THE SIZE OF THE TRANSFORMED ZONE DURING STEADY-STATE CRACKING IN TRANSFORMATION-TOUGHENED MATERIALS

An Exact expression is derived for the steady-state zone size, in materials susceptible to transformation toughening, assuming the transformation strain to be purely dilatational and to be uniform within the transformed zone around a crack. This expression predicts that the zone size, and hence the toughness, increase beyond bounds when the critical stress for inducing transformation decreases to a definite value, proportional to the constrained transformation stress, rather than when the critical stress tends to zero, as it had been implicitly assumed in previous work.