An experimental investigation of residual stresses in an epoxy–steel laminate

This paper describes an investigation of residual stresses in epoxy–steel systems using a bi-material beam. Residual stresses due to curing shrinkage, differential thermal contraction and hygroscopic expansion were investigated. Curvature in the bi-material beams was used as a measure of residual stress and simple beam theory was used to relate curvature to stresses and strains in the beam. Good agreement was achieved between predicted and experimental curvature of the beam. It was shown that in the system studied, the shrinkage stresses associated with the initial cure reaction were small, but considerable stresses developed on cooling due to differential thermal contraction of the two materials. However, on repeat thermal cycling, the residual stresses increased, which was interpreted as evidence of a post-cure effect. Samples exposed to moisture demonstrated significant stress relaxation, which was proportional to the weight of water absorbed. This was attributed mainly to the hygroscopic expansion of the adhesive.