RE-0756-01: Effectiveness of Mechanical Treatment of Crack-Arrest Holes Subject to Distortion-Induced Fatigue

Abstract

This report details a study that was aimed at assessing the effectiveness of mechanically-treating crack arrest holes used at the tips of distortion-induced fatigue cracks. Different mechanisms are possible for producing expansion of crack-arrest holes drilled at the tips of fatigue cracks, and such treatments have shown to be effective in improving the fatigue performance of cracks subjected to in-plane loading. However, prior research has not established the effectiveness of crack-arrest hole cold-expansion for distortion-induced fatigue applications. Because the majority of fatigue cracks in bridges are caused by distortion-induced fatigue mechanisms, this study aimed to explore whether cold-expansion of crack-arrest holes can be expected to produce any benefit to fatigue life.

An analytical investigation was undertaken in which C(T) specimens were modeled using 3D finite element analysis. The study included the modeled specimens being loaded in Mode I (in-plane loading) and Mode III (out-of-plane loading). In addition to different loading directions, the suite of finite element models included cracks of different lengths, as well as different diameter crack-arrest holes. The models included nonlinear material properties to capture inelastic effects. In some of the models, the crack-arrest holes were subjected to cold expansion and allowed to develop compressive residual stresses.

Stresses around the crack-arrest holes were examined for models with and without the cold-expansion treatment. The study clearly showed that while there can be expected a beneficial influence from cold-expansion for in-plane loading, no such beneficial effect existed for distortion-induced fatigue (out-of-plane loading). Based on these results, the authors concluded that crack arrest hole treatment can be expected to have limited to no practical benefit when considering cracks caused by distortion-induced fatigue.