Abstract
A model developed to predict the time-dependent build-up of residual stresses induced during processing of continuous-fiber semicrystalline thermoplastic com posite materials has been developed. The model is employed to conduct a parametric study investigating the influence of material, geometric, and processing variables on residual stress distribution in AS4 graphite/PEEK composites. The model includes such parame ters as plate thickness, cooling rate, thermal conductivity, and ply orientation. The study indicates that all of these parameters play a role in the development and distribution of re sidual stresses. Two competing mechanisms of residual stress build-up are studied as a function of process history: stacking sequence and spatial solidification effects. The stack ing sequence effect yields traditional, discontinuous changes in in-plane stress components resulting from the mismatch in stiffness and thermal expansion of plies within the lami nate. Spatial solidification resulting from nomsothermal cooling of the thermoplastic com posites yields a parabolic residual stress distribution through the laminate thickness. Stacking sequence effects are found to dominate the residual stress distribution of cross- ply laminates for the cooling rates and laminate thickness investigated. Residual stress dis tnbution is shown to be highly sensitive to two difficult-to-measure properties: relaxation behavior and the volumetric shrinkage due to crystallization of the matrix. Based on a dou ble cantilever beam test conducted to measure performance via Mode I fracture toughness,the presence of residual stress in cross-ply laminates was shown to reduce the apparent toughness as much as 35% relative to a unidirectional baseline.