Material Parameters for Life Prediction in Ceramics

Abstract

A detailed experimental study of critical material parameters needed for life prediction methodology in hot-pressed silicon nitride, NC132, has been made. The primary experimental techniques were double torsion and indentation induced flaw methods to determine the relationship between crack velocity, V, and stress intensity, K, during subcritical crack growth.

The subcritical crack growth exponent ‘n’ was determined using flexural stress and strain rate methods and stress rupture methods, and showed a wide scatter in magnitude. When all the relevant life prediction parameters such as inherent flaw size, strength, critical stress intensity factor, and K-V relationship for slow crack growth are known, an estimate of time to failure for a given applied stress, temperature and environment can be made using the numerical relation-ships outlined by Evans and Wiederhorn earlier. Care should be taken in selecting the appropriate parameters since these parameters are a function of evaluation technique otherwise the predicted time to failure will show a large variation. Future work in the current program will be designed to verify this life prediction methodology by comparing data obtained from simple uniaxial tensile stress rupture testing done in the temperature regimes of fast fracture (< 1200°C) and slow crack growth (> 1200°C).

The life prediction methodology as outlined in this study should be equally applicable to other ceramic materials which show a time dependent fracture behavior.