Microscopic deformation in a heated unidirectional graphite-epoxy composite

Out-of-plane displacements caused by heating of a unidirectional P75S/1962-ERLX graphite-epoxy composite were measured on a surface cut normal to the fibres after thermal cycling. On heating to 44° C from room temperature, the epoxy confined within clusters of three fibres sank in a trough beneath the plane of the fibre ends by 50–120 nm. The sense of the deformation was the opposite of what one would expect from differential thermal expansion. The role of residual stresses in this deformation was studied by measuring the depth of the trough for several different thermal histories applied before the free surface was cut. When the maximum temperature achieved in the prior thermal cycles exceeded 100° C, the depth of the trough increased. However, if cycles exceeding 100° C were followed by thermal cycles of decreasing amplitude, chosen to induce interfacial stress relaxation, the depth of the trough decreased, as expected. The experiments illustrate the feasibility of deducing quantitative information about local deformation in the interior of a specimen from high spatial resolution strain measurements on cut surfaces.