Effects of magneto-mechanical coupling on enhanced magnetic memory signals of defective coiled tubing

To investigate the evolution characteristics of enhanced magnetic memory signals in defective coiled tubing under magneto-mechanical coupling, CT130 coiled tubing specimens containing crack, pit, and perforation defects were tested under different stress states. The enhanced magnetic memory signals and corresponding characteristic parameters in defect regions were analyzed. The results show that the enhanced magnetic memory signals can effectively reflect the internal stress and damage evolution of defective coiled tubing. With the increase in crack length and depth, the characteristic parameters of magnetic signals exhibit an approximately linear increasing trend. When the crack depth increased from 0.5 mm to 4 mm, the tangential and normal characteristic parameters increased by 494.9% and 233.6%, respectively. When the crack length increased from 1 mm to 8 mm, the corresponding increases reached 263.7% and 161.9%, indicating that crack depth has a more significant influence on local magnetic field anomalies. In addition, the characteristic parameters exhibited distinct staged evolution behavior with increasing stress amplitude and reached local peak values near the elastic-plastic transition stage at 800 MPa. The characteristic parameters under unloading conditions were generally higher than those under online loading conditions, demonstrating that enhanced magnetic memory signals possess obvious historical memory effects. The results provide a reference for defect evaluation and stress state assessment of coiled tubing.