Analytical and experimental methods were developed to measure the structural and fracture properties of a linear elastic epoxy and a linear viscoelastic polyurethane material (Solithane). These properties were used in stress and fracture analysis to predict the initiation, growth, and trajectory of cracks in circular port and star grain designs. Experimental confirmation of these predictions was obtained by testing thin two-dimensional (plane stress) and three-dimensional grains of both materials under thermal and pressurization loading conditions. Crack initiation conditions with the epoxy material were in excellent agreement with theoretical predictions using elastic numerical analysis methods. Crack initiation and velocity measurements with the Solithane two-dimensional grains were in good agreement with numerical analysis techniques combined with a time-dependent crack analysis which was programmed for efficient use. An approximate three-dimensional fracture analysis for circular port grains with elliptical flaws was developed and programmed. Results of the crack prediction computer program were in good agreement with three-dimensional grain test data. Experimental work with the epoxy and Solithane grains showed that cracks in two- and three-dimensional grains will propagate under thermal and pressurization loading conditions and these fracture conditions can be predicted using analytic methods presented in this report. Photoelastic analysis was used to confirm the stress intensity factors at the crack tip for thermal and pressurization loaded grains. Photoelastic results were in good agreement with both the numerical predicitons and the crack velocity measurements with Solithane grains. (Author).