Process Modeling of Composite Materials: Residual Stress Development during Cure. Part II. Experimental Validation

In a companion paper [1] a process model was developed for investigation of residual stress development during autoclave or hot press processing of thermosetting polymer matrix composites. Several material property characterization studies are required as input to this model. The present paper summarizes the results of the characterization studies required for input to the model and validation of the model is accomplished by the intermittent cure of unsymmetric cross-ply laminates in which the processing-induced residual curvatures are measured. An IM6/3100 graphite/bismaleimide composite system was chosen for the study. Longitudinal and transverse mechanical properties were shown to increase during the cure cycle due to increase in matrix strength and stiffness and development of the fiber/matrix bond. Thermal strains were shown to remain relatively constant during cure. Chemical strains occur early in the cure cycle and are completed before cure is fully developed. The viscoelastic mechanical response of BMI is strongly dependent on the cure state. At low cure states the creep response is quite significant. As full cure is approached, the material becomes predominantly elastic. Based on the measured input data, the viscoelastic analysis showed that the contribution of chemical strains to residual stress was less than 4% for a typical cure cycle. A good correlation was obtained between model predictions and experimental warpage data for the cure cycle investigated.