Uncertainty and risk associated with the Charpy impact energy of multi-run welds

Abstract A model for determining the uncertainty and risk associated with the Charpy impact energy of multi-run welds is developed. Using the model, it is demonstrated that a ductile-to-brittle transition region determined from fitting Charpy impact energy at several test temperatures is associated with large uncertainty. The uncertainty in the location of the transition region is strongly dependent on the number of test temperatures, the way the multi-run weld is sampled by the Charpy V-notch, the choice of the test temperatures and the variation of the impact energy at the test temperatures. With increasing the number of test temperatures, the uncertainty associated with the location of the transition region can be reduced significantly. A conservative model of the variation of the Charpy impact energy at a specified test temperature is also developed based on determining an upper bound of the variance of the Charpy impact energy. On this basis, a risk assessment model is developed for detecting impact toughness degradation with a high degree of confidence. A general equation is derived regarding the probability of failure from multiple impacts following a homogeneous Poisson process in a finite time interval. On its basis a method for setting risk-based reliability requirements is also proposed regarding the maximum acceptable number density of impacts in the time interval. The reliability requirements guarantee that if the number density of the impacts is within a specified envelope, the risk of impact failure in the time interval will remain below a maximum acceptable level.

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