Prediction of cracking in Ti–6Al–4V alloy under fretting-wear: use of the SWT criterion

Abstract

Fretting occurs when two contacting surfaces are submitted to small amplitude displacements (few micrometers to few hundreds of micrometers). Depending on the normal stresses and the displacement amplitudes, two types of damage may occur: formation and ejection of debris or crack nucleation and propagation. In this study, we focus on the nucleation of cracks in Ti–6Al–4V alloy under fretting-wear loading (i.e. without bulk fatigue loading in the specimen). Our goal is to predict crack nucleation knowing the normal and tangential stresses and the materials properties. Cylinder on flat fretting-wear tests are performed with three different normal forces, two different cycles’ numbers and, for each cases, few values of the displacement amplitudes. All cylindrical and flat specimens are made of alpha–beta Ti–6Al–4V alloy. The effect of shot peening has also been investigated and, in this case, the two contacting specimens were shot peened. In all cases, the cracks in the flat specimen were analysed in polished cross sections. Cracks appear in the partial slip and the mixed slip regimes and these tests allow us to determine the minimal tangential force leading to crack nucleation for the given normal force and number of cycles. In order to predict the crack nucleation, the Smith–Watson–Topper criterion is used. This critical plane criterion is based on the calculations of the maximal normal stress and the maximal strain amplitude in the considered plane. The predicted location of the crack nucleation in the fretting scar coincides with the one observed in the fretting tests. To take into account the size effect in fretting, stresses are averaged on an elementary volume, which corresponds to the volume of an alpha grain. Shot peening is considered in the SWT criterion through an additional compressive stress, which reflects the compressive stress state induced by shot peening in the surface.