First Principles Calculation Of Magneto Mechanical Effect Of ?-Fe

The ferromagnetic materials such as iron,are widely used in all walks of life,especially in the petrochemical industry,aerospace,railway construction and other fields.These mechanical devices are subjected to the action of fatigue load during service,the metal materials will be processed the elastic deformation,yield and plastic deformation,which results the produce of the crack defect,and leads to the failure of ferromagnetic metal components.Plastic deformation,as an early form of the crack defect,the effective detection of the plastic deformation area can avoid the occurrence of large accident.The metal magnetic memory testing technology,as one of the most promising new nondestructive testing technology in 21 century,can be used to detect the stress and deformation concentration of metal components quickly and effectively,and it has been successfully applied in many fields,such as pipe,boiler and rail stress detection,etc.However,the magnetic memory signal is easily affected by the parameters of metal component and the external environment because of the complexity of the magnetic effect,the characteristics of the magnetic memory signal after and before the critical yield point are not clear,and the service life of the ferromagnetic metal component can not be evaluated and predicted accurately.In this paper,the magneto mechanical coupling model of ferromagnetic crystal is established based on the theory of solid band theory,the crystal structure and magnetic memory signal characteristics of ferromagnetic crystal before and after the critical yield point are studied from the microscopic angle,and the mechanism of magnetic memory is analyzed systematically.The first principles method based on density functional theory is used to calculate the change situation of lattice structure,atomic magnetic moment,the electron spin density of states,the charge density difference,energy band structure with stress,and the critical pressure value of Fe transform from bcc structure to hcp structure is calculated.The theoretical experimental results show that: the stability of crystal structure is affected by the binding force between atoms,with the increase of stress,the bonding force between atoms in the crystal is decreased,and the crystal structure becomes weaker;when the stress concentration reaches a certain degree,the phase transition of Fe crystal will be occurred,the critical stress value of phase transformation is about 10.5GPa,the crystal structure is changed,and the magnetic properties of the system will be mutated.In order to verify the results of theoretical research,in this paper,a steel pipe pressure test is designed,and the high precision magnetic flux gate sensor is used to collect the magnetic memory signal,the change of magnetic memory signal under the different pressure,before and after strong magnetic magnetization and under the different lift off values are measured;and the system experimental study is carried out,the change characteristics of magnetic memory signal of the component before and after yield is analyzed quantitatively.The experimental results show that: there is an one-to-one correspondence between the magnetic memory signal and stress;the external strong magnetic field can cover the magnetic memory signal,but it will not destroy the magnetic memory signal;the lift off value can not affect the detection results in a certain range;when the concentration degree of the stress concentration reaches the yield strength of the material,the magnetic memory signal curve appears the inflect point,the slope of curve changes.The theoretical results are in good agreement with the experimental results in this paper.