Prediction of creep crack initiation time in steel pipes with embedded spherical defects

Creep crack initiation time (t_i) of P92 pipes with embedded spherical defects was investigated by using finite element (FE) method. 108 models were built and they covered a wide variety of geometry factors such as inner radius to the wall thickness ratio (R_i/T), defect depth over the wall thickness ratio (a/T), ratio of the distance from the center of the defect to the inner surface of the pipe over the wall thickness (e/T) and loading factors (internal pressure (p)). A new parameter-Average Equivalent Stress (AES) was proposed to characterize the stress level and damage evolution throughout the whole creep process. The mathematic relationship between ti and AES and that between R_i/T, a/T, e/T and AES were obtained by nonlinear regression. Finally, the proposed prediction functions for creep crack initiation time was deduced from the above functions.