Crack Size and Shape Considerations Related to Near-Threshold Fatigue Crack Growth Rate Behavior

It is well recognized that crack size or load history or both have an important influence on near-threshold fatigue crack rate behavior at a low stress ratio. This investigation will compare the behavior of a radial crack, such as a surface flaw or a corner crack, to that of a linear crack to see if crack shape plays a role in physically small crack to long crack behavior. Recent research has unveiled a methodology to experimentally partition plasticity from roughness- and oxide-induced crack closure, showing that nonplasticity-induced closure mechanisms have a greater influence at low Kmax by orders of magnitude. Knowledge of this is key to understanding differences between long crack and physically small crack behavior in the near-threshold regime. In this study, the well behaved 2024-T351 aluminum alloy was used to generate threshold data using numerous specimen geometries, flaw geometries, crack sizes, and test procedures to highlight not only the significance of crack size but also the role of crack shape. It will be shown both experimentally and analytically that a radial crack (surface flaw, corner crack) has significantly less remote closure and, therefore, a lower threshold than a through crack (compact tension, middle crack tension), even if the crack size is the same.