ON RESIDUAL STRESSES AND STRESS INTENSITY FACTORS IN COMPOSITE MATERIALS (ANALYSIS, RELAXATION, METAL MATRIX)

As a composite material is fabricated, residual stresses are produced. Residual stresses can arise because of thermomechanical property differences between the constituents of the composite. Substantial residual stresses are induced during the curing or quenching process. In metal matrix composites, residual stresses exceeding the yield strength of the metal may accumulate as the metal matrix cools, due to the difference in linear thermal expansion coefficients between the constituents.; Analyses are presented for the calculation of residual stresses generated in an isolated spherical inclusion metal matrix composite. The interfacial contact pressures and plastic zone sizes for various cooling rates imposed on an alumina sphere embedded in a cast aluminum alloy are evaluated. The results show that stress relaxation and alloy viscosity are important factors affecting the final composite residual stress state.; Elastic residual stresses in multiple-reinforcement composites are measured photoelastically, with stiff cylinders in compliant photoelastic plates. The cylinders are positioned in the photoelastic samples to measure the influence of volume fraction and reinforcement configurations. At a volume fraction of fibers of one-half, in a hexagonal array, the coupled residual stresses are minimized in the areas of greatest stress concentration.; Stress intensity factors are measured photoelastically for cracks in flat plates loaded by a solitary force-fitted cylindrical inclusion. The stress intensity factors are characterized for a range of matrix-inclusion interfacial pressures and inclusion to crack tip distances.; An isolated-reinforcement composite with an artificial crack is subjected to fluctuating applied stress. A theoretical crack growth rate is determined from the residual stress and stress intensity factor solutions mentioned above, in the Forman fatigue relation. The theoretical fatigue crack growth rate solution is compared to the experimental fatigue crack growth rate in the composite sample.