THE INTERRUPTED STATIC FATIGUE TEST FOR EVALUATING THRESHOLD STRESS INTENSITY FACTOR IN CERAMIC MATERIALS - A NUMERICAL-ANALYSIS

The interrupted static fatigue (ISF) test can be used to evaluate the threshold for sub-critical crack growth, K-th. A critical theoretical analysis of this technique is presented in order to elucidate its adva ntages and limitations and to furnish some suggestions for a more defi nitive procedure for evaluating K-th. It was concluded that a critical parameter in the ISF procedure is the duration of the static hold tim e. In particular, one should demonstrate that the calculated value of K-th does not change when this holding time is increased. Moreover, th e applied stress must be chosen such that a significant fraction of sa mples fail in the hold period. Two different approaches ape suggested for calculating the value of K-th from experimental data. The first ap proach is considered prefer able for materials with larger Weibull mod uli and the threshold is determined from the applied stress intensity factor at which 50% of samples fails during the stress hold. In the se cond approach, the stress intensity factor applied to the weakest spec imen during the stress hold is calculated for various hold rimes. Once this value becomes independent of hold time, it is equivalent to the threshold.