Theoretical and numerical analysis of the creep crack initiation time considering the constraint effects for pressurized pipelines with axial surface cracks

A theoretical approach and numerical simulation were conducted to investigate the creep crack initiation (CCI) time and the effect of constraints induced by the geometrical sizes of pipelines with axial surface cracks. The theoretical enhancement model of the C*-Q* approach under the transient creep condition, which considers the load-independent constraint parameter Q*, was proposed to predict the CCI time around the crack front. The results revealed that the distribution regulation of Q* along the crack front for circumferential internal surface cracks and external surface cracks was similar. The maximum constraint level occurred near the deepest crack front part for cracks with small a/c (a/c <0.4), while it occurred near the free surface for cracks with large a/c (a/c > 0.4). The constraint values at the same position (20/z) increased with the increasing of the crack depth when a/c kept constant. In addition, the axial internal surface cracks of pipelines were proved more dangerous than the external surface cracks with the same geometrical size. Furthermore, the CCI times were decided by the peak values of constraint, or the CCI firstly occurred at the position where the constraint level was maximum. Additionally, the variation of hydrostatic stresses and triaxiality considering the constraint was discussed. The suitability of the analytical C*-Q* approach was verified to predict CCI. The comparison of CCI times between the analytical approach and the BS 7910 as well as the FE results demonstrated that the solutions under stress intensity factor Riedel Rice (K-RR) control (initially by K, then by transient creep stress or Riedel Rice conditions) were more accurate when internal pressure P < 15 MPa, but the solutions under Hutchinson-Rice-Rosengren Riedel-Rice (HRR-RR) control (initially by plastic HRR control, then by RR conditions) were more appropriate when P>15MPa.