Impact failure characteristics in sandwich structures: Part I: Basic failure mode selection

In the present work we present a systematic experimental investigation of the generation and subsequent evolution of dynamic failure modes in sandwich structures subjected to low-speed impact. Model sandwich specimens involving a compliant polymer core sandwiched between two metal layers were designed and subjected to impact loading to simulate failure evolution mechanisms in real sandwich structures. High-speed photography and dynamic photoelasticity were utilized to study the nature and sequence of such failure modes. A series of complex failure modes was documented. In all cases, inter-layer (interfacial) cracks appeared first. These cracks were shear-dominated and were often intersonic even under moderate impact speeds. The transition from inter-layer crack growth to intra-layer crack formation was also observed. The shear inter-layer cracks kinked into the core layer, propagated as opening-dominated intra-layer cracks and eventually branched as they attained high enough growth speeds causing core fragmentation.