Influence of the cover plate thickness on the ballistic penetration of re-entrant auxetic structures

This study investigated how mild steel cover plates of various thicknesses affected the collapse of re-entrant auxetic structures under penetration at 500 m/s by o16 mm spherical projectiles. A combination of experimental and numerical methods was used to capture this effect with auxetic structures made from 3D printed titanium alloy (Ti6Al4V) and ABS plastic (acrylonitrile butadiene styrene), and wire cut 304 stainless steel (SS304). Experimentally, structures were tested with a 2.90 mm cover plate and without the cover plate. Numerically, the effect of the cover plate thickness on the auxetic response of the structure was tested with 0.95 mm, 1.50 mm, and 2.90 mm cover plates (as well as no cover plate). The auxetic densification was found to be greatest under the thickest cover. Material characterisation tests were conducted to develop strength and failure models for the mild steel cover and back plates in this study, which produced simulated correlation to the ballistic experiments. Although the simulated auxetic effect generally increased with cover plate thickness, similar to the experiments, the structure material significantly affected the range of this dependence. For the ductile SS304 structures, a thin (0.95 mm) cover plate showed a small improvement in the auxetic response, with a marked improvement with a 1.50 mm cover plate. Whereas the Ti6Al4V structures showed a steady improvement in auxetic densification as the cover plate thickness was increased. Overall densification of the ABS structures was significantly improved by adding cover plates, however collapse was delayed compared to the other materials. These findings therefore indicate that the thickness of cover plates in auxetic sandwiches can be specifically engineered to maximise auxetic effect while considering the ranges of this dependence with different auxetic materials.