POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2007; 18: 90–96

terscience.wiley.com) DOI: 10.1002/pat.806

Published online 21 November 2006 in Wiley InterScience (www.in

Fracture and failure behavior of fabric-reinforced

all-poly(propylene) composite (Curv1)y

Gabor Romhany1*, Tamas Barany1, Tibor Czigany1 and Jozsef Karger-Kocsis2

1Department of Polymer Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics,

Muegyetem rkp. 3, H-1111 Budapest, Hungary2Institut fur Verbundwerkstoffe GmbH, TU Kaiserslautern, PO Box 3049, D-67653 Kaiserslautern, Germany

Received 29 June 2005; Revised 4 April 2006; Accepted 20 April 2006

*Correspoeering, Faof TechnBudapestE-mail: royPaper pthe PolymBratislava

The in-plane static fracture of a fabric reinforced all-poly(propylene) (all-PP, Curv1) composite was

studied at ambient temperature using the concept of the linear elastic fracture mechanics. The

apparent fracture toughness was determined on single-edge notched tensile specimens (SEN-T)

considering themaximum load. The related values did not differ much from those determined by the

resistance curve (KR) method. The crack growth, requested to construct the KR curves, was traced by

the movement in the center of gravity of the cumulative amplitude of the located acoustic emission

(AE) events. The quality of consolidation of the all-PP composite was reflected by the force-

displacement curve (appearance of pop-in), course of the cumulative AE events during loading,

extension and change of the estimated damage zone during fracture. The failure behavior was

studied also by fractography and is discussed. Copyright # 2006 John Wiley & Sons, Ltd.

KEYWORDS: homocomposites; poly(propylene) (PP); fracture; failure; acoustic emission

INTRODUCTION

In poly(propylene) (PP) homocomposites (also termed

‘‘all-PP’’ or ‘‘self-reinforced’’ composites) the reinforcing

phase (highly stretched fibers, tapes and their textile

architectures) are embedded in the matrix. The matrix is

usually a random PP copolymer having a lower melting

temperature than the reinforcement (isotactic PP homo-

polymer).1,2 The pioneering work with all-PP components

was done by the group of Ward and coworkers.3–6 Via their

method, called hot compaction, a portion of the reinforcing

fiber has been transformed into the matrix. The related

products are now available commercially under the trade

name Curv1 (www.curvonline.com). This material is

gaining acceptance in automotive applications because of

its beneficial mechanical properties, ultimate recyclability

and low density (�0.9 g/cm3). However, little information is

available on the fracture mechanical and failure behavior of

this composite which are the topics of the present

contribution.

EXPERIMENTAL

MaterialCurv1 composites sheets with woven fabric reinforcement

(plain-wave) and having a thickness of ca. 3 mm were

ndence to: G. Romhany, Department of Polymer Engin-culty of Mechanical Engineering, Budapest Universityology and Economics, Muegyetem rkp. 3, H-1111, Hungary.mhany@pt.bme.hu

resented as part of an issue containing papers fromeric Materials for Automotive conference held in

, May 2005.

delivered by BP, Amoco Fabrics GmbH (Gronau, Germany).

The tensile mechanical properties of the composite are listed

in Table 1. The discrepancy between the nominal and

measured stiffness can be attributed to the presence of less

consolidated areas as shown on metallographically polished

thickness sections in Fig. 1.

TestsStatic tensile mechanical properties were determined on

dumbbells (1B, according to EN ISO 527) at room

temperature and at a crosshead speed of v¼ 2 mm/min on

a Zwick 1474 universal testing machine.

Single edge-notched tensile (SEN-T) specimens cut from

the Curv1 sheets were also subjected to tensile loading at a

crosshead speed of v¼ 2 mm/min. The initially sawed notch

(a0) was sharpened by razor blade tapping. In order to obtain

information about the failure mode, the acoustic emission

(AE) activity was registered in situ using one microphone

(Fig. 2a).

To trace the damage development, static fracture mech-

anical tests were also performed on SEN-T specimens at

room temperature at a crosshead speed of v¼ 2 mm/min.

Their dimensions allowed the location of the AE via a four

sensors array (Fig. 2b). The notch was prepared as

aforementioned. The fracture toughness values (KQ,max)

were determined by the formula cited in Ref. 7, considering

the maximum load.

Copyright # 2006 John Wiley & Sons, Ltd.

Table 1. Tensile mechanical properties of the Curv1 composite

Tensile modulus (MPa) Tensile strength (MPa) Tensile strain to failure (%)

Producer’s data 4200 120 20Measured 2950 117 23

Producer’s data—based on the datasheet of the producer. Measured—according to DIN EN ISO 527 standard.

Figure 1. (a) Light (LM) and (b) scanning electron microscopy (SEM) pictures taken

from the cross-section of the all-PP composites after polishing. Note: the high

magnification picture represents a ‘‘void’’.

Figure 2. Single edge-notched tensile specimen with a width of (a) 30mm (SEN-T(A)) and (b) 70mm

(SEN-T(B)).

Fracture behavior of Curv1 91

The damage zone was estimated by location of the AE

events, collected during loading of the SEN-T(B) specimens.

The AE activity was recorded in situ by a Defektophone NEZ

220 device (AEKI, Budapest, Hungary). Wide bandwidth

(100–600 kHz) microsensors (Micro 30D of Physical Acoustic

Co., Princeton, NJ) were used as AE sensors. Localization of

the acoustic events (cf. Fig. 2b) was determined by a built-in

Copyright # 2006 John Wiley & Sons, Ltd.

algorithm of the AE device in the knowledge of the acoustic

wave speed. To estimate the size of the damage zone, a

mathematical weighing procedure, described elsewhere,8–10

was used. Briefly, the located map (Fig. 1) was scanned by a

circle of 6 mm diameter in 1 mm steps in both the x and the y

directions and for each surface point the value in the

z-direction was determined by a bell-type weighing function.

Polym. Adv. Technol. 2007; 18: 90–96

DOI: 10.1002/pat

92 G. Romhany et al.

This procedure was adopted to the cumulative amplitudes

(reported here) and cumulative AE events, as well. The

damage zone was assigned to the surface that contained 90%

of all the located AE events. The located AE events served to

trace the increase in the crack length (Da¼ ax�a0 according to

Fig. 2). This occurred by considering the movement of the

center of gravity of the cumulative AE events in subsequent

but not intersecting time intervals. The related procedure is

described in detail in the literature.8–10

The fracture surface of the SEN-T specimens was inspected

in a scanning electron microscope (Jeol JSM 5400, Japan). Prior

to scanning electron microscopy (SEM) observations the

specimen surface was sputter-coated with an Au/Pd alloy.

Figure 4. Cumulative AE events as a function of displace-

ment showing the quality of consolidation on SEN-T(A)

specimens.

Figure 5. Change of load and apparent crack length as a

function of time during loading of a SEN-T(B) specimen.

RESULTS AND DISCUSSION

Fracture mechanical responseSelected load–displacement curves registered on the

SEN-T(A) specimens are displayed in Fig. 3. One can see

that the curve shape differs considerably from that of an

‘‘elastic’’ material. This makes questionable the use of a linear

elastic fracture mechanical approach. In order to consider

this aspect the fracture toughness linked with the maximum

load (KQ,max) was termed as apparent. Its value was found to

be 11.8� 0.5 MPa m1/2. The apparent fracture energy

(GQ,max) estimated by K2Q;max/E, yielded 47.2� 0.1 kJ/m2.

Apart of these apparent values, an attempt was made to

derive the initiation fracture mechanical parameters using

the resistance curve method (KR)—see later. An interesting

feature of the SEN-T(A) specimens was the onset of pop-in.

Note that this is characterized by load drops prior to Fmax. Its

cause is not fully cleared but some authors assign the pop-in

to crack bifurcation.11 Crack bifurcation, however, should be

favored by poor consolidation. Plotting the cumulative

number of the AE events as a function of displacement for

the SEN-T(A) specimens displayed in Fig. 3, corroborates the

speculation about poor consolidation—cf. Fig. 4. So, one can

quote that the consolidation quality of the all-PP composite is

well reflected by the appearance of pop-in and course of the

cumulative AE events.

Figure 3. Load-displacement curves of selected SEN-T(A)

specimens.

Copyright # 2006 John Wiley & Sons, Ltd.

To estimate the initiation or critical fracture toughness the

KR-concept was followed. This necessitates the assessment of

the crack tip growth. As for that purpose neither visual

impaction nor compliance, crack growth calibration func-

tions (as all specimens are unique), can be used, the located

AE maps on SEN-T(B) specimens were used. This was done

by determining the center of gravity of the located

cumulative AE amplitudes in consecutive time intervals as

Figure 6. The KR-curve for a SEN-T(B) specimen. Note:

specimen is identical with that of Fig. 5.

Polym. Adv. Technol. 2007; 18: 90–96

DOI: 10.1002/pat

Fracture behavior of Curv1 93

described in earlier studies in detail.8–10 The related points,

connected by straight lines, represent the position of the

running crack tip at a given time. The outcome of this

procedure is depicted in Fig. 5.

Note that the crack length does not increase (initial value is

ca. 20 mm—cf. Fig. 2) until the load reaches a plateau (Fig. 5,

range I). This is linked with the formation of steady

(equilibrium) damage zone. After that the slight increase

(Fig. 5, range II) in the load is associated with some increase in

Figure 7. Force–displacement curves of SEN

cumulative AE amplitudes registered until the lo

the z-axis is dB/mm2.

Copyright # 2006 John Wiley & Sons, Ltd.

the crack length (Da� 5 mm). Fast but still resolvable crack

growth took place in range III. By constructing the KR-curve,

i.e. plotting K as a function of the increasing crack length

(defined by Da¼ ax�a0; cf. Fig. 2b), the initiation value (KI,in) at

Da¼ 0 can be estimated by linear extrapolation (cf. Fig. 6).

Note that the related value (KI,in� 11 MPa m1/2) agrees well

with that ofKQ,max derived on SEN-T(A) specimens. However,

the corresponding fracture energy (47.2 kJ/m2) is still much

higher than usual for unreinforced PP (�30 kJ/m2).12

-T(B) and 3D contour plots showing the

ad positions indicated. Note that the unit of

Polym. Adv. Technol. 2007; 18: 90–96

DOI: 10.1002/pat

94 G. Romhany et al.

Damage development and growthThe damage zone has been defined by considering the

located cumulative AE amplitudes, as described in Ref. 8.

The three-dimensional (3D) cumulative AE amplitude

contour plots in Fig. 7 show not only the crack propagation

along the free ligament of SEN-T(B) but also that the

extension of the damage zone did not change practically

Figure 8. Force–displacement curves of SEN

cumulative AE amplitudes registered until the lo

Copyright # 2006 John Wiley & Sons, Ltd.

during crack growth (ca. 8 mm). That is why the term

‘‘equilibrium’’ damage zone was used before.

The earlier scenario proved to be valid for well

consolidated all-PP composites. For a poor consolidated

specimen the damage development and growth show

somewhat other features—cf. Fig. 8. The crack propagation

in this SEN-T(B) specimen occurred by stick-slip mechanism

-T(B) and 3D contour plots showing the

ad positions indicated. For note see Fig. 7.

Polym. Adv. Technol. 2007; 18: 90–96

DOI: 10.1002/pat

Figure 9. SEM pictures at various magnifications taken from the fracture surface of a SEN-T(B)

specimen. Note: the load–displacement and damage behavior of this specimen were shown in

Figs 5–7. Arrows show brittlely fractured rovings.

Fracture behavior of Curv1 95

(see load drops in the post-maximum range). Each load

drops resulted in a peak linked with the crack progress.

Comparing Figs 7 and 8 one can see that for the poor

consolidation specimen the apparent damage zone is some-

what larger (ca. 12 mm instead of 8 mm). This is due to

differences in the stress redistribution at the crack tip. The

large damage zone becomes, however, smaller with

increasing crack length as the time is not enough for an

efficient stress redistribution to compensate the adverse

effects of the voided areas (cf. Fig. 1).

Similar results were published on poor consolidated

glass-mat reinforced thermoplastic PP. It is worth noting

that the accuracy of the crack tip estimation by AE has been

checked by infrared thermography (IT)—cf. infrared win-

dow in Fig. 2. Between the IT (not reported here) and AE

results a good agreement was found.

Failure behaviorSEM pictures taken from the fracture surface of SEN-T(B)

specimens show that rovings aligned to the loading direction

fractured mostly brittlely. However, those laying perpen-

dicular to the loading direction underwent massive fibrilla-

tion—see Fig. 9(a). At the higher magnification (cf. Fig. 9b)

one can observe that the in fracture plane laying fibers and

their bundles were splitting from the matrix in a very ductile

manner. This is a clear hint for the good interface produced

by hot compaction. One can suppose that the good bonding

was owing to the transcrystallization phenomena (for

example, see Ref. 6) caused by the reinforcing PP fibers.

The overall failure mode shows similarities with that of

unidirectional fiber reinforced thermoplastic components

having a ductile matrix after transverse fracture.

CONCLUSION

Based on this work devoted to the study of the in-plane

fracture behavior of a fabric reinforced all-PP composite

(Curv1), the following conclusions can be drawn:

� Fracture mechanical data determined on SEN-T specimens

by considering the maximum load did not differ from that

Copyright # 2006 John Wiley & Sons, Ltd.

of those deduced by the resistance curve methods. The

crack growth can well be traced by considering the located

cumulative AE amplitudes or events (i.e. shift in their

center of gravity during the test). This has been supported

by infrared thermographic results.

� The consolidation quality is well reflected by the apparent

damage zone size and its change during loading. A small

and stable damage zone traveling through the ligament

suggests good consolidation. The onset of pop-in

phenomena during loading of SEN-T specimens was

ascribed to the presence of voids in the all-PP composite.

AcknowledgmentsThis study was supported by Hungarian Scientific Research

Fund (OTKA F60505 and NI62729) and German Science

Foundation (Ka 1202/17). T. Barany is thankful for the

funding of the Hungarian Governmental Eotvos Scholar-

ship and Janos Bolyai Research Scholarship of the Hungar-

ian Academy of Sciences.

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