Each year, a few cracking of small bore piping systems occurs in French Nuclear Power Plant (NPP) due to vibratory fatigue. The particular design of such small bore piping system (basically composed of a socket, a small pipe and a valve with or without tubing) allows the crack initiation at two different locations of the socket weld toes: on the main pipe or on the small bore piping. The favored location depends on specific features, from geometrical and material characteristics to the weld quality.
The knowledge of the favored location with its related vibratory limit is an important issue in terms of inspection plans, plant availability and integrity of safeguard systems.
The main purpose of the present work is to provide analytical expressions for estimating the location of a potential failure by cracking fatigue and its initiation. This threshold is given by a vibratory limit expressed in terms of root mean square velocity. Inherent uncertainties (e.g. preload, weld quality…) impose to consider the proposed expressions as guidelines for the prediction of the crack location and initiation.