Towards prediction of the fatigue life of Ni microbeams under extreme stress gradients

This work investigated the effect of two extreme normalized stress gradients (η=17% vs. 36% μm−1) on the fully-reversed bending fatigue behavior (fatigue life and fatigue crack propagation curves) of 20-μm-thick, electroplated Ni microbeams, in humid air environments. The results highlight the significant challenge in predicting the bending fatigue life of microbeams subjected to extreme stress gradients, which was measured to be three order of magnitudes larger for η=36%μm−1 at a stress amplitude of ∼450 MPa. The fatigue life is dominated by the ultraslow growth of microstructurally small cracks, which is a strong function of the normalized stress gradient. For η=17%μm−1, the crack growth rates are initially about one order of magnitude larger than for η=36%μm−1 and, in contrast to the larger stress gradient microbeams, do not decrease with increasing crack size. This singular behavior results in low Basquin and Coffin-Manson exponents (in absolute value) compared to η= 0. As a result, the fatigue endurance limit increases from 35% to 50% of the tensile strength for η increasing from 17% to 36% μm−1, compared to 30% in the absence of stress gradients. The environmental effects are also discussed.