Debonding fracture of foam core sandwich structure

A new test specimen, named tilted sandwich debond specimen (TSD), has been Introduced to promote face/core debonding over crack kinking and enable characterization of an important failure mode of sandwich structures. An experimental compliance calibration procedure was developed for evaluation of debond fracture toughness in a straight-forward manner. The specimen has been evaluated through kinematics analysis, elastic foundation model, finite element analysis and a comprehensive experimental investigation. An elastic foundation model of the TSD specimen was developed to obtain analytical expressions for specimen compliance and strain energy release rate. A design equation for the maximum tolerable crack length was derived. Finite element analysis of various configurations of the TSD specimen was conducted to obtain the mixed mode stress intensity factors, crack kinking angle, specimen compliance and strain energy release rate. The results revealed that the bimaterial character of the TSD specimen influences the mode mixity for the specimen and that crack kinking was more likely for thick and low density cores. The presence of the interphase layer only slightly influenced the mode mixity and kinking angle. The debonding characteristics of several sandwiches consisting of glass/vinylester face sheets and PVC foam cores of various densities were examined using the TSD specimen. Crack propagation from the beelcore precrack involved "micro-kinking" or kinking deeply in the core for all specimens at the first crack propagation increment(s). Crack kinking in the intermediate density core could be suppressed by selecting a certain range of tilt angles. After kinking, crack returned to a path parallel and close to the interface in agreement with the analysis of sub-interface cracks. Cracks propagated in a stick/slip manner. Measurements of the debond fracture toughness, Gc, using the TSD specimen revealed that Gc is fairly independent of crack length and increases with increasing core density. The debond toughness was of similar order as the mode I toughness of the core.