What can we learn from R-curve measurements?

In the last 30 years, crack growth resistance curves (R curves) have been measured and analyzed for many ceramic materials. This body of literature is reviewed critically to address the utility of these measurements. Three effects have been held responsible for the existence of a rising R curve: crack face interaction in the wake of the crack tip, microcracking and crack branching ahead of the tip, and phase transformation ahead of the crack tip. For the crack face interaction, the relation between bridging traction and crack opening displacement characterizes the material behavior. Most experimental investigations have concentrated on specimens with macrocracks and on indentation-induced cracks. Very few results for R curves starting from natural flaws have been published. From these results, it is concluded that the behavior of specimens with natural flaws cannot be predicted directly from the results of tests with macrocracks and no established framework exists to correlate the two. The analysis of indentation cracks contains many uncertainties and cannot close the gap between macrocracks and natural flaws. The effect of a rising R curve on the strength and on the scatter of the strength is also discussed in the paper.