Fatigue Crack Growth Behavior in Silicon Nitride under Constant and Variable Amplitude Load Sequences

Abstract

In order to investigate cyclic fatigue crack growth behavior of a gas-pressure-sintered silicon nitride under constant and variable amplitude load sequences, fatigue crack growth tests were carried out using compact type (CT) specimens, The crack length and macroscopic crack closure were measured using an unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test conditions, which implied fatigue crack growth of this material was associated with progressive degradation of the grain interlocking by cyclic loading. The fatigue crack growth rate, da/dn, under constant amplitude loading was controlled not only by the maximum stress intensity factor, K max , but also the stress amplitude, while K max was the most important factor in cyclic fatigue crack growth. The overload and high-level load excursion produced the acceleration of fatigue crack growth and the downward shift of crack closure point under low-level loading. This decrease was thought to result from much severer crash or frictional wear of grain interlocking and the crack closure could explain the acceleration behavior qualitatively.