Fatigue Notch Sensitivity of High Entropy Alloy CoCrFeNiMn Tested Under Ultrasonic Loading Conditions

Abstract

The fatigue notch sensitivity of miniaturized samples of the Cantor alloy CoCrFeNiMn has been investigated. Notched specimens with lengths from 5 to 10 mm were tested in high-cycle fatigue at an ultrasonic frequency of 20 kHz. Thereby, the notch radius varied in the range from 0.125 to 2 mm. The remaining sample width at the height of the notches was between 0.3 and 0.4 mm. During testing, the specimens were fixed on a vibrating Ti6Al4V sample holder of 125 mm length with a hole of 2 mm diameter in the middle. The samples were aligned and glued across that hole to increase the stress level acting on the samples. In a first attempt, the influence of the notches on the stress concentrations in the samples was determined by conventional Finite Element Analysis. It was found that despite stress concentrations, samples with a sharp notch showed remarkably long fatigue life. An interpretation of this effect is elaborated in the frame of Mindlin’s strain gradient theory. For this purpose, the constitutive equations of the model were implemented in the software ABAQUS through the user subroutine UEL. The simulations show that strain gradient elasticity reduces the stress concentrations at the notch. Accordingly, the expected fatigue life of the notched specimens increases in comparison to predictions of conventional continuum mechanics.