Extreme stress gradient effects on the fatigue behavior of Ni notched microbeams

Abstract

The fully-reversed bending fatigue behavior of 20-μm-thick electroplated Ni notched microbeams actuated at resonance (∼8 kHz) was characterized in humid air environments, in an effort to investigate the effects of extreme stress gradients (normalized stress gradients of 36%/μm over the first 2 μm at the notch root) in small-scale fatigue. Compared to our previous study, larger driving forces (stress amplitudes up to 510 MPa (60% of the ultimate tensile strength), corresponding to plastic strain amplitudes up to ∼0.1%) were applied, leading to the propagation of microstructurally small cracks ahead of the notch. The endurance limit reaches 50% of the ultimate tensile strength, and the Basquin and Coffin–Manson fits for the stress and strain-life curves, respectively, exhibit much lower (in absolute value) fatigue exponents that typical values for bulk metals. This singular behavior is explained by the ultraslow and decelerating crack growth rates, calculated based on the measured resonance frequency evolution during the fatigue tests, which appear to characterize the growth of microstructurally small cracks under extreme stress gradients. This study highlights the need to further characterize the effects of different stress gradients values on crack growth rates and fatigue lives in order to accurately predict the small-scale fatigue damage in metallic microbeams.