X-ray Fractography on Fatigue Fractured Surface of Solution and Aging Treated Ti-6Al-4V Alloy.

Abstract

Fatigue tests were carried out under two kinds of stress ratios in constant amplitude loading of Ti-6Al-4V alloy after heat treatment of solution at 1223K and aging at 813 K (STA). Retardation behavior of fatigue crack propagation was observed. It is considered that this retardation is caused a stress relieving at front of crack tip by growth of secondary cracks. X-ray fractography, which is a technique to estimate the stress conditions and to be used for analysing a mechanisms when a fracture of structures occurs, is applied on fractured surface. Distributions of residual stress and halfvalue breadth by X-ray technique on the fractured surface along the crack propagation direction as a function of a stress intensity factor. It was found that the residual stress on the fatigue fractured surface had a peak value when maximum stress intensity factor was nearly equal to 25MPa√(m). Strain induced transformation was observed on the fatigue fractured surface from X-ray diffraction pattern. The half-value breadth on fractured surfaces increased with increasing maximum stress intensity factors. Depth of monotonous plastic zone size was estimated from a change of the distribution of the residual stress in direction of depth near fractured surface. Depth of monotonous plastic zone size ωy was expressed by following equation ; ωy=a(Kmax/σy)2, where the value of a was 0.17. By using X-ray fractography, maximum stress intensity factor Kmax in fatigue crack propagation of Ti-6Al-4V alloy can be predicted.