Investigation of the mechanical and ballistic properties of hybrid carbon/ aramid woven laminates

Ji-wei Bo ,Yng-wei Wng ,b,c,* ,Rui An ,Hun-wu Cheng ,b ,Fu-chi Wng ,b

a School of Materials Science and Engineering,Beijing Institute of Technology,Beijing,100081,China

b National Key Laboratory of Science and Technology on Materials under Shock and Impact,Beijing,100081,China

c Beijing Institute of Technology Chongqing Innovation Center,Chongqing,401120,China

Keywords:Carbon fiber Aramid fiber Hybrid composites LS-DYNA Numerical simulation

ABSTRACT High-performance ballistic fibers,such as aramid fiber and ultra-high-molecular-weight polyethylene(UHMWPE),are commonly used in anti-ballistic structures due to their low density,high tensile strength and high specific modulus.However,their low modulus in the thickness direction and insufficient shear strength limits their application in certain ballistic structure.In contrast,carbon fiber reinforced epoxy resin matrix composites (CFRP) have the characteristics of high modulus in the thickness direction and high shear resistance.However,carbon fibers are rarely used and applied for protection purposes.A hybridization with aramid fiber reinforced epoxy resin matrix composites (AFRP) and CFRP has the potential to improve the stiffness and the ballistic property of the typical ballistic fiber composites.The hybrid effects on the flexural property and ballistic performance of the hybrid CFRP/AFRP laminates were investigated.Through conducting mechanical property tests and ballistic tests,two sets of reliable simulation parameters for AFRP and CFRP were established using LS-DYNA software,respectively.The experimental results suggested that by increasing the content of CFRP that the flexural properties of hybrid CFRP/AFRP laminates were enhanced.The ballistic tests’results and the simulation illustrated that the specific energy absorption by the perforation method of CFRP achieved 77.7% of AFRP.When CFRP was on the striking face,the shear resistance of the laminates and the resistance force to the projectiles was promoted at the initial penetration stage.The proportion of fiber tensile failures in the AFRP layers was also enhanced with the addition of CFRP during the penetration process.These improvements resulted in the ballistic performance of hybrid CFRP/AFRP laminates was better than AFRP when the CFRP content was 20 wt% and 30 wt%.

1.Introduction

High-performance ballistic fibers,their advantages of low density,high specific strength and high specific modulus,made it broadly applied in anti-ballistic structures,such as flexible body armor,combat helmets,backing plates for helicopters,military vehicles.UHMWPE fibers,aramid fibers and glass fibers are the most commonly used ballistic fibers[1-5].Glass fiber has low cost but also relatively lower specific strength and specific modulus than the other two fibers.UHMWPE fibers and aramid fibers are the main materials used for soft bulletproof vests and combat helmets[6-8],these materials possess superior ballistic performance when protecting against the impact of high-speed projectiles(＞100 m/s).

In addition to the above three types of fibers,carbon fiber,which was used as a structural material in the aerospace field,was also applied in the field of protection,such as the protection of the hypervelocity debris in space [9],the protection of low-speed (＜100 m/s) impact of fragments,birds,ice on aircraft [10-14].CFRP has unique advantages different from AFRP and UHMWPE laminates.Table 1 presents the typical properties of these three materials.The characteristics of UHMWPE laminates are the highest specific strength and the lowest in-plane and out-of-plane shear strength.The modulus and shear properties of CFRP are better than the other two fiber composite laminates.

Table 1 Mechanical Properties of typical composites.

It was uncommon for CFRP laminates to be used alone forprotection or as a ceramics support panel in the protection against penetration of bullets.Crouch [17] reported a typical human body protection module,the CFRP layer was used as the first support plate of the ceramic layer,as shown in Fig.1.Besides,more research focused on hybridizing CFRP with other fiber composites to obtain better ballistic performance [18].Bandaru [19] reported a rational arrangement for ballistic composites.That was: arranging the Kevlar layer at the rear side,glass fiber layer in the exterior and carbon fiber layer on the striking face.Reddy [20] found that comprising carbon and E-glass in the ratio of 50:50 had shown maximum performance in terms of energy absorption.It presented 17% and 30% higher energy absorption than carbon fiber composites and E-glass composites respectively.F.Zulkifli [21] reported that arranging a 0.5 mm thick CFRP in the front of a 4 mm UHMWPE plate increased the flexural strength of the laminate(from 30.5 MPa to 155.2 MPa),and the flexural modulus upgraded from 9.9 GPa to 28.1 GPa.A significant reduction in the UHMWPE plate＆apos;s back convex appeared (from 20 mm to 14mm).Qu [22] proposed that arranging a high-strength and high-hardness alloy on the striking surface of UHMWPE improved the ballistic properties of the multilayer structure.

The above researches have shown that combining CFRP,metal or the other high-strength and high-rigidity materials with UHMWPE or AFRP laminates improved the ballistic performance of the composite structure,which was of great significance to the fiber composite bulletproof panels.In addition,for the most widely used ceramic/UHMWPE structures [23-26],the addition of the CFRP layer before UHMWPE and after the ceramic layer would benefit the restraint effect on the ceramic layer [27].

Fig.1.Schematic cross-sectional view of a HAP(Hard armor plate),showing numerous sub-elements of a typical modular body armor system [17].

The fiber composite made of a single type of fiber cannot meet all the requirements of a ballistic structure.The hybridization of fiber laminates is bound to be one of the ways to improve ballistic performance and reduce the weight of the composite structures.The protection for bullets is an important part in the field of protection.There were few reports on the application of hybrid composite materials in bullet protection structures.

This research aimed at the application prospects of CFRP in hybrid fiber composite structures and hybrid fiber composite interlayers for supporting ceramics.CFRP and AFRP were hybridized to study the flexural property and ballistic property of composite laminates.The research proposed a certain guiding role in the optimization for the single material laminates and the improvement of the ballistic property for the ceramic/fiber composite structures.

2.Experiments and numerical simulation

2.1.Materials

The woven plain weave aramid fabric was supplied by The China National Bluestar(Group)Co,Ltd.The fiber type was STARAMID-F2,and the fiber denier was 3000D.The areal density of a single layer was 400 g/m-420 g/m,and the thickness of the single layer was about 0.5 mm.Aramid/epoxy prepreg was fabricated by AVIC Composite Technology Co.,Ltd.The resin content was 40 wt%±2 wt%.The woven plain weave carbon fabric/epoxy prepreg was also provided by AVIC Composite Technology Co.,Ltd.The resin content was also 40 wt%±2 wt%.The carbon fiber type was T300 and the fabric type was 3K200.

2.2.Preparation and mechanical tests of composites

The laminates were prepared by the autoclave method,and the preparation process was as follows: (1) Vacuuming (2)The temperature was raised to 90C at a rate of 3C/min and kept for 40 min.Then the temperature was raised to 120C at a rate of 3C/min and held for 1 h.During the heating period,the pressure was increased at a rate of 40 kPa/min to 1.2 MPa,and the pressure was maintained for 7 h for ensuring that the pressure was maintained during the naturally cooled down process.(3) When the temperature was below 60C,the workpiece was taken out.

For testing the tensile property,the in-plane shear property and the out-of-plane shear property,2 mm thickness AFRP sheets and CFRP sheets were prepared,and high-pressure water jet cutting technology was used to cut the test samples.

Table 2 shows the followed standards of the mechanical tests’in this research.At least 5 valid data were obtained for each test.

Table 3 shows the specific information of hybrid CFRP/AFRP laminates,which were used to test the flexural property and ballistic property.Fig.2 displays the cross-sectional views of the laminates.In flexural tests,the AFRP was on the tension surface,and the support span-to-thickness ratio was 16:1.

Table 2 Tests’ standards.

Table 3 Information of hybrid CFRP/AFRP composites.

Fig.2.Cross-sectional view of specimen: (a) AFRP，(b) Hybrid-20%，(c)hybrid -30%，(d) hybrid -40%，(e) hybrid -50%，(f) CFRP.

2.3.Ballistic tests

In ballistic tests,the spherical projectiles with a diameter of 10.30 mm ± 0.02 mm,a hardness of 63 ± 3 HRC,and a mass of 4.50 g±0.02 g were used to conduct the experiments.The distance between the muzzle and the target was 5 m.Fig.3 shows the schematic diagram of the ballistic tests.

The specific energy absorption by perforation method was calculated by Eq.(1):

Fig.3.A schematic diagram of specific energy absorption by perforation tests.

：the specific energy absorption by perforation method,the unit is J/(kg·m).m: the mass of the projectile,the unit is kg.v:the initial velocity of the projectile.v: the residual velocity of the projectile,the unit are both m/s.: the areal density of the laminate,the unit is kg/m.

In ballistic tests,CFRP was at the striking face and AFRP was at the rear side.The dimension of the target was 300 mm×300 mm.The preset initial velocity was 400 m/s,500 m/s,600 m/s and 800 m/s,respectively.

2.4.Numerical simulation

Based on LS-DYNA software,the process of projectile penetrated the laminates were simulated and analyzed.The projectile hardly deformed during the penetration process,therefore a rigid body model was adopted for the projectiles.The material model of AFRP and CFRP was MAT 54 [28],that was,the enhanced composite damage model.The failure criterion was Chang-Chang composite material failure criterion.

The 1/4 model was used in the simulation.The size of the target plate in the 1/4 model was 75 mm×75 mm.According to the actual situation,the thickness of a single AFRP layer was 0.50 mm,and the thickness of a single CFRP layer was 0.25 mm.The grid size of the 20 mm × 20 mm area in the center of the plate was 0.5mm × 0.5 mm.The grid size at the edge of the plate was 1 mm × 1 mm.Fig.4 presents the simulation model.

The contact between the projectile and the laminates adopted the eroding single surface.Due to delamination that occurred under real penetration conditions,tiebreak contact was selected between composite layers.The criterion for tiebreak contact shows in Eq.(2) [29],:

σand σare normal tensile stress and shear stress of the contact surface respectively.NFLS and SFLS are the normal tensile strength and shear strength of the surface respectively.When the relationship in the formulation is met,delamination failure occurs.

3.Results and discussion

3.1.Mechanical tests’ results

Fig.5 presents the stress-strain or stress-displacement curve of the mechanical tests.The results of the tests on the mechanical properties of AFRP and CFRP are shown in Table 4.The macroscopic failure morphologies of AFRP and CRPP are displayed in Fig.6.The tensile strength of AFRP was determined to be 53.2% of CFRP,and the elongation at breaking point was 2.15 times that of CFRP.The resulting fractures demonstrated that the fibers were fibrillated at the tensile fracture of AFRP,while the tensile fracture of CFRP was a brittle fracture.In the in-plane shear performance tests,the AFRP specimen mainly failed in form of fibers＆apos; extrusion,fibers＆apos; pull-off and fibers＆apos; breakage,while for CFRP,the failure occurred in form of fibers’ breakage.In the interlaminar shear tests,CFRP internally delaminated under minimal deformation,while AFRP experienced interlaminar failure until significant deformation occurred.The various modulus and strength indexes of AFRP were lower than CFRP,but the deformability of AFRP was better than CFRP.

Table 4 Results of AFRP and CFRP mechanical properties tests.

Fig.4.Geometric model in simulation.

Fig.5.AFRP and CFRP mechanical properties test curves:(a)tensile stress-strain curve,(b) in-plane shear stress-strain curve,(c) out-of-plane stress-displacement curve.

Fig.6.Macroscopic fracture morphology of AFRP and CFRP mechanical properties tests.

Fig.7 shows the flexural properties of hybrid CFRP/AFRP laminates.With the increase of CFRP,the flexural modulus and flexural strength of the laminates both increased.When an additional amount of CFRP reached 50 wt%,the flexural modulus achieved 36.59 GPa,and the flexural strength achieved 593.5 MPa.In the flexural tests,fibers’breakage and delamination occurred inside the CFRP layer.There were no obvious failures identified in the AFRP layer,as can be seen in Fig.8.

3.2.Ballistic properties of AFRP and CFRP

Fig.9 presents the findings regarding the ballistic performance of AFRP and CFRP.The residual velocity and the Ep values both increased linearly with the initial velocity of the projectiles.The Ep values of CFRP reached 77.7%of AFRP at each velocity,it suggested that the CFRP has certain ballistic property.

Fig.10 shows the cross-sectional views of AFRP after ballistic tests.The damage morphology of AFRP was in the shape of an“hourglass”.The fibers at the center of the penetration hole on the striking surface were a shear failure.In the middle thickness of the laminate,the fiber not only showed the characteristics of shear failure but also displayed the phenomenon of tensile deformation caused by downward bending.Therefore,the failure mode of the fibers was a mix of shear failure and tensile failure.At the lower part of the “hourglass” damage morphology,delamination and a bulge on the rear side were evident.This change indicated that the fibers have undergone a stretching process during the penetration process.As the velocity increased,the proportion of the shear failure part at the striking face also increased.

Fig.11 displays the cross-sectional failure views of CFRP under different velocities.The bulging phenomenon of CFRP was not evident.It was only noticeable when the projectile penetrated at 417.4 m/s.The other laminates had no rear convex,it demonstrated punching and shearing failure were the main failure behavior in CFRP laminates.Delamination also occurred between the back layers of the laminates.The diameter of the bullet hole at the rear showed a decreasing trend when increasing the velocities.Compared with the damage morphology of AFRP,the deformability of CFRP under dynamic conditions was much lower than that of AFRP,showing strong impact brittleness characteristics.Nevertheless,due to the relatively high strength of CFRP,the Ep values of CFRP still could reach 77.7% of AFRP.

Fig.7.Flexural property of each specimen: (a) Flexural modulus,(b) Flexural strength.

Fig.8.Cross-sectional views of damaged flexural test specimen: (a) Hybrid-20%,(b) Hybrid-30%,(c) Hybrid-40%.

Fig.9.Ballistic property comparison of AFRP,CFRP: (a) Residual velocity，b: Ep.

Fig.10.Cross-sectional views of AFRP after penetration: (a) 408.2 m/s,(b) 531.2 m/s,(c) 604.1 m/s,(d) 800.0 m/s.

Combined with the mechanical properties tests＆apos;results and the ballistic tests’results,the constitutive parameters of AFRP and CFRP were shown in Table 5.

Table 5 Model parameters for AFRP and CFRP.

Fig.12 presents a comparison of the actual cross-sectional damage views and the simulated cross-sectional damage views of AFRP.The simulation reflected the failure behavior of AFRP,such as shear failure,fibers’ breakage and delamination.The bulge height and delamination area of the laminates were consistent with the experimental results.When the projectile velocity was at 408.2 m/s-604.1 m/s,the maximum deviation between the simulation and the actual residual velocity was 3.5 m/s,with the error between the simulated residual velocity and the experimental velocity at 0.7%.At the initial velocity of 800.0 m/s,the deviation was 8.5 m/s and the error was 1.2%.In general,the simulation model could satisfactorily predict the failure behavior and the residual velocity of AFRP.

Fig.13 shows the comparison between the actual damage crosssectional views and the simulation damage cross-sectional views of the CFRP.The delamination,fibers’ breakage and shear failures of CFRP in the simulations corresponded well with the experimental results.

Fig.11.Cross-sectional views of CFRP after penetration: (a) 417.4 m/s,(b) 502.0 m/s,(c) 628.8 m/s,(d) 807.9 m/s.

The simulated residual velocities were slightly higher than the residual velocities identified during the experiments.When the initial velocity was 400 m/s-600 m/s,the maximum difference between the simulated residual velocities and the actual residual velocities was 6.2 m/s,with the error of the experimental residual velocities at 1.3%.When the initial velocity was 803.9 m/s,the deviation between the simulation and the experiments was 15.5 m/s,with the error of the residual velocity from the tests at 2.2%.The simulation parameters of CFRP were accurate in predicting the velocity of the projectiles.

Fig.12.Comparison of the cross-sectional damaged views in simulations and in experiments of AFRP: (a) 408.2 m/s,(b) 531.2 m/s,(c) 604.1 m/s,(d) 800.0 m/s,(e)comparison of residual velocities in simulation and in experiments.

3.3.Ballistic properties of hybrid CFRP/AFRP laminates

Fig.13.Comparison of the cross-sectional damaged views in simulations and in experiments of CFRP：(a):405.4 m/s,(b):503.8 m/s,(c): 608.6 m/s,(d): 803.9 m/s,(e):comparison of residual velocities in simulation and in experiments.

Fig.14.Ballistic properties of hybrid CFRP/AFRP laminates: (a) Hybrid-20%，(b) Hybrid-30%，(c) Hybrid-40%，(d) Hybrid-50%.

Fig.14 presents the ballistic performance of the hybrid CFRP/AFRP laminates.The residual velocity of the four hybrid CFRP/AFRP laminates had a close linear relationship with the initial velocity.When the initial velocity was 374.4 m/s-623.6 m/s,the maximum deviation between the simulated residual velocity and the experimental values was 10.5 m/s,with the error from the experimental residual velocity at 2.0%.The maximum deviation between the simulated residual velocity and the test value was 19.3 m/s at an initial velocity of 800 m/s.The errors generated from the experimental results were 2.8%.In general,the simulation parameters were reliable in predicting the ballistic properties of hybrid CFRP/AFRP laminates.

Fig.15.Cross-sectional damaged views in experiments and simulation after penetration:(a)Hybrid-20%at 398.6 m/s,(b)Hybrid-50%at 382.8 m/s,(c)Hybrid-20%at 801.1 m/s,(d)Hybrid-50% at 793.3 m/s.

Fig.16.5 mm thick hybrid CFRP/AFRP laminates in simulation.

Fig.17.Ballistic properties of different hybrid CFRP/AFRP laminates in simulation: (a) Residual Velocity,(b) Ep.

Fig.15 displays the cross-sectional damage views in the experiments and the simulations.The simulations restored the deformation behavior of the hybrid CFRP/AFRP laminates.Since the CFRP layer was on the striking surface and the deformability under dynamic conditions was poor,the CFRP layers all demonstrated shear failure.The deformation characteristics of the AFRP were familiar with the pure AFRP laminates.When the projectile penetration at about 800 m/s,the deformation of the AFRP layer was greatly reduced and showed punching failures.The arrow in the figure shows that the aramid fiber was sheared,broken and then separated from the laminates.In the simulation,the deformation of the CFRP and AFRP layers were the same as the experimental results,such as the height of the rear bulge,the delamination,and the diameter of the bullet hole on the rear side.

3.4.Numerical simulation studies

In regard to the experimental tests,the thickness of the laminates and the initial velocity were not identical,therefore,it was difficult to accurately compare the ballistic properties of each laminate.Based on the simulation model presented in this paper,the thickness of the laminate was set to 5 mm,and the initial velocities were the same.Fig.16 presents the structure of the hybrid CFRP/AFRP laminates in the simulation.The thickness of the CFRP single layer at 0.25 mm,and the thickness of the AFRP single layer at 0.50 mm.Fig.17 presents the ballistic properties of each laminate.When the initial velocity was 400 m/s-800 m/s,the residual velocities of each laminate were nearly the same,but differences appeared in Ep due to the different areal density of the laminates.With the addition of CFRP,the Ep values of all hybrid CFRP/AFRP could reach 92% of AFRP,and all Ep values were higher than CFRP.Among the hybrid laminates,when the CFRP addition amount was 20 wt%and 30 wt%,the Ep values were equal to,or higher,than that of AFRP.

Fig.18.Internal energy of the first 1 mm layer in laminates.

Fig.18 shows the internal energy of the first 1 mm layers on the striking side of the laminates.Simulation of internal energy is the total energy of the deformation behavior of the laminates,which includes the energy caused by in-plane tensile,in-plane shear and in-plane compression behavior.Shear failure was the main energy absorption form of the layers on the striking face.The internal energy of the first 1 mm layer in AFRP laminate was 8.34 J.With the addition of CFRP,this value subsequently increased to about 11.50 J.This indicated that the addition of CFRP enhanced the shear resistance of the layers on the striking face,which was conducive to the improvement of the ballistic properties of the laminates.

Fig.19 displays the effective stress contour diagram of the projectiles and the laminates when the initial velocity was 400 m/s.When the projectile was in contact with the surface of the laminate for 3 μs,the effective stress of the four CFRP layers in the hybrid-20%,hybrid-50% and CFRP laminates all exceeded 3 GPa.However,the maximum effective stress on the striking layers of AFRP was between 2.4 GPa and 2.7 GPa.After 4 μs,the layers on the striking face began to fail and the effective stress decreased.In addition,the effective stress of the projectile was also higher when the projectile interacted with hybrid CFRP/AFRP laminates compared with the AFRP.The above analysis suggested that the addition of CFRP increased the resistance penetration force of the laminate to the projectiles during the initial penetration stage.

Fig.19.Effective stress contour of different laminates at 400 m/s.

Fig.20.Deformation of laminates at 11μs: (a) AFRP,(b) Hybrid-20%,(c) Hybrid-50%.

Fig.21.Tensile damage contour of different laminates at 400 m/s: (a) AFRP,(b) Hybrid-20%,(c) Hybrid-30%,(d) Hybrid-40%,(e) Hybrid-50%,(f) CFRP.

From the analysis of energy consumption and resistance force to penetration of the projectiles,the addition of 20 wt% CFRP could improve the ballistic property compared with AFRP.However,the further increase of CFRP content would reduce the ballistic performance of the hybrid CFRP/AFRP laminates.Therefore,there is a balance between enhancing the shear resistance of the laminates and not reducing the energy consumption of the remaining part of the structure.In this study,from the perspective of energy consumption ability,hybrid-20 wt% was the best choice.

Except for the influence of the initial stage of penetration,the addition of CFRP also affected the penetration process.Fig.20 shows the deformation of the laminates when the projectile was penetrated at 400 m/s at 11μs.The back convex height in the AFRP laminate was 3.97 mm and the fiber breakage phenomenon appeared.In contrast,the back-convexity value of the hybrid laminates was less than 3.97 mm,and the fiber on the rear side did not break.This result indicated that with the addition of CFRP,the convex height of the laminates could be reduced and the tension time of the aramid fibers was prolonged simultaneously.

Meanwhile,the addition of CFRP also altered the damage morphology of AFRP.Fig.21 presents the tensile damage contour of different laminates at 400 m/s.The red section stands for complete tensile failure.The proportion of red areas in the AFRP laminate was 40.2%,and the proportion of the red areas in the AFRP layer of hybrid-20%laminate was increased to 57.8%.The value of the other laminates was also greater than 51%.This demonstrated that the addition of CFRP increased the proportion of fibers’tension failure in the AFRP layers.It was conducive to wielding and applying effective strong tensile strength characteristics of the fibers.

3.5.Discussion

Combining the test results from the flexural tests and the simulation research on the ballistic performance,hybrid CFRP/AFRP laminates were found to possess higher flexural properties than AFRP,and better ballistic properties than CFRP.From the perspective of flexural performance,the flexural strength of hybrid-50%laminate was 78.4% of CFRP,and the flexural modulus was 80.9%of CFRP.The Ep value of hybrid-50%laminate was 1.28 times that of CFRP.

Compared with AFRP,hybrid CFRP/AFRP laminates could obtain higher rigidity than AFRP without significantly reducing or altering the ballistic performance.And the hybrid CFRP/AFRP containing 20 wt%and 30 wt%CFRP gained higher flexural strength and better ballistic performance than AFRP.This research provides a potential method for optimizing the ballistic performance of soft panels,helmets and ceramic/UHMWPE structures.

4.Conclusion

By conducting mechanical properties tests and specific energy absorption using perforation techniques,useful and exploratory models of AFRP and CFRP were produced.The prediction of the residual velocity in the simulation method aligned with the experimental results.The model was able to restore the fiber stretch,shear,and delamination behaviors of the laminates.The damaged morphology of the laminates in the simulation model also aligned with the results of the tests.With the addition of CFRP in AFRP,the flexural performance of the hybrid laminates was higher than in the singular AFRP.When the CFRP content reached 50 wt%,the flexural strength of the laminate was 78.4% of CFRP,and the flexural modulus was 80.9% of CFRP.The Ep value of hybrid-50%laminate was 1.28 times that of CFRP.

The specific energy absorption by the perforation method of CFRP achieved 77.7% of AFRP.The addition of CFRP enhanced the shear resistance and resistance penetration force of the laminates during the initial penetration stage and significantly reduced the deformation of the AFRP layers during the penetration process.The ratio of tensile failure also increased in AFRP layers with the addition of CFRP,which improved the energy capacity of the hybrid CFRP/AFRP laminates.The ballistic performance of hybrid-20%and hybrid-30% laminates was equal to or better than AFRP when the initial velocity of the projectiles was 400 m/s to 800 m/s.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The research received no external funding.

Table ABallistic Experimental Information

Fig.B1 The specimen dimension of tensile tests.

Fig.B2 The specimen dimension of in-plane shear tests.

Fig.B3 The specimen dimension of out-of-plane shear tests.

Fig.B.4 The specimen dimension of flexural tests.