Internal stress and damage assessment in thick epoxy by laser induced shock wave: Effects of the plasticity

High performance epoxies are more and more used for structural design in naval and offshore applications. A trend for thick carbon fibre reinforced plastics application appears recently for severely loaded structures, especially for deep water oil extraction. On the other hand it is well known today that internal residual stresses appear during the curing of thermosetting materials, as shown by defects like long fibre wavinesses, cracks and bubbles within the epoxy matrix, especially with increased thickness The understanding of the undergoing phenomena during the curing of thick epoxy was described in a previous work by a FEM modelling of the thermal, chemical and mechanical couplings [1]. However, to enable a better quality control, pulsed lasers have recently been used in non destructive inspection and characterization of composites [2]. Nevertheless, to assess the experimental set-up of the laser induced shock wave technique, a description of material parameters is required. Because of the complexity of performing experiments to identify material parameters for any degree of cure, models were developed for a spatial description of epoxy material parameters obtained at the end of the process, assuming a dependency of those with the degree of cure. In this framework, this study presents the effects of epoxy plasticity modelling for internal stress and damage prediction induced by the curing process of a thick matrix. As a first approach, a simple cut-off fracture model based on the maximal tensile stress is used to identify the weakest parts of the solid matrix. The material behavior of the matrix is first described by a perfect elasto-plastic and hydrodynamic constitutive law. In a second step, a Ramberg-Osgood model was used to improve the description of the epoxy plasticity. Identifications of damage area within thick epoxy samples after a laser induced shock wave are performed to assess simulation results for damage prediction.