Influence of inhomogeneous stress on biaxial 3D magnetic flux leakage signals

Magnetic memory method (MMM) is a newly developed passive nondestructive testing method, mainly used to detect the ferromagnetic materials. Because of the faster testing speed and the more convenient testing equipment, MMM has attracted many scholars' attention. This paper designed wide plate tensile specimens without and with a defect to study the influence of stress distributions on magnetic-flux-leakage (MFL) signals. The threedimensional (3D) MFL signals on the specimen surface along the directions parallel (x-axis) and vertical (y-axis) to the loading direction were measured. Finite element method (FEM) simulation and digital image correlation (DIC) testing were performed as comparison. Then a theoretical model was applied in finite element method (FEM) software to calculate the MFL signals. The experimental results are essentially in agreement with the calculated results. It is shown that the MFL signals on the measuring lines both parallel and vertical to the loading direction are capable of describing the load magnitude and stress inhomogeneity. Besides, the MFL gradient is in agreement with the equivalent stress distribution along the loading direction. The magnitude of MFL gradient vector developed in this paper shows a good correlation with the equivalent stress distribution vertical to the loading direction. Finally, five characterization parameters were extracted to evaluate the stress inhomogeneity.