Contact geometry effects on fretting fatigue crack initiation behaviour of Ti–6Al–4V

Fretting fatigue is complex phenomenon which depends on various factors. The present study focuses on the contact geometry dependence of fretting fatigue crack initiation behaviour by combining tests and their analysis. Various geometries were investigated which ranged from cylindrical shape of different radii to essentially flat including flat with rounded edges in Ti alloy Ti–6Al–4V. The measured fretting fatigue life data were compared using stress range, effective stress and a critical plane based parameter, modified shear stress range (MSSR) for the eight pad geometries where first two are the variations of the applied cycling stress on the substrate while the third one is the combination of normal and shear stresses on the critical plane in the contact region. Fatigue life relationships from all contact geometries as well as from the plain fatigue (i.e.without fretting) were within a scatter band as commonly seen in the fatigue tests. Further, MSSR predicted the crack initiation location and its orientation at the contact surface, which were in good agreement with their experimental counterparts. Fretting fatigue crack initiation behaviour is thus governed by a combination of shear and normal stresses on the critical plane in the tested material. A methodology to design against the fretting induced crack initiation damage can be thus developed from the approach of this study using the plain fatigue data only, especially in the high cycle fatigue regime which is primarily governed by the threshold considerations.