Abstract
An indentation fracture mechanics analysis is developed to characterize the toughening effects of a compressive surface layer in brittle materials. The analysis is used to describe the enhanced toughness of cordierite glass–ceramic laminate composites, in which thermal-expansion mismatch effects induced uniform stress in the exterior layers of the symmetric exterior:interior:exterior structures. Interpretation of indentation crack length and inert strength tests via the analysis shows that cracks can be viewed as experiencing discrete regions of decreasing stabilization on propagation from small cracks and complete containment within the compressive layer to large cracks and partial extension into the compensating tensile interior. The observations are described using a stress-intensity factor for circular cracks in linear stress fields that includes different base and surface values for extended cracks. Deconvolution of inert strength data for the model cordierite system studied suggested an increase in toughness from 1.4 MPa·m1/2 for the base material to a peak of about 5 MPa·m1/2 for a 1:18:1 composite structure, with attendant increases in strength and flaw tolerance.
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