Objective Metal Magnetic Memory (MMM) stress detection technology, facilitating non-contact and on-line detection, has demonstrated significant potential for application in stress damage detection on ferromagnetic materials, particularly steel pipelines used for oil and gas transmission. However, existing research lacks a comprehensive analysis of the magneto-mechanical relationship. Therefore, this study seeks to establish a comprehensive magneto-mechanical relationship system and clarify the correspondence between MMM detection signals and stress concentration during both the elastic and plastic stages.

Methods In this study, numerical models were developed to represent the relationship between magnetization and strain in pipelines, using the J-A theory as the basis. Computational analysis was conducted to investigate the characteristics of MMM detection signals at various stages of pipeline deformation and to examine the impact of shape parameters of hysteresis-free magnetization curves and pinning coefficients on MMM signals under stress influence. To verify the obtained relationship between strain and magnetic signals, a full-scale hydrostatic experiment was conducted.

Results The experimental results revealed a gradual increase in strain with the applied stress in the pipeline. During the elastic deformation stage, the magnetization curve exhibited its first reversal near the yield point, and this reversal occurred earlier with higher shape parameters of hysteresis-free magnetization curves. In the plastic deformation stage, the presence of dislocations caused alternations in the magnetic characteristics of the pipeline material, leading to the second reversal on the magnetization curve near the yield point, which was significantly influenced by pinning coefficients. Specifically, higher pinning coefficients resulted in delayed reversals. The strain values corresponding to the reversals decreased quadratically, and the two reversals occurred in opposite directions.

Conclusion By incorporating phase contrast analysis between MMM signal reversals and calibration results obtained from the same material as the pipelines, this model, which utilizes the aforementioned characteristics, has been proven effective in identifying plastic strain occurrence in stress concentration zones of steel oil and gas pipelines and offers a practical solution for rapid qualitative detection of stress concentration in pipelines.