Fretting fatigue of deep rolled shrink fitted samples

The shrink fitted shaft to hub connection, loaded under bending, experiences Fretting Fatigue at the end of the contact where both stress concentration and microslip are acting. Shrink fitted testing is not standard and common as the bridge type test, however, the round geometry implies no edge effect, the shrinkage and the bending loads can be carefully controlled with strain gages, and the stress distribution easily calculated with finite element analyses. Though the shrink fit compressive state, the cyclic bending introduces tensile stresses, hence the introduction of compressive residual stresses can play a significant role to prevent the initiation and the propagation of the fretting fatigue cracks. Experimental results are reported in the paper where the fretting fatigue strength was remarkably improved after a deep rolling treatment. Numerical analyses even predicted the complete mode I closure of potential fretting fatigue cracks by taking into account the compressive residual stresses. SEM investigation showed how the crack pattern was significantly different with deep rolling, having multiple crack initiation sites and then either coalescence or crack branching. From the modelling point of view, the paper shows that the self-arrest theory cannot be used to assess the fretting fatigue strength, for deep rolled specimens, where the crack remains closed. A better prediction can be obtained with a critical plane multiaxial fatigue criterion, based on the shear stress amplitude, which under a compressive stress state can be more effectively assumed as the driving force of the crack. Consistently the orientation of the plane experiencing the maximum shear stress amplitude was found in agreement with the observed initial crack direction.