Propagation of stress corrosion cracks in steam generator tubes

Abstract

A model suitable to describe the propagation of stress corrosion cracks in steam generator tubes made of Inconel-600 is proposed in this paper. It concentrates on axial cracks located in the tube expansion transition zones which are assumed to be through-wall. The residual stress field is therefore considered as the major contributing factor driving short cracks while operational stresses dominate the growth of longer cracks. An estimate of residual hoop stresses is obtained using a non-linear finite element simulation of the tube to tube-sheet rolling process. Scatter of the residual stresses due to the stochastic variations of the dominant influencing parameters was studied. The crack propagation model utilizes linear-elastic fracture mechanics theory. In particular, both crack tips are modelled to propagate with different velocities due to the highly asymmetric stress field. Provisions are also made to account for the reactor coolant temperature and chemical composition effects. The model performance is demonstrated by a numerical example considering the crack propagation data from D4 steam generators during a 15 month operational cycle as recorded by subsequent non-destructive tube examinations.