Quantitative Study Of Micromagnetic Nondestructive Testing For Fatigue Damage In Ferromagnetic Materials

Ferromagnetic materials are widely used in various fields such as railway transportation,aerospace,petroleum transportation,etc.,because of their good mechanics and mechanical properties.However,these ferromagnetic structures have a long service period.Therefore,these structures are prone to a large amount of fatigue damage in the stress concentration area or the micro-defect area due to the repeated action of the load during the service period,which may eventually lead to the failure of the structure.How to accurately evaluate the fatigue damage of ferromagnetic materials during long-term service is a key factor to ensure the safe operation of ferromagnetic structures in service.For ferromagnetic structures,the coupling between the external load and the environmental magnetic field will cause the internal magnetization to change during the long-term service process,which in turn leads to identifiable changes in the magnetic field distribution on the surface of the structure.Therefore,it can be concluded from theoretical analysis that the change in the magnetic field distribution on the surface of the ferromagnetic structure can be used to accurately assess the degree of fatigue damage,which can effectively monitor the damage development process of the structure during the entire service life and avoid engineering accidents due to accumulation of fatigue damage.However,the current research on the magneto-mechanical coupling mechanism of ferromagnetic structures under fatigue loads is not comprehensive,and the relationship between the magnetic field distribution on the surface of the material and its fatigue damage is still unclear.As a result,the method of using surface magnetic field distortion to evaluate the fatigue damage state of ferromagnetic structures is still in the exploratory stage.This paper focuses on the relationship between the magneto-mechanical coupling behavior of ferromagnetic materials and the evolution of the surface magnetic field distribution and the fatigue damage state under fatigue loading.Based on the thermodynamic framework and the suppression effect of plastic strain on the magnetic domain movement,an anhysteresis magnetization model for ferromagnetic materials is established,and then a stress magnetization model of ferromagnetic materials is proposed based on the local equilibrium state of magnetization and the principle of proximity.On the basis of the stress magnetization model,combined with the classical elastoplastic theory,a constitutive model of the magneto-mechanical coupling fatigue damage of ferromagnetic materials is established.By comparing with existing models and experimental results,it appears that the model in this paper can more accurately reflect the influence of cyclic loading on the magnetization behavior of ferromagnetic materials.By considering the effect of magnetocrystalline anisotropy on the anhysteresis magnetization behavior of ferromagnetic materials,a magnetocrystalline anisotropy magneto-mechanical coupling fatigue damage model for ferromagnetic materials is proposed,and the angle effect on the stress magnetization behavior of ferromagnetic materials is analyzed.In the stress magnetization model part,classical loss and anomalous loss are introduced,the ratedependent magneto-mechanical coupling fatigue damage model of ferromagnetic materials is established,and the influence of load rate on the stress magnetization behavior of ferromagnetic materials is analyzed and discussed.Based on the classical elastoplastic theory,the magneto-mechanical coupling fatigue damage model and the magnetostatics theory,a micro-magnetic detection forward model for the fatigue damage for ferromagnetic materials is established.The model can accurately reflect the phenomenon that the spontaneous magnetic flux leakage(SMFL)gradually increases with the increase of the elastic load,and it can also describe the change law of the SMFL first rapidly increasing then becoming stable and finally appearing abrupt with the number of loads under cyclic loading.At the same time,the SMFL evolution of materials containing initial damage(stress concentration zone)under cyclic loading is also analyzed,and the effect of the initial damage size on the SMFL is also studied.The results show that there is a quantitative relationship between the damage size and the SMFL,and the buried depth is an important factor that cannot be ignored in the micro-magnetic detection.Based on the magnetocrystalline anisotropy magneto-mechanical coupling fatigue damage model,the magnetocrystalline anisotropy of the material and the effect of the magnetic field and magnetization angle on the evolution of the SMFL can also be described.Through the ratedependent magneto-mechanical coupling fatigue damage model,the effect of load loading rate on the evolution of the SMFL is studied.The research results show that the SMFL of the material after considering the load loading rate shows a gradual decrease change law with the increase of the load loading times.Based on the conjugate gradient inversion algorithm,combined with the forward analysis model of the SMFL,the material fatigue damage state is quantitatively inverted.Based on the results of the SMFL forward analysis model,the degree of plastic damage and the size of the plastic deformation zone were analyzed inversely,and the influence of the magnetocrystalline anisotropy of the material and the load loading rate on the quantitative evaluation of the fatigue damage state was discussed.In this paper,the quantitative research on the fatigue damage micro-magnetic detection of ferromagnetic materials,the established magneto-mechanical coupling fatigue damage model will help to further understand the cyclic stress magnetization mechanism of ferromagnetic materials,and improve the theoretical framework of micro-magnetic detection and evaluation of the fatigue damage state of materials.It has a certain impetus to the engineering application of micro-magnetic detection to quantitatively assess the fatigue damage of ferromagnetic materials,and it also provides research ideas for other nondestructive detection methods.