Effect of stress relief groove on fretting fatigue strength and index for the selection of optimal groove shape

The applicability of small stress relief groove for the improvement of fretting fatigue strength was studied. Fretting fatigue tests were done using several kinds of grooved specimens. The shape of groove was systematically changed with parameters of groove radius R and tangential angle θ. The improvement of fatigue limit by a stress relief groove depended on both R and θ. The fretting fatigue limit with stress relief groove was increased with the increase of R and θ. The parameter θd (d: groove depth) was selected for the unified evaluation of the improvement. FEM stress analyses were done to investigate the stress condition. In a simple elastic FEM analysis assuming that the contact edge is ideally shaped, a highly compressive stress field was generated near the contact edge, where small cracks could never propagate. This suggested that such a simple analysis was not enough to solve this problem. Thus, an assumption to relieve the highly compressive contact pressure near the contact edge was introduced to explain the experimental fact that a crack could propagate. The profile change was simulated by the local plastic deformation at the contact edge calculated by elasto-plastic FEM deformation analysis. This deformation reduced the highly compressive contact pressure and enabled the crack propagation. As a result, it was found that fretting fatigue limit of grooved specimen could be evaluated on the basis of the maximum axial stress near the contact edge. The estimation of fretting fatigue limit using a relationship between Kt/Kt0 and θd provided a good estimation with the experimental results and it would be a useful method to select the optimal groove shape.