Fatigue Crack Driving Force for Axial Surface Cracks in Pipes

Fatigue life assessment procedures require knowledge of the fatigue crack driving force, such as stress intensity factor range (ΔK) and cyclic J-integral (ΔJ), for the flaw geometry detected during inspection. Because three-dimensional closed-form crack driving force solutions are not available for typical flaws in pipelines, it is common practice to obtain these solutions from finite element analysis (FEA) or to adopt a closed-form crack driving force solution for the equivalent flawed plate and include a correction factor to take account of the pipe bulging effect. In the present study, pipes and plates with an axial rectangular crack with filleted corners under fatigue loading are simulated by FEA. The initial results show that the stress intensity factor range (ΔK) for a thin-walled pipe with a shallow crack (a/t < 0.5) is given reasonably well by the bulging factors given in BS 7910 combined with the stress intensity factor equation given by Newman and Raju for a plate with a semi-elliptical crack. However, the stress intensity factor is significantly over-estimated for a long and deep crack using this procedure. Different parameters for elastic-plastic fatigue are calculated and are proposed to be correlated with the rate of crack growth for thin-walled pipes with an axial rectangular crack with filleted corners. It is intended to use the results presented here in combination with full scale experimental fatigue data to obtain pipeline fatigue crack growth formulations, to accurately predict the rate of crack growth within a pipeline due to fluctuating internal pressure.