Small, short and long fatigue crack growth in an advanced silicon nitride ceramic material

Abstract

In metallic materials, a number of workers have reported that the growth rates of small fatigue cracks cannot be correlated with the stress intensity factor range, {Delta}K. Small cracks normally exhibit faster growth rates than long cracks and often show growth rate minima. This anomalous behavior has been attributed to the failure of the linear elastic fracture mechanics parameter {Delta}K to characterize small, or short fatigue crack growth. Ceramic materials combine a lack of dislocation deformation and a very small grain size and thus the reasons for any observed anomalous small or short crack growth effect are less clear. Previous work on small or short fatigue crack growth in ceramics is limited, and work on silicon nitride which is one of the most promising structural ceramics is particularly sparse. As the majority of the fatigue lifetime of any silicon nitride component will be controlled by the propagation of a preexisting small flaw to a critical size, the presence of any short or small crack effect in this material is of engineering importance. Thus, the objective of the work presented here is to investigate the small, short and long crack growth in an advanced silicon nitride material.