Assessment of hygrothermal ageing and damage in sandwich foam using TSA

The present paper is an introduction to the use of Thermoelastic Stress Analysis (TSA) to assess the response to closed-cell PVC structural foams from damage sustained during service in a marine environment. The effects of damage were accelerated by hygrothermal ageing at a temperature of 60ºC in distilled water. The CTS-type foam specimens contained a scalpel induced ‘edge-crack’ which were loaded at 0º, 10º, 30º, 45º, 60º,80º and 90º to obtain fracture toughness properties. A correlation was observed between increasing foam density and resistance to moisture uptake, with moisture acting as a plasticizer replacing that which has been leached away during immersion. After exposure to moisture specimens exhibited increased brittleness with significant reductions in energy to fracture. Linear foams were found to absorb almost twice the moisture of a cross-linked equivalent, and experience the greatest change in energy to fracture, most notably in shear, the most vulnerable loading mode for a sandwich structure. A CEDIP ‘Silver’ infrared system comprising an array of 320 x 240 detectors was used to perform TSA and evaluate the effects of fracture damage. The system recording capability allows crack propagation to be captured in real-time. The thermoelastic signal data at the crack-tip was related to stress using a calibration constant obtained from a specimen in its aged or unaged states when loaded with a known uniform stress. In addition, a high-resolution lens attachment allowed the cellular response to a crack-tip to be magnified, revealing the complex stress distribution on the thin struts around individual foam cells