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epoxy composites subjected to static and dynamic loading by acoustic emission monitoring
Composites Science and Technology 59 (1999) 201±208
Initiation and growth of delamination in glass/epoxy composites subjected to static and dynamic loading by acoustic emission monitoring S. Benmedakhene *, M. Kenane, M.L. Benzeggagh Universite de Technologie de CompieÁgne, Laboratoire de GeÂnie MeÂcanique-PolymeÂres et composites ± LG2ms URA CNRS 1505, B.P. 649 60200 CompieÁgne Cedex, France Received 8 May 1996; received in revised form 29 January 1998; accepted 27 February 1998
Abstract Delamination is one of the most common failure modes of composite materials. It may result from imperfections in the production process or may be caused by service life conditions, such as impact by foreign objects. The presence of delamination in the composite material may reduce the overall stiness. The geometrical parameters, material proprieties, and loading conditions are important factors which aect the initiation, development and growth of delamination (Chen HP, David L. Comp. Sci. and Technol. 1993; 46:325±33). Experimental results obtained from monotonic tests and instrumented drop tests have shown good agreement between monotonic and dynamic critical load and crack-growth data. Previous studies made on dierent composite materials (Meraghni F, Benmedakhane S, Benzeggagh ML. In: Poursartip A, Street K, editors. Proc. ICCM 10, vol. I Vancouver, Canada: Woodhead Publishing, 1995:359±66; Barre S, Benzeggagh ML. Comp. Sci. and Technol. 1994; 54:369±76; Benzeggagh ML, Benmedakhene S. Comp. Sci. and Technol. 1995; 55:1±11) have allowed us to set up a schematic model of acoustic emission. In this model the dierent levels of amplitude signals emitted by materials under dierent types of loading are assigned to dierent damage mechanisms. This experimental methodology has been applied to this work and has allowed continuous monitoring of damage growth through delamination tests. This type of analysis has also allowed us to highlight the complementary aspect of acousticemission amplitude distributions with microscopic observations. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: D. Acoustic emission; D. Scanning electronic microscope; Glass/Epoxy; Mode I; Strain energy release rate; Velocity
1. Introduction Impact events are known to be potential causes of damage in composite materials and this damage can seriously impair the engineering properties of a composite structure. Consequently, the more information that can be obtained from an impact evaluation, the more certain we can be about the ultimate performance of material. Impact tests can generate many types of failure events such as interlaminar fracture. Several studies have been conducted on this type of failure, because it is a realistic situation and is the dominant failure mode in composite laminate structures. This is the case mainly in the mixed mode (I+II) [1,2], in which some hackles are present and are oriented at less than 45 with respect to the fracture surface. It should be noted that the orientation * Corresponding author.
of these hackles is proportional to the extent of participation of mode II [3]. Interlaminar fracture under pure mode I loading is the most dangerous mode. This is due to the fact that the delamination initiation energy is low compared to that of the shearing mode. The purpose of this study is to investigate the eect of velocity on delamination initiation and growth. The use of analytical and numerical approaches proposed in the literature is currently too complex, and much attention has therefore been focused on the characterisation of delamination behaviour by using fracture mechanics approaches: fracture toughness in term of the stress intensity factor K or strain energy release rate G. The literature contains studies of the eect of velocity which present contradictory results. Freidrich et al. [4], Mall et al. [5], and Smiley and Pipes [6] have observed that GIIC decreases when the velocity increases, but Chailou [7] found that there is no eect of velocity
0266-3538/99/$Ðsee front matter # 1999 Elsevier Science Ltd.. All rights reserved. PII: S0266 -3 538(98)00063 -3
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S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
between of 1.6 10ÿ5 and 1.33 m/s. In addition, Hunston [8], Aliyu et al. [9], Boudjemaa [10], and Aboura [11] show that GIIC increases when the velocity increases. The experimental approach used in this study is based on acoustic-emission analysis and microscopic observations. It has allowed us to determine and correlate the dierent fracture mechanisms with the corresponding velocity levels. It is found that, for low velocities, fractures are localised principally in the resin and in the resin/®bre interface, whereas for high velocities, fractures are concentrated in the resin/®bre interface. 2. Experiment 2.1. Material The composite used in this study is the Epoxy M10 resin (VICOTEX) reinforced with 52% by volume of Eglass, 5% of the ®bres being woven perpendicularly to hold the parallel ®bres together. The laminates were prepared by compression moulding of CIBA GEIGY prepregs. The starter crack was formed by inserting a Te¯on ®lm at mid-thickness during moulding. The material was considered to be elastically homogenous and orthotropic. The elastic constants of this material were obtained experimentally [12]: E11=36.2 GPa E22=10.6 GPa E33=7.2 GPa
G12=5.6 GPa G13=3.7 GPa G23=3.2 GPa
12=0.26 13=0.33 23=0.48
Experimental curves of tension and compression tests are quite linear up to rupture. This corresponds to the behaviour of composites containing of a brittle matrix. 2.2. Specimen Double-cantilever beam (DCB) specimens are frequently used under monotonic pure mode I conditions (Fig. 1). Specimen 180 to 220 mm long by 20 mm wide and of 6 mm thickness were cut from each experimental panel. The tests were carried out in an Instron 1186 machine at a displacement speed of 2, 100 and 500 mm/mn.
Fig. 1. DCB specimen (double-cantilever beam).
Fig. 2. SCB specimen (simple cantilever beam).
The simple-cantilever beam (SCB) specimen (Fig. 2) developed by Boudjemaa [10], and Benzeggaggh and Aboura [13] was used for dynamic tests (1±5 m/s). These tests were conducted in a drop-weight impact tester (Fig. 3). A Kesler load sensor permits simultaneous recording of the load and time. The initial velocity, V0, agreed well with the relation, p 1 V0 2gh; g being the acceleration due to gravity and h the drop height. The velocity is supposed constant if the mass of the drop weight is signi®cant, then, 1 mV2 F S; 2
2
where S is the cross-sectional area of the specimen and F is surface energy. The experimental data were recorded by means of a microcomputer system providing data acquisition, handling and analysis. The sampling period was taken as 10 ms, which corresponds to 100 kHz frequency. 2.3. Test conditions Each specimen was instrumented with an acousticemission receptor and strain-gage at the crack tip
Fig. 3. Drop-weight tester con®guration.
S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
(Fig. 1). This instrumentation permits us to detect crack initiation and to study the crack growth mechanisms. Work on a variety of composites with dierent ®bres and resins [14±17] have con®rmed that the acousticemission amplitude range corresponds to dierent damage mechanisms (Fig. 4). Damage growth detection and monitoring were performed by using the amplitude analysis of acoustic emission (AE) signals generated during the occurrence of damage mechanisms. The acquisition and the processing of the acoustic activity were carried out by means of an acoustic emission system (DUNEGAN/ ENDEVCO 3000) possessing the following physical characteristics: Preampli®er gain: 39 dB Preampli®er ®lters: 450 B and 190 B (according to the system used) A.E threshold: Selectable: ®xed at 30 dB for the study Interval of amplitude channels is from 0 to 100 dB Channel width (resolution) 1 dB Dead time 1 m (emission by bursts) Acoustic emission signals were detected by a piezoelectric transducer (PAC MICROPHONE 80) and have a large range of frequencies going from 200 kHz to 1 MHz. A coupling ¯uid (Dough 428 Rhodorsil Silicone) is used to provide a ¯awless contact between the transducer and the specimen. The A.E investigation method was combined with successive microscopic observations (in a scanning electron microscope). This provides optimum understanding of the occurrence and the evolution of the basic damage mechanisms. 2.4. Data reduction methods The determination of composite toughness of delamination in term of the strain energy rate was performed by applying the compliance based on the Irwin± Kies [18] equation: GC
P2C dC ; 2B da
Fig. 4. Typical amplitude distribution and areas de®nition.
3
203
where PC is the critical load, B the width, C the compliance de®ned by the displacement over the load, and a the crack length. In the case of DCB specimen, the compliance calibration introduced by Berry [19] and adapted for composite materials by de Charentenay et al. [20] can be written as C
an : h
4
Applying Eq. (3), the energy, GIC corresponding to delamination initiation can be calculated by GIC
nPC C 2Ba
5
where PC and C must be the load and displacement at the initiation of delamination. The stable growth ¯aws can be characterised using the crack-growth resistance curve (Rcurve). The eective crack length, aeff and the corresponding energy values, GIP can be obtained by the formulae, an P P 1=n eff ) aeff h ; 6 C PP h PP GIP
nPP P : 2Baeff
7
Note that PP and P represent the load and corresponding displacement during delamination propagation 3. Results 3.1. Delamination initiation Fig. 5 shows the mechanical behaviour (load versus displacement) and the corresponding acoustic emissions. The mechanical behaviour is characterised by the linear part of the load versus displacement curve up to the point A. The crack initiation at the crack tip appears at the point A. This corresponds to the simultaneous appear-
Fig. 5. Load and cumulative counts versus displacement (v=2 mm/ mn).
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S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
Fig. 6. Load and microstrain versus displacement (v=2 mm/mn).
ance of acoustic emission and to the change of the response of the strain gauge (Figs. 5 and 6). We notice that the crack initiation point is followed by a light instability of the load. In the case of high velocities (V=3 and 6 m/s), the recording of acoustic emission is not possible. In fact, it is not possible to detect the crack initiation precisely, because tests are very short, and curves are disturbed by impact tester vibrations. To overcome this problem, each specimen was instrumented with two strain gauges. One on the face of the specimen, and the other through of the thickness at crack tip (Fig. 2). As we see on Fig. 7, the crack initiation is dicult to detect on the load/displacement curve. However it is very easy to detect with responses of strain gauges, because it corresponds to a deviation in the slope of the curve. Fig. 8 shows the critical strain energy release rate increases with the velocity. The curve GIC f v is composed of two phases having dierent slopes, the transition is around v=104 mm/mn. The same variation was obtained by Boudjmaa [10], Aboura [11], and Smiley and Pipes [6]. They interpreted this variation by the nature of the damage initiation which is developed in the resin-rich zone. However, Shaw and Kinloch [21] showed that the critical strainenergy release rate, GIC , of adhesively bonded joints with dierent thicknesses, depends on the height of the plastic zone (Fig. 9).
Fig. 7. Load and micro-strain versus displacement (v=3 m/s).
Fig. 8. Critical strain energy release rate, GIC, versus velocity.
In our case, the increase in velocity results in brittle behaviour of the resin and so the plastic zone size decreases, whereas, the critical strain-energy release rate, GIC increases. In fact, according to the microscopic observations, we can see that crack initiation appears in the resin at low velocities. When the velocity increases, crack initiation occurs in the interface instead (Figs. 10± 12). 3.2. Delamination growth According to the load/displacement curves, the composites have a considerable resistance to delamination propagation compared to delamination initiation. The delamination process is the result of a sequence of several damage mechanisms. These include matrix cracking, interface fracture, the ®bre pull-out phenomena and ®bre fracture, which are all easy to evoke as damage mechanisms, but their participation is dicult to ascertain in the case of delamination phenomena. For this account, it becomes interesting to determine the fracture energy by using the R curves, and to identify fracture mechanisms during crack growth by the correlation between acoustic emission monitoring and microscopic observations.
Fig. 9. Critical strain-energy release rate GIC as a function of plastic zone size.
S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
205
Fig. 13. A fracture energy GIP versus crack length (ae).
Fig. 10. Crack initiation in the resin v=2 mm/mn (x25).
Fig. 11. Crack initiation in the in resin and the ®ber/matrix interface v=500 mm/mn (x40).
Fig. 12. Crack initiation in the ®ber/matrix interface (v=6 m/s).
R curves are shown in Fig. 13 in which GIP versus the corresponding eective crack length aeff is plotted. First, the delamination extension resistance increases rapidly from an initial value as the crack grow until it attains a plateau, characterised by a cloud of points. Fig. 14, which represents the change in the fracture energy as a function of velocity, shows that the fracture energy increases rapidly from about v=104 mm/mn as we have seen in the case of the critical strain-energy release rate. Eectively, this is explained by the presence of dierent fracture mechanisms at low or high velocities. The interlaminar fracture process is due to the coating microcracks which corresponds to increased matrix deformation. In order to understand the interlaminar fracture mechanisms, the fracture surfaces were observed in the scanning electron microscope (SEM). We note that the pure mode I at a velocity of 2 mm/mn (Fig. 15) is characterised by a propagation localised principally in the resin with the formation of ®ber bridging [22], only a few zones where the crack path is propagating along the interface, this is the case when the crack meets woven ®bers (Fig. 16). In Fig. 16, we observe microcracks which are perpendicular to the normal principal stress 33, that appears in the resin. The fracture surface presents cleavage paths characterising the opening mode (Fig. 17). The acoustic amplitude distribution (Fig. 18) shows that events are generally situated between 40 and 55 dB
Fig. 14. Fracture energy GIP versus velocity.
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S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
Fig. 18. The acoustic emission amplitude distribution at 2 mm/mn.
Fig. 15. Crack propagation in the resin (v=2 mm/mn).
Fig. 19. Fracture appearance for v=2 mm/mn.
Fig. 16. Crack propagation along the interface when it meets woven ®bers.
characterising the matrix micromicracking and matrix / matrix friction. It also shows some events around 65 dB characterising the interface decohesion. Moreover, compared to those tested in static tests (Fig. 19Fig. 20) we see more fragments of resin on the fracture from specimens tested at 3 and 6 m/s (Fig. 21).
Fig. 17. Mode I fracture surface (presence of cleavage paths, v=2 mm/mn).
Another phenomenon due to the eect of velocity is observed with the simultaneous use of microscopic observation and acoustic emission analysis. This concerns the failure path at 100 and 500 mm/mn the crack path appears to propagate through the resin and along the interply interface (Fig. 22). Concerning the acoustic emission amplitude distributions, at 100 mm/mn (Fig. 23) there are some events in the range of 40±55 dB and around 65 dB, but at 500
Fig. 20. Fragments of resin on the fracture surface (v=100 mm/mn).
S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
207
Fig. 21. More fragments of resin on the fracture surface (v=6 m/s).
Fig. 25. Crack propagation in the ®ber/matrix interface (v=6 m/s).
Fig. 22. Crack propagation in the resin and the interface (v=100 mm/ mn).
Fig. 26. The crack path passes round the packet of woven ®bers (v=3 m/s).
Fig. 23. The acoustic emission amplitude distribution at 100 mm/mn.
Fig. 27. Rupture of some ®bers under eect of undulations created by weaving.
Fig. 24. The acoustic emission amplitude distribution at 500 mm/mn.
mm/mn their number in the same range decreases (Fig. 24). In the case of the specimen tested at 3 and 6 m/s, decohesion in the interply interface and ®ber/matrix interface are the predominant fracture mechanisms (Fig. 25). As it is shown on Fig. 26, the crack path passes round the packet of woven ®bers, and cracks ®bers/ matrix interfaces.
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S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
We also see, as shown in Fig. 27, that the packet of woven ®bers seems to be cracked completely by a local shearing stress and principal normal stress, 33; this is related to the fact that weaving creates undulations. 4. Conclusion Investigation of composite laminates is carried out in this study in order to characterise delamination growth mechanisms under pure mode I conditions over large range of velocities (2, 100, and 500 mm/mn, 3 and 6 m/ s). The dynamic-testing apparatus has been improved to permit us simultaneously to record the applied load and the corresponding displacement. This permits the determination of the critical strain-energy release rate, GIC , and the fracture energy GIP for dierent velocity values. The experimental results obtained in this study shows an increase of GIC and GIP as a function of velocity. In fact, the resin becomes more brittle as the velocity increases. The analysis of the fracture surfaces and the delamination pro®les provide an explanation of the changes in fracture mechanisms as a function of the velocity: at low velocity values, failure propagates principally in the resin, and progressively as the velocity increases, the failure propagates in the ®ber/matrix and interply interfaces. Results obtained in the dierent studies are sometimes contradictory. This may be due to dierent materials and experimental conditions used by the different authors. References [1] Takeda N. Experimental studies of the delamination mechanisms in impacted reiforced composite plates. Ph.D. Thesis, University of Florida, USA, 1980 [2] Beaumont N. Contribution aÁ l'eÂtude de l'impact d'une bille sur une plaque en mateÂriau composite. TheÁse de doctorat, EÂcole Nationale SupeÂrieure des Mines de Paris Centre des MateÂriaux, France, DeÂcembre 1990 [3] Bradley WL, Cohen RN. Matrix deformation and fracture in graphite reinforced epoxy. ASTM Symposium on delamination and debonding of materials, Pittsburg (Pennysylvania, USA), 1983 [4] Friedrich K, Walter R, Carlsonn LA, Smiley AJ, Gillespie JW. Mechanisms for rate eects on interlaminar fracture toughness of carbon/epoxy and carbon/PEEK composites. J of Mat Sci 1985;24(9):3387±98
[5] Mall S, Law EG, Katouzian M. Loading rate eect on interlaminar fracture toughness of thermoplastic composite. J of Comp Mat 1987;21:569±79 [6] Simley AJ, Pipes RB. Rate eect on mode I interlaminaire fracture toughness in composites materials. J of Comp Mat 1987;21:670±87 [7] Chaillou F. EÂtude des endommagements au choc de plaques composites. EÂtablissement Technique Centrale de l'Armement, Centre de Recherche d'Arcueil, DeÂpartement Comportement des MateÂriaux. Rapport interne, ETCA-92-R-068, France, 1992 [8] Hunston DL. Composite interlaminaire fracture: eect of fracture energy. Comp Techno Rev 1984;6(4):176±80 [9] Aliyu AA, Daniel IM. Eects of strain rate on delamination fracture toughness of graphite/epoxy. Delamination and Debonding Material, ASTM STP 876 Philadelphia, 1985:336±48 [10] Boudjemaa N. DeÂlaminage des mateÂriaux composites strati®eÂs aÁ faibles et grandes vitesses de sollicitation en mode I. Ph.D. thesis de Universite de Technologie de CompieÁgne, March 1987 [11] Aboura Z. Etude du processus de deÂlaminage mode I, mode II et en mode mixte (I+II) de mateÂriaux composites a renforts tisses aÁ dierentes vitesses de sollicitation. Ph.D. thesis de Universite de Technologie de CompieÁgne, November 1993 [12] Gong XJ. Rupture Interlaminaire en Mode Mixte I+II du Composite Strati®e Verre/Epoxy Unidirectionnel et Multidirectionnel. Ph.D. thesis, Universite de Technologie de CompieÁgne, April 1992 [13] Benzeggagh ML, Aboura Z. DeÂlaminage mode I et II de composites aÁ renfort tissu solliciteÂs aÁ faibles et grandes vitesses. J de Phys III France 1991;1:1927±51 [14] Meraghni F, Benmedakhane S, Benzeggagh ML. Identi®cation and modelling of damage mechanisms in short glass ®bre reinforced polypropylene composite. In: Poursartip A, Street K, editors. Proceedings of The Tenth International Conference on Composite Materials (ICCM. 10), vol. I. Vancouver, Canada: Woodhead Publishing, 1995:359±66 [15] Barre S, Benzeggagh ML. On the use of acoustic emission to investigate damage mechanisms in glass ®ber-reinforced polypropylene. Comp Sci and Technol 1994;52:369±76 [16] Benzeggagh ML, Benmedakhene S. Residual strength of glass/ polypropylene composite material subjected to impact. Comp Sci and Technol 1995;55:1±11 [17] Laksimi A, Gong XL, Benzeggagh ML. Analysis of damage mechanisms in centrally notched glass-®ber/epoxy plate. Comp Sci and Technol 1994;52:85±91 [18] Irwin GR, Kies JA. Critical energy rate analysis of fracture strength. Welding Res Suppl 1954;19:193 [19] Berry JP. Determination of fracture surface energies by cleavage technic. J of Appl Phys 1963;34:62 [20] De Charentenay FX, Bethmont M, Benzeggagh M, ChreÂtien JF. Delamination of glass ®ber reinforced polyester, an acoustic emission study, vol. 3. Cambridge, UK: ICM, 1979 [21] Shaw SJ, Kinloch AJ. The fracture resistance of a toughened Epoxy adhesive. Journal of Adhesion 1981;12:59±77 [22] Benzeggagh M, Gong XJ, Laksimi A, Roelandt JM. On the mode I delamination test and the importance of laminate lay-ups. Polymer Engineering and Science 1991;31(17):1286±92
Initiation and growth of delamination in glass/epoxy composites subjected to static and dynamic loading by acoustic emission monitoring S. Benmedakhene *, M. Kenane, M.L. Benzeggagh Universite de Technologie de CompieÁgne, Laboratoire de GeÂnie MeÂcanique-PolymeÂres et composites ± LG2ms URA CNRS 1505, B.P. 649 60200 CompieÁgne Cedex, France Received 8 May 1996; received in revised form 29 January 1998; accepted 27 February 1998
Abstract Delamination is one of the most common failure modes of composite materials. It may result from imperfections in the production process or may be caused by service life conditions, such as impact by foreign objects. The presence of delamination in the composite material may reduce the overall stiness. The geometrical parameters, material proprieties, and loading conditions are important factors which aect the initiation, development and growth of delamination (Chen HP, David L. Comp. Sci. and Technol. 1993; 46:325±33). Experimental results obtained from monotonic tests and instrumented drop tests have shown good agreement between monotonic and dynamic critical load and crack-growth data. Previous studies made on dierent composite materials (Meraghni F, Benmedakhane S, Benzeggagh ML. In: Poursartip A, Street K, editors. Proc. ICCM 10, vol. I Vancouver, Canada: Woodhead Publishing, 1995:359±66; Barre S, Benzeggagh ML. Comp. Sci. and Technol. 1994; 54:369±76; Benzeggagh ML, Benmedakhene S. Comp. Sci. and Technol. 1995; 55:1±11) have allowed us to set up a schematic model of acoustic emission. In this model the dierent levels of amplitude signals emitted by materials under dierent types of loading are assigned to dierent damage mechanisms. This experimental methodology has been applied to this work and has allowed continuous monitoring of damage growth through delamination tests. This type of analysis has also allowed us to highlight the complementary aspect of acousticemission amplitude distributions with microscopic observations. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: D. Acoustic emission; D. Scanning electronic microscope; Glass/Epoxy; Mode I; Strain energy release rate; Velocity
1. Introduction Impact events are known to be potential causes of damage in composite materials and this damage can seriously impair the engineering properties of a composite structure. Consequently, the more information that can be obtained from an impact evaluation, the more certain we can be about the ultimate performance of material. Impact tests can generate many types of failure events such as interlaminar fracture. Several studies have been conducted on this type of failure, because it is a realistic situation and is the dominant failure mode in composite laminate structures. This is the case mainly in the mixed mode (I+II) [1,2], in which some hackles are present and are oriented at less than 45 with respect to the fracture surface. It should be noted that the orientation * Corresponding author.
of these hackles is proportional to the extent of participation of mode II [3]. Interlaminar fracture under pure mode I loading is the most dangerous mode. This is due to the fact that the delamination initiation energy is low compared to that of the shearing mode. The purpose of this study is to investigate the eect of velocity on delamination initiation and growth. The use of analytical and numerical approaches proposed in the literature is currently too complex, and much attention has therefore been focused on the characterisation of delamination behaviour by using fracture mechanics approaches: fracture toughness in term of the stress intensity factor K or strain energy release rate G. The literature contains studies of the eect of velocity which present contradictory results. Freidrich et al. [4], Mall et al. [5], and Smiley and Pipes [6] have observed that GIIC decreases when the velocity increases, but Chailou [7] found that there is no eect of velocity
0266-3538/99/$Ðsee front matter # 1999 Elsevier Science Ltd.. All rights reserved. PII: S0266 -3 538(98)00063 -3
202
S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
between of 1.6 10ÿ5 and 1.33 m/s. In addition, Hunston [8], Aliyu et al. [9], Boudjemaa [10], and Aboura [11] show that GIIC increases when the velocity increases. The experimental approach used in this study is based on acoustic-emission analysis and microscopic observations. It has allowed us to determine and correlate the dierent fracture mechanisms with the corresponding velocity levels. It is found that, for low velocities, fractures are localised principally in the resin and in the resin/®bre interface, whereas for high velocities, fractures are concentrated in the resin/®bre interface. 2. Experiment 2.1. Material The composite used in this study is the Epoxy M10 resin (VICOTEX) reinforced with 52% by volume of Eglass, 5% of the ®bres being woven perpendicularly to hold the parallel ®bres together. The laminates were prepared by compression moulding of CIBA GEIGY prepregs. The starter crack was formed by inserting a Te¯on ®lm at mid-thickness during moulding. The material was considered to be elastically homogenous and orthotropic. The elastic constants of this material were obtained experimentally [12]: E11=36.2 GPa E22=10.6 GPa E33=7.2 GPa
G12=5.6 GPa G13=3.7 GPa G23=3.2 GPa
12=0.26 13=0.33 23=0.48
Experimental curves of tension and compression tests are quite linear up to rupture. This corresponds to the behaviour of composites containing of a brittle matrix. 2.2. Specimen Double-cantilever beam (DCB) specimens are frequently used under monotonic pure mode I conditions (Fig. 1). Specimen 180 to 220 mm long by 20 mm wide and of 6 mm thickness were cut from each experimental panel. The tests were carried out in an Instron 1186 machine at a displacement speed of 2, 100 and 500 mm/mn.
Fig. 1. DCB specimen (double-cantilever beam).
Fig. 2. SCB specimen (simple cantilever beam).
The simple-cantilever beam (SCB) specimen (Fig. 2) developed by Boudjemaa [10], and Benzeggaggh and Aboura [13] was used for dynamic tests (1±5 m/s). These tests were conducted in a drop-weight impact tester (Fig. 3). A Kesler load sensor permits simultaneous recording of the load and time. The initial velocity, V0, agreed well with the relation, p 1 V0 2gh; g being the acceleration due to gravity and h the drop height. The velocity is supposed constant if the mass of the drop weight is signi®cant, then, 1 mV2 F S; 2
2
where S is the cross-sectional area of the specimen and F is surface energy. The experimental data were recorded by means of a microcomputer system providing data acquisition, handling and analysis. The sampling period was taken as 10 ms, which corresponds to 100 kHz frequency. 2.3. Test conditions Each specimen was instrumented with an acousticemission receptor and strain-gage at the crack tip
Fig. 3. Drop-weight tester con®guration.
S. Benmedakhene et al. / Composites Science and Technology 59 (1999) 201±208
(Fig. 1). This instrumentation permits us to detect crack initiation and to study the crack growth mechanisms. Work on a variety of composites with dierent ®bres and resins [14±17] have con®rmed that the acousticemission amplitude range corresponds to dierent damage mechanisms (Fig. 4). Damage growth detection and monitoring were performed by using the amplitude analysis of acoustic emission (AE) signals generated during the occurrence of damage mechanisms. The acquisition and the processing of the acoustic activity were carried out by means of an acoustic emission system (DUNEGAN/ ENDEVCO 3000) possessing the following physical characteristics: Preampli®er gain: 39 dB Preampli®er ®lters: 450 B and 190 B (according to the system used) A.E threshold: Selectable: ®xed at 30 dB for the study Interval of amplitude channels is from 0 to 100 dB Channel width (resolution) 1 dB Dead time 1 m (emission by bursts) Acoustic emission signals were detected by a piezoelectric transducer (PAC MICROPHONE 80) and have a large range of frequencies going from 200 kHz to 1 MHz. A coupling ¯uid (Dough 428 Rhodorsil Silicone) is used to provide a ¯awless contact between the transducer and the specimen. The A.E investigation method was combined with successive microscopic observations (in a scanning electron microscope). This provides optimum understanding of the occurrence and the evolution of the basic damage mechanisms. 2.4. Data reduction methods The determination of composite toughness of delamination in term of the strain energy rate was performed by applying the compliance based on the Irwin± Kies [18] equation: GC
P2C dC ; 2B da
Fig. 4. Typical amplitude distribution and areas de®nition.
3
203
where PC is the critical load, B the width, C the compliance de®ned by the displacement over the load, and a the crack length. In the case of DCB specimen, the compliance calibration introduced by Berry [19] and adapted for composite materials by de Charentenay et al. [20] can be written as C
an : h
4
Applying Eq. (3), the energy, GIC corresponding to delamination initiation can be calculated by GIC
nPC C 2Ba
5
where PC and C must be the load and displacement at the initiation of delamination. The stable growth ¯aws can be characterised using the crack-growth resistance curve (Rcurve). The eective crack length, aeff and the corresponding energy values, GIP can be obtained by the formulae, an P P 1=n eff ) aeff h ; 6 C PP h PP GIP
nPP P : 2Baeff
7
Note that PP and P represent the load and corresponding displacement during delamination propagation 3. Results 3.1. Delamination initiation Fig. 5 shows the mechanical behaviour (load versus displacement) and the corresponding acoustic emissions. The mechanical behaviour is characterised by the linear part of the load versus displacement curve up to the point A. The crack initiation at the crack tip appears at the point A. This corresponds to the simultaneous appear-
Fig. 5. Load and cumulative counts versus displacement (v=2 mm/ mn).
204
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Fig. 6. Load and microstrain versus displacement (v=2 mm/mn).
ance of acoustic emission and to the change of the response of the strain gauge (Figs. 5 and 6). We notice that the crack initiation point is followed by a light instability of the load. In the case of high velocities (V=3 and 6 m/s), the recording of acoustic emission is not possible. In fact, it is not possible to detect the crack initiation precisely, because tests are very short, and curves are disturbed by impact tester vibrations. To overcome this problem, each specimen was instrumented with two strain gauges. One on the face of the specimen, and the other through of the thickness at crack tip (Fig. 2). As we see on Fig. 7, the crack initiation is dicult to detect on the load/displacement curve. However it is very easy to detect with responses of strain gauges, because it corresponds to a deviation in the slope of the curve. Fig. 8 shows the critical strain energy release rate increases with the velocity. The curve GIC f v is composed of two phases having dierent slopes, the transition is around v=104 mm/mn. The same variation was obtained by Boudjmaa [10], Aboura [11], and Smiley and Pipes [6]. They interpreted this variation by the nature of the damage initiation which is developed in the resin-rich zone. However, Shaw and Kinloch [21] showed that the critical strainenergy release rate, GIC , of adhesively bonded joints with dierent thicknesses, depends on the height of the plastic zone (Fig. 9).
Fig. 7. Load and micro-strain versus displacement (v=3 m/s).
Fig. 8. Critical strain energy release rate, GIC, versus velocity.
In our case, the increase in velocity results in brittle behaviour of the resin and so the plastic zone size decreases, whereas, the critical strain-energy release rate, GIC increases. In fact, according to the microscopic observations, we can see that crack initiation appears in the resin at low velocities. When the velocity increases, crack initiation occurs in the interface instead (Figs. 10± 12). 3.2. Delamination growth According to the load/displacement curves, the composites have a considerable resistance to delamination propagation compared to delamination initiation. The delamination process is the result of a sequence of several damage mechanisms. These include matrix cracking, interface fracture, the ®bre pull-out phenomena and ®bre fracture, which are all easy to evoke as damage mechanisms, but their participation is dicult to ascertain in the case of delamination phenomena. For this account, it becomes interesting to determine the fracture energy by using the R curves, and to identify fracture mechanisms during crack growth by the correlation between acoustic emission monitoring and microscopic observations.
Fig. 9. Critical strain-energy release rate GIC as a function of plastic zone size.
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Fig. 13. A fracture energy GIP versus crack length (ae).
Fig. 10. Crack initiation in the resin v=2 mm/mn (x25).
Fig. 11. Crack initiation in the in resin and the ®ber/matrix interface v=500 mm/mn (x40).
Fig. 12. Crack initiation in the ®ber/matrix interface (v=6 m/s).
R curves are shown in Fig. 13 in which GIP versus the corresponding eective crack length aeff is plotted. First, the delamination extension resistance increases rapidly from an initial value as the crack grow until it attains a plateau, characterised by a cloud of points. Fig. 14, which represents the change in the fracture energy as a function of velocity, shows that the fracture energy increases rapidly from about v=104 mm/mn as we have seen in the case of the critical strain-energy release rate. Eectively, this is explained by the presence of dierent fracture mechanisms at low or high velocities. The interlaminar fracture process is due to the coating microcracks which corresponds to increased matrix deformation. In order to understand the interlaminar fracture mechanisms, the fracture surfaces were observed in the scanning electron microscope (SEM). We note that the pure mode I at a velocity of 2 mm/mn (Fig. 15) is characterised by a propagation localised principally in the resin with the formation of ®ber bridging [22], only a few zones where the crack path is propagating along the interface, this is the case when the crack meets woven ®bers (Fig. 16). In Fig. 16, we observe microcracks which are perpendicular to the normal principal stress 33, that appears in the resin. The fracture surface presents cleavage paths characterising the opening mode (Fig. 17). The acoustic amplitude distribution (Fig. 18) shows that events are generally situated between 40 and 55 dB
Fig. 14. Fracture energy GIP versus velocity.
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Fig. 18. The acoustic emission amplitude distribution at 2 mm/mn.
Fig. 15. Crack propagation in the resin (v=2 mm/mn).
Fig. 19. Fracture appearance for v=2 mm/mn.
Fig. 16. Crack propagation along the interface when it meets woven ®bers.
characterising the matrix micromicracking and matrix / matrix friction. It also shows some events around 65 dB characterising the interface decohesion. Moreover, compared to those tested in static tests (Fig. 19Fig. 20) we see more fragments of resin on the fracture from specimens tested at 3 and 6 m/s (Fig. 21).
Fig. 17. Mode I fracture surface (presence of cleavage paths, v=2 mm/mn).
Another phenomenon due to the eect of velocity is observed with the simultaneous use of microscopic observation and acoustic emission analysis. This concerns the failure path at 100 and 500 mm/mn the crack path appears to propagate through the resin and along the interply interface (Fig. 22). Concerning the acoustic emission amplitude distributions, at 100 mm/mn (Fig. 23) there are some events in the range of 40±55 dB and around 65 dB, but at 500
Fig. 20. Fragments of resin on the fracture surface (v=100 mm/mn).
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Fig. 21. More fragments of resin on the fracture surface (v=6 m/s).
Fig. 25. Crack propagation in the ®ber/matrix interface (v=6 m/s).
Fig. 22. Crack propagation in the resin and the interface (v=100 mm/ mn).
Fig. 26. The crack path passes round the packet of woven ®bers (v=3 m/s).
Fig. 23. The acoustic emission amplitude distribution at 100 mm/mn.
Fig. 27. Rupture of some ®bers under eect of undulations created by weaving.
Fig. 24. The acoustic emission amplitude distribution at 500 mm/mn.
mm/mn their number in the same range decreases (Fig. 24). In the case of the specimen tested at 3 and 6 m/s, decohesion in the interply interface and ®ber/matrix interface are the predominant fracture mechanisms (Fig. 25). As it is shown on Fig. 26, the crack path passes round the packet of woven ®bers, and cracks ®bers/ matrix interfaces.
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We also see, as shown in Fig. 27, that the packet of woven ®bers seems to be cracked completely by a local shearing stress and principal normal stress, 33; this is related to the fact that weaving creates undulations. 4. Conclusion Investigation of composite laminates is carried out in this study in order to characterise delamination growth mechanisms under pure mode I conditions over large range of velocities (2, 100, and 500 mm/mn, 3 and 6 m/ s). The dynamic-testing apparatus has been improved to permit us simultaneously to record the applied load and the corresponding displacement. This permits the determination of the critical strain-energy release rate, GIC , and the fracture energy GIP for dierent velocity values. The experimental results obtained in this study shows an increase of GIC and GIP as a function of velocity. In fact, the resin becomes more brittle as the velocity increases. The analysis of the fracture surfaces and the delamination pro®les provide an explanation of the changes in fracture mechanisms as a function of the velocity: at low velocity values, failure propagates principally in the resin, and progressively as the velocity increases, the failure propagates in the ®ber/matrix and interply interfaces. Results obtained in the dierent studies are sometimes contradictory. This may be due to dierent materials and experimental conditions used by the different authors. References [1] Takeda N. Experimental studies of the delamination mechanisms in impacted reiforced composite plates. Ph.D. Thesis, University of Florida, USA, 1980 [2] Beaumont N. Contribution aÁ l'eÂtude de l'impact d'une bille sur une plaque en mateÂriau composite. TheÁse de doctorat, EÂcole Nationale SupeÂrieure des Mines de Paris Centre des MateÂriaux, France, DeÂcembre 1990 [3] Bradley WL, Cohen RN. Matrix deformation and fracture in graphite reinforced epoxy. ASTM Symposium on delamination and debonding of materials, Pittsburg (Pennysylvania, USA), 1983 [4] Friedrich K, Walter R, Carlsonn LA, Smiley AJ, Gillespie JW. Mechanisms for rate eects on interlaminar fracture toughness of carbon/epoxy and carbon/PEEK composites. J of Mat Sci 1985;24(9):3387±98
[5] Mall S, Law EG, Katouzian M. Loading rate eect on interlaminar fracture toughness of thermoplastic composite. J of Comp Mat 1987;21:569±79 [6] Simley AJ, Pipes RB. Rate eect on mode I interlaminaire fracture toughness in composites materials. J of Comp Mat 1987;21:670±87 [7] Chaillou F. EÂtude des endommagements au choc de plaques composites. EÂtablissement Technique Centrale de l'Armement, Centre de Recherche d'Arcueil, DeÂpartement Comportement des MateÂriaux. Rapport interne, ETCA-92-R-068, France, 1992 [8] Hunston DL. Composite interlaminaire fracture: eect of fracture energy. Comp Techno Rev 1984;6(4):176±80 [9] Aliyu AA, Daniel IM. Eects of strain rate on delamination fracture toughness of graphite/epoxy. Delamination and Debonding Material, ASTM STP 876 Philadelphia, 1985:336±48 [10] Boudjemaa N. DeÂlaminage des mateÂriaux composites strati®eÂs aÁ faibles et grandes vitesses de sollicitation en mode I. Ph.D. thesis de Universite de Technologie de CompieÁgne, March 1987 [11] Aboura Z. Etude du processus de deÂlaminage mode I, mode II et en mode mixte (I+II) de mateÂriaux composites a renforts tisses aÁ dierentes vitesses de sollicitation. Ph.D. thesis de Universite de Technologie de CompieÁgne, November 1993 [12] Gong XJ. Rupture Interlaminaire en Mode Mixte I+II du Composite Strati®e Verre/Epoxy Unidirectionnel et Multidirectionnel. Ph.D. thesis, Universite de Technologie de CompieÁgne, April 1992 [13] Benzeggagh ML, Aboura Z. DeÂlaminage mode I et II de composites aÁ renfort tissu solliciteÂs aÁ faibles et grandes vitesses. J de Phys III France 1991;1:1927±51 [14] Meraghni F, Benmedakhane S, Benzeggagh ML. Identi®cation and modelling of damage mechanisms in short glass ®bre reinforced polypropylene composite. In: Poursartip A, Street K, editors. Proceedings of The Tenth International Conference on Composite Materials (ICCM. 10), vol. I. Vancouver, Canada: Woodhead Publishing, 1995:359±66 [15] Barre S, Benzeggagh ML. On the use of acoustic emission to investigate damage mechanisms in glass ®ber-reinforced polypropylene. Comp Sci and Technol 1994;52:369±76 [16] Benzeggagh ML, Benmedakhene S. Residual strength of glass/ polypropylene composite material subjected to impact. Comp Sci and Technol 1995;55:1±11 [17] Laksimi A, Gong XL, Benzeggagh ML. Analysis of damage mechanisms in centrally notched glass-®ber/epoxy plate. Comp Sci and Technol 1994;52:85±91 [18] Irwin GR, Kies JA. Critical energy rate analysis of fracture strength. Welding Res Suppl 1954;19:193 [19] Berry JP. Determination of fracture surface energies by cleavage technic. J of Appl Phys 1963;34:62 [20] De Charentenay FX, Bethmont M, Benzeggagh M, ChreÂtien JF. Delamination of glass ®ber reinforced polyester, an acoustic emission study, vol. 3. Cambridge, UK: ICM, 1979 [21] Shaw SJ, Kinloch AJ. The fracture resistance of a toughened Epoxy adhesive. Journal of Adhesion 1981;12:59±77 [22] Benzeggagh M, Gong XJ, Laksimi A, Roelandt JM. On the mode I delamination test and the importance of laminate lay-ups. Polymer Engineering and Science 1991;31(17):1286±92