Strength and Life Prediction for Hot‐Pressed Silicon Nitride

Abstract

The strength of hot‐pressed Si 3 N, was evaluated for constant stress rate, linear cyclic stress, and constant‐stress loading. A stress‐rupture rig was used to simultaneously test 10 samples under constant‐stress loading. Exponential crack‐growth‐rate expressions were numerically integrated for constant stress rate and cyclic‐stress loading and an approximate expression for the time‐to‐failure for these loading cases was developed. For either the power law or the exponential crack‐growth‐rate formulation, linear cyclic‐stress and constant‐stress‐rate loading can be treated with the same fracture stress‐time‐to‐failure representation. This correspondence was demonstrated for hot‐pressed Si 3 N 4 . The exponential crack‐growth formulation provided a consistent interpretation of the strength‐degradation results, whereas the power‐law formulation was inadequate.