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Due to the lack of the efficient and accurate detection method, the axial crack of pipeline seriously threatens the safe operation of oil pipeline, and therefore the application of micromagnetic nondestructive testing technology in the field of pipeline axial crack detection is of practical significance. To study the characterization of axial crack by micromagnetic signal, a numerical model of micromagnetic signal to detect the axial crack of pipeline is established based on the micromagnetic theory in this article. The characteristics of micromagnetic detection signals of cracks with different sizes and directions are calculated. The propagation law of micromagnetic detection signal is analyzed, and the experimental study of X70 pipeline is carried out to verify the correctness of theoretical model and simulation. The results show that at each tip of the axial crack, the axial component of the micromagnetic detection signal has a peak and a valley, and the radial component has an extreme value. The amplitude of the micromagnetic internal detection signal at the axial crack linearly increases as the depth or length of the crack increases. The propagation law of micromagnetic signal conforms to the falling exponential function, specifically, the smaller the crack depth, the faster the signal decays and eventually stabilizes. The micromagnetic detection signal of the probes to detect the crack having a certain angle with the axial direction is arranged with equal displacement spacing.

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Magnetic memory stress detection technology has broad application prospects in the field of stress detection in long‐distance oil and gas pipelines because it supports non‐contact and dynamic on‐line detection. However, the magnetic memory signal is weak and susceptible to an external magnetic field. A J‐A model is established to quantitatively study the distribution characteristics of the magnetic memory signal in the stress concentration area of the pipeline. In this study, the mathematical model of magnetomechanics of the pipeline is established. The variation law of magnetic memory signal in the stress concentration area with the external magnetic field is calculated and analysed. A systematic experimental study is carried out. The results show that the magnetic memory signal linearly changes with stress in the geomagnetic field, and magnetic memory signal characteristics of circumferential stress with internal pressure is larger than the axial one. The amplitude of the magnetic memory signal in the stress concentration area increases with the increase in magnetic field intensity of the weak magnetic field, meantime the magnetic sensitivity decreases. The magnetic memory signal in the stress concentration area of the pipeline in magnetic saturation is covered by the strong magnetic field resulting in an undetected stress concentration area.