Performance enhancements of polymer–matrix composites by changing of residual stresses

Manufacturing induced residual stresses of polymer–matrix composites (PMC) reduce the tensile load at which first ply failure occurs. Thermomechanical treatments offer the potential to change these residual stresses, but their application is hindered because the shape stability of PMC components is limited at treatment temperatures, which must be above the glass transition temperature of matrix. This study describes successful localized treatments performed on a stress concentration induced by a circular hole in a laminate. Localization of the treatment allows significant property improvements at much lower treatments loads. Investigation of the influence of moisture pick up on treatment effectiveness revealed it reduces the benefits of treatment to a large extent. Calculations of local stresses between single fibres and predictions of laminate properties based on laminate plate theory using a quadratic failure criterion confirm the experimental results and provide confidence in the physical explanations offered for the measured effects.