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Fiber/matrix (F/M) cracking was analyzed to provide better understanding and guidance in developing F/M interface fracture toughness tests. Two configurations, corresponding to F/M cracking at a broken fiber and at the free edge, were investigated. The effects of mechanical loading, thermal cooldown, and friction were investigated. Each configuration was analyzed for two loadings: longitudinal and normal to the fiber. A nonlinear finite element analysis was performed to model friction and slip at the F/M interface. A procedure for fitting a square-root singularity to calculated stresses was used to determine stress intensity factors (KI and KII) for a bimaterial interface crack. For the case of F/M cracking at a broken fiber with longitudinal loading, crack tip conditions were strongly influenced by interface friction. As a result, a F/M interface toughness test based on this case was not recommended because nonlinear data analysis methods would be required to calculate frictional energy. For the free edge crack configuration, both mechanical and thermal loading caused crack opening, thereby avoiding frictional effects. A F/M interface toughness test based on this configuration would provide data for KI/KII ratios of about 0.7 and 1.6 for fiber and radial normal loading, respectively. However, thermal effects must be accounted for in the data analysis.
Senior researcher, NASA Langley Research Center, Hampton, VA
Research scientist, Analytical Services & Materials, Inc., c/o NASA Langley Research Center, Hampton, VA
Stock #: CTR10086J