Static and Dynamic Characterization of Polymer Foams Under Shear Loads

When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest component of the structure and is the first one to fail in shear. In this study the shear fatigue behavior of two closed-cell cellular PVC foams, Divinycell HD130 (linear) and H130 (cross-linked), with the same nominal density of 130 kg/m3, were investigated. Static shear tests reveal that HD130 foams are more ductile, have almost twice the energy absorption capability, and an extraordinary crack propagation resistance when compared to the H130 foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio, R=0.1 on the HD130 and H130 foams. S–N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the linear foams was substantially higher than that of the cross-linked PVC foams. HD foams have smaller cells with thicker faces and edges. This microstructure supports absorption of larger amounts of liquid resin forming a resin rich subinterface zone in the core. The high-intrinsic toughness of the subinterface delays the initiation of fatigue cracks and thereby increases the fatigue life. For both foams, shear deformation occurs without volume change and the materials fail by shearing in the vicinity of the centerline along the longitudinal axis. In both cases numerous 45° shear cracks form along the length and across the width of the specimen immediately prior to the final failure event. Details of the experimental investigation and the evaluation of the fatigue performance are presented.