Flat and Rising R‐Curves for Elliptical Surface Cracks from Indentation and Superposed Flexure

Abstract

Flat and rising R‐curves, fracture resistance versus crack extension, were determined for a sintered 99%α‐silicon carbide and for a hot‐pressed composite of 25 wt% silicon carbide whiskers and alumina, respectively. The R‐curves were evaluated from a combination of measured crack lengths, which were produced over a range of Vickers indentation loading, and of measured strengths, which were correlated either with the indentation flaws or with the most severe natural flaws on flexure specimens. A published analysis of the stress‐intensity factor for a surface crack in flexure was interpreted to show that the crack front takes the form of a semiellipse where both the ratio of the minor to major radii and the configuration coefficient itself decrease with increasing crack extension. A power‐law function of the indentation load was fitted to the product of an effective configuration coefficient and the flexural strength to evaluate the R‐curves. When the configuration coefficient is assumed constant, a customary practice, the R‐curves appear to have steeper rises. The assumed constancy of the coefficient of the indentation driving force may also have an effect on R‐curves, but the effect would be much less.