This paper provides a guideline to the literature of magnetic techniques for nondestructive evaluation. Compared with other NDE methods such as ultrasonics or eddy currents, the literature for magnetic methods is relatively small, but one of the difficulties is that it is widely scattered. This review therefore presents a fairly comprehensive summary of works on Barkhausen effect, magnetoacoustic emission, magtenic hysteresis, residual field and magnetically induced velocity change methods that have appeared to date.

Review of magnetic methods for nondestructive evaluation

This paper is an overview of the studies into the effect of weak magnetic fields on the structure and mechanical properties of nonmagnetic solids of various nature (ionic, covalent, molecular, and metallic crystals, polymers, etc.). The various effects and aftereffects initiated by static, pulsed, and microwave magnetic fields that have been discovered over the past 15 years are classified and critically analyzed. The thermodynamic and kinetic aspects of the magnetic-field sensitivity of real solids with structural defects containing paramagnetic centers (electrons, holes, radicals, excitons, etc.) are discussed. Possible mechanisms for the effect of a weak magnetic field on the defect structure of crystals are considered. Special attention is given to the most developed chemical-physical theory of spin-dependent reactions between mobile particles and unpaired electrons. Interpretation of magnetoplastic effects is proposed in terms of the spin, electron, molecular, and dislocation dynamics of the complex multistage processes initiated by a magnetic field in a system of metastable structural defects.

Magnetoplastic effects in solids

Non-destructive evaluation (NDE) of materials for characterising various key microstructural features, mechanical properties (tension, creep, fatigue crack growth, hardness and fracture toughness), deformation and damage mechanisms has attracted considerable attention in the past 20 years as a primary step towards ensuring structural integrity of components. However, until recently, the correlations between the various NDE parameters and material properties have been only empirical and based on physical principles. The interaction between the NDE probing medium and the mechanical behaviour is not yet fully understood. The purpose of this review is to discuss the progress made in the application of non-destructive testing (NDT) techniques in evaluating various microstructural features and mechanical properties with emphasis on recent studies. Reinterpretation of older data, in the light of present understanding of the interaction of the NDE probing medium with material parameters, is carried out selectively. The NDT techniques evaluated include acoustic emission, ultrasonic attenuation and velocity, magnetic hysteresis parameters, magnetic Barkhausen emission, acoustic Barkhausen emission, laser interferometry, positron annihilation, X-ray diffraction and small angle neutron scattering. Critical assessments of the applicability of the various NDE techniques for the material parameters are provided.

Assessment of microstructures and mechanical behaviour of metallic materials through non-destructive characterisation

This paper reports a significant decrease on residual stress by combined treatment of a pulsed magnetic field and a pulse current on steel samples with pre-induced residual stress conditions, compared to a separately single treatment by either the pulsed magnetic field or the pulsed current. Briefly, 10% stress decrease by pulsed magnetic field treatment and 20% increase by pulsed current treatment were observed respectively. While 60% stress release is achieved by the combined treatments, in which the same magnetic field and current parameters were applied. It is supposed that the magnetic field facilitates dislocations depinning and pulsed current provides conduction electrons to drive dislocations to move further and faster. The combined effects lead to electro-magneto-plasticity and further residual stress release.

Residual stress reduction by combined treatment of pulsed magnetic field and pulsed current

An investigation was carried out to analyze and quantify the mechanical property changing mechanism of high speed steel by pulsed magnetic treatment. Specifically, a pulsed magnetic treatment system was designed and applied to the tool material treatment. The micro-hardness and red hardness were measured to quantitatively investigate the influence of magnetic treatment on the mechanical properties. Results showed that high speed steel material possessed better mechanical properties using the proposed pulsed magnetic treatment, of which the maximum increase of micro-hardness was 50 HV. Also, the red hardness with low temperature could be improved. Moreover, the influence of magnetic treatment conditions on the micro-hardness change rate was investigated. It was shown that the micro-hardness change rate first increases and then stabilizes with the increase of magnetic field intensity, and it decreases with the increase of magnetization time. Ultimately, the dislocation densities were calculated by XRD data analysis according to the Williamson–Hall method and the interaction between magnetic field and dislocation was discussed in detail. Results revealed that the dislocation density of high speed steel material increased by 24% after pulsed magnetic treatment. The dislocation multiplication and motion induced by magnetic field led to the increase of dislocation density, which was the main reason of the increase in micro-hardness. The dislocation pile-up mechanism was responsible for the treatment effects of different magnetic treatment conditions. The dislocation configuration determined from TEM micrographs was in good agreement with the X-ray results and the discussion of dislocation mechanisms.

An investigation on the mechanical property changing mechanism of high speed steel by pulsed magnetic treatment

Fatigue life of a component is influenced by a number of factors such as corrosion, temperatur, stress concentration, residual stress, etc. Yet another factor, namely the magnetic field, is shown to influence the fatigue life of ferromagnetic material. Some qualitative explanation is offered on the influences during fatigue process. Experimental results are presented to show the influence on the total fatigue life.



Die Lebensdauer von Maschinenteilen hangt von vielen Faktoren ab, zum Beispiel Korrosion, Temperatur, Spannungskonzentration, Restspannung, usw. Es wird von einem anderen Faktor, namlich dem magnetischen Feld berichtet, das die Lebensdauer ferromagnetischer Materialien beeinflust. Einige qualitative Erklarungen zu den Wirkungen dieser Einflusse wahrend der Ermudungsverfahren werden dargelegt. Versuchsergebnisse werden mitgeteilt, die den Einflus auf die gesamte Lebensdauer zeigen.

Effect of saturated magnetic field on fatigue life of carbon steel

Magnetic hysteresis technique is employed to evaluate fatigue damage in ferromagnetic specimens. The changes in the magnetic properties during fatigue loading are measured with the help of a specially fabricated hysteresis loop tracer. The measurements are taken with ac magnetizing fields in the Rayleigh region. The changes in magnetic properties are found to be related to the damage by fatigue loading.



Um die Ermudungsschaden ferromagnetischer Proben auszuwerten, wird das Verfahren der magnetischen Hysterese angewendet. Die Anderungen der magnetischen Eigenschaften werden wahrend der Dauerbelastung mit einer speziell fur diesen Zweck angefertigten Hysteresesonde gemessen. Die Mesergebnisse werden im Rayleigh-Bereich mit Hilfe eines Wechselmagnetfeldes registriert. Es wird festgestellt, das die Anderung der magnetischen Eigenschaften mit den durch Dauerbelastung entstandenen Schaden zusammenhangen.

M. S. C. Bose

Papers

Effect of saturated magnetic field on fatigue life of carbon steel

Magnetic NDT technique to evaluate fatigue damage