Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

Abstract

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant Δ K. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si 3N 4 grains within the crack wake.