Stress gradient effect on the crack nucleation process of a Ti–6Al–4V titanium alloy under fretting loading: Comparison between non-local fatigue approaches

• Crack initiation is studied using a combined experimental and numerical methodology.
• To predict the cracking risk, non-local multiaxial fatigue approach must be adopted.
• Square averaging method of the Crossland’s equivalent stress is used for predictions.
• Stress gradient effects on crack nucleation are studied on plain fretting tests.
• For a given pressure, the larger the contact radius, the higher the cracking risk.

This study focuses on the stress gradient effect regarding the crack nucleation of a cylinder/plane Ti–6Al–4V titanium alloy contact under low cycle fatigue (LCF) fretting loading. Several local and non-local analytical approaches were compared to predict experimental results. The first part of the study presents fretting nucleation boundaries for three different cylinder radii in the partial slip regime. In the next part, the Crossland and Papadopoulos multi-axial fatigue criteria are computed and compared. Finally, local and non-local fatigue approaches are compared. Square constant volume, critical distance and weighted function approaches have been compared.The methodology used covers a large range of stress gradients. The impact of varying the stress gradients is that the larger the stress gradient, the larger the difference between experiments and local stress fatigue predictions. A Crossland local form was applied to confirm that a local stress fatigue analysis cannot predict the fretting cracking risk. Three non-local approaches were carried out, and the results allowed the proper prediction of the empirical thresholds with a 3–5% margin of error. The positive results obtained helped to select a multi-axial fatigue criterion and a non-local approach which take into account the gradient effect of contact fretting behavior.