Dimensional accuracy of thermoset composites: Simulation of process-induced residual stresses

The warpage of thermoset composite structures during the manufacturing process is a direct consequence of residual stress development. The capability to predict residual stresses is crucial to the manufacture of dimensionally accurate composite structures. This paper is focused on understanding the fundamental issues leading to residual stresses in thermoset polymer composites and their effect on the dimensional accuracy of the manufactured components. Unlike most existing work, which use simplified geometrical and/or material models or focused merely on the cooldown process, a three-dimensional coupled thermo-chemo-viscoelastic model is developed to simulate the heat transfer, curing, residual stresses and deformation of a composite part during the entire cure cycle. The predicted values of curvature for cross-ply graphite-epoxy laminates agree well with experimental observations. The numerical result indicates that a significant fraction of the residual stress develops before cooldown. Detailed studies are also performed to examine the springforward phenomenon in L-shaped composite parts. The finite element results show that mold design (male vs. female mold), mold thermal expansion, part thickness, and fiber orientation all play an important role on the final shape of the parts, while cure cycle parameters such as dwell temperature, pressure, and cooling rate have less effect on springforward.