FLAT AND RISING R-CURVES FOR ELLIPTIC SURFACE CRACKS FROM INDENTATIONAND SUPERPOSED FLEXURE

Flat and rising R-curves, fracture resistance versus crack extension, were determined for a sintered 99% alpha-silicon carbide and for a hot -pressed composite of 25 wt% silicon carbide whiskers and alumina, res pectively. 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 flexur e 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 inc reasing crack extension. A power-law function of the indentation load was fitted to the product of an effective configuration coefficient an d the flexural strength to evaluate the R-curves. When the configurati on coefficient is assumed constant, a customary practice, the R-curves appear to have steeper rises. The assumed constancy of the coefficien t of the indentation driving force may also have an effect on R-curves , but the effect would be much less.