Process Modeling of Composite Materials: Residual Stress Development during Cure. Part I. Model Formulation

In the production of composite parts the pertinent processing parameters are time, temperature, and pressure. A judicious choice of these three parameters pro duces composites which are fully cured, compacted, and of high quality. Slight deviations from the recommended processing conditions can result in unacceptable quality. One of the most significant problems in the processing of composites is residual stresses. Processing-induced residual stresses can be high enough to cause cracking within the matrix even before mechanical loading. This microcracking of the matrix can expose the fibers to degradation by chemical attack. Strength is adversely affected by residual stresses since a pre-loading has been introduced. The topics considered and discussion presented in this paper have been chosen to ad dress the issue of understanding how residual stresses develop during processing and how they can be predicted. A process model has been developed which can be used to predict the residual stress history during the curing of composite laminates. This model includes the effects of chemical and thermal strains and assumes the material to exhibit linear, vis coelastic behavior. A phenomenological model is used to predict the degree of cure history during the cure cycle. Mechanical properties are allowed to develop based on a functional dependence on the cure state (degree of cure) and the transverse compliance is taken as the only time-dependent compliance. Simultaneous application of the cure kinetics and a vis coelastic stress analysis yields the residual moments and curvatures for unsymmetric cross-ply laminates. An experimental correlation is provided in an accompanying paper.