Abstract: The contact stress and the critical stress distribution were analyzed, and some paradoxes related to contact crack initiation were explained through the effect of stress gradient. Based on the Mean-Strength criterion, a testing method for determining the local strength and residual stress in brittle materials was developed. And also a simple technique of proof test was created, that is useful for non-destructive evaluation of the strength for brittle components.

Key words: sphere indentation, local strength, proof test, brittle materials