Magnetic Signal Characteristics and Damage Assessment of Pipeline Fatigue Cracks Based on the Weak Magnetic Field Method

The classical magnetic charge model overlooks the influence of stresses at the crack tip along the wall thickness direction, which compromises its accuracy in fatigue crack detection. To address this, we incorporate the phenomenon of stress accumulation at the crack tip into a redistribution model of magnetic charges on the defect sidewall, thereby establishing an enhanced magneto-mechanical model for fatigue crack analysis. This model is used to investigate the characteristics of weak magnetic signals both at the instant of crack propagation and throughout the crack evolution process under cyclic loading. Furthermore, a fatigue damage factor D is proposed by synthesizing the stress ratio, loading frequency, crack size, inflection features of the tangential magnetic component, and the evolution rate of the normal component, B_y(N). This factor serves to characterize both the degree and stage of fatigue damage. Results show that the improved model, which accounts for stress effects, increases the peak values of the normal and tangential magnetic components by 56.9% and 90.7%, respectively, and more accurately captures the variation in weak magnetic signals before and after crack extension under cyclic loading. The tangential component exhibits a double-inflection-point feature at crack initiation and when the crack extends to 50% of the wall thickness, whereas the normal component increases monotonically with the number of loading cycles. The proposed fatigue damage factor D can effectively identify the damage stage in pipelines: 0<D<0.006 indicates no crack propagation; 0.006<D<0.250 corresponds to a crack depth within 50% of the wall thickness; D>0.250 signifies a crack depth ranging from 50% of the wall thickness to the point of fatigue failure.