Reconsiderations of fatigue crack growth at negative stress ratios: Finite element analyses

• A variable to control the fatigue crack tip loading in compression is verified.
• Deformation mechanism of fatigue cracks due to negative loads are identified.
• Mechanisms of variable amplitudes at negative stress ratios are explained.
• Plasticity induced crack closure is compared with crack tip plasticity.

Two important aspects of fatigue crack growth at negative stress ratios are reconsidered within this study. First, the controlling crack tip loading parameters are discussed. As already shown in former studies K (Kmax or ΔK) and R are not suitable to describe properly the crack tip loading for the negative part. It is proven that an alternative stress based parameter correlates with the crack tip loading for negative loads. Second, the deformation mechanisms at negative stress ratios are intensively discussed. Therefore a new method is proposed in order to visualize the plastic deformations before and behind the crack tip, even at the negative loading part. Phenomena of constant and variable amplitude loading are investigated.