Cycle and Crack Length Dependence of Crack Growth Characteristics in Silicon Nitride.

Crack growth tests on silicon nitride have been made to clarify the crack growth characteristics under static and cyclic loading. The test results have shown that, under static loading, the crack growth rate decreases with an increase of crack extension until the crack is arrested, while under cyclic loading, the crack grows with almost constant velocity without being arrested, even in the range of K values where no growth is observed under static loading. Crack length dependence of “K-V relation” (K: stress intensity factor, V: crack velocity) and crack velocity dependence of “K-Δa relation” (Δa: crack extension) have been obtained from the crack growth data concerning the relations between crack velocity and crack extension under static loading and cyclic loading. Crack velocity is higher for smaller cracks than larger cracks at the same K values and the effect of stress cycling in accelerating crack growth is much larger at smaller K values. The effect of stress cycling is obvious in crack growth tests even in the material in which no effect of stress cycling is observed on cyclic fatigue tests on smooth specimens.