o f th e

t e n s i l e a x i s . T h is p r o c e s s had b e e n s t u d i e d c a r e f u l l y b y

C ia l.on e and A sro ( 2 9 , 3 0 , 3 1 ) * H ow ever , f o r p r e c h a r g e d 1090

ro u n d b a r s p e c im e n s s t u d i e d h e r e , c r a c k s a l s o can form on t h e

s u r f a c e a s shown i n f i g u r e 2 3 - b . T h is c r a c k in g i s a c o n s e q u

e n c e o f more t r a p p e d h y d ro g e n b e i n g a v a i l a b l e d u r in g s t r a i n i n g

o f th e 1090 s p e c im e n s a s d i s c u s s e d l a t e r .

T here a r e more l a r g e d im p le s i n th e c e n t r a l r e g i o n o f th e

f r a c t u r e s u r f a c e o f p r e c h a r g e d 1020 round b a r s th a n a t th e

p e r ip h e r y ( f i g u r e 1 8 - b ) . F or p r e c h a r g e d 1090 rou n d b a r s , t h e r e

a r e many s e c o n d a r y c r a c k s and b r i t t l e r e g i o n s d e n o te d i n th e

c e n t r a l p a r t o f t h e f r a c t u r e s u r f a c e ( f i g u r e 19)* The s e c -

o n c a r y c r a c k s a r e i n g e n e r a l a t g r a i n b o u n d a r ie s where c a r b id e

p a r t i c l e s are l a r g e r and c l o s e r to e a c h o t h e r . The s e c o n d a r y

c r a c k s w ere n o t a s s o c i a t e d w i t h MnS i n c l u s i o n s a s shown i n

f i g u r e 1 9 - b . The b r i t t l e r e g i o n s a re o f t h e s i z e o f a g r a i n .

T hey l o o k t h e same a s th e f r a c t u r e s u r f a c e s r e v e a l e d a t th e

s e c o n d a r y c r a c k s . T h i s c a n be i n t e r p r e t e d a s sh o w in g t h a t

t h e h y d ro g e n t r a p p e d a t g r a i n b o u n d a r ie s and c a r b i d e - f e r r i t e

i n t e r f a c e s above a c e r t a i n c o n c e n t r a t i o n a s s i s t e d t h e i r d e c o

h e s i o n . Yokogawa e t a l ( 4 7 ) fo u n d t h a t th e s t e e l w i t h t h e

h i g h e s t c a r b o n c o n t e n t ( 0 . 57# ) showed i n t e r g r a n u l a r f r a c t u r e

u n d er th e h i g h e s t h y d r o g e n p r e s s u r e (3 9 * 3 MPa) i n t h e i r work.

As shown i n t a b l e 9 , f o r p r e c h a r g e d p la n e s t r a i n s p e c im e n s ,

c r a c k s c a n i n i t i a t e e i t h e r on th e s u r f a c e o r i n t e r n a l l y .

A l s o , t h e y c a n p r o p a g a te a lo n g th e c h a r a c t e r i s t i c s ( 4 5 ° to

151

t h e t e n s i l e a x i s ) o r norm al t o th e t e n s i l e a x i s . S u r fa c e

c r a c k s a r e o f t e n a s s o c i a t e d w i t h MnS p a r t i c l e s n e a r t h e s u r

f a c e w h i l e c r a c k s form ed i n t e r n a l l y may o r may n o t be a s s o c i

a t e d w i t h MnS p a r t i c l e s .

T h ese s e e m in g ly com p lex o b s e r v a t i o n s c a n be i n t e r p r e t e d on

t h e b a s i s o f th e f o l l o w i n g r e a s o n i n g ! ( 1 ) . H yd rogen p r o m o te s

s h e a r i n s t a b i l i t y , ( 2 ) . W ith a l a r g e r amount p r e s e n t , h y d r o g e n

r e d u c e s th e c o h e s i v e s t r e n g t h o f s t e e l s , (3 )* I n p r e c h a r g in g

t e s t s , d e e p h y d ro g e n t r a p s a c c u m u la te h y d r o g e n d u r in g c h a r g in g

and r e l e a s e i t d u r in g p l a s t i c d e f o r m a t io n .

D u r in g t h e t e s t p e r i o d m ost o f t h e u n tr a p p e d h y d ro g e n h a s

b e e n d i f f u s e d o u t . The o n l y h y d r o g e n a v a i l a b l e n e a r t h e s u r

f a c e i s a t th e h yd rog en t r a p s . T h e r e f o r e , s u r f a c e c r a c k s are

o f t e n a s s o c i a t e d w i t h MnS p a r t i c l e s w i th t h e i r l a r g e a r e a o f

se c o n d p h a se p a r t i c l e - m a t r i x i n t e r f a c e t o t r a p h y d r o g e n . The

t r a p p e d h y d r o g e n c a n prom ote s h e a r i n s t a b i l i t y r e s u l t i n g i n

t h e f o r m a t io n o f a c r a c k a lo n g th e c h a r a c t e r i s t i c s , o r , i f th e

h y d r o g e n c o n c e n t r a t i o n i s h ig h en o u g h , t h e c o h e s i v e s t r e n g t h

ca n be lo w e r e d t o su ch a d e g r e e t h a t a mode I c r a c k c a n be

form ed a s shown i n f i g u r e 4 6 - b .

F or i n t e r n a l c r a c k s , t h e r e s i d u a l h y d ro g e n i n t h e l a t t i c e

i s h i g h e r th a n t h a t n e a r t h e s u r f a c e , so t h e p l a s t i c i n s t a b

i l i t y and mode I I c r a c k in g c a n ta k e p l a c e w i t h o u t th e s u p p ly

o f h y d ro g e n from t h e MnS-Fe i n t e r f a c e . H ow ever , s i n c e th e

mode I c r a c k n e e d s a h i g h e r h y d r o g e n c o n c e n t r a t i o n , i t s t i l l

m ust be a s s o c i a t e d w i t h MnS p a r t i c l e s a s shown i n f i g u r e 4 7 .

152

T h is p o i n t a l s o can b e s e e n from t h e f r a c t u r e s u r f a c e s

( f i g u r e 37 and 3 8 )*

F or 1020 s p e c im e n s , b e c a u s e o f t h e s m a l l e r vo lum e f r a c t i o n

o f c a r b id e p a r t i c l e s , t h e r o l e o f MnS p a r t i c l e a s h y d r o g e n

r e s e r v o i r s i s more p r o n o u n c e d . I n f i g u r e 37» th e b r i t t l e r e

g i o n s r e s e m b le f i s h - e y e f r a c t u r e s , i n t h a t MnS p a r t i c l e s a c t e d

a s f o c i f o r z o n e s o f h y d ro g e n a s s i s t e d b r i t t l e f r a c t u r e . A

s i m i l a r r e s u l t was o b s e r v e d by F ren ch e t a l ( 1*J4) .

F or p r e c h a r g e d bend s p e c im e n s , b e c a u s e o f t h e a p p l i e d s t r e s s

s t a t e , a l l c r a c k s w ere i n i t i a t e d on th e n o t c h s u r f a c e and grew

a lo n g t h e c h a r a c t e r i s t i c s . T h is had b e e n r e p o r t e d i n d e t a i l

i n e a r l i e r work b y Lee e t a l ( 2 8 ) .

153

5-2 -J f C ra ck s i n D y n a m ic a l ly C harged S p ec im en s

I n d y n a m ic a l ly c h a r g e d s p e c im e n s a l l c r a c k s a r e s t a r t e d

from o r n e a r t h e s u r f a c e and p r o p a g a t e i n a d i r e c t i o n norm al

t o t h e maximum p r i n c i p a l t e n s i l e s t r e s s a x i s . F ig u r e 20 show s

t h e f r a c t u r e s u r f a c e o f a d y n a m ic a l ly c h a r g e d 1020 round b a r

s p e c im e n . N ear t h e s u r f a c e i s a r e g i o n o f b r i t t l e f r a c t u r e ,

w h ich a l s o can be fou n d i n d y n a m ic a l ly c h a r g e d p la n e s t r a i n

t e n s i o n and b,end s p e c im e n s ( f i g u r e 3 9 - c and 6 3 -b ) . The mor

p h o lo g y o f t h e n e a r s u r f a c e b r i t t l e r e g i o n i s d i f f e r e n t from

t h a t o f t h e c l e a v a g e f r a c t u r e p r o d u c e d a t lo w t e m p e r a t u r e s

( f i g u r e s 6 l - c , 62 - d ) o r h ig h s t r e s s i n t e n s i t y f a c t o r s ( f i g u r e

6 3 - c ) , b u t s i m i l a r t o th e b r i t t l e r e g i o n fo u n d i n p r e c h a r g e d

sp e c im e n s ( f i g u r e 37 - c ) .

S i m i a l r o b s e r v a t i o n was made f o r 1090 s p e c im e n s . F ig u r e s

2 1 - c , 4 0 - c and 6 6 - b are th e h y d ro g e n a s s i s t e d b r i t t l e f r a c t u r e

s u r f a c e s f o r round b a r , p la n e s t r a i n t e n s i o n and b end s p e c i

m en s , r e s p e c t i v e l y . F ig u r e 3 8 - c i s a b r i t t l e r e g i o n on th e

f r a c t u r e s u r f a c e o f a p r e c h a r g e d sp e c im e n .

F ig u r e 50 shows t h a t a t i n i t i a t i o n , s u r f a c e c r a c k s s t a r t a t

p o t e n t i a l p l a s t i c a l l y i n c o m p a t ib le s i t e s su ch a s c a r b i d e -

f e r r i t e i n t e r f a c e s and g r a i n b o u n d a r i e s . The c r a c k s t h e n

grow i n a d i r e c t i o n p e r p e n d i c u l a r t o t h e maximum p r i n c i p a l

t e n s i l e s t r e s s a x i s . The c r a c k s w ou ld e v e n p r o p a g a t e th r o u g h

c a r b id e p a r t i c l e s i n t h e i r p a th i n o r d e r t o m a in t a in th e n o r

m al d i r e c t i o n o f g r o w th . S i m i l a r o b s e r v a t i o n s were made on

1020 s p e c im e n s . F or 1020 s t e e l , c r a b k s a l s o fo u n d i n i t i a t e d

1 5 ^

a t a s h o r t d i s t a n c e (< 1 0 0 um) from t h e s u r f a c e . T h i s m ig h t

be a c c o u n te d f o r b y one o f two r e a s o n s . F i r s t , t h e number o f

p o t e ' n t i a l p l a s t i c a l l y i n c o m p a t i b l e s i t e s ( c a r b i d e s ) a v a i l a b l e

f o r c r a c k i n i t i a t i o n on t h e s u r f a c e i s l e s s f o r 1020 sp e c im e n

t h a n f o r 1090 s p e c im e n s . S e c o n d , t h e d i f f u s i o n o f h y d r o g e n

i s more r a p i d i n 1020 s p e c im e n s an d , s i n c e t h e r e a r e fe w e r

h y d ro g e n t r a p s i n i t , t h e r e i s a r e g i o n n e a r s u r f a c e w h ich

h a s a l a r g e enough h y d ro g e n c o n t r a t i o n , t h a t t h e c o h e s i v e

s t r e n g t h i s lo w e r e d and mode I c r a c k s c a n be i n i t i a t e d .

The n e a r - s u r f a c e , h y d r o g e n - a s s i s t e d b r i t t l e f r a c t u r e r e

q u i r e s b o th a l a r g e enough h y d r o g e n s u p p ly and a s u f f i c i e n t l y

g r e a t a p p l i e d t e n s i l e s t r e s s . I n d y n a m ic a l ly c h a r g e d 1020

b en d s p e c im e n s , a f t e r a mode I s u r f a c e c r a c k grew t o some

d e p t h , i t ch an ged i n t o a mode I I c r a c k and s t a r t e d t o f o l l o w

th e c h a r a c t e r i s t i c s , w i t h v o i d s fo r m in g i n f r o n t o f i t j u s t

a s i n th e u n ch a rg e d o r p r e c h a r g e d c a s e . A s i m i l a r o b s e r v a t i o n

was made by Lee e t a l (2 8 ) on 1090 b end s p e c im e n s . T h is can

b e e x p l a i n e d a s f o l l o w s . B e c a u se o f c h a n g e s i n th e form o f

t h e e l e c t r i c f i e l d w i t h c r a c k gro w th d u r in g c h a r g i n g , t h e

amount o f h y d r o g e n b e in g c h a r g e d i n t o th e s p e c im e n l o c a l l y

n e a r t h e c r a c k t i p become l e s s a s th e c r a c k grew d e e p e r i n t o

t h e s p e c im e n . When t h e h y d r o g e n s u p p ly a t th e c r a c k t i p i s

l e s s th a n a c r i t i c a l v a l u e , t h e mode I f r a c t u r e i s no l o n g e r

f a v o r e d and th e c r a c k s w i t c h e s b a c k t o a mode I I f r a c t u r e .

I n a s i m i l a r e x p e r im e n t on a HSLA NiCop s t e e l , th e mode I

c r a c k ch a n g ed t o mode I I a t a s h o r t e r l e n g t h when th e c h a r g in g

155

c u r r e n t was lo w e r ( 5 0 )*

F or d y n a m ic a l ly c h a r g e d s p e c im e n s d u r in g th e b r i t t l e ,

h y d r o g e n - a s s i s t e d , mode I c r a c k gro w th p e r i o d , t h e r e was no

e v i d e n c e o f v o i d s form ed i n t e r n a l l y o r i n f r o n t o f th e c r a c k s .

The c r a c k s r e s e m b le d t h o s e w h ich form i n a v e r y b r i t t l e ma

t e r i a l . T h is f i n d i n g f u r t h e r s u p p o r t s t h e p o i n t o f v i e w t h a t

a l a r g e enough h y d r o g e n s u p p ly c a n make t h e c o h e s i v e s t r e n g t h

o f s t e e l lo w e r and f a v o r i t s b r i t t l e f r a c t u r e . The c r i t i c a l

h y d r o g e n f u g a c i t y n e e d e d to c a u s e mode I c r a c k i n g o r b r i t t l e

f r a c t u r e o f a m a t e r i a l d e p e n d s o n th e r e l a t i v e r e s i s t a n c e s to

t h e d i f f e r e n t modes o f c r a c k i n g o f th e m a t e r i a l .

The ab ove r e a s o n i n g a l s o e x p l a i n s th e o b s e r v a t i o n t h a t un der

p r e c h a r g i n g c o n d i t i o n s t h e r e a r e more s u r f a c e c r a c k s on 1090

s p e c im e n s th a n on 1020 s p e c im e n s , w h i l e u n d er d y n a m ic a l ly

c h a r g in g c o n d i t i o n s th e r e v e r s e i s t r u e ( e g . f i g u r e s 23 and

2*0 . Under p r e c h a r g i n g c o n d i t i o n s t h e r e i s more tr a p p e d h y

d r o g e n a v a i l a b l e i n th e 1090 sp e c im e n s d u r in g s t r a i n i n g

s i n c e t h e r e a r e more c a r b id e p a r t i c l e s t o t r a p h y d r o g e n .

T h e se t r a p p e d h y d r o g e n atom s h e l p prom ote t h e s h e a r i n s t a b i

l i t y and lo w e r t h e c o h e s i v e s t r e n g t h , so c r a c k s a r e e a s i e r

t o form on 1090 s p e c im e n s . On th e o t h e r h a n d , i n th e d yn a

m i c a l l y c h a r g e d c o n d i t i o n , th e s u p p ly o f h y d ro g e n on s u r f a c e

i s g r e a t , so w hat d e c i c e s th e number and s i z e s o f s u r f a c e

c r a c k s i s th e c o n s t r a i n t s t r e s s on th e s u r f a c e from th e m a te

r i a l b e lo w th e s u r f a c e . T h is c o n s t r a i n t was l a r g e r i n 1020

s p e c im e n s b e c a u s e th e f r a c t u r e s t r a i n was l a g e r i n them .

As d i s c u s s e d a b o v e , th e n e a r s u r f a c e mode I c r a c k i s a

r e s u l t o f h y d r o g e n l o w e r i n g t h e c o h e s i v e s t r e n g t h o f t h e ma

t e r i a l . One more i t e m o f e v id e n c e f o r t h i s h y p o t h e s i s i s

t h a t i n bend t e s t s , c r a c k s i n i t i a t e on th e s u r f a c e and f o l l o w

th e c h a r a c t e r i s t i c s u n d er b o th u n ch a rg e d and p r e c h a r g e d c o n

d i t i o n s . Then a f t e r t h e y grow t o a c r i t i c a l s i z e , t h e y p r o

p a g a t e u n s t a b l y a lo n g th e s m a l l e s t s e c t i o n o f t h e sp e c im e n

and c a u se t h e f i n a l f r a c t u r e . The c r i t i c a l s i z e o f a mode

I I c r a c k n e e d e d to t r i g g e r t h e u n s t a b l e f r a c t u r e i s lo w e r i n

t h e p r e c h a r g e d c a s e th a n i n th e u n c h a r g e d c a s e . As r e p o r t e d

i n s e c t i o n 4 - 3 - 3 f o r 1090 bend s p e c im e n s , th e c r i t i c a l s i z e

i s 1 mm f o r u n ch a rg e d and 0 . 5 mm f o r p r e c h a r g e d s p e c im e n s ,

r e s p e c t i v e l y .

As m e n t io n e d e a r l i e r , when c r a c k s grow d eep i n t o th e m a te

r i a l , t h e h y d r o g e n s u p p ly to th e c r a c k t i p w i l l be r e d u c e d

d r a s t i c a l l y . The f i n a l f r a c t u r e o f d y n a m ic a l ly c h a r g e d

s p e c im e n i s d e te r m in e d by th e n o t c h e f f e c t o f th e s u r f a c e

c r a c k s and th e r e s i s t e n c e o f th e m a t e r i a l t o d i f f e r e n t modes

o f f r a c t u r e .

6 . CONCLUSIONS

( 1 ) . H ydrogen i n s t e e l p r o m o te s p l a s t i c i n s t a b i l i t y a s m an i

f e s t e d i n t h e e a r l y fo r m a t io n o f s h e a r b an d s and a l s o

d e g r a d e s th e s t e e l by fo r m in g q u a s i - c l e a v a g e f r a c t u r e s .

Which one o f t h e s e i s o b s e r v e d d ep en d s on t h e s u p p ly o f

h y d ro g e n t o t h e c r i t i c a l r e g i o n and th e p l a s t i c s t r a i n

s t a t e t h e r e . W ith t h e c o m b in a t io n o f a l a r g e t e n s i l e

s t r e s s and a l a r g e s u p p ly o f h y d r o g e n , a mode I c r a c k

can form w i t h q u a s i - c l e a v a g e f r a c t u r e s u r f a c e s . I f th e

a p p l i e d t e n s i l e s t r e s s a n d /o r h y d r o g e n s u p p ly i s s m a l l ,

f a i l u r e i n form o f a mode I I s h e a r f a i l u r e f o l l o w i n g

s h e a r l o c a l i z a t i o n i n a form o f p l a s t i c i n s t a b i l i t y i s

o b s e r v e d .

( 2 ) . The p la n e s t r a i n s t a t e , b e c a u s e i t p r o m o te s p la n a r s l i p

o r , f o r l a r g e r s t r a i n s , p l a n a r s h e a r f l o w , p r o d u c e s

g r e a t e r s u s c e p t i b i l i t y t o h y d r o g e n - in d u c e d , p l a s t i c


( 3 ) . The y i e l d p o i n t phenom enon i s s u p p r e s s e d i n p la n e s t r a i n

s p e c im e n s .

(J+) . S l i p b a n d s form a s p l a s t i c d e f o r m a t io n s t a r t s , and t h e y

d e v e lo p i n number and e x t e n t w i t h an i n c r e a s e o f s t r a i n .

F or u n ch a rg e d and p r e c h a r g e d p la n e s t r a i n s p e c im e n s , th e

157

158

s h e a r b an d s and c r a c k s d e v e lo p a lo n g t h e s l i p b a n d s .

For d y n a m ic a l ly c h a r g e d s p e c im e n s , th e c r a c k s i n i t i a t e .

- a t t h e s u r f a c e f o r t h e 1090 s t e e l and a t th e s u r f a c e o r

i n a r e g i o n w i t h i n 100 um o f t h e s u r f a c e f o r th e 1020

s t e e l . The c r a c k s i n i t i a t e a t any p r e f e r e n t i a l d e f e c t

s i t e and t h e n p r o p a g a t e i n a d i r e c t i o n w i t h th e c r a c k

p la n e n orm al to th e t e n s i l e a x i s u n d er d y n a m ic a l ly

c h a r g e d c o n d i t i o n s .

(5 )» MnS p a r t i c l e s w i t h t h e i r l o n g d im e n s io n l y i n g a lo n g th e

t e n s i l e a x i s c r a c k e d r e a d i l y . W ith o u t h y d ro g e n c h a r g

i n g , t h e s e c r a c k s w ou ld n o t e x t e n d i n t o th e f e r r i t e ma

t r i x . W ith h y d r o g e n c h a r g in g , th e c r a c k s w ou ld p r o p a g a te

i n t o th e f e r r i t e i n e i t h e r a mode I o r mode I I m anner.

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1 6 2

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5 1 . -J .E . C o s t a and A.W. Thompson, M et. T r a n s . A , 1 9 8 2 , V o l .13 A , p . 1 3 1 5 .

5 2 . J .E . C o s t a and A.W.' Thompson, M et, T r a n s . A , I 9 8 I , V o l .12A, p . ? 6 1 .

53* P r o c e e d i n g s o f th e 2nd I n t e r n a t i o n a l C o n fe r e n c e o n H ydrogen i n M e t a l s , P a r i s , 1977» v o l s . 1 - 1 0 , P ergam on, O x fo r d , 1 978 .

5 4 . 2nd J a p a n e s e I n s t i t u t e o f M e t a l s Symposium on H ydrogen i n M e t a l s , T o k yo , November 1979*

55* "H ydrogen E f f e c t s i n M e t a l s " , I .M . B e r n s t e i n and A.W. Thompson, e d s . , TMS-AIME, New Y ork, I 9 8 I .

5 6 . R.M. L a n t a n i s o n , O.H. G a s t i n e , and G.M. Compeau, " E n v iro n ment S e n s i t i v e F r a c t u r e o f E n g in e e r i n g M a t e r i a l s " , Z .A . F o r o u l i s , e d . , TMS-AIME, 1 9 7 9 , p . 4 8 .

57* S .P . L ynch , M e t a l s Forum, 1 9 7 9 , v o l . 2 , p . I 8 9 .

5 8 . M.R. L o u th a n , J r . and R .P . M c N i t t , R e f . 1 7 , P* 4 9 6 .

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6 6 . G.G. H ancock and H.H. J o h n so n , T r a n s . TMS-AIME, 19^ 5 , v o l . 2 3 6 , p . 5 1 3 .

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7 1 . R .A . O r ia n i and P .H . J o s e p h i c , R e f . 5 6 . p . 2 3 2 .

7 2 . A.W. Thompson and I .M . B e r n s t e i n , Adv. C o r r o s . S c i . T e c h n o l . , 1 9 7 9 , v o l . 7 , P» 53*

73* T. G o ld e n g e r g , T .D . Lee and J . P . H i r t h , M et. T r a n s . A, 1 9 7 8 , v o l 9A, p . I 6 6 3 .

7 4 . A.W. Thompson, M et. T r a n s . A , 1 9 7 9 , v o l . 10A, p . 7 2 7 .

7 5 . C .J . McMahon, J r . and V. V i t e k , A c ta M e t . , 19 7 9 $ v o l . 2 7 , p . 5 0 7 .

7 6 . C .J . McMahon, J r . , V. V i t e k and G.R. B e l t o n , S c r . M e t . , 1 9 7 8 , v o l . 1 2 , p . 7 8 5 .

7 7 . N .J . P e t c h , P h i l . M a g ., 1 9 5 6 , v o l . 1 , p . 3 3 1 *

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89* W.E. C a r r in g t o n and M arie L .V . G a y le r , P r o c . R oy. S o c . , 1 9 4 8 , v o l . A 194 , p . 323*

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