Dynamic response of hybrid carbon fibre laminate beams under 1 ballistic impact 2

9 This novel hybrid fibre composites combining stiff composites with soft composites are 10 developed to improve the ballistic impact resistance of composite beams while maintaining 11 good quasi-static loading bearing capacity. The ballistic impact performance of the hybrid 12 beams have been investigated experimentally at a projectile velocity range of 13 1 1 0 50 ms 300 ms v     , including ballistic limits, failure modes, energy absorption capacity 14 and the interaction between stiff and soft composite parts. For each type of monolithic beams, 15 i.e. stiff, soft and hybrid monolithic beams, three categories of failure modes have been 16 identified: minor damage with rebound of projectile at the low impact velocities, fracture of 17 beam at the medium impact velocities and perforation of beam at the high impact velocities. 18 The critical velocity of hybrid monolithic beam was similar to that of the soft monolithic beam 19 under the same failure mode, and higher than that of the stiff monolithic beam. For the 20 sandwich beams with stiff, soft and hybrid face sheets, the failure modes were similar to those 21 of the monolithic beams. Among the monolithic beams, the hybrid and soft monolithic beams 22 exhibited better energy absorption capacity than the stiff monolithic beams. As for the 23 sandwich beams, the hybrid-face sandwich beams absorbed more kinetic energy of projectile 24


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Fibre reinforced composites have been attractive in both military and civilian applications due 33 to their outstanding mechanical properties [1]. It has been demonstrated that the lightweight  is able to infuse through fibre reinforcements quickly, and becomes hard and brittle after full 147 cure. Hence, it is suitable for manufacturing resin infusion composites. As for the EF80 flexible 148 epoxy resin, it exhibits higher mixed viscosity (500-1200 mPa·s) than the IN2 epoxy resin. In 149 addition, it has the capacity of maintaining flexibility after full cure, and is therefore suitable 150 for the applications where the flexibility of fibre reinforced composite parts are required. 151 Throughout the paper, the fibre composites with IN2 epoxy infusion resin are termed stiff 152 composites and the ones with EF80 flexible epoxy resin are termed soft composites. 153 Owing to the different bending stiffness and structural applications from those of monolithic 154 composite beams, the sandwich beams were also investigated in this study. The phenolic resin-155 impregnated aramid paper honeycombs, commercially known as Nomex ® honeycombs, were 156 employed as the cores of the sandwich beams in this study owing to its high ratio of 157 strength/stiffness to density [22][23][24][25]. The manufacturing process of the Nomex honeycombs is 158 summarized as follow: the Nomex aramid paper layers made from random fibres are stacked 159 on each other and adhered by the thermoset epoxy adhesive strips at intervals. The hexagonal 160 unit cells were formed by expanding the paper layers along the stacking direction. Finally, the 161 expanded geometry was impregnated into phenolic resin to be coated and obtain the specific 162 density of the honeycombs. The density and out-of-plane thickness of the Nomex honeycomb 163 core were h  =54 kgm -3 and H =10 mm, respectively. Figure 1 ( as shown in Fig. 1 (b). The steel mould had one outlet port located at the centre and four inlet 174 ports located at the four corners, both of diameter 2.5 mm. Eight bolts at the edges of the mould 175 were tightened to provide sufficient seal. Degassing of resin and gas tightness checking of VA-

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RTM system were conducted before resin injection. A vacuum pump connected with the outlet 177 port created a vacuum environment in the mould to infuse the resin through the dry fibre layers.

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For soft matrix, the compressed air of pressure 8 bars within a catch-pot was imposed to 179 facilitate the infusion of liquid resin. The ratios of resin to hardener by weight were 100 : 30 180 and 100 : 145 for manufacturing stiff composite panels and soft composite panels, respectively.

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The infused composite panels were then cured for 7 h at 65 ℃. To reduce the flaws caused by 182 cutting dry fibre layers, approximately 10 mm was removed from each edge of the panels after specimens are listed in Fig. 1  whereas had a gauge length of 12 mm for compression test in order to prevent Euler buckling.  beams more clearly. It should be noted that we suppose the soft composite parts, which are in 293 hybrid monolithic and hybrid-face sandwich beams, act as a cushion that avoids the direct stiff contact between non-deforming projectile and stiff composite part. Based on this assumption, 295 we set the projectile firstly impact the soft composites part of the hybrid beams.

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The experimental measurements for the six types of composite beams have been summarized 298 in Table 2, including the initial projectile velocity, residual projectile velocity, kinetic energy 299 of projectile transmitted to beams, and failure modes of beams.  Figure 5 (b) shows that the fracture in the middle 320 develops from the back face of the beam, thus the fracture mechanism is stretch governed. The 321 fracture at the clamped ends is also stretch governed, as indicated in the photograph of Fig. 4.

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At impact velocity of 67 ms -1 , the fracture mainly focuses on the middle of the beam while a 323 part of fracture also occurs at the clamped ends (Fig. 4). At higher impact velocity of 100 ms - images for three different failure modes are shown in Fig. 7. As the beam has a long response 339 history at low impact velocity of 72 ms -1 , the response history at this velocity ( Fig. 6 (a)) is 340 separated from others at higher velocities ( Fig. 6 (b)) for clarity. The ballistic behaviour is ): At the velocity of 72 ms -1 , the projectile is rebounded along with a 343 part of beam fracture in the width direction, as shown in Fig. 7 (a).  ): When the impact velocity is high enough, the projectile perforates 370 the beam with a negligible deflection. As shown in Fig. 9 (c), the debonding is not observed 371 before perforation, but develops after that. It is concluded that the debonding is due to the wave  sheet, leading to the debonding between back face sheet and honeycomb core, and finally 414 trapped into the beam (Fig. 11 (c)). This may due to the fact that the initial kinetic energy of

Ballistic resistance of beams characterised by the initial-residual velocity relation of the 441
projectile.
442 Figure 15 shows the initial projectile velocity 0 v as a function of residual projectile velocity i.e. higher slope and intercept correspond to better impact resistance of beams. This figure   446 indicates that the lowest intercept and slope of fitting lines are from the stiff monolithic beam 447 and stiff-face sandwich beam, respectively. In addition, the slopes of the stiff, soft and hybrid 448 monolithic beams are higher than those of the corresponding stiff-face, soft-face and hybrid-449 face sandwich beams, respectively. This is because the number of fibre layer for monolithic 450 beams is more than that for sandwich beams in order to achieve identical areal mass, and carbon 451 fibre laminated composites play a far more significant role than the Nomex honeycomb core 452 in resisting ballistic impact.
where k_0 E and k_r E are the initial and residual kinetic energy of projectile, respectively. The energy absorption capacity of the monolithic and sandwich beams have also been studied.

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For the monolithic beams, the energy absorption capacity of the hybrid and soft monolithic 533 beams were better than that of the stiff monolithic beams, whereas the stiff monolithic and