The micromechanics involved in increased crack growth resistance, KR, due to the addition of TiBz particulate in a SIC matrix was analyzed both experimentally and theoretically. The fractography evidence, in which, the advancing crack was attracted to adjacent particulates, was attributed to the tensile region surrounding a particulate. Countering this effect is the compressive thermal residual stress, which results in the toughening of the composite, in the matrix. This thermal residual stress field in a particulate-reinforced ceramic-matrix composite is induced by the mismatch in the coefficients of thermal expansion of the matrix and the particulate when the composite is cooled from the processing to room temperature. The increase in K R of the composite over the monolithic matrix, which was measured by using a hybrid experimental-numerical analysis, was 77%, and compared well with the analytically predicted increase of 52%. The increase in K R predicted by the crack deflection model was 14%. Dependence of K R on the volume fraction of particulates, &, and of voids, fv, is also discussed. [

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The fracture toughness of a sintered dense a-Sic was estimated by the Vickers indentation microfracture method in the low-load Palmqvist crack regime. It was observed that the use of simultaneously obtained Vickers hardnesses does not yield reliable fracture toughness values, nor does application of the median-crack-derived equations. It is necessary to utilize a load-independent, crack-free hardness value with this toughness estimation method. Although several of the curvefitting equations yield similar toughnesses, it is concluded for the Palmqvist crack system in this a-Sic that the Niihara-Morena-Hasselman equation is the only one which yields fracture toughness values in agreement with conventional measurement techniques. [

A comparative study on crack curving and branching criteria in dynamic fracture mechanics shows that the criteria based on "advanced cracking" concept correlated best with available experimental data. The crack branching criterion requires as a necessary condition, a critical dynamic stress intensity factor, K'b' and a sufficient condition involving the crack curving criterion. The criteria are used to predict crack curving and crack branching in dynamic photoelastic experiments involving Homalite-100 and polycarbonate fracture specimens, as well as bursting steel and aluminum pipes.

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This paper deals with a procedure to calculate the elastic stress intensity factors for arbitrary-shaped cracks in plane stress and plane strain problems. An assumed displacement.hybrid finite element model is employed wherein the unknowns in the final algebraic system of equations are the nodal displacements and the elastic stress intensity factors. Special elements, which contain proper singular displacement and stress fields, are used in a fixed region near the crack tip; and the interelement displacement compatibility is satisfied through the use of a Lagrangean multiplier technique. Numerical examples presented include: central as well as edge cracks in tension plates and a quarter-circular crack in a tension plate. Excellent correlations were obtained with available solutions in all the cases. A discussion on the convergence of the present solution is also included.

The stress intensity factor for a semicircular edge crack is derived. Numerical values for axial, bending, and thermal loads in half spaces and plates are presented. The results show that a magnification of the stress intensity factor of about 20 percent occurs at the free surface.