Optimisation of an ultrasonic torsion fatigue system for high strength materials

Abstract

The bearable shear stress amplitude in very high cycle fatigue regime (VHCF) is of interest for many applications in mechanics (springs, crankshafts, ball bearings, etc.) when a very high repetition of load cycles occur. This paper proposes to optimise an existing torsion gigacycle fatigue testing system, developed fifteen years ago by the Pr C. Bathias’ research team. The aim is to increase the shear stress amplitude applied to the investigated specimen and, consequently, meet the expectations from industries about fatigue life for high strength materials under shear loading. The system differs from other ultrasonic torsion fatigue machines that can be found in the literature, by the combination of two amplification horns, transforming a translation movement into a rotational one. In this work, there are two optimisation objective functions: (i) maximize the shear stress level in the specimen and (ii) minimize the stress level in the fatigue system and mainly in the pin connecting the two horns. The development of the optimised device is essentially carried out by a parametric study and numerical simulations through modal and harmonic analysis. Numerical results are compared with the analytical 1D solution and with experimental results obtained from the new real experimental set-up. Finally, new results in the gigacycle domain are presented concerning the torsion fatigue strength of high strength steels (50CrV4 and 16MnCr5) and discussed.