A numerical analysis has been performed on a system involving a crack near a single particle with the objective of finding a general relation for the size and shape of the elastic, crack-particle interaction zone which necessarily exists near particles in two-phase composites. In order to quantify the zone boundaries, various crack-particle geometries were modelled and a single characterization parameter was developed. Results show that a zone in which the energy release rate and direction of crack propagation are significantly affected can be simply defined near a particle as a function of the crack-particle geometry and elastic mismatch. A wide range of elastic combinations was examined with the result that for any significant crack deflection to occur away from the particle the magnitude of the first Dundurs parameter, I~1, must be greater than ,,~ 0.2. Numerical results show good agreement with experimental fatigue crack path data.

A plane-strain Finite Element (FE) analysis has been performed on a composite model consisting of a homogeneous, side-cracked elastic material with a single, symmetrically located elastic particle under pure mode-I loading, in an attempt to simply characterize the crack-particle interaction for a general two-phase composite. In order to uniquely characterize the geometry of a given model (crack length, particle size and crack-particle separation) it is necessary to introduce a new comprehensive 'geometric' parameter. For the purpose of making this analysis broadly applicable, a wide range of elastic moduli for both the matrix and the reinforcement are incorporated into the analysis. The results indicate that the particle has a strong influence on the crack-tip stress intensity factor (SIF) only when the particle is relatively near the tip as determined by the geometric parameter. Within this crack-tip region it is found that particles elongated parallel to the crack are more able to affect the crack-tip SIF than identically sized particles elongated perpendicular to the crack. Finally, the differential SIF of the composite is given as a general function of the geometric variables, particle shape (aspect ratio) and Dundurs' parameter ~ which characterizes the elastic mismatch of the constituents. With this relation, a simple and accurate estimate of the elastic interaction between a crack and particles of various shapes can be made on many combinations of materials without an extensive numerical analysis.

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A SiC-whisker-reinforced alumina composite was crept in compression at 1200" to 1400°C in an air ambient and in nitrogen. The data were described by a power-law-type constitutive relation. The measured value of the stress exponent was n = 1 at 1200°C and n = 3 at 1300" and 1400°C in both ambients. TEM observations were correlated with the measured creep response to determine active deformation mechanisms. Values of n = 1 were associated with diffusional creep and unaccommodated grain-boundary sliding, while values of n = 3 were associated with increased microstructural damage in the form of cavities. Experiments conducted in circulated air resulted in higher creep rates than comparable experiments in nitrogen. The accelerated creep rates were caused by the thermal oxidation of Sic and the resultant formation of a vitreous phase along composite interfaces. The glassy phase facilitated cavitation, weakened interfaces, and enhanced boundary diffusion. [