Research and Development
Geometry variation and life prediction of biaxial/multiaxial fatigue components

This project's inspiration is to increase the understanding of fatigue life dependency of service components on geometrical variation. In particular, the project has used laboratory specimens data and numerical simulation to investigate the contribution of the subsurface stress or strain conditions to the fatigue life. This involved the development and the testing of biaxialy loaded fatigue specimens employing several geometries and thickness variations. The experimental results were correlated using detailed numerical simulation and the outcome was used to investigate the contribution of the subsurface strain conditions to the fatigue life and included further development in subsurface damage analysis. A simple life prediction procedure has been developed that aspires to be independent of both multiaxial stress or strain states, and variation in the component's geometry.

Subsurface Strain Model
1) Multiaxial strains are calculated through a subsurface finite element strain path.
2) These values are used to calculate the lives at each point using life-strain relation.
3) The contribution to the total fatigue damage under the surface depends on its distance from surface and strain gradient.
4) Contributions of each subsurface point is weighted and summed to estimate the total life along that path.

Test fixtures simply support a rhombic plate specimen at its corners by the use of four rollers that are placed and free to rotate inside holders. The applied uniaxial cyclic reversed load is converted into rotation of the rollers that, in turn, apply an anticlastic bending on the specimen corners. Biaxial fatigue cracks initiate from a centre slot.

FEA analysis
To investigate the stress and strain distribution near the crack, a fairly complex mesh model was used that consists of nodes arranged in such an order that enabled results to be obtained for several paths into the thickness and at different planes.