Analysis of Damage's Influences on Metal Magnetic Memory Signal

Metal Magnetic Memory (MMM) technique has been extensively used as a qualitative method to test the position of possible damages in ferromagnetic metal structures, a bottleneck in further development of MMM technique is the quantitative relationship between MMM signal and damage characteristic has not been fully revealed. In this paper, analysis of influences of stress and discontinuous structure on MMM signals is presented. Samples of diverse types of structural defects were made and then loaded with different series of stress. The resulting residual deformations and gradients of the magnetic field were measured. Analysis of the quantitative relationship between MMM signals and stress was conducted. The results show: 1) The magnetic field gradient above the structure-continuous zone increases with stress. Therein lies the potential to estimate the stress level point by point. 2) The second-order gradient of the magnetic field near the structure-discontinuous zone increases with the maximum strain in a similar way for different types of discontinuous structures, implying a possibility that the second-order gradient of the magnetic field may be conceive some information about the maximum stress at the edge of discontinuous structures independent of their geometry characteristics.