Probabilistic Predictions for Piping Fatigue Including the Effects of Ultrasonic Examinations

This paper presents a methodology for estimating failure probabilities of piping welds that experience cyclic stresses and that are subject to ultrasonic examinations designed to detect growing fatigue cracks. Fatigue cracks can start as either preexisting fabrication flaws or as cracks initiated after an accumulation of stress cycles. Low levels of cyclic stresses and/or small numbers of cycles produce low failure probabilities, with the failures caused mainly by fabrication flaws. More severe cyclic stress conditions produce higher failure probabilities, with the failures caused mainly by fatigue cracks that initiate during the life of the component. Numerical results are presented to address both crack initiation and crack growth. The calculations cover both stainless and ferritic steels, inservice inspections with different inspection intervals, and stress states with and without high levels of through-wall stress gradients. It is shown that effective inspection programs can significantly reduce failure probabilities, and that such programs require suitable NDE sensitivities and adequate inspection frequencies.