Mechanisms of Fatigue Damage in Foam Core Sandwich Composites with Unsymmetrical Carbon/Glass Face Sheets

This paper attempts to describe the fatigue response of a foam the core sandwich composite with unsymmetrical carbon/glass face sheets under tension-tension cyclic loading. The laminate is a critical element of the structure of a light full composite airplane which was determined through the stress analyses for numerous load cases. The face sheets, over a PVC foam the core, consist of two similar woven E-glass fibers following unidirectional carbon fibers with epoxy resin. Extensive fatigue data were generated for the S-N diagram of this laminate. Based on the static FEM analyses, two designs for the configuration of foam to solid laminate bonding zone have been investigated. Hence, the effects of using each of these different proposed designs have been determined. The damage events causing final failure were traced during fatigue tests. Four distinct damage events were found. Two of them were initiated and propagated in the foam the core, which could result in lower dispersion in comparison with other composite materials, because of foam the core homogeneity. The first damage event in the core is crack initiation and propagation along thickness, followed by the second damage event, which is crack propagation parallel to the length of the gauge. The parallel crack length is shortened by increasing the load ratio.