Stereological characterization of crack path transitions in ceramic matrixcomposites

All ceramic composites involve a mismatch in physical properties the extent of which differs from one composite to another, Mismatch in thermal expans ion (Delta alpha) and elastic modulus (DeltaE) is known to produce stresses that influence the path of a propagating crack, Thus, the relative effect of thermal and elastic mismatch on the crack path is expected to change wit h change in stress intensity. We propose that the crack path in ceramic com posites should undergo a transition with the crack being strongly influence d by the thermal mismatch stresses at low stress intensity and elastic mism atch stresses at high stress intensities. Thus, a material in use under dif ferent applications each with its own loading conditions is expected to exh ibit different crack propagation tendencies which may be reflected in the n u -K characteristics of the composite material, In the present work several model composites with different combinations of thermal and elastic mismat ch have been considered. Cracks propagating at different sub-critical stres s intensities (velocities) were generated by a novel indentation technique. Each indentation was performed at a constant displacement rate and a peak load. A range of displacement rates were used to produce cracks propagating at different velocities. The indentations were made using a Vickers indent or fitted in a universal mechanical testing machine. The crack paths in com posites were quantified by stereological technique and the proposed theory was verified.