Recently we showed that, under nonhydrostatic loading, the F_r1→ A_0. polymorphic transformation of unpoled lead zirconate titanate 95/5-2Nb (PNZT) ceramic began when the maximum compressive stress equaled the hydrostatic pressure at which the transformation otherwise occurred. More recently we showed that this criterion seemed not to apply to poled ceramic. However, unpoled ceramic is isotropic whereas poled ceramic is not. If we further assume that the transformation depends on both the stress magnitude and its orientation relative to PNZT's structure, these disparate results can be resolved. This modified hypothesis makes two predictions for transformation of unpoled ceramic under uniaxial compression((i) it will begin when the compressive stress equals the hydrostatic pressure for transformation, and (ii) steadily increasing stress will be required to drive it to completion. Here we present experimental results that confirm these predictions. We then revisit our earlier results fOr poled and unpoled PNZT. The new hypothesis quantifies the observed effect of shear stress on the mean stress for onset of the transformation of unpoled ceramic and explains previously reported kinetic effects.
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