Effect of Environment and Multiaxial Stress States on Subcritical Crack Growth of Machinable Ceramics.

Abstract

In order to study the effect of environment and multiaxial stress states on subcritical crack growth behavior of machinable ceramics containing mica flakes in a glass matrix, dynamic fatigue tests (constant stressing rate tests) were conducted by four point bending of plates (uniaxial tension), concentric ring loading of disks (equibiaxial tension), and diametral compression of disks (tension-compression) under different environments of air, water, and vacuum. The subcritical crack growth parameters, n, were determined from the stressing rate dependence of the fracture toughness value in both air and water environments. In any case of multiaxial stress states, the n value obtained in a water environment was obviously higher than that in an air environment. Furthermore, multiaxial stress effects were exhibited with a somewhat higher n value under equibiaxial tensile stress state in an air environment, and with a significantly lower n value under a tension-compression stress state in both air and water environments. These tendencies can be explained by the facts that in the biaxial stress state crack branching and formation of multiple cracks occurs easily with the absence of a strong orientation dependence for crack growth and with the existence of mica grains dispersed in the matrix, whereas in the tension-compression stress state the strong orientation dependence of the stress intensity at the crack tip prevents the crack from going around the mica grains.