An Experimental Study on Fatigue Life Variance, Residual Stress Variance, and Their Correlation of Face-Turned and Ground Ti 6Al-4V Samples

It is well known that there is a large variance associated with fatigue life. However, in literature little is found on the relationship between manufacturing processes and fatigue variance of the manufactured components. In this research, the influence of machining processes on the fatigue variance of the machined Ti 6Al-4V samples is studied experimentally. The impact is evaluated by comparing the safety ratios of face-turned samples with those of ground samples. The safety ratio is defined as the average fatigue life over the fatigue life with a reliability of 95%. In the computation, it is assumed that fatigue life follows Weibull distribution. Two sets of faced samples and two sets of ground samples are tested for bending fatigue. The test is under constant amplitude in high cycle fatigue regime under room temperature. The results show that the safety ratios of face-turned samples are significantly smaller than are those of ground samples. Consequently, it is suggested that fatigue life variance be considered as a new process capability. This capability serves as a basis in choosing manufacturing processes for making fatigue critical products such as aircraft. Another finding is the positive correlation between the residual stress variation and fatigue life variation. This correlation suggests that a better understanding and prediction of residual stress lead to a better prediction of fatigue life.