Drop-weight impact response of hybrid composites impacted by impactor of various geometries

Materials and Design 52 (2013) 67–77

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Drop-weight impact response of hybrid composites impacted by impactor of various geometries Ercan Sevkat a,⇑, Benjamin Liaw b, Feridun Delale b a b

Department of Mechanical Engineering, Meliksah University, 38280 Talas, Kayseri, Turkey Department of Mechanical Engineering, The City College of New York, 140th and Convent Avenue, NY 10031, USA

a r t i c l e

i n f o

Article history: Received 26 December 2012 Accepted 6 May 2013 Available online 23 May 2013 Keywords: Hybrid Impact behavior Ultrasonics Impactor shape Finite element

a b s t r a c t Drop-weight impact response of hybrid woven composite plates was studied. Hybrid S2 glass-IM7 graphite fibers/toughened epoxy composites with two lay-up arrangements were impacted using spherical, flat-ended cylindrical and straight-line Charpy impactors. The time-histories of impact-induced dynamic strains and impact forces were recorded. The damaged specimens were inspected using ultrasonic C-Scan methods. Experimental results exhibited that hybrid composites with glass outer skins had higher resistance to impact compared to second type. It also delaminated more than hybrid composites with graphite outer skins. The 3-D dynamic finite element software, LS-DYNA, was then used to simulate the experimental result of drop-weight tests. Good agreement between experimental and FE results was achieved when comparing dynamic force, strain histories and damage patterns between experimental measurements and finite element simulations. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Fibers in textile forms are being introduced for use in composites because of their better impact resistance. These materials offer flexibility for designing innovative and damage tolerant systems subjected to extreme changes in pressure, temperature and strain rate [1]. Composites might be exposed to impacts loading during their service life. For instance, a tool can be dropped onto a composite during maintenance or flying fragment with low velocity can impact the composites structure. Resultant damage of such impacts is usually in the form of delamination. If the impact energy is high enough, fiber breakage and matrix failure can also be observed. Majority of the low-velocity impact studies conducted using hemispherical impactor [2]. However the objects dropped onto composite structure or the flying fragment might have various shapes. Hence the effect of impactor shape on the drop-weight response of composites is important. Mitrevski et al. [3,4] studied the effect of impactor shape on the drop-weight impact performance of thin woven carbon/epoxy composites. They concluded that specimen absorbed more energy when impacted by a conical impactor. Impacting specimen using hemispherical impactors produced highest contact force and lowest contact time. Mitrevski et al. in another study [5] studied the effect of impactor shape on the low velocity impact response of preloaded carbon/epoxy ⇑ Corresponding author. Tel.: +90 3522077300. E-mail address: [email protected] (E. Sevkat). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.05.016

composites. It was found that impactor shape had little effect under preloaded conditions. Zhou [6] conducted low-velocity impact on glass-reinforced woven fabric laminates using flat-ended impactor. It was reported that structural characteristics of these impact damage mechanisms are affected by geometry. Hybrid composites have been used in many applications that are exposed to impact loadings. Combining two or more fibers in the same composite will take the advantages of the merits of each fiber system. In most cases, one of the fibers in hybrid composite is high modulus/high-cost fiber such as carbon or boron and the other is low modulus/low-cost fiber such as glass or Kevlar. The high modulus fiber provides the stiffness and load bearing capability, whereas the low modulus fiber makes the composite more damage tolerant and reduces the cost. Some of the advantages in this approach are the balance of strength and stiffness, reduced weight and/or cost, improved fatigue resistance, improved fracture toughness, and impact resistance. One of the attractiveness of hybridization approach is the synergy effect that is also called ‘hybrid effect’. The ‘positive hybrid effect’ is to obtain a composite property whose value is higher than the value predicted from the rule of mixtures. Several researchers found the hybrid effects of the tensile modulus and the strain at failure of glass-rich hybrids [7–10] to be positive. Studies on drop-weight response of hybrid composite are very limited. Hosur et al. [11] conducted lowvelocity impact tests on thin hybrid composites by using only hemispherical impactor. They concluded that considerable improvement was observed in the load carrying capability of hybrid composites as compared to carbon/epoxy laminates with

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slight reduction in stiffness. Finite element calculations have been applied to composites of different shapes, sizes, compositions, loadings and boundary conditions without the expense and time required for actual testing. Once validated using experimental data; FE models can generate very valuable results for many different cases [12,13]. Zang et al. [14] developed A numerical model for composite laminate plates to predict the matrix crack and delamination damage initiation and propagation in low velocity impact tests. In this study, two hybrid composites with same amount of glass and graphite fabric but with different lay-up sequences were tested using various shapes of impactors. Effect of lay-up sequence on the impact resistance of composites was investigated. Furthermore; four types of impactor were employed to see whether effect caused by lay-up sequence was consistent when different impactor is used. Dynamic impact-time, strain–time histories and resultant delaminations were studied. Numerical studies were conducted using LS-DYNA [15], which is a commercially available generalpurpose finite element code for analysis of 3D large deformation dynamic response of structures based on explicit-time integration scheme. Chang–Chang linear-orthotropic damage model [16,17] was used for simulating composite material. Good agreement between experimental FE results was achieved. 2. Experimental procedure Woven S2-glass-IM7-graphite fibers/toughened epoxy hybrid composites with two lay-up sequences were used. First type had glass fabrics outside and graphite fabrics inside. The glass outer skins had 9 layers of glass fabrics at each side for a total of 18 layers. The graphite core was made of 16 layers of graphite fabrics. This specimen was designated as the GL/GR/GL specimen, Fig. 1a. The second type had graphite face sheets outside and glass core inside. The graphite sheet contained 8 layers of fabrics at each side and the glass core had 16 layers. In total 32 layers of fabrics were used. This type was identified as the GR/GL/GR specimen, Fig. 1b. Although the thickness was the same for two types, the number of fabric layers used in each composite varied due to the different thicknesses of graphite and glass fabrics. Vacuum assisted resin transfer molding (VARTM) technique was used to stack the plainwoven fabrics together and process the composites. The specimens were cured at 177 °C. Fiber volume fraction for all types was 55%. The final thickness of the composite panels was approximate 6.35 mm. The thickness of the core and skin material for both hybrid composites was the same. It was approximately 3.2 mm for the core and 1.6 mm for the skin on each side. Drop-weight tests were carried out using a pressure-assisted Instron-Dynatup 8520 instrumented drop-weight impact tester, which is equipped with a pneumatic break to avoid multiple strikes. The ASTM: D-7136 test standard was followed. The speci-

men was clamped circumferentially along a diameter of 76.2 mm in a pneumatic-actuated clamping fixture. The composite specimens with +45°/45° fiber orientation were then impacted at a velocity of 4.1 m/s by impactors of four different shapes with a mass of 6.15 kg. The four different impactors used during this study are shown in Fig. 2. In addition, in order to study the effect of impactor angle on the low velocity impact response of woven composites, Charpy-type straight-line impactor (Fig. 2a), was used by rotating 45° around z-axis. Thus five results were presented for each lay-up configuration. The specimens were then scanned using an immersion ultrasonic system. The through-transmission technique using a pair of 5 MHz focused and flat transducers were employed to detect the internal delamination in the impacted composite panel. Two strain gages (one in longitudinal direction and the second in transverse direction) were mounted 25.4 mm away from the center (Fig. 3). After each impact test, specimen was removed from fixture and pictures of front and back surface were taken. 3. Experimental results and discussion Fig. 4 shows optical pictures of hybrid composite panels impacted with impactor of various shapes. When hybrid composites were impacted by Charpy impactor at 0°, front-surface cracks were observed. While the front-surface crack was only along 0° for GL/ GR/GL composites, additional cracks along +45°/45° at the tip of the horizontal one were observed on GR/GL/GR (Fig. 4a). The back-surface splitting was not severe during these impacts. Even though some splitting was observed on GR/GL/GR composite, force–time histories did not capture these behaviors (Fig. 5a). In case of significant back-surface splitting, force–time histories usually show drops right after initial peak. Impacting composites along 45° by Charpy impactor, created front surface cracks along 45° (Fig. 4b). Back surface splitting were only observed on GR/GL/GR composite. Impacting composites by 25.4 mm spherical impactor resulted in some indentation at impact location and front-surface splitting along +45° and 45° Fig. 5c. Some back-surface splitting initiated on GR/GL/GR composite (Fig. 4c). The 12.7 mm spherical impactor resulted in deeper indentation at front surface of GR/ GL/GR composite and no significant damage was observed at the back side of the GL/GR/GL composite (Fig. 4d). The 10 mm flatended cylindrical impactor created circular indentation at front surface of both composites (Fig. 4e). The indentation on GL/GR/ GL composite was not deep, compared to GR/GL/GR composite. In terms of load sustainment, GL/GR/GL composite sustained more load than GR/GL/GR composite during all tests conducted (Fig. 5). Using glass fabrics as an outer skin was not only effective during the impacts by relatively blunt impactors such as Charpy and 25.4 mm spherical, it was also very effective during the impacts by 12.7 mm spherical and 10 mm cylindrical impactors.

Fig. 1. Two lay-up sequences (a) glass skin graphite core (GL/GR/GL) and (b) graphite skin, glass core (GR/GL/GR).

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Fig. 2. Impactors: (a) Charpy-straight line, (b) 25.4 mm spherical, (c) 12.7 mm spherical and (d) 10 mm flat-ended cylindrical.

3.1. Initial peak force When a typical force–time history of drop-weight impact is reviewed, three parameters appear to be significant: initial peak force, maximum force and contact duration. Initial peak force represents the first damage initiation. For all impactor shapes GR/GL/ GR composite had higher initial peak compared to that of GL/GR/GL composite. While Charpy impactor produced highest initial peak force, spherical impactors produced lowest. The initial peak produced by flat-ended cylindrical impactor was between that of Charpy and spherical ones. It should be noted that spherical impactor with 12.7 mm diameter produced little less initial peak compared to the other spherical impactor (Fig. 6a). Contact area between composite and impactor found to be critical parameter. During the impact by Charpy impactor the contact between composite and impactor is along the slender surface and many fibers are active in terms of load sustainment. However when using spherical impactors, contact starts at a point and few fibers are active in terms of load sustainment. The slight initial peak force difference between two spherical impactors can be explained with the same concept. Using spherical impactor with 25.4 mm diameter, the contact area is slightly greater than the contact area created by 12.7 mm spherical impactor. Initial peak force obtained using flat-ended cylindrical impactor lay somewhere in between

the initial peaks of Charpy and spherical impactors. Once again, the contact area of cylindrical impactor is between the contact area of the Charpy and the spherical impactor. 3.2. Maximum impact force The impact forces produced by hybrid GL/GR/GL composites were higher than the impact forces of GR/GL/GR composites in all cases (Fig. 6b). It should also be noticed that while the Charpy impactor produced the highest impact force, spherical impactors produced the lowest and the force produced by cylindrical impactors was somewhere in between. The tests conducted using Charpy impactor at 45° orientation produced little higher maximum force than the maximum force produced by Charpy impactor at 0° orientation. On the other hand, 25.4 mm spherical impactor produced higher maximum force than the force produced by 12.7 mm spherical impactor. It should also be noticed that force–time histories of GR/GL/GR composite had greater angle of slope all the time. Stiff graphite fibers in tension side better resisted to deflection at the beginning compared to less stiff glass fibers (Fig. 4). 3.3. Contact duration Hybrid GR/GL/GR composites produced little longer contact durations compared to GL/GR/GL composite Fig. 6c While the Charpy impactor produced shortest contact duration, spherical impactors produced longest contact duration. Contact durations produced by flat-ended cylindrical impactor were somewhere between those produced by Charpy and spherical impactors. Charpy impactor at 0° orientation produced little higher contact duration compared to that produced by Charpy at 45°. Between two spherical impactors, the one with 12.7 mm diameter produced little longer contact durations. It was observed that small contact area between composite and impactor tend to increase contact durations. 3.4. Delamination

Fig. 3. Strain-gage mounted square composite specimen for drop-weight impact test.

Fig. 7 shows C-Scan images of impacted specimens. Delaminated area was measured and then compared in Fig. 6d. Delamination in GL/GR/GL hybrid composite was more severe than delamination in GR/GL/GR composite. Charpy impactors created more delamination than other impactors. Rotating Charpy impactor 45° around its axis did not change the size of the delamination significantly but resulted in different shape of delamination. Spherical impactor with 25.4 mm diameter created more delamination compared to

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SIDE

GL/GR/GL

GR/GL/GR

FRONT

SHAPE OF IMPACTOR

FRONT

BACK

(a) CHARPY STRAIGHT LINE AT 0o

FRONT

BACK

(b) CHARPY STRAIGHT LINE AT 45o

FRONT

BACK

(c) 25.4 MM SHPER CAL

FRONT

BACK

(d) 12.7 MM SHPER CAL

BACK

(e) 10 MM FLAT-ENDED CYLINDRICAL

Fig. 4. Optical pictures of impacted GL/GR/GL (left) and GR/GL/GR (right) composites impacted by 6.15-kg impactor of various shapes.

second spherical impactor with 12.7 mm diameter. Delamination created by flat-ended cylindrical impactors between that was created by Charpy and spherical impactors. Following the impact tests, some of the impacted specimens were cut in halves for visual inspection. It was observed that captured delamination mostly occurred between dissimilar layers. Once again contact area between impactor and composite found to be critical parameter for delamination. Impactors with bigger contact area caused more delamination. However this is mostly true for non-perforation cases.

Perforation was not observed during any of the tests conducted in this study. In case of perforation; delamination size might be much smaller regardless of contact area. During perforation, the specimen may not deflect as much as it can during no perforation cases. The effect of impactor shape on impact force, initial peak, and contact duration was obvious and detailed above. The findings were in agreement with the findings of similar studies [3–6]. The low-velocity impact response of hybrid composites are limited

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Fig. 5. Impact force histories of hybrid and non-hybrid composite panels impacted with (a) Charpy straight-line 0°, (b) Charpy straight-line 45°, (c) 12.7 mm spherical, (d) 25.4 mm spherical and (e) 10 mm flat-ended cylindrical impactors.

[11]. In those studies usually resultant damage was evaluated and effect of impactor shape was not studied specifically. It was observed that using different reinforcement as skin or core in composite, the behavior of composite can be altered. Prediction of damage using FE approach was used by many researchers [13,14]. It was proved here that Composite Damage Model [16,17] can simulate low-velocity impact response of hybrid composites with sufficient accuracy.

4. Dynamic finite element simulations The mechanical properties of the woven glass and woven graphite composites were obtained from tensile tests [18]. Mechanical properties of glass and graphite composite were given in Table 1. These material properties were then entered into LSDYNA for the Chang–Chang model (Mat_22) which is used to simulate linearly orthotropic behavior of composites.

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(a) Initial peak force

(b) Maximum impact force

(c) Contact duration

(d) Delamination

Fig. 6. Comparison of key characteristics of composites subject to drop-weight impact tests conducted using impactors of various shapes.

4.1. Material models Steel impactors were modeled using rigid material MAT_20. Composite materials were modeled using MAT_COMPOSITE_DAMAGE (MAT_022). This model is also called the ChangChang composite damage model. It is an orthotropic material where optional brittle failure for composites can be defined. 4.2. Delamination and contact models

FE simulations. ERODING_SURFACE_TO_SURFACE contact model was used between the steel impactor and composite. In this study strain-based failure criterion was used for element erosion: that is, when e P eerosion, element was eroded and removed from calculation. In this study, the much higher Young’s modulus and tensile strength of the steel impactor than those of the woven composites enabled us to model it as a rigid body with a density of the steel while eerosion = 0.04 and 0.03 for S2-glass/SC-79 and IM7-graphite/SC-7 woven composites were chosen, respectively.

The criterion for delamination between the composite layers is governed by:

4.3. Finite element mesh

 2  maxð0; rn Þ rs 2 þ P1 NFLS SFLS

Fig. 8a shows the LS-DYNA finite element meshes simulating drop-weight impact onto GL/GR/GL with 12.7 mm spherical impactor. Fig. 8b shows finite element meshes of model that is used for GR/GL/GR that was impacted by 25.4 mm spherical impactor. Depending on the fabric number layers were created. Two fabrics in composite represented as one layer in FE model. It should be noticed that fine mesh around the center and course mesh away from center was created.

ð1Þ

where n and s are the normal and shear stresses acting on the layer interface, respectively, while NFLS and SFLS are the normal and shear strengths of the layer interface, respectively. This criterion was incorporated into LS-DYNA through the command: CONTACT_AUTOMATIC_SURFACE_TO_SURFACE_TIEBREAK. For the interfaces of glass/glass and graphite/graphite, the above experimentally measured S12 values were taken as their delamination shear strengths, SFLS, which are in line with the values reported in open literature [19–21]. Since the interface of glass/ graphite was more susceptible to delamination, an SFLS of 0.05 GPa was chosen to match the experimental results through

4.4. Verification of finite element predictions FE predictions in this study were verified using experimentally obtained impact force, dynamic strain histories and post-impact damage patterns.

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Fig. 7. Ultrasonic C-scan images of GL/GR/GL and GR/GL/GR composites impacted by impactor of various shapes.

Table 1 Initial engineering elastic constants, Poisson’s ratios and densities of woven glass and graphite fibers-reinforced toughened epoxy composite.

Glass/epoxy prepreg Graphite/epoxy prepreg

E1 = E2 (GPa)

E3 (GPa)

G12 (GPa)

G13 = G23 (GPa)

m12

m13 = m23

q (g/cm3)

17.04 36.43

6.51 5.18

2.742 3.080

2.305 1.816

0.13 0.12

0.413 0.427

1.756 1.458

Fig. 8. Finite element simulation models. (a) drop-weight impact model for the GL/GR/GL (12.7 mm spherical impactor) and (b) drop-weight impact model for the GR/GL/GR (25.4 mm spherical impactor).

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15000 12500 10000 7500 5000 2500 0 -2500 0 -5000 -7500 -10000 -12500 -15000

EXPERIMENT(SG1) EXPERIMENT(SG2) LS-DYNA(SG1) LS-DYNA(SG2)

1

2

3

4

5

18 16

EXPERIMENT LS-DYNA

14

FORCE (KN)

MICROSTRAIN (µε)

74

12 10 8 6 4 2 0 0

1

2

3

TIME (ms)

7500 5000 2500 0 -2500

0

1

2

3

4

5

16 14

FORCE (KN)

MICROSTRAIN (µε)

10000

12 10 8 6

-5000

4

-7500

2

-10000

EXPERIMENT LS-DYNA

18

EXPERIMENT(SG1) EXPERIMENT(SG2) LS-DYNA(SG2) LS-DYNA(SG2)

12500

5

(a)

(a) 15000

4

TIME (ms)

0

-12500

0

1

2

3

4

5

TIME (ms)

-15000

TIME (ms)

(b) Fig. 9. Comparison of dynamic strain histories of (a) GL/GR/GL composite impacted by a 12.7 mm spherical impactor and (b) GR/GL/GR composite impacted by a Charpy impactor.

(a) Comparison of Dynamic strain histories As it was mentioned earlier, two strain gages (one in longitudinal direction and the second in transverse direction) were mounted 25.4 mm away from the center (Fig. 3) on each composite tested. In finite element calculations two elements at the same distances where two strain-gages mounted were selected for strain output. Fig. 9 shows the comparison of dynamic strain histories obtained from experiments and finite element models. Experimental results and FE predictions are in good agreement. The experimental curves of strain were smoother than the curves produced by FE simulations. This can be explained with the fact that experimental strain measurements were obtained strain gage amplifiers, where actual readings might have been filtered. (a) Comparison of Force–time histories During the drop-weight impact tests, force–time histories are obtained through a load-cell which is attached to impactor. In finite element calculations, force–time histories are obtained through CONTACT_AUTOMATIC_SURFACE_TO_SURFACE model. The force time histories of GL/GR/GL specimen from experiment and

(b) Fig. 10. Comparison of force–time histories of (a) GL/GR/GL composite impacted with 12.7 mm spherical impactor and (b) GR/GL/GR composite impacted by a Charpy impactor.

FE exhibited slight difference (Fig. 10a). Predicted initial peak force was higher than the experimental one. Initial angle of slope was also bigger in predictions. In terms of maximum force and contact duration; good agreement was achieved. The force time-histories of GR/GL/GR specimen showed only fair agreement in terms of impact force (Fig. 10b). While maximum impact force obtained from experiment was 13 kN, FE calculation predicted 15 kN. Initial slopes were in good agreement. Predicted contact duration came out little shorter compared to experimental one. Fig. 11 shows damage progression in GR/GL/GR composite impacted by Charpy impactor. (a) Comparison of damage patterns Fig. 12 shows predicted and experimentally obtained damage patterns for GL/GR/GL composite impacted by Charpy impactor. Horizontal front surface indentation and back surface splitting was successfully captured by simulations. Fig. 13 shows the damage patterns of impacted GR/GL/GR composite both from experiments and FE simulations. Due to the impact by 25.4 mm spherical impactor, some front surface crack and back surface splitting occurred. Because of +45°/45° fiber orientation, the ‘‘X’’ shape front surface crack and back surface splitting was observed. FE simulations predicted similar results very accurately for front and back surface splitting.

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t=0 ms

t=1 ms

t=2 ms

t=3 ms

t=4 ms

t=5 ms

t=6 ms

t=7 ms

Fig. 11. Damage progression and stress counters of FEM simulation for hybrid woven GR/GL/GR composite plate impacted at 4.1 m/s by a 6.15 kg 100 diameter Charpy impactor. (Time interval between each plot is 1 ms).

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FE

BACK SURFACE

FRONT SURFACE

EXPERIMENT

Fig. 12. Comparisons of post impact damage patterns of FEM and experimental results for drop-weight tests where GR/GL/GR specimen was impacted using Charpy straight line impactor at 0°.

FE

BACK SURFACE

FRONT SURFACE

EXPERIMENT

Fig. 13. Comparisons of post impact damage patterns of FE and experimental results for drop-weight tests where GR/GL/GR specimen impacted by 25.4 mm spherical impactor.

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5. Conclusions Drop-weight impact response of hybrid composites impacted by impactor of various geometries was studied experimentally and numerically. Based on this study, the following conclusions can be drawn: (1) GL/GR/GL composite sustained greater impact loads, though GR/GL/GR composite had bigger initial slope and bigger initial peak load. (2) Delamination in GL/GR/GL hybrid composite was always more than the delamination in GR/GL/GR composite. (3) Effect of lay-up sequence on the impact resistance of composites was significant. However effect caused by lay-up sequence was consistent and did not vary once impactor was changed. In example; initial peak load of the GR/GL/ GR composite was changed depending on impactor geometry but it was always higher than that of GL/GR/GL composite. (4) Contact area between impactor and composite found to be critical for the response of composites. Impactor with larger contact surface produced higher initial peak, higher maximum force, bigger delamination between hybrid layers and shorter contact duration. (5) Linear-orthotropic Chang–Chang material model simulated the behavior of hybrid and non-hybrid composites with sufficient accuracy. Dynamic force, strain histories and post impact damage patterns obtained from experiment and finite element calculations were in good agreement.

References [1] Zukas JA. Impact dynamics. Malabar, Florida: Krieger Publishing Company; 1992. [2] Deng Y, Zhang W, Cao Z. Experimental investigation on the ballistic resistance of monolithic and multi-layered plates against hemispherical-nosed projectiles impact. Mater Des 2012;41:266–81.

77

[3] Mitrevski T, Marshall IH, Thomson R, Jones R, Whittingham B. The effect of impactor shape on the impact response of composite laminates. Compos Struct 2005;67:139–48. [4] Mitrevski T, Marshall IH, Thomson R, Jones R. The influence of impactor shape on the damage to the composite laminates. Compos Struct 2006;76:116–22. [5] Mitrevski T, Marshall IH, Thomson RS, Jones R. Low-velocity impacts on preloaded GFRP specimens with various impactor shapes. Compos Struct 2006;76:209–17. [6] Zhou G. Damage mechanisms in composite laminates impacted by a flat-ended impactor. Compos Sci Technol 1995;54:267–73. [7] Kalnin IL. Evaluation of unidirectional glass-graphite fiber/epoxy resin composites. In: Composite materials testing and design (second conference), ASTM STP 497. American Society for Testing and Materials, Philadelphia; 1972 p. 551–563. [8] Zweben C. Tensile strength of hybrid composites. J Mater Sci 1977;12:1325–37. [9] Kretsis G. A review of the tensile, compressive, flexural and shear properties of hybrid fibre-reinforced plastics. Composites 1987;18(1):13–23. [10] Maron G, Fischer S, Tuler FR, Wagner HD. Hybrid effects in composites: conditions for positive or negative effects versus rule-of-mixtures behaviour. J Mater Sci 1978;13:1419–26. [11] Hosur MV, Adbullah M, Jeelani S. Studies on the low-velocity impact response of woven hybrid composites. Compos Struct 2005;67:253–62. [12] Moura MFSF, Marques AT. Prediction of low-velocity impact damage in carbon–epoxy laminates. Compos Part A-Appl S 2002;33:361–8. [13] Sridhar C, Rao PK. Estimation of low-velocity impact damage in laminated composite circular plates using nonlinear finite element analysis. Comput Struct 1995;54(6):1183–9. [14] Zhang Y, Zhu P, Lai X. Finite element analysis of low-velocity impact damage in composite laminated plates. Mater Des 2006;27(6):513–9. [15] LS-DYNA 971, Livermore Software Technology Corporation (LSTC), Livermore, CA. . [16] Chang FK, Chang KY. Post-failure analysis of bolted composite joints in tension or shear-out mode failure. J Compos Mat 1987;21:809–33. [17] Chang FK, Chang KY. A progressive damage model for laminated composites containing stress concentrations. J Compos Mat 1987;21:834–55. [18] Sevkat E, Liaw B, Delale F. Tensile behavior of woven hybrid composites. In: 2010 ASME international mechanical engineering congress and exposition. Proceedings of IMECE2010, Vancouver, British Colombia, 12–18 November, 2010, Imece 2010-38368. [19] Gillespie Jr JW, Gama BA, Cichanowski CE, Xiao JR. Interlaminar shear strength of plain weave S2-glass/SC79 composites subjected to out-of-plane high strain rate compressive loadings. Compos Sci Technol 2005;65:1891–908. [20] Reinhart TJ et al., editors. High-strength medium-temperature thermoset matrix composites. Engineered materials handbook, 1. Metals Park (OH): ASM International; 1987. [21] Xiao JR, Gama BA, Gillespie Jr JW. Progressive damage and delamination in plain weave S-2 glass/SC-15 composites under quasi-static punch-shear loading. Compos Sci Technol 2007;78:182–96.