Epoxy resins and other thermosetting polymers change from liquids to solids during cure. A precise process model of these materials requires a constitutive model that is able to describe this transformation in its entirety. In this study the viscoelastic properties of a commercial epoxy resin were characterized using a dynamic mechanical analyzer (DMA). Specimens were tested at several different cure states to develop master curves of stress relaxation behavior during cure. Using this experimental data, the relaxation modulus was then modeled in a thermorheologically complex manner. A Prony (exponential) series was used to describe the relaxation modulus. An original model was developed for the stress relaxation times based on similar work by Scherer (16) on the relaxation of glass. Shift functions used to obtain reduced times are empirically derived based on curve fits to the data. The data show that the cure state has a profound effect on the stress relaxation of epoxy. More important, the relaxation behavior above gelation is shown to be quite sensitive to degree of cure.

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In this study the cure-dependent stress relaxation of 3501-6 epoxy resin was analyzed. To formulate the constitutive equations, several stress relaxation tests were performed at different degree of cure states. The master curve and shift function for each degree of cure case were obtained by time-temperature superposition. On the basis of this data, the thermorheologically complex behavior of 3501-6 epoxy resin was modeled. The resin model was then used to formulate the constitutive equations for AS4/3501-6 composite. A finite element method was then used to calculate the interlaminar stress relaxation at the free edge of AS4/3501-6 cross-ply laminates during cool down from the cure temperature. The relaxation behavior of 3501-6 epoxy was shown to be dependent on the degree of cure and significant reduction in residual stress was found to occur at a modest (2.5°C/min) cool down rate.

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