· Table of Contents
· Methods & Instrumentation
Magnetization of ferromagnetic materials in geomagnetic field by mechanical strain - Prinicple of metal magnetic memory testing and diagnostic techniqueWei-Chang Zhong
Nanjing Gas Turbine Research Institute, 47 Northern Central Road,
Nanjiing, 210037, People's Republic of China
According to Faraday's law on electromagnetic induction, this paper proves that at the mechanical strain of ferromagnetic materials in the geomagnetic field there will appear induced electromotive force, induced electric current and induced magnetic field in the materials to magnetize themselves, and only at the unsymmetrical strain the two opposite magnetizing effects can not be all offset against each other, further along with the increase of the strain cycle index a considerable scattering magnetic field strength will be accumulated out for non-destructive testing and technical diagnostics.
In October 1999 Professor A. A. Doubov from Russia came to China to participate The Chinese Society for Non-Destructive Testing 7th Conference on NDT and International Research Symposium, and he introduced a new method of NDT and technical diagnosis - metal magnetic memory technique , to which great interest and profound attention were given rise. Not long after in China a relevant instrument was developed and experimental investigation was carried on, and a gratifying primary result has been obtained .
Professor Doubov in his article  puts forward only in a general way that this method is based on magneto- mechanical effect, but not explains its mechanism - Why the ferromagnetic materials will be spontaneously magnetized in the geomagnetic field ? The author thinks that only this problem is answered first, then this new technique in the 21 century can be fully recognized, grasped, extended and applied.Keywords: Metal magnetic memory, Elastic-plastic strain, Ferromagnetic materials, Spontaneous magnetization.
At the end of the 70's and the beginning of the 80's of the last century the author met a series of spontaneous magnetization phenomena in production practice - some parts made from ferromagnetic materials primarily don't show magnetism, but the parts themselves and cutting tools are intensely magnetized after cutting process, and some parts and components of a machine, which originally don't have magnetism, show magnetism after a period of time of operation . The author names the former "magnetization by machining" and the latter "magnetization through operation". For removing the harmful effects caused by these spontaneous magnetization, through preliminary analysis the author believes that there are 9 probable reasons causing spontaneous magnetization: the effect of geomagnetic induction, the effect of directional magnetization, the effect of thermoelectricity by cutting, the effect of magnetostriction, the transformation of stress-structure, the thermal effect of cutting, the effect of vibrational orientation and some other accidental factors .
Obviously the magneto- mechanical effect, which is the basis of the metal magnetic memory method should belong to the effect of geomagnetic induction in the magnetization through operation, but a further concrete analysis must be made.
Every body knows that generally at a site of the Earth the geomagnetic field strength is uniform and weak, except in the particular spots, where ferromagnetic deposit or ferromagnetic articles locate. So a common part or an assembly of a machine can be considered in a uniform magnetic field.
Suppose the crossed sectional area of a work piece made by ferromagnetic material, perpendicular to the geomagnetic field He (A/m) is S. By the action of a directional stress through a period of time Dt, strain takes place at S, and S becomes S+DS. So the magnetic flux F through the transverse section of the work piece also from BS changes into B ( S+DS ), here B is the magnetic induction ( Tesla ), that is, the increment of the magnetic flux DF is :
|DF =BDS=mrmoHeDS||( 1 )|
here,mr is the relative magnetic permeability for the material, it is a pure number.
mois the magnetic permeability for vacuum, and equal to 4p´10-7 H/m
According to Faraday's Law on electromagnetic induction, the induced electromotive force E is .
|E = -DF/Dt = -mrmoHeDS/ Dt||( 2 )|
Under the action of E, an induced electric current i must appear on this cross section. Thereby, the induced magnetic field H is excited out, the work piece being magnetized - the residual magnetization Mr and residual magnetic induction Br appear , that is, the work pieces made by ferromagnetic materials under strain in the geomagnetic field naturally produce the phenomenon of spontaneous magnetization.
It is known from the Strength of Materials  that the characteristics of stress s- strain e of plastic materials such as low carbon steel under extension and compression are alike in the elastic limits, but different in the plastic range ( Fig.1.a ). And the extension diagram and compression diagram for the brittle materials such as cast iron are very much different from each other ( Fig.1.b ).
Fig 1: Stress-strain diagram of materials extended and compressed.|
a. Low carbon steel b. Cast iron
When the materials, whose extension diagram and compression diagram coincide with each other are acted by a pure alternative stress, because the yielded extensional strain equals to compressional strain, DS, DF, E, i and H in positive and negative two directions are all the same, that is, the magnetic field strength induced by the strain offset each other. So in this condition there not appears spontaneous magnetization. As the same principle, if an arbitrary material mentioned above is acted by an alternative stress plus a directional stress, and the yielded positive, negative strain are all in the elastic limits of extension ( or compression ), in this condition there also not appears spontaneous magnetization. Only when a pure alternative stress is acting upon a material, whose extension diagram differs from its compression diagram, and when an alternative stress and a directional stress are acting in concert upon a common material and an unsymmetrical (that is unequal) elastic-plastic strain is yielded, those materials will just produce spontaneous magnetization.
The magnetization-demagnetization character of a ferromagnetic material is demonstrated by its magnetic hysteresis loop (Fig. 2. The original magnetizing stage under a weak field strength is shown in the right bottom corner). The semi-branch of ascension in type a, b, c, d is magnetizing curve, and the semi-branch of descension in type d, e, f, a is the corresponding demagnetizing curve. And in the original magnetization it is alike (see the detail drawing in the right bottom corner of Fig.2).
|Fig 2: The magnetizing-demagnetizing process of the ferromagnetic materials - magnetic hysteresis loop.|
From electromagnetics  we know :
|H ¥ i ¥ E||(3)|
According to expression (2) there is
|Ds = S (2e+e2)»2s e||(5)|
here,e is the linear strain of the material, whose value is small, and is a proportional constant.
|\ H µe||(6)|
Because of the unequal elastic-plastic strain of ferromagnetic material in the geomagnetic field, the induced magnetic induction first ascends along the curve OP of the small picture in the right bottom corner of Fig.2, and descends along the curve PR as shown by the curve oa and ab in the Fig.3, and then it will ascend along curves type abcd, and descend along curves type defa in Fig.2. Due to the detour and saturation characters of magnetic hysteresis loop, after every cycle of magnetization-demagnetization, the magnitude of B will increase cycle by cycle, at last approaches to a fixed maximum Br, as shown in Fig.3.
The theoretical deduction is identical with the Russian research result (Fig.4) given in reference , only the abscissa of the former is |e |,the latter is Ds and the ordinate of the former is |B|, and the latter is DB : the shapes and tendency of these two groups of curves are all the same.
|Fig 3: The spontaneous magnetization process of ferromagnetic materials.||
Fig 4: The diagram of magneto-elasticity phenomenon development|
DB - residual induction change
Ds- cyclic load change
H e - externald magnetic field
The author gratefully acknowledges the financial support of this work by The National Natural Science Foundation of China.
|© AINDT , created by NDT.net|||Home| |Top||