A new magnetic structural algorithm based on virtual crack closure technique and magnetic flux leakage testing for circumferential symmetric double-crack propagation of X80 oil and gas pipeline weld

Based on the virtual crack closure technique for finite element numerical simulation, a new magnetic structural algorithm is proposed to analyze fracture signals from nondestructive testing. As an example of solving engineering problems, this algorithm is employed to investigate the circumferential symmetric double-crack propagation in an X80 oil and gas pipeline welding zone. The material property of the welding zone is treated as bilinear kinematic hardening elastic–plastic, and the fluid pressure load on the inner wall of the pipeline weld is dynamically applied. In our magnetic structural multi-physics field model, every time when the incremental crack propagation is completed, the mesh is reconstructed. As a result, the crack propagation and magnetic field is analyzed cyclically. Six characteristic quantities (P, \(G_{ I}\), \(L_\mathrm{g}\), CTOA, \({Bx}^p\), \({\hbox {Ms}}^\mathrm{p}\)) in the process of crack propagation are computed, forming the magnetic structural algorithm for circumferential symmetric double-crack propagation. The results show that using the algorithm can judge the damage location and damage degree of the pipeline weld by calculating the crack growth process. The algorithm has high sensitivity which can distinguish the double cracks whose circumferential spacing is greater than or equal to 0.05 times of the circumferential arc length of the weld. When the circumferential spacing of double cracks is less than or equal to 0.4 times of the circumferential arc length of the weld seam, the single crack grows faster than the double crack due to the interference effect of double cracks, and the existence of double cracks inhibits the crack growth. Through this practical example, it is proved that the implementation of the proposed algorithm can provide a theoretical basis for guiding the actual safety assessment of engineering materials and structures containing microcracks.