Assessment of a multiscale fatigue damage model associated with stress gradient effects

The aim of this research work is to develop a finite element numerical tool able to predict accurately the fatigue life of mechanical components. These components can have complex geometries, they can be submitted to a complex loading, leading to a specific stress field with possible stress concentration. Additionally, the successive cycles of loading are not necessarily identical. It is expected that the numerical tool can handle these demanding constraints.

In this respect, a multiaxial fatigue damage model was implemented in our home-made finite element code Lagamine. The finite element method permits to account for the actual geometry of the mechanical part and the loading for the stress computation in the whole structure. The formulation of the multiaxial fatigue model is able to capture:
- The non linear damage accumulation for multiblock and variable cyclic loading,
- The effect of the mean (hydrostatic) stress,
- The effect of the cycles below the fatigue limit if the damage was previously initiated.

Finally, the occurrence of stress concentration will significantly reduce the life time of the studied piece. However, it is well-known that the subsequent local degradation of the material will be partly compensated by an enhanced load carrying contribution of the surrounding material, favourably leading to a reduction of the crack propagation. The stress gradients computed with different techniques are incorporated in the model so as to account for such beneficial influence.

The physical roots of this model depart from the mesoscopic length scale, where the damage evolution is related to the mesoscopic accumulated plastic strain.
Therefore, the variables of the model are defined at both macroscopic and mesoscopic scales and a specific scale transition method was implemented, based on the well-known simplified Zarka method but used at the multiscale level.

The predictive capabilities of this multiscale multiaxial model are assessed by means of comparison with the classical Lemaitre-Chaboche model (implemented in the same FE code with stress gradient effects).

For both models, the material parameters were identified from SN tests on smooth specimens of Ti-6Al-4V alloy, while the predictions of the models are validated thanks to comparison with experimental tests on notched samples, with stress gradient effects.