Evaluation of hygrothermal ageing damage in structural foam cored marine sandwich materials and the potential of thermoelastic stress analysis for its detection

This work seeks to address concerns voiced in the marine industry over the confidence of design safety factors applied to sandwich cores following reported failures in service and the introduction of new lower standards for linear foams. Design calculations largely define the safety factor for the sandwich structure based upon the performance characteristics of the laminate skins with only basic criteria for the core, in terms of section modulus, moment of inertia, core shear and density, all intended to limit skin buckling at the compressive face. Typically, the core is the most vulnerable component of the sandwich, and is assigned the lowest factor of safety with no allowance for change in the material properties over time. Much of the research on durability and the effects of environmental ageing into the degradation of sandwich materials has been generated by the aeronautical industry and has focused upon laminate composites with environmental exposure specific to flight. Further information is required by marine designers to address the knowledge gap in the estimation of risk posed by the altered response of aged cores, especially in light of the special dispensation offered to linear foams of equivalent density. The approach adopted by this work was an experimental investigation into the effects of ageing on two medium density foam types, typical of the marine industry; a cross-linked C70.130 and a linear R63.140. Their responses to exposure and ageing were chronicled by changes in weight and fracture toughness. Fracture toughness values were obtained for a progression of mixed modes from pure tensile to pure shear, mapping the deterioration of mechanical properties over a period of accelerated ageing using a sharp edge- crack CTS-type specimen with the same configuration for all modes. Mechanical testing revealed increasing brittle behaviour for both foam and micrographs depicted evidence of pitting and leaching of material from the cell of the R63.140. Long-term environmental exposure was simulated by