Real Time Experimental Investigation of Dynamic Failure Mode Selection in Sandwich Structures

Abstract

Past studies of dynamic failure in sandwich structures have mostly been concerned with post-mortem investigations of failure patterns resulting from the dynamic loading of such structures through projectile impact. The final fracture patterns have typically been very complex and their time sequence, interdependence, and detailed nature (opening versus shear) were unclear. In the present work we concentrated on the real-time investigation of the generation and the subsequent evolution of dynamic failure that occurred within microseconds after impact. Model sandwich structures involving a combination of transparent polymers and metals (typically tri-layers bonded together by weak or strong adhesives) were designed and subjected to impact loadings to study the failure evolution mechanism. High-speed photography and dynamic photoelasticity were utilized to study the nature and the sequence of failure modes. A series of complex failure modes was observed. In most cases, interfacial cracks (dynamic delamination) appeared first. These cracks were shear dominated and were often intersonic. Thus, initial delamination was found to be shear driven and happen at extremely short time scales even at moderate impact speeds ranging between 20 to 50 m/s. The transition behavior between interlayer crack growth and intralayer crack formation was also observed. Opening (subsonic) intralayer cracks kinking from the dynamic shear delamination propagated into the core layer of the model sandwich structure and eventually branched as they reached a high enough growth speed.