PREDICTION OF FAILURE DUE TO CRACK COALESCENCE IN STEAMGENERATOR TUBES

摘要

This paper investigates the failure mode of steam generator tubes under internal pressure in the presence of differentcracks and ligament (between cracks) configurations. Leak and rupture for single crack configuration in tubes arewell documented in the literature. However, when the cracks interact, the tube pressure under which the ligamentbetween cracks fails would be different than the critical burst pressure for a tube with an individual equivalent crack.This failure pressure is termed the "coalescence pressure". The burst pressure is the pressure at which the cracks getruptured. The ligament failure is caused by local instability of the axial or circumferential ligaments. Thecoalescence pressure increases as the ligament length increases.In this study, the fracture mechanics approach is used to investigate crack coalescence. The stress intensity factoralong the crack front is used to determine the coalescence (failure of the ligament between cracks) pressure. Thefracture mechanics parameters (J-integral and stress Intensity factor, KI) are determined using finite elementsimulation by introducing Elastic and Elastic-Plastic Line spring elements along the crack. The fracture parametersare found for different pressures and different ligament lengths and configurations. This is an inexpensive way todetermine J and consequently KI. These parameters are compared with the material toughness properties JIC IC and KICto asses failure and find the coalescence pressure. The results are presented for part-through wall (crack depth of90% of tube thickness) under different pressure loadings. These results are compared with the coalescence pressurevalues presented in the literature using strength-based failure approach.