Comparison of damage development in C/epoxy laminates during isothermal ageing or thermal cycling

The aim of this study is to characterize and compare damage processes in carbon/epoxy laminates submitted to either isothermal ageing or thermal cycling in neutral (vacuum or nitrogen) and oxidative (air) atmospheres. During a thermal cycling test performed in an oxidative atmosphere like air, there is a coupling effect between matrix oxidation, occurring at the highest temperatures of the cycle, and matrix cracking due to thermo-mechanical ply stresses induced by the prevented differential expansions of the plies. In order to separate those two damage mechanisms, a model of oxidation is used to determine the experimental conditions of an isothermal test ‘equivalent’ to a thermal cycling one, in terms of mass loss due to oxidation. After having checked this equivalence, a quantitative analysis of the damages induced by the two types of tests, is carried out. It is shown that the oxidation of a laminate in isothermal conditions results in damages, which concern only the skin of the sample. On the contrary, the coupling of such damage mechanisms with cyclic stresses in thermal cycling accelerates the damage processes and especially the matrix crack propagation from the surfaces to the core of the laminate.