Proof testing of ceramics

Theoretical estimates are made of strength distributions after proof testing. Assuming that the crack velocity can be expressed as a power function of the stress intensity factor,v=AK _I ⁿ , an analysis of the amount of strength loss during a load cycle is presented for single-region crack propagation. For multi-region crack propagation, a numerical analysis is used to describe strength loss. In both analyses, the effects of environment and loading rate are studied. For single region crack propagation, the strength after proof testing can be represented by two Weibull curves: one with a slope ofm at high cumulative failure probability levels; the other with a slope ofn–2 at low failure probability levels. Truncation of the strength distribution always occurs as the result of proof testing; the truncation strength depends on the rate of unloading. Multi-region crack propagation results in a more complicated strength distribution after proof testing. Bimodal strength distributions occur as a consequence of region II type crack growth (i.e.n=0). Theoretical results confirm experimental findings that proof tests must be conducted at rapid unloading rates and with good environmental control to be effective.