Fatigue of Crosslinked and Linear PVC Foams under Shear Loading

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 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 sub interface zone just below the actual core skin interface. The high intrinsic toughness of the sub interface delays the initiation of fatigue cracks and thereby increases the fatigue life of the HD foams. For both foams, shear deformation occurs without volume change and the materials fail by shearing in the vicinity of the centerline of the specimen along the longitudinal axis. In both cases numerous 45° shear cracks form across the width of the specimen and are equidistantly spaced along the length of the specimen. The occurrence of these through the thickness shear cracks signals the final failure event during fatigue. Details of the experimental investigation and the evaluation of the fatigue performance are presented.