Prediction of creep crack initiation behavior considering constraint effects for cracked pipes

The creep crack initiation (CCI) location and time of axial surface cracks with different sizes in cracked pipes have been comparatively predicted by finite element calculations based on creep ductility exhaustion model and creep fracture mechanics considering constraint effects by using two new creep constraint parameters (R∗ and Ac). The effects of calculation methods of the parameters C∗, R∗ and Ac along the crack fronts on the prediction accuracy of CCI time have been analyzed. It has been found that with decreasing crack length, the CCI location changes from the deepest part to near surface part along the crack front, and the composite parameters composed of C∗ and constraint parameters can predict the CCI location. The CCI time of pipe cracks increases with decreasing crack depth and length due to the decrease of crack-tip constraint level. The excessive conservatism in conventional CCI life assessments using single parameter C∗ can be significantly reduced by using the two-parameter concept (C∗-R∗ and C∗-Ac) considering constraint effects. When the average values of C∗, R∗ and Ac along the crack fronts (instead of the values at the deepest locations of crack fronts) are calculated and used in CCI life predictions of cracked pipes, the prediction accuracy with respect to the finite element calculations based on creep ductility exhaustion model can be significantly improved. The C∗ values calculated by reference stress method can lead to extra conservatism in CCI life predictions.