Investigation Of SMFL Signals Of Ferromagnets Under Different Deformation Stages

Ferromagnetic materials are consisted of numerous magnetic domains. A residual or applied stress can change the macro-magnetic properties of ferromagnetic materials due to the magnetic moments rotation and domain wall displacement under the mechanical action. Such a magnetic/mechanical coupling effect is the physical basis for developing various magnetic non-destructive techniques in assessing mechanical properties of ferromagnetic structures. Traditional magnetic NDT techniques are effective in diagnosing developed defects, but providing little information about the mechanical degradation and early damage of the tested objects.Metal magnetic memory (MMM) method is a novel, passive magnetic method in inspecting the mechanical degradation of ferromagnetic components. Such technique owns several advantages over other developed magnetic tests. For example, it is a passive test method and the self-magnetic-flux-leakage (SMFL) signal of objects is measured. The Earth's magnetic field instead of an applied strong field is received as the stimulus source. In addition, it is regarded to be effective in assessing the early damage as well as developed defects. However, as a new test method, systematic experimental data and theoretical analyses are lacked. Until now only two qualitative criteria were provided. MMM test can only determine the possible dangerous regions. The defect characteristics must be further justified by other measurements. This paper aims to present an experimental and numerical research on the characteristics of magnetic flux leakage (MFL) signals near the defect zone. The details include the follows.Experimentally, the normal spontaneous stray field component and its gradient of Q235-steel specimens were measured during the uniaxial tensile and compressive loading processes. The results show that the normal spontaneous stray field component and its gradient are effective in capturing different deformation stages under tension, but no detectable change can be found during the whole compressively loading process. Compared with the amplitude of the normal spontaneous stray field component, the gradient is a more sensitive parameter. Additionally, the results exhibit that it is easy to differentiate the macro-crack and plastic deformation because the measured spontaneous stray field signals are obviously different.Numerically, ANSYS FE simulation was performed on the MFL distribution in the defect zone. Affecting factors were analyzed systematically, which includes deformation stages (elastic, plastic and fracture) and Geometry characteristics (width, thickness and location of the defect) and testing condition (lift-off value of the senor) et al. Numerical simulation results can effectively explain testing data.The results have much significance in promoting the development and applications of MMM technique.