The influence of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires

Abstract

Dwell sensitivity in titanium alloys is generally attributed to the phenomenon of load shedding, which is a time dependent redistribution of stress from weak grains, with their c-axis perpendicular to the loading direction, to strong grains, with their c-axis approximately parallel to the loading direction. This leads to the formation of internal quasi-cleavage facets on the basal planes of strong grains. In this paper, the effect of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires is investigated, because these wires do not contain strong α grains. It has been found that this leads to a different dwell fatigue behaviour compared to what has been described in literature. In the case of drawn wires, introducing load holds promoted crack initiation at the surface, through the formation of a facet on a prismatic plane of a surface grain that was oriented for very easy prismatic slip. This was confirmed by sectioning facets, using focused ion beam milling, and electron backscatter diffraction measurements. Because of the specific crystallographic texture of drawn wires, the phenomenon of load shedding is less pronounced, and there is no formation of internal facets. The amount of cycles to failure was reduced by two to three orders of magnitude compared to fatigue tests without load holds. The time to failure remained similar, and was even higher for some dwell fatigue tests. There was a significant amount of strain accumulation during dwell fatigue tests. The maximum strain increased more rapidly in tests with 30s load holds compared to tests with 120s load holds, due to creep recovery during the periods in between load holds. During these periods, the strain decreased, even though a small tensile load was still applied.