Metal magnetic memory (MMM) testing (also known as micro-magnetic testing) is a new non-destructive electromagnetic testing method that can diagnose ferromagnetic materials at an early stage by measuring the MMM signal directly on the material surface. Previous experiments have shown that many factors affect MMM signals, in particular, the temperature, the elastoplastic state, and the complex environmental magnetic field. However, the fact that there have been only a few studies of either how these factors affect the signals or the physical coupling mechanisms among them seriously limits the industrial applications of MMM testing. In this paper, a nonlinear constitutive relation for a ferromagnetic material considering the influences of temperature and elastoplastic state is established under a weak magnetic field and is used to establish a nonlinear thermo-magneto-elastoplastic coupling model of MMM testing. Comparing with experimental data verifies that the proposed theoretical model can accurately describe the thermo-magneto-elastoplastic coupling influence on MMM signals. The proposed theoretical model can predict the MMM signals in a complex environment and so is expected to provide a theoretical basis for improving the degree of quantification in MMM testing.

In this paper, some progresses in numerical techniques mainly made in our research group for the forward and inverse simulation of electromagnetic nondestructive evaluation (ENDE) signals are introduced. For the first part, efficient forward analysis schemes for the simulation of eddy current testing (ECT), remote field ECT (RFECT) and magnetic flux leakage testing (MFLT) signals are described respectively, in addition to some numerical examples. Fast and accurate ECT signal simulation is realised by introducing a database type strategy using precalculated unflawed potential field data. To meet the high accuracy requirement of the simulation of RFECT signals, a hybrid scheme using 2D and 3D geometry and a new formula for pickup signal are proposed. To improve the efficiency of MFLT signal simulation, a fast scheme is developed based on a FEM–BEM hybrid code of polarisation method. In addition, a phenomenological method is also described in the first part, which is developed for the qualitative estimation of eddy current distribution and pickup signals. The second part of this review paper is on the reconstruction of defect from the detected ENDE signals (mainly ECT signals). Reconstruction schemes based on conjugate gradient (CG) method of deterministic category and NN method, metaheuristic methods of stochastic category are developed and sizing of both artificial and natural cracks are performed by using measured signals. It is clarified through applications that a deterministic optimisation method is more efficient for treating simple cracks, while a stochastic way is prefer for defects of complicated geometry such as a stress corrosion crack and multiple cracks. In the crack modelling and parameterisation, an element of discontinuous material property is introduced to treat crack of arbitrary shape based on a given regular mesh. Several numerical models are proposed for natural cracks, which makes the reconstruction of some natural cracks become possible.

Ferromagnetic materials are widely used in engineering structures. Damages caused during manufacture and the use of ferromagnetic materials may seriously undermine the safety of the engineering structure, and even lead to serious industrial accidents. For the early damage testing problem of ferromagnetic materials, it is imperative to propose and adopt new nondestructive testing methods. The metal magnetic memory (MMM) method, which is also known as the micromagnetic testing method, can achieve the early detection of damages in ferromagnetic materials such as steels. However, due to the lack of the quantitative research, the MMM method is only applicable for damage localization. In this paper, the MMM method is utilized to quantitatively evaluate the stress and crack in ferromagnetic materials. The inverse model including the objective function and optimization parameters is proposed. The reconstruction approach is established based on the conjugate gradient inversion method and the exact line search algorithm. In the theoretical analysis, the effectiveness of the MMM method in quantitative determination regarding the position and size of defects is verified with the experimental signal of a hole defect. Following this, the theoretical analyses of localization and sizing of stress and crack are conducted by using the simulation results as the measured signals. It can be found from the results that the MMM method can be applicable for quantitative evaluation of surface rectangular cracks, surface stress concentration zone, convex-type defect, and so on. In addition, the influences of the signal selection, sampling rate, lift-off value, stress level, and noise amplitude on the quantitative analysis of early stress concentration are discussed in details.