Quantitative Research On Stress And Defect Based On Magnetic Memory Method

In the process of the production,welding and assembly,there are always defects(e.g.holes and cracks)in the ferromagnetic materials,where stress concentration is common in service.The stress concentration will cause the decrease of ferro-magnetic materials' mechanical properties and may result in some catastrophe failure,which is a threat for people's life and the economic development.The Metal Magnetic Memory(MMM)method has been developed as a new passive nondestructive testing(NDT)technique.It can be applied to detect the stress states(e.g.elastic stress,residual stress and stress concentration)and the damage states(e.g.stiffness degradation,holes and cracks).And MMM testing process is relatively fast and convenient.H_owever,the quantitative interpretation for MMM has not been studied thoroughly due to the lack of a simple and effective magneto-elastic model.In this thesis,a new hysteretic magnetoelastic coupling model based on the micro-statistical method is proposed to interpret the physical mechanism of MMM.This model is relatively intuitive with a clear physical meaning and a wide range of application.And it requires few material parameters for calculation,thus it is easy to use.Based on the proposed model,the MFL signals under homogeneous stress and stress concentration with hole and cracks are studied experimentally and theoretically,and the characterizations of MFL signals are obtained.The comparison between the experimental and theoretical results indicates that the proposed model is effective to describe the MMM phenomenon.With the experimental data as the objective function,the Gene Algorithm(GA)and Finite Element Method(FEM)based on the proposed hysteretic magnetoelastic coupling model are used to inverse and reconstruct the size and position of the crack.Detailed contents and main conclusions are as follows:(1)Based on the linear magnetoelastic energy,the Boltzmann distribution is improved to construct the anhysteretic micro-statistical magnetization and magnetostriction models for ideal ferromagnetic cubic crystal materials.The models can describe the nonlinear behavior of anhysteretic magnetization and magnetostriction with clear physical meaning.The solution of interaction coefficients between magnetization and deformation is simplified by using the micro-statistical method.Based on a new basic function with exponential function,the magnetoelastic energy is expressed by a Taylor's series expansion of strain with the form of the magnetocrystalline anisotropy energy.The interaction coefficients are solved by using the above simplified method.And the nonlinear magnetoelastic energy is obtained.Combining the nonlinear magnetoelastic energy,the anhysteretic micro-statistical magnetization model and the improved hysteretic model,the hysteretic magnetoelastic coupling model under the external magnetic field and stress is constructed finally.This model can describe the nonlinear behavior of hysteretic magnetization accurately for imperfect ferromagnetic cubic crystal material under stress and magnetic fields.(2)Based on the proposed hysteretic magnetoelastic coupling model,the biaxial 3D MFL signals for wide plat specimen with and without a defect are studied experimentally and theoretically.The coincidence between calculated and measured results indicates that the proposed model can describe the MFL signals under homogeneous stresses and stress concentration.Besides,the following conclusions for MFL signals under homogenous stresses are obtained: The tangential MFL signals along the specimen surface and the loading direction(H_x)almost keep constant values along and perpendicular to the loading direction,and its value increases with the load increasing.The normal MFL signals(H_z)show an approximately linear variation along the loading direction,and almost keep constant values perpendicular to the loading direction.The remaining MFL signals are not sensitive to the stress.Thus there are three characterization parameters that can evaluate the homogeneous stress: the gradient of the normal component H_z,and the amplitude of the tangential componenet H_x along and perpendicular to the loading direction.The characteristics of MFL signals under stress concentration are also obtained: H_x exhibits a peak variation along and perpendicular to the loading direction and the peak position is at the center of the defect.Besides,the amplitude of the peak increases with the load increasing.H_z exhibits a peal-to-peak variation along the loading direction and a peak variation perpendicular to the loading direction.Therefore,there are five characterization parameters that can evaluate the stress concentration: the peal-to-peak value of H_z,the peak value of H_x along and perpendicular to the loading direction and the peak value of H_z on the two sides of the defect perpendicular to the loading direction.Among them,the peal-to-peak value of H_z is the best one to evaluating the stress concentration.(3)The relationship between the effective stresses and MFL signals are discussed.It is indicated that the evaluation parameter proposed in the literature can describe the effective stress distribution along the loading direction.Besides,a new evaluation parameter is proposed,which can describe the effective stress distribution perpendicular to the loading direction.They can be served as the comprehensive evaluation parameters along and perpendicular to the loading direction,respectively.(4)The MFL signals for the specimens with different crack defects are studied experimentally and theoretically.The coincidence between the calculated and measured results indicates that the proposed model can predict the MFL signals when the tensile and compressive stresses are present simultaneously.Besides,the following conclusions of MFL signals for surface-and buried-cracks are obtained: for both the surface-and buried-cracks,H_x shows a peak variation,and H_z shows a peak-to-peak variation.The signals for the buried crack are smaller than thoses for the surface crack,but their behaviors are the same.There are four characterization parameters to evaluate the cracks: the peak-value and peak-width of H_x,the peak-to-peak value and peak distance of H_z.Among them,the peak-to-peak value of H_z is more sensitive than the others.(5)Taking the the experimental data as the objective function,the surface and buried-cracks are inversed and reconstructed by GA and FEM based on the proposed hysteretic magnetoelastic coupling model.It is demonstrated that MFL signals can be used to inverse the geometric dimensions of the surface-or buried-cracks.The inversed results based on H_z are more accurate for the surface-crack,and the inversed results based on H_x are more accurate for the buried-crack.It provides the guidance for the inversion of more complicated damage.