Strength and Life Prediction for Hot-Pressed Silicon Nitride

The strength of hot-pressed Si₃N, 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 Si₃N₄. The exponential crack-growth formulation provided a consistent interpretation of the strength-degradation results, whereas the power-law formulation was inadequate.