The scatter of surface residual stresses produced by face-turning and grinding

Conventional studies on residual stresses induced by manufacturing processe s have focused on the average residual stress value over the processed surf ace area. However, what dictates the fatigue life of a manufactured surface is its weakest point. Thus, it is not the average value of the stress but the local extreme that is most relevant for safety considerations. Therefor e, it is very important to study the variations of residual stresses over t he machined surface. This paper is the continuation of the work (1) investi gating the magnitude of surface residual stress scatter between the face-tu rned and ground samples. The objective of this research is to test the hypo theses that the scatter of surface residual stresses over the faced samples is smaller than that of the ground ones and that the scatter of surface re sidual stresses varies significantly among ground samples while it does not vary in a statistically significant sense among faced specimens for the gi ven cutting conditions. In order to compare the surface residual stress var iations, two sets of the specimens of Ti 6Al-4V bar are ground while the ot her two sets faced. The residual stresses over a small surface area (5mm X 8mm) are measured at four locations of each machined sample using an X-ray diffraction technique. Statistical analysis of the measured residual stress es shows that the proposed hypotheses hold. Experimental data also show tha t a different number of grinding passes may induce a different scatter of m icrohardness. The possible causes and ramifications of the foregoing result s are discussed. It is suggested that the variations of residual stress be included as a surface integrity parameter, joining its average value.