Process Induced Viscoelastic Residual Stress Analysis Of Graphite Epoxy Composite Structures

In this research the effect of viscoelastic behavior of thermoset matrix fiber-reinforced composites on process-induced residual stresses was investigated. The material selected for study was AS4/3501-6, a commercial prepreg system manufactured by Hercules, Inc. To investigate the cure-dependent viscoelastic behavior, a series of stress relaxation tests was performed on fully and partially cured 3501-6 epoxy specimens. The master relaxation curves were modeled as functions of time and degree of cure in a thermorheologically complex manner. From this information, time and degree of cure dependent viscoelastic constitutive equations were developed for the composite. A process model was then formulated to predict residual stresses during cure. Two separable problems were analyzed with the process model. First, a finite difference scheme was used to solve the thermochemical problem to obtain degree of cure and temperature fields. The results of this analysis were then used as input for a finite element model of rectangular laminates and circular cylinders. A recursive formulation was adopted to solve the computationally intensive time-superposition integration. Both thin and thick plates and cylinders were analyzed, subjected to the manufacturer's recommended cure cycle. In the cylinder case, the effect of the mandrel structure on residual stress fields was also investigated. Stress histories and distributions were analyzed to understand how residual stresses develop and what mechanisms are most important. The common assumption of stress free laminates before cooldown was shown to be invalid. Chemical shrinkage effects, often neglected, can be quite significant. In cylinder geometries the geometrical constraint together with the influence of the mandrel makes a direct correlation difficult between process-induced stresses for laminates and cylinders.