Factors Affecting the Statistical Strength of Alumina

Abstract

This program was undertaken to determine the applicability of statistical fracture theories to inorganic ceramics, and to define the major parameters affecting fracture strength. Specimen shapes were developed, adaptable to a broad range of loading conditions and volumetric variations. The effect of five variables was investigated on the strength of Wesgo AL995: prior thermal history, specimen finish, test temperatures, environment, and specimen size. Of these parameters, specimen size, testing temperature, surface treatment, and thermal history were found to have a primary influence on fracture strength. Environmental effects (moisture content) were found to be significant only for ground specimens tested at 20°C; at 1000°C all environmental influences became negligible. Fracture in Wesgo AL995 at room temperature is governed purely by surface-induced failure mechanisms. Both Weibull constants—the flaw density parameter m and the zero strength σ _u —are highly sensitive to surface treatment and thermal history. Specifically, grinding increases the value of m but leaves σ _u unaltered; annealing increases m while also dropping the value of σ _u to zero. Both conditions act to weaken the material. This weakening effect is particularly pronounced in the case of annealed specimens; this is thought to be ascribable to the destruction of a beneficial residual stress distribution originally existing in the material due to the subsequent annealing treatment.