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Mechanism study of metal magnetic memory testingRen Jiling, Song kai,Wu Guanhua,
Nanchang Institute of Aeronautical Technology
Lin Junming, EDDYSUN (Xiamen) Electronic Co., Ltd.
From the magnetic theorem the mechanism of a metal magnetic memory (MMM) phenomenon of ferromagnetic items engendered under the action of magneto-elasticity and magneto-mechanical effect has been investigated in this paper.
Keywords: Magnetic memory testing, magnetic elasticity effect, magneto-mechanical effect
In recent years more and more attention has been paid to a newly developing technique of metal magnetic memory testing (MMMT) for it is able to effectively diagnose early damage (particularly hidden discontinuity) in ferromagnetic items.
The principle of MMMT is based on that when a ferromagnetic item is in operation, under the combined action of working load and the earth field the orientation and irreversible reorientation of magnetic domains texture with magneto-striction will take place. After the working load is removed, the irreversible change of this magnetic status will not only be remained, but also related to maximum acting stress, i.e. the magnetic memory in correspondence with the acting stress is produced .
The further theoretical study and testing prove that the change and redistribution of the residual magnetism in the magnetic item induced by its operation result from the action of magnetic elasticity. The mechanism of the above mentioned magnetic memory testing is discussed in this paper.
The magnetic memory principle con be expressed as that in the iron made item located in the earth field the direction and irreversible reorientation of the magnetic domain textures with magnetic-striction will take place under the action of working load, and in the stress as well strain concentration zones the change of the maximum leakage magnetic field, Hp(x), is formed, i.e., the tangent component of the magnetic field, Hp(x), has its maximum value and the normal component, Hp(y), changes polarity and acquires zero value. The irreversible change of this magnetic status will be kelp after the working load is removed. Through measurement of the normal component of the leakage magnetic field the stress concentration positions in the tested item can accurately be deduced.
The principle of MMMT can be verified by tension testing. A steel made tension specimen is installed in a tension testing apparatus and the distribution of the leakage magnetic field, Hp(y), along with the item surface axis is measured. As shown in Fig. 1 (a), in it at Hp(y)=0 the stress concentration line is determined by the tension testing. After this the loading, P, perpendicular to the measured stress concentration line continues to be imposed for the tension testing. From Fig. 1 (b), the fracture of the tested item takes place along with the stress concentration line [Hp(y)=0].
|Fig 1: Principle testing of MMMT.|
It was proved that the magnetic elastic effect early by ferromagnetic theoretical studies. The main contribution of MMMT consists in that only through measuring the normal component of the leakage magnetic field of the tested surface, Hp(y), in the earth environment the stress concentration zones can be determined.
The study of the magnetics indicates that the magnetic elastic effect means that when an elasticity stress acts on a ferromagnetic material, the ferromagnetic material will no only produce the elastic strain, but also the magnetic-striction strain, thus the displacement of the magnetic domain barriers takes place and the self-magnetization direction is changed . Studying the reason, the magnetic elasticity effect is caused by the stress increment within the ferromagnetic crystal inner by the action of the external stress. For instance, taking an item with isotropic magnetic-striction, its stress energy is given by (1) and is .
Without the action of external stress and external magnetic field, the total free energy of magnetic crystal inner in stable status E is 
where, Ek is anisotropy of the magnetic crystal, Ems is magnetic elasticity, and Eel is elasticity.
Under the action of the external stress the stress energy portion induced by the external stress shall be added to the total free energy of the magnetic crystal inner. Therefore the total free energy is
At the moment, in accordance with the principle of "practically existing status must be the status with a minimum energy." only if the stress energy is reduced and made to approach to minimum or the original magnetic elastic performance etc of the magnetic item is altered, the total free energy can trend towards minimum and thus the new stable status of the ferromagnetic item be retained.
It can be seen from formula (1) that the approach of decreasing the stress energy is to change the direction of the magnetization intensity. When the external force exists and for the material with isotropic magnetic-striction, when ls>0 and if q = 0° or p, the stress energy will be minimum. This expresses that for the material with positive magnetic-striction imposing a tension force on it has its magnetization intensity direction approach toward that of the tension force, i.e. for the ferromagnetic items with positive magnetic-striction the tension direction is easily magnetized direction and therefore the ferromagnetic items become easily to be magnetized in this direction. When ls<0 and if q = p/2 or q = 3p/2, the stress energy will become minimum. This means that imposing a tension force on the ferromagnetic item with negative magnetic-striction will make its magnetization intensity direction trend toward that perpendicular to the tension force. In other words, in the tension force direction it is difficult to magnetize the tested item with negative magnetic striction.
From this it can be seen that the external force existing will affect the direction of the magnetization intensity in the ferromagnetic item and has it not arbitrarily be oriented. Under the action of the stress the direction of the magnetization intensity will approach to that parallel or perpendicular to the stress direction with the magnetic-striction coefficient variation so that it can reduce the stress energy increment. This anisotropy caused by the stress is called as the stress anisotropy and it is one of important reasons inducing the magnetic elasticity.
When ferromagnetic items are acted by the external stress, although the magnetization intensity of the ferromagnetic item inner will be forced to change its direction so that the stress energy will be decreased, but the stress existing or the residual stress in the ferromagnetic items after the stress removal will always raise the stress energy. This certainly will break the original balance status and promote the increment of the magnetic elastic energy.
The ferromagnetic item will not only produce the elastic strain, but also deformation with the magnetic-striction when the elastic stress is acting on the ferromagnetic item. From magnetism study it is known that the magnetic elasticity energy Ems related to the magnetic-striction of the ferromagnetic item inner and the elastic energy caused by the simple deformation Eel are respectively.
|Eel= 1/2 C11(exx2+eyy2ezz2)+2c44(exy2+eyz2+ezx2)+c12(exxeyy+ eyyezz+ezzexx)||(5)|
where, exx,eyy,ezz,exy,eyz,ezx are deformation components; c11,c44,c12 are modulus of elasticity. From the above mentioned formulas it can be seen that when the external force exists, not only deformation induced by the stress will raise the magnetic elastic energy, but also the stress will change the spontaneous magnetization direction of the magnetic domains in the ferromagnetic item so that increase the magnetic elastic energy for compensating the increment of the stress energy. It can be seen that under the action of the external force the deformation of the magnetic-striction is produced in the ferromagnetic item, thereupon the displacement of the magnetic domain barriers is induced, and the change of the spontaneous magnetization direction is important factor causing the magnetic elasticity effect as well.
The inner cause producing the magnetic memory phenomenon in ferromagnetic items under the action of loading and weak magnetic field of the earth is determined by microstructure characteristics of its crystal grains. Generally, when a ferromagnetic item is undergoing processing technologies of melting, forging and heat-treatment etc, its temperature exceeds Curie point, the magnetic domain textures of the item inner is destroyed and its magnetism disappears. In the process of cooling the item below Cuy point, on the one hand, ferromagnetic crystal gains and magnetic structure are simultaneously reformed in it, on the other due to various heterogeneity of the item inner (for instance, shape, structure as well inclusion and defect etc) heterogeneous textures and structures with inheritance are formed. These heterogeneous positions of the textures and structures are always defect and stress concentration positions, generally they exist in form of dislocation, and under the action of magneto-mechanic effect in the earth magnetic field fixed nodes of the magnetic domains will appear, thus magnetic poles are produced and demagnetized field is formed, and it appear on the surface of the item in form of scattering magnetic field and manifests itself as metal magnetic memory. At this time if a load is imposed on a ferromagnetic item, dynamical stress existing will make it deform and thus the dislocations of the item slide and move along with the sliding plane determined by dislocation line and Burgers vector. In the sliding the dislocations need to overcome resistance of crystal grid lattice as well interacting force among impurities or defects. As a result of dislocations sliding the density of crystal dislocations will increase, i.e. the enriched dislocations will engender high stress energy and form the stress concentration zones .
It is worth to pay an attention to that the formation of the stress concentration zones in the item will converge quite high stress energy. Therefore, in order to let the total free energy in the ferromagnetic item reach to minimum the displacement and even irreversible reorientation permute of the magnetic domains in the item inner will be caused under the action of magneto mechanical effect in the earth magnetic field, and mainly are in form of increasing the magnetic elasticity for compensating the increment of the stress energy. Thereby, the magnetic field intensity larger than that of the earth is produced in the ferromagnetic item. According to the study of metal mechanics even if in the elastic deformation zones of a metal material a complete elasticity zone with no energy loss does not exist. Since many inner consumptions effects (for instance, elasticity viscosity, dislocation etc) exist in the metal item and they certainly make the stress concentration zones already formed during imposing load remain after dynamical loading removal, in particular, special outstanding under dynamical loading, large deformation and high temperature conditions. The remained stress concentration zones possess high stress energy at the same and thereby in order to remove the stress energy, the reorientation permute of the magnetic domain textures induced under the action of magnetic mechanical effect will be preserved simultaneously as well, and leakage magnetic field distribution shape similar to defects in the stress concentration zones are formed, i.e. the tangent component of the leakage magnetic field, Hp(x), is maximum and the normal component, Hp(y), changes polarity and gets zero value.
Up to today, the metal magnetic memory testing technique is only, feasible NDT method being able to diagnose early damage of ferrometal items. The study proves that the principle of producing metal magnetic memory is the magnetic elasticity effect. For this, from further studies it will be anticipated that it is possible to find out a correspondent relationship between the internal residual stress of a ferromagnetic item and its surface leakage magnetic field, thus the quantitative analysis against the results of the magnetic memory testing can be brought into effect.
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