New Metrics for Pit Shape Characterization

Currently a variety of approaches are used to match stress concentration factors with corrosion pit geometry. The majority of these approaches use standardized stress concentration factors, such as concentration factors for circles or ellipses, to estimate the maximum stress values along the pit front. These factors are based on regular geometric shapes. Pits that form in a microstructure are influenced by the individual grains surrounding the pit. These pits often do not have simple shapes. Use of standardized geometric factors do not capture the geometric complexity of the pit. Rather than a single parameter, such as a the aspect ratio of an ellipse, multiple parameters may be required to define the extent and variation in localized curvature along a pit front within a microstructure. Maximum depth and curvature are just two possible candidate metrics. In addition the authors looked to the medical field for potential metrics to adequately describe the convoluted nature of the pit front. Several methods have been developed to mathematically define the serpentine twists in diseased retinal blood vessels. In this work the authors present a methodology for determining characteristics including tortuosity of computationally predicted pit shapes embedded in microstructures. Ultimately it is hoped that maximum curvature, pit tortuosity and other geometric based metrics can be combined to predict the maximum rise in stress associated with a pit embedded in a microstructure.