Stage II crack propagation direction determination under fretting fatigue loading: A new approach in accordance with experimental observations

Cracking is a dangerous degradation mode under fretting loading and the understanding of crack initiation and propagation is thus a necessity. A double experimental and theoretical approach has been undertaken to deal with crack initiation during Stage I [1,2], Stage I/Stage II transition and Stage II propagation. The work presented here is related to the latter point and aims at determining the direction and the propagation mode of cracks, A new approach is proposed here to account for non-proportional mixed mode conditions such as those encountered under fretting conditions at crack tips. Propagation directions during Stage II are derived from Delta sigma(theta theta)*(max), the maximum effective amplitude of the tangential stress perpendicular to the crack trajectory. They correlate well with experimental data. The stress field analysis shows that the trajectory of cracks borders the tensile-compressive and the tensile zones existing around the crack tip over a loading cycle.