INVESTIGATION OF THE SURFACE TOPOLOGY CHANGES ASSOCIATED WITH FATIGUE

Abstract

Metals have long been known to change their surface topology when subjected to a plastic strain. Given that a plastic zone exists at the tip of a propagating crack, it would seem natural to quantitatively study surface topology changes associated with nucleating and propagating fatigue cracks. So far the workers in this study have only found one measurement of the surface topology changes associated with a nucleating crack. The current research seeks to study and refine, a possible correlation between crack growth and surface topology changes, with the object to develop a damage index for fatigue cracking from surface topology measurement. The research uses a mechanical testing machine with an in-situ white light interferometer (WLI) which allows for high temporal resolution of surface roughness data. This research has demonstrated that certain aspects of crack propagation are associated with unique surface topology effects. It has also shown that WLI images can be used to develop a damage parameter which can predict the path of a propagating crack. Finally, using Electron Back-Scatter Diffraction (EBSD) techniques it has been shown that the damage parameter for propagating cracks is related to strain localization on the surface of a sample. For cracks initiating from a stress concentration the WLI methods developed in the current research have shown the ability to identify the location of a future crack even. This method has been shown to work for initiation location even when no traditional strain localization features are present on the surface visible to the WLI, e.g. slip banding.