SHEAR-INDUCED TRANSFORMATION TOUGHENING IN CERAMICS

There is experimental evidence that the tetragonal to monoclinic transformation in zirconia (ZrO2) can be triggered not only by hydrostatic pressure but also by shear stresses. In this paper we consider the implications of a transformation criterion which depends on both mean stress and shear stress on transformation toughening in ceramics reinforced with zirconia inclusions. We treat zirconia toughened ceramics (ZTC) as dilute composites of randomly oriented inclusions in an elastic matrix and develop the constitutive model for the composite from the behaviour of single inclusions by orientation averaging. We then study quasistatic crack propagation and predict the transformation zone, distribution of transformed inclusions and toughness enhancement.