Application of finite element method to effect of weld overlay residual stress on probability of piping failure

Abstract

This investigation probabilistically assesses the fracture mechanics of piping welds in a nuclear power plant considering the effects of welding residual stresses. To assess the structural integrity of the welds objectively, the computer code for probabilistically analyzing the fracture mechanics of nuclear piping is utilized to estimate the leak probabilities of the welds. Aging-related degradation and the rates of growth of the cracks in the welds that are subjected to fatigue and stress corrosion cracking are considered in the Monte Carlo Simulations. To confirm the improvement in reduction of stress state that is achieved by overlay welding, a series of residual stress distributions in the welds were determined using the finite element method. The geometries, material properties and operating loads of the piping system are based on those in the stress report of the piping system. The presented results demonstrate that the leak probabilities increase with operating loads, and are significantly related to welding residual stresses. Imposing compressive stresses at the inner surface of the original pipe, a weld overlay efficiently inhibits crack propagation during subsequent operation. Furthermore, the leak probability distributions are important in evaluating the structural safety and operational stability of a piping system.