R. I. Stephens Professor,

Mechanical Engineering Department.

S. G. Lee Research Assistant.

H.W.Lee Research Assistant.

The University of Iowa, Iowa City, Iowa 52242

Threshold and Near Threshold Fatigue Crack Growth of Five Cast Steels at Room Temperature and - 4 5 ° C Threshold and near threshold fatigue crack growth behavior was obtained for five carbon or low alloy cast steels at room temperature and —45°C ( —50°F) with the load ratio R = 0.05 and 0.5. Mean stress effects at both temperatures appear to be substantially dependent upon crack closure. AKlh did not correlate well with other mechanical properties such as S,„ S^, Sy or Sf. Threshold and near threshold fatigue crack growth resistance was generally improved at —45°C despite the fact that this temperature was below the NDT temperature for three of the five cast steels. SEM fractography also established that the NDT temperature is not associated with a possible ductile-brittle constant amplitude fatigue crack growth transition in cast steels. AKlh values for these five cast steels appear to be quite favorable relative to values found in the literature for wrought steels.

Introduction

The room and low temperature fatigue behavior of five cast steels commonly used in the ground vehicle industry was recently reported for both constant and variable amplitude behavior [1-4]. This paper is a continuation of the previous published research and concentrates on threshold and near threshold constant amplitude fatigue crack growth behavior at room temperature and at the typical low climatic tem­perature of -45°C.

Materials

The five cast steels were normalized and tempered SAE 0030 and SAE 0050A steels and quenched and tempered C-Mn, Mn-Mo and 8630 steels. Their chemical compositions are given in Table 1. SAE 0030 and 0050A are fine grained (ASTM number 8 to 9) ferritic-pearlitic cast steels while C-Mn, Mn-Mo and 8630 are tempered martensitic cast steels. Additional information on pouring, heat treatment, and microstructure can be found in references [1 and 4].

Monotonic tensile properties and cyclic yield strengths are given in Table 2 for both room temperature and -45°C. Room temperature ultimate strengths ranged from 496 to 1144 MPa (72 to 166 ksi) and thus the five cast steels represent a wide range of carbon or low alloy cast steels .At - 45 ° C, the 0.2 percent yield strength (Sy) ultimate strength (Su) and cyclic yield strength (Sy) increased between about 5 and 15 percent while the true fracture ductility (ty) decreased about 3 to 40 percent.

Standard Charpy V notch impact tests indicated that the low test temperature, -45°C, was in the lower shelf CVN region for 0030 and 0050A steels and in the low CVN tran-

Contributed by the Materials Division for publication in the JOURNAL or ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received by the

Materials Division, September 27, 1983.

sition region for C-Mn, Mn-Mo and 8630 steels [1-4]. Standard drop weight tests were also obtained and the nil ductility transition (NDT) temperatures are given in Table 2 [1-4].

Test Procedures

Compact type (CT) Chevron notched specimens as shown in Fig. 1 were machined from cast steel blocks. The 3-hole grip configuration was used with a monoball gripping system which assured axial ram loading. The machined notch depth where the chevron intersects the surface was 18 mm which corresponds to an initial machined a/w ratio of 0.27. Specimen thickness was 8.2 mm (0.324 in). The threshold and near threshold tests were performed using an incremental load shedding technique which is one of two procedures in the ASTM proposed recommended practice [5]. Tests were conducted with the load ratio (R) set at either 0.05 or 0.5 for each of the five cast steels at both room temperature and -45°C. The threshold stress intensity factor, AKth, can be defined as the limiting value of AK where crack growth does not occur. A more realistic functional definition of AKth, as suggested by Bucci [5], defines AKlh at a fatigue crack growth rate (FCGR) of 10 "10 m/cycle. This functional definition was used in this research.

All tests were run in an 89 kN closed-loop electrohydraulic test system using an 8.9 kN load cell. The low temperature tests were run in an automated COz temperature chamber. The crack growth was measured with a 33 x magnification traveling telescope with a least reading of 0.01 mm (0.0004 in). The precision of the crack length measurements was typically between 0.03 and 0.05 mm. Tests were run unin­terrupted in load control at 30-40 Hz and each test lasted between 100 and 125 hours.

166/Vol.107,APRIL1985 Transactions of the ASME

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Table 1 Cast steel chemistry—% by wt

0030

O050A

C-Mn

Mn-Mo

8630

C

.24

.49

.23

.34

.30

Mn

.71

.93

1.25

1.32

.84

Si

.44

.61

.39

.40

.53

S

.026

.023

.028

.035

.022

P

.015

.024

.036

.024

.021

Cr

.10

.11

.10

.11

.51

Ni

.10

.08

.09

.11

.61

Mo

.08

.04

.04

.22

.17

Al

.06

.08

.02

.06

.08

NDT-°C(°F)

S„ -MPa (ksi)

S,,-MPa (ksi)

ef S'^-MPa (ksi)

0030

- 2 9 (

RT

496 (72)

303 (44)

.62

324 (47)

-25)

-45°C

542 (79)

320 (46)

.36

359 (52)

Table 2 Reference material properties

0050A

+ 27

RT

787 (114)

415 (60)

.21

400 (58)

( + 80)

-45°C

834 (121)

436 (63)

.17

421 (61)

C-Mn

- 5 7 (

RT

583 (85)

402 (58)

.34

372 (54)

-70)

H-45°C

612 (89)

464 (67)

.21

379 (55)

Mn-Mo

- 6 2 ( -80)

RT -45°C

702 (102) 758 (110)

542 (79) 562 (81)

.37 .36

427 (62) 469 (68)

8630

- 2 9

RT

1144 (166)

985 (143)

.35

682 (99)

-25 )

-45°C

1178 (171)

999 (145)

.33

799 (116)

8.2 mm (.324 in)

Fig. 1

9.58 mm-(.377 in) 6 HOLES

CT fatigue crack growth test specimen

Precracking from the chevron machined notch was done using the procedure recommended in ASTM standard E647 [6]. This is the same procedure recommended in reference [5]. The precracking was done with 10 percent load decreases for incremental crack growth of between 0.5 to 1 mm for each load level. Final precrack extension was 3 to 10 mm which gave initial a/w values between 0.32 and 0.43. The initial AA" values for data acquisition with the decreasing A" tests were from 10 to 20 MPavm. To obtain the FCGR below 10 "8

m/cycle the load shedding was reduced from 10 percent at the beginning to 2 to 3 percent near AA"th. The percent decrease was lowered as the FCGR decreased. The proposed test method [5] suggests a minimum crack growth of 1 mm with a 10 percent drop of AA'. To minimize the testing time, d(AK)/AK and Aa values were specifically reduced as follows, where d{AK) is the increment of AA"at each step and Aa is the crack growth at each step:

(1) For da/dN between 1Q"8 and 10~9 m/cycle, d(AK/AK) was between -0 .08 and -0 .05 and Aa was between 0.5 and 1 mm.

(2) For da/dN between 1 0 9 and 10~10 m/cycle, d{AK)/AK was between -0 .06 and -0 .03 and Aa was between 0.25 and 0.5 mm.

For the i? = 0.5 tests A"max and A"min were reduced in the same manner. One to three crack length measurements were taken per step at the higher crack growth rates, while just a single, or occasionally a second, crack measurement was taken per step at the lower crack growth rates.

To ensure that fatigue crack growth retardation did not occur in the decreasing AA" tests, an increasing AK test was often run after the required AA"th was obtained. To reduce the time for these tests, the loads were increased 2 to 3 percent and the FCGR was measured during 0.25 to 0.5 mm of crack growth at each step. For AA" decreasing tests, retardation may occur in the monotonic plastic zone size ry. The FCGR thus was calculated from the point where the new monotonic plastic zone passed over the previous plastic zone at each load step. The plastic zone size was calculated under plane stress conditions. The 8630 cast steel was used to check for possible retardation during the decreasing step test. For the first test, the average crack growth rates and the crack growth rates considered free from retardation effects were measured. The crack growth rates during the AK increasing test were also measured. To verify the first test result, a duplicate test was done and the average crack growth rate at each step was checked. As the comparison of the three different methods showed no significant differences, all the following tests were done only with the decreasing test procedure except when the FCGR showed possibly suspect behavior. In these cases the increasing test was done to assure the data obtained during the decreasing test were correct. The AA" increasing tests were done for 8630 at room temperature and -45°C for both fl = 0.05 and fl=0.5, 0050A at room temperature for both R = 0.05 and R = 0.5 and Mn-Mo at -45°C for both R = 0.05 and 0.5. These results were very consistent with decreasing AK results. To clarify this, increasing AK data points are marked with horizontal ticks to distinguish them from decreasing AA" data points in a later figure which presents the 8630 FCGR data.

Test Results

The stress intensity factor range, AA", for the CT specimen for H/W= 0.6 is [7]:

AP (2 + a/W) AA" = r- ,,, [0.886 + 4.64(4-,)

where AP is the applied load range, B is the specimen thickness, and Wis the specimen width. This equation is valid

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AK-ksi v in

20

AK- MPaTm

AK-ksi^ in 6 S | p̂

• -RT, R=0.5 v-RT, R"0.05 o- -45°C(-50°F) , R = 0.5 v- -45°C(-50° F),

10

AK-MPa -/m

Fig. 2 Threshold and near threshold fatigue crack growth behavior, Fig. 4 Threshold and near threshold fatigue crack growth behavior, C-0030 cast steel Mn cast steel

AK- ksi vin AK-ksi^/fn"

• -RT, R=0.5 T-RT, R o - - 4 5 ° C ( v - - 4 5 ° C

6 8

AK-MPa

10

io-7

Fig. 3 Threshold and near threshold fatigue crack growth behavior, 0050A cast steel

for the three hole grip configuration for the a/ W ratios in­volved according to Newman's findings [8]. The AK values were calculated at the center of the crack extension increment Aa for each step. The fatigue crack growth rate da/dN was calculated with the secant method by averaging the crack growth increment Aa over the elapsed number of cycles AN for each step. The crack lengths measured on one side of the

3

2 -

o 10-E

-RT, R = 0.5

" 7 ~ 10 8

• MPaynT

Fig. 5 Threshold and near threshold fatigue crack growth behavior, Mn-Mo cast steel

surfaces were corrected from the measurements of crack lengths on both sides and the curvature of the crack tip, by taking the average of the three crack lengths after the test was over. Neither side to side variation nor crack tip curvature were excessive.

Threshold and near threshold da/dN versus AK data (region I) are plotted in Figs. 2 through 6 for each cast steel.

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Table 3 AKlh for Five Cast Steels

Material

0030 0050A C-Mn

Mn-Mo 8630

R-

Room Temperature MPaVm (ksiVin)

9.1 (8.3) 10.2* (9.3) 8.3 (7.6) 8.1 (7.4) 9.4* (8.6)

= 0.5

- 4 5 ° (-50°F) MPaVm (ksiVin)

14.2 (12.9) 12.3 (11.2)

. 14.4 (13.1) 10.7* (9.7) 9.4 (8.6)

R = 0.5

Room Temperature MPavm (ksiVin)

5.3 (4.8) 5.2 (4.7) 3.9* (3,5) 4.1 (3.7) 4.1 (3.7)

- 4 5 ° C ( - 5 0 ° F ) MpaVm (ksiVm)

9.3* (9.5) 6.8* (6.2) 7.1* (6.5) 6.5 (5.9) 5.7 (5.2)

•extrapolated values

Table 4 Near threshold crack closure values Material

0030

C-Mn

Temperature

R.T. R.T.

-45°C -45°C

-45°C -45°C -45°C

R

0.05 0.5 0.05 0.05

0.05 0.05 0.05

Kmm- MPavm 21 23 14 24

18 21 18

Pc ' " max

.45

.52

.50

.55

.54

.47

.48

Pci is the closure load. Pmax is the maximum load.

median curves drawn by visual examination in Figs. 2 to 6. If there were no available data points below 2x 10 "10 m/cycle the median trend curve was extrapolated to 10 ~10 m/cycle along the slope and the threshold stress intensity factor was estimated at this growth rate. Seven AA"th values have this small extrapolation. Thus all AKth values were obtained for da/dN equal to 10 "10 m/cycle.

The median visual values of AATth from Figs. 2 to 6 are given in Table 3. Extrapolated values are shown with an asterik. AKth for the five cast steels at room temperature ranged from 8.1 to 10.1 MPavm and at R = 0.5 from 3.8 to 5.2 MPa/Vm. At -45°C, AATth increased from zero to 90 percent compared to room temperature values. At R = 0.5 for both temperatures, the AKth values were generally about 50 percent of the values at i? = 0.05 (actual percentage varied from 42 to 58) which indicates the importance of apparent applied mean stress. This behavior is consistent with many materials and can be closely associated with crack closure at both room temperature [9] and at low temperatures [10-12]. Near threshold crack growth rates were higher at R = 0.5 than at R = 0.05 for a given AK value for both temperatures. They were also higher at room temperature compared to those at -45 °C for both R ratios. However, all the curves in Figs. 2 to 6 tend to somewhat converge or cross over at the higher crack growth rates. Thus a general improved, near threshold, fatigue-crack-growth resistance occurred at -45°C for these five cast steels which is consistent with previously reported region II higher crack growth rates and smooth axial fatigue behavior [1-4],

Discussion of Results

Crack opening and closing displacements were measured for several of the test conditions with 0030 and C-Mn cast steels in the near threshold region using a COD clip gage mounted at the free end of the CT specimens (right side of Fig. 1). The crack opening load could be obtained from the load versus crack opening displacement curves. The loading and unloading curves showed essentially no difference be­tween the crack opening and closing loads. The crack opening or closure loads were observed to be approximately 0.5 Pmax. Actual values were between .45 and .55 as shown in Table 4. These values thus suggest that mean stress influence can be closely related to crack closure at both room and low tem­perature. This concept is also reinforced by the greater amount of fretting often found on the fracture surface for i? = 0.05 tests compared to that for 7? = 0.5 tests. Insufficient

APRIL 1985, Vol. 107/169

1~ 10

6 8

AK-MPaym"

Fig. 6 Threshold and near threshold fatigue crack growth behavior, 8630 cast steel. (The horizontal tick refers to increasing AK data.)

Four test conditions are included in each figure namely R = 0.5 and 0.05 for both room temperature and -45°C. Solid data points are for room temperature (RT) and open data points are for -45°C. Circles are for R = 0.5 and triangles are for R = 0.05. Generally it is hard to obtain data evenly distributed in the 10 "9 to 10~10 m/cycle region because the FCGR near the threshold drops sharply. Bucci [5] recommended that a straight line regression analysis of at least five data points in the 10~9 to 10^10 m/cycle region might be utilized to decide the threshold stress intensity factor range AA"th. This method seems unreasonable for these cast steels as the crack growth rate curves show knees in that region and a sudden drop near the threshold level. In this research, the threshold values were determined from the

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(a) 0030 (e) C-Mn

(b) 0050A

Fig.7 SEM fractographs at near threshold, R = 0.5

crack closure loads were taken however, to attribute the lowtemperature crack growth resistance improvements com­pletely to crack closure. Improved low temperature thresholdand near threshold fatigue resistances, however, have beenattributed principally to crack closure by Esaklul et al. [10]with HSLA and Fe-Si steels, Liaw et al. [II] with stainlesssteel and lata et al. [12] with a ,B-Ti alloy.

The -45°C test temperature was below the NDT tem­perature for 0030, 0050A, and 8630 steels. This low testtemperature was also in the lower shelf region CVN impactenergy region for 0030 and 0050A steels arid in the lowtransition region for the other three steels. Despite this, thelow temperature threshold and near threshold fatigue crackgrowth resistance of these five cast steels was generallysubstantially better than that at room temperature. 0030 andC-Mn which are two of the lower strength cast steels showedthe greatest increases in I1Kth at - 45°C for both R ratios. Forthe five cast steels, improved or equivalent fatigue-crack­growth resistance also existed in region II (the log-log linearregion) but to a lesser extent [I, 2, 4].

The generally improved low temperature threshold andnear threshold fatigue resistance found with these five caststeels is in complete agreement with the findings fromreference [13]. There it was noted that under all low tem­perature conditions reported, the applied I1Kth at low tem­perature was never less than the applied I1Kth at room tem­perature. This improved, or at least equivalent, behavior wasrepprted for aluminum, magnesium and titanium alloys,ferritic and martensitic steels, and austenitic stainless steels.

A macro and micro fractographic analysis was made of theCT specimen threshold and near threshold crack growth testsfor both temperatures. All fatigue crack growth was flat modeI with fretting often evident for R=0.05. SEM fractographsin the threshold and near threshold region are given in Fig. 7for the five steels with R =0.5 for both room temperature and-45°C. Cracks grew from the bottom to the top in Fig. 7.There is little difference between the room temperature and ­45°C crack growth appearance for a given cast steel. For alltests, very few striations were found. Some did exist, but ingeneral the surfaces revealed a ductile type transcrystaIlinefatigue-crack-growth behavior at both temperatures. Nobrittle micro crack growth was observed at either tem­perature. The similarity of the fractographic results indicate

Fig. 7 (cant.)

why the low temperature. was not detrimental to this constantamplitude threshold and near threshold behavior. Thissimilarlity also existed in region II except just prior to finalfracture, where some cleavage was found in the 0050A steel[I, 2, 4]. This suggests that for these cast steels, the NDTtemperature is not associated with a possible ductile-brittletransition for both regions I and II constant amplitude fatiguecrack growth behavior. Under variable amplitude loading,however, the 45 °C temperature did cause decreased fatiguecrack growth resistance [I, 3, 4]. Thus constant amplitudeloading may not provide the entire influence of low tem­perature on fatigue crack growth behavior.

It has been shown by others [9] that I1Kth tends to decreaseand that da / dN tends to increase in the near threshold regionas the ultimate strength or yield strength increases. This trendwas not apparent for these five cast steels at either tem­perature. In fact, I1Kth showed no consistent correlation withother properties such as yield strength, ultimate strength,cyclic yield strength or fatigue limit. A comparison of 11K1hfor R = 0.05 with summaries by Tanaka [14] and Lindley andRichards [15] for wrought steels however, shows that l1K'h forthese five cast steels are in the upper scatter band of thereported data. This is a very positive indication for cast steelusage.

(e) 8630

Conclusions

I. Crack closure appears to be a predominant factor forthreshold and near threshold mean stress effects in cast steels.

2. Properties such as SII' SY' S;, or Sf could not becorrelated with 11K1h values for these five cast steels at eithertest temperature.

3. I1Kth for these cast steels at room temperature comparedvery favorably with values found in the literature for wroughtsteels.

-45°C(-50° F)RT

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4. AA"th for these five cast steels increased from zero to 90 percent at — 45 °C compared to that at room temperature. Near threshold da/dN values were also generally lower at -45°C than at room temperature. This increased fatigue crack growth resistance occurred despite the fact that the NDT temperature was above the low test temperature for three of the five cast steels.

5. The NDT temperature is not an indication of a ductile-brittle constant amplitude crack growth transition for cast steels.

Acknowledgments

The authors would like to thank the Steel Founder's Society of America, The University of Iowa, and contributing companies for financial sponsorship of this research.

References

1 Stephens, R. I., Chung, J. H., Fatemi, A., Lee, H. W., Lee, S. G., Vaca-Oleas, C , and Wang, C. M., "Constant and Variable Amplitude Fatigue Behavior of Five Cast Steels at Room Temperature and - 4 5 ° C , " ASME JOUR­NAL OF ENGINEERING MATERIALS AND TECHNOLOGY, Vol. 106, No. 1, 1983, p. 25.

2 Stephens, R. I., Chung, J. H., Lee, H. W., Fatemi, A., and Vaca-Oleas, C , "Constant Amplitude Fatigue Behavior of Five Carbon or Low Alloy Cast Steels at Room Temperature and -45°C ( -50°F) , " in Fatigue at Low Temperatures, Edited by R. I. Stephens, ASTM STP 857, to be published, 1985.

3 Stephens, R. I., Fatemi, A., Lee, H. W., Lee, S. G., Vaca-Oleas, C , and Wang, C. M., "Variable Amplitude Fatigue Crack Initiation and Growth of Five Carbon or Low Alloy Cast Steels at Room and Low Climatic

Temperatures," in Fatigue at Low Temperatures, Edited by R. I. Stephens, ASTM STP 857, to be published, 1985.

4 Stephens, R. I., "Fatigue and Fracture Toughness of Five Carbon or Low Alloy Cast Steels at Room and Low Climatic Temperatures," Steel Founders' Society of America, Report 94A/94B, 1982/83.

5 Bucci, R. J., "Development of a Proposed Standard Practice for Near Threshold Fatigue Crack Growth Measurement," in Fatigue Crack Growth Measurement and Data Analysis, ASTM STP 738, 1981, p. 5.

6 ASTM E647-83 "Standard Test Method for Constant Amplitude Fatigue Crack Growth Rates Above 10~8 m/Cycle," ASTM Annual Book of Stan­dards, Section 3, Vol. 03.01, 1983, p. 710.

7 ASTM E399-83 "Standard Test Method for Plane-Strain Fracture Toughness of Metallic Material," ASTM Annual Book of Standards, Section 3, Vol. 03.01, 1983, p. 518.

8 Newman, J. C , "Stress Analysis of the Compact Specimen Indicating the Effects of Pin Loading," in Fracture Analysis, ASTM STP 560, 1974, p. 105.

9 Backlund, J., Blom, A. F., and Beevers, C. J., Editors, Fatigue Thresholds Fundamentals and Engineering Applications, Engineering Materials Advisory Services Ltd., United Kingdom, 1982.

10 Esaklul, K. A., Yu, W., and Gerberich, W. W., "The Effect of Low Temperature on Apparent Fatigue Threshold Stress Intensity Factors," in Fatigue at Low Temperatures, Edited by R. I. Stephens, ASTM STP 857, to be published, 1985.

11 Liaw, P. K., and Logsdon, W. A., "Computerized Near-Threshold Fatigue Crack Growth Rate Testing at Cryogenic Temperatures: Technique and Results," in Fatigue at Low Temperatures, Edited by R. I. Stephens, ASTM STP 857, to be published, 1985.

12 Jata, K. V., Gerberich, W. W., and Beevers, C. J., "Low Temperature Fatigue Crack Propagation in a /3-Ti Alloy," in Fatigue at Low Temperatures, Edited by R. I. Stephens, ASTM STP 857, to be published, 1985.

13 Stephens, R. I., Editor, Fatigue at Low Temperatures, ASTM STP 857, to be published, 1985.

14 Tanaka, K., "A Correlation of AKth Values With the Exponent, m, in the Equation of Fatigue Crack Growth for Various Steels," International Journal of Fracture, Vol. 15, No. 1, Feb. 1979, p. 57.

15 Lindley, T. C , and Richards, C. E., "Near Threshold Fatigue Crack Growth in Materials Used in the Electrical Supply Industry," in Fatigue Thresholds Fundamentals and Engineering Applications, Vol. 1, Engineering Materials Advisory Services Ltd., United Kingdom, 1982, p. 1087.

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