Time-Dependent Strength Degradation and Reliability of an Alumina Ceramic Subjected to Biaxial Flexure

Abstract

Fracture stresses of a sintered alumina ceramic were assessed in qualified uniaxial (three- and four-point) and biaxial (uniform pressure-on-disk) flexure tests under inert conditions (dry N2, 100 MPa/s stressing rate) and in deionized water at a low stressing rate (1 MPa/s). The size and stress-state effects on the inert fracture stresses of the alumina ceramic could be explained by a reliability analysis based on randomly oriented surface flaws and a mixed-mode fracture criterion. The decreased fracture stresses measured in both uniaxial and biaxial flexure tests in water were consistent with subcritical crack growth behavior inferred from dynamic fatigue tests in water. The reliability analysis of the size- and stress-state effects on and time-dependent degradation of fracture stresses included consideration of the statistical uncertainties (90 % confidence bands) of the estimated Weibull (Weibull modulus, m, and characteristic strength, σɵ) and slow-crack-growth (stress-intensity exponent, N, and critical crack growth rate, Vc) parameters. Results suggested that subcritical crack growth and strength degradation were more severe in biaxial flexure than in uniaxial flexure, which implies an interaction between stress state and subcritical crack growth.