Evaluation of residual stress during fatigue test in a FSW joint

At present, friction stir welding (FSW) represents one of the most interesting techniques in the field of welding. The process is has been implemented in industrial practice for joining aluminium alloys, while the welding of the titanium alloy and the steels is still primarily in a developmental stage. The thermo-mechanical action of the tool causes a residual stress field in the FSW joint. Although, the peak temperatures of the process are relatively low; residual stresses similar to the traditional welding technique may develop in the FSW joint. Moreover, the restraints of clamps, which are used for fixing the plates during the process, impede the relief of the heated zones that causes longitudinal and transversal residual stresses. The fatigue crack growth behaviour in FSW joints was investigated by several authors [1-4] and most of them [1, 3-4] have shown through experimental tests that the residual stresses play a predominant role on the fatigue crack growth rate as compared to the effects of microstructure and hardness changes. Therefore, an accurate method has to be adopted for measurement of the residual stress field in order to evaluate the influence of the same residual stresses on the fatigue crack growth behaviour of FSW joints. Several experimental techniques can be used to evaluate the residual stress field developing in FS welded joints. The non-destructive techniques of X-rays and neutrons have enabled the determination of the longitudinal and transverse residual stress profiles The destructive cut compliance method described by Prime [5] is widely used to provide the through-thickness residual stress intensity factor of a supposed crack front. Subsequently, the residual stress profile can be analytically extrapolated by the knowledge of the residual stress intensity factor, Kres. Similarly, the hole drilling method provides the residual stress distribution by means of incremental hole-drilling steps and numerical routines. Nevertheless, both these techniques require the knowledge of the influence functions which depend from the cut depth, the strain gage location and the geometry of the investigated body. Moreover, it has been recently illustrated that the innovative techniques of the contour method described by Prime et al. [6] and the on-line crackcompliance method [7] combined with the adjusted compliance ratio [8,9], ACR, can be used to measure the residual stress field in FSW joints. In this paper, an application of the ACR method and of the on-line crack-compliance technique is shown for a FSW joint in titanium alloy. The on-line crack-compliance is used to obtain the residual stress intensity factor in real time from a fatigue crack growth test. Thus, the postprocessing ACR methodology is adopted to separate the closure and residual stress effects from the fatigue crack growth rate data. The on-line crack-compliance method is similar to the cut compliance method. This technique uses the load-displacement curve directly provided during the fatigue test, so that a specific test for the determination of the residual stresses is avoided. Moreover, the practical application of this methodology does not require the knowledge of the influence functions; only a sufficient signal stability and linearity of displacement need to compute the residual stress intensity factor. At the same way, on the basis of ratio of displacement, the ACR