PROCESS-INDUCED STRESS AND DEFORMATION IN THICK-SECTION THERMOSET COMPOSITE LAMINATES

A study of process-induced stress and deformation in thick-section thermosetting composite laminates is presented. A methodology is proposed for predicting the evolution of residual stress development during the curing process. A one-dimensional cure simulation analysis is coupled to an incremental laminated plate theory model to study the relationships between complex gradients in temperature and degree of cure, and process-induced residual stress and deformation. Material models are proposed to describe the mechanical properties, thermal and chemical strains of the thermoset resin during cure. These material models are incorporated into a micromechanics model to predict the effective mechanical properties and process-induced strains of the composite during cure. Thermal expansion and cure shrinkage contribute to changes in material specific volume and represent important sources of internal loading included in the analysis. Temperature and degree of cure gradients that develop during the curing process represent fundamental mechanisms that contribute to stress development not considered in traditional residual stress analyses of laminated composites. Model predictions of cure dependent epoxy modulus and curvature in unsymmetric graphite/epoxy laminates are correlated with experimental data. The effects of processing history (autoclave temperature cure cycle), laminate thickness, resin cure shrinkage and laminate stacking sequence on the evolution of process-induced stress and deformation in thick-section glass/polyester and graphite/epoxy laminates during cure are studied. The magnitude of process-induced residual stress is sufficient to initiate transverse cracks and delaminations. The results clearly indicate that the mechanics and performance of thick-section thermoset laminates are strongly dependent on processing history.