Study of the Surface Roughness Evolution of Pinned Fatigue Cracks, and its Relation to Crack Pinning Duration and Crack Propagation Rate Between Pinning Points

Abstract

Changes in surface morphology have long been thought to be associated with crack propagation in materials. In this paper, we study the changes in the surface profile of the crack-tip plastic zone with an attempt to understand the relationship between the plasticity-induced surface profile changes and the crack growth behavior. Center crack specimens were electropolished and etched to reveal the grain structure for white light interferometer (WLI) imaging prior to and during fatigue testing. After growing the crack to a predetermined pre-crack length, a viewing zone was selected outside of the plastic zone of the pre-crack. The surface profile of the viewing zone was imaged using a WLI microscope at selected fatigue cycle intervals. An image processing algorithm was developed to evaluate the changes of the surface profile. We observed that the crack growth rate is not uniform at the microscopic scale; the crack growth was retarded at crack pinning points and the crack grows at a faster rate while propagating between the pinning points. Relatively large surface topology changes were observed to be constrained to the area surrounding the tip of pinned cracks. However, there was an avalanche of surface changes covering the entire monotonic zone upon the crack being released from a pinned location. Interestingly enough, minor or no measurable surface changes could be seen for propagating cracks. These results indicate a surface roughness change threshold may exist for predicting the duration during which a crack is pinned. Results suggest the threshold and crack propagation rate between pinning locations may be functions of the amplitude of the stress intensity factor.