VISCOELASTIC ANALYSIS OF PROCESSING-INDUCED RESIDUAL STRESSES IN THICK COMPOSITE LAMINATES

Residual stresses induced during the processing of thick composite laminates are analyzed. A two-dimensional thermochemical analysis is used to obtain temperature and degree of cure distributions during cure. A two-dimensional finite-element model is developed to predict the residual stress history in rectangular laminated plates. A cure-dependent visco-elastic material model is used in the analysis. Both unidirectional and cross-ply laminates are investigated. Unidirectional laminates are studied because residual stresses in this class of laminates is driven purely by thickness effects. Regardless of the layup,for moderately thick (2.54-cm) laminates the nonuniformities in temperature and degree of cure are mild. As a result, the development of residual stress is very similar to the analysis of thin laminates. For laminates of large thickness (7.5 cm), stronger variations in temperature and degree of cure develop during the cure cycle. The development of residual stress in this case is more complex. The transverse stress distribution in unidirectional laminates is tensile at the center and compressive on the edge at the end of cool-down. The profile is inverted twice during the cure cycle as competing mechanisms of thermal expansion, chemical shrinkage, and chemical hardening interact. The transverse residual stress for 7.5-cm thick unidirectional laminates was shown to reach a value of 3.5 MPa after cure, compared to a value of only 0.65 MPa for 2.54-cm-thick laminates. In all of the cases analyzed, the laminates exhibited a center-to-surface cure behavior. Thicker laminates and laminates constructed of materials with different cure kinetics could show a surface-to-center cure behavior. In such cases the residual stress development will be significantly changed from the results presented in this article.