In this work the effect of inclusions such as alumina on the structure-sensitive magnetic properties of nickel under prolonged exposure to stress and high temperature was studied. Samples containing varying amounts of alumina inclusions were prepared by powder metallurgical techniques. The tests were conducted under a stress range of 0–20 MPa at a temperature of 650°C, and the magnetic properties were measured at predetermined time intervals. Results from the study indicate that the product of coercivity and remanence is a good indicator of the evolution of the microstructure.

Creep damage in steel causes a reduction of magnetic properties. A mathematical model, previously formulated, accounts for this. Recently, this model was used in finite element modeling (FEM) of a magnetic C-core signal due to creep damage at a seam weld in Cr-Mo steam pipe. The FEM assumed unrealistically that in the absence of creep damage, the weld material and heat-affected zone (HAZ) and base metal all had the same magnetic properties, in this paper, new finite element simulations are presented for worst case relative permeabilities of 1271, 784 and 571 for base metal, HAZ, and weld material. Reduced permeability at the weld results in a considerably reduced emf at low probe magnetic fields. However, creep damage does produce an additional emf reduction that is large enough to be detected, even when the creep damage does not extend to the pipe wall surface. A method is suggested for calibrating the magnetic signal for weld, HAZ, and base metal effects

Introduction In this investigation magnetic measurements have been used to evaluate the accumulation of microscopic voids due to creep in pure nickel and nickel alloys. Creep is the slow plastic flow of material under high stress and high temperature. Creep rupture failure is common for many power plant components. An assessment of the creep damage in such components by a nondestructive technique such as magnetic inspection is essential for plant life extension strategies. It has been shown that magnetic properties of steels are sensitive to microstructural changes induced by mechanical and thermal treatments [I]. During high temperature creep there is significant change in microstructure such as the formation of voids, dislocation networks and grain boundary cavities. The evolution of these defects during creep affects the magnetic properties by changing the impedance to magnetic domain wall motion and also by introducing intemal demagnetizing fields associated with cavities. Earlier investigations by Devine et al. [2] have shown that coercivity and remanence can be used to determine creep degradation in steels. In this work, by monitoring the changes in macroscopic magnetization curve with creep damage, the effect of microstructural condition of the material on the magnetic properties has been demonstrated.

Magnetic hysteresis measurements have been used to evaluate creep damage in power plant weldments. This method relies on the sensitivity of the magnetic properties of steels, such as coercivity, remanence and hysteresis loss, to microstructural changes occurring during creep. During high temperature creep there is a significant change in microstructure such as the formation of voids, dislocation networks and grain boundary cavities. The evolution of these defects during creep affects the magnetic properties by changing the impedance to magnetic domain wall motion and also by introducing internal demagnetizing fields associated with cavities. The present paper discusses results obtained from on-site inspection of creep damaged Cr-Mo steel welds at two thermal power plants. One of the objectives of this research was to establish whether there were any trends in the magnetic properties as a result of creep damage which could be used later as part of a more comprehensive screening procedure for monitoring the progress of creep damage.

Cyclic loading causes cumulative microstructural changes in materials. The magnetic properties of A533B steel are determined by both initial microstructures and microstructural changes induced by fatigue damage. From the results of a series of strain-controlled fatigue tests, the magnetic properties were found to change systematically with fatigue damage throughout the fatigue life. A linear relationship between magnetic remanence and mechanical modulus was observed. The fatigue lifetimes were also dependent on the initial microstructure, and a relationship between these lifetimes and pre-fatigue magnetic properties such as coercivity was observed. Therefore this study has demonstrated that magnetic measurements can be exploited to evaluate the progress of fatigue damage in steel.

Recent work was undertaken to identify the possibility of using composite Terfenol, consisting of Tb–Dy–Fe in a nonmagnetic, nonmetallic binder, for magnetoelastic sensor applications in which the material needs to be formed into complex shapes, and in which the high levels of magnetostriction obtained in Tb–Dy–Fe alone are unnecessary. Recent results reported by Sandlund et al.1 have indicated the possible use of such a material in high frequency applications. In this work we have studied the material for low frequency, or even dc applications, in which the material could be used to sense an applied torque. The results showed that these composite materials need magnetic fields of typically 800 kA/m to obtain the magnetostriction levels of 350 ppm reported elsewhere.2 Similar work by Peters3 needed fields of over 1 MA/m to obtain magnetostrictive strains beyond 100 ppm. Such field strengths are impractical for low power devices. From the present work, 38 μm Terfenol powder in an epoxy or polymer binder gave magnetostrictive strains of 120 ppm and above for field strengths of 100 kA/m, which is much more appropriate for small low power sensors. A benchmark sensitivity of 18 μ T/N m for measurement of torque levels over the range ± 10 Nm was set based on the response of nickel which has previously been used in magnetoelastic sensors. In the present composite material axial magnetic induction sensitivities of 32 μ T/Nm and tangential field sensitivities of 12 A/Nm2 (=15 μT/Nm) were obtained, which were 50% better than nickel.

Magnetic hysteresis and Barkhausen emissions have been measured for amorphous Fe82B10Si8 samples with positive magnetostriction of λs = 27 × 10−6 under tensile stress of up to 35 MPa. The root mean square voltage of the Barkhausen signal and the number of events per cycle increased monotonically with the applied stress. The results are explained in terms of a theory which includes a stress-dependent hysteresis model and a stochastic process model for the Barkhausen emissions.

A study of the effect of non-magnetic particles on the magnetic properties of nickel is reported, The presence of inclusions is known to affect the structure sensitive magnetic properties of materials, In this work, two kinds of inclusions, namely, alumina particles and voids were studied and their effects on the magnetic properties were investigated. Powder metallurgy techniques were used to produce nickel compacts with varying amounts of alumina present. While coercivity increased with the volume of inclusions present, initial permeability decreased. Other properties such as remanence and hysteresis loss did not show a significant variation due to their sensitivity to the demagnetizing effects of the inclusions, An attempt has been made in this paper to explain quantitatively the variation in the magnetic properties in terms of the amount of inclusions present. Magnetic property measurements could be a useful non-destructive technique for determining porosity in magnetic materials.

The current study was undertaken to explore the possibility of detecting hydrogen cavitation in magnetic materials through magnetic property measurements. It is known that dissolved hydrogen in a material causes microvoids. These voids may affect the structure-sensitive magnetic properties such as coercivity and remanence. In this study, hydrogen was introduced into nickel and iron by two processes, namely thermal charging and cathodic charging. The effect on the magnetic properties was measured. In addition, the variation of the magnetic properties with porosity was studied.

Laser scribing of 3% silicon steel laminations was carried out using three different lasers: a KrF excimer laser, a pulsed Nd:YAG laser and a continuous wave CO2 laser. The processing parameters included the energy fluence at the surface of the workpiece, pulse repetition rate and pulse separation distance (for the pulsed lasers), scan separation distance and scan direction. The samples were tested for hysteresis loss, permeability, coercivity, remanence and saturation induction before and after laser treatment. An overall improvement in the core loss was observed in the laser-scribed samples. The best improvement in core loss was obtained in excimer laser scribing on the rolling direction and CO2 laser scribing in the transverse direction. Three mechanisms were proposed to explain the improvement in energy efficiency characteristics of the silicon-steel samples: magnetic domain refinement, stress relaxation and inhibition of domain-wall movement. Domain refinement, namely the formation of subdomains, results from the shocks induced by the beam. Laser scribing also relieves the stresses that are induced in the material during manufacture. The scribe lines increase the surface resistivity of the material, resulting in reduced eddy current loss. Tensile stresses are created between the laser scribe lines that elongate the domains and serve to refine the domain-wall spacing thus inhibiting the wall movement and reducing core losses.

Fatigue damage in steel structural components affects their magnetic properties. In this study, a series of strain-controlled low cycle fatigue tests were conducted on samples prepared from A533B nuclear reactor pressure vessel steel. Magnetic hysteresis and Barkhausen measurements were made at predetermined intervals of fatigue life under zero strain, fixed compressive and tensile load conditions. The results indicate that magnetic hysteresis parameters, such as remanence, and Barkhausen signals show significant changes as the material approaches the end of its fatigue life.

The effects of microstructure on the micromagnetic Barkhausen signal emissions in AISI 4140 steel are reported. The Barkhausen signal amplitude is known to be highly sensitive to the type and distribution of microstructural inhomogeneities, such as grain boundaries, precipitates and dislocations. The Barkhausen measurements were taken on samples having pearlitic, spheroidized and bainitic microstructures. The Barkhausen emissions were measured in terms of rms voltage peak-to-peak voltage and number of events per cycle. It is shown that Barkhausen signals can be used to distinguish between pearlitic and bainitic microstructures

The integrity of large scale structural systems such as bridges is an area of prime concern at this time because many of these bridges currently in use have been in service longer than originally intended (Fisher, 1989). Failure of these structures is often caused by fatigue cracks as a result of cyclic loading. Hence there exists a need to develop nondestructive evaluation techniques to evaluate the structural integrity of these bridges in order to ensure that the bridges are safe for continued operation. The development of a viable nondestructive evaluation (NDE) technique has the benefit of being able to detect the progress of fatigue damage at likely trouble spots before the damage becomes critical and threatens the integrity of the structure. Once the damage has been detected at an early stage, corrective action can be taken either in the form of repairs to the damaged part or by replacement. Ferritic steels are widely used as a constructional material and it has been shown that the magnetic properties of these steels are highly sensitive to microstructural changes (Bozorth, 1951). Magnetic NDE techniques are therefore capable of detecting changes in mechanical and microstructural condition. Hence these measurements have the potential for monitoring structural integrity of such systems. A new portable device, the Magnescope, has been developed for measuring bulk magnetic properties of steel structures and components (Eichmann et al. 1992). The Magnescope has been used to successfully evaluate the effects of stress and cyclic loading under laboratory conditions and also for the on-site measurements on bridges in active service. The objective of this paper is to demonstrate the unique capabilities of the Magnescope in measuring the magnetic properties of structural components under laboratory conditions and on-site inspection.

Results of inspection creep damage by magnetic hysteresis measurements on Cr-Mo steel are presented. It is shown that structure sensitive parameters such as coercivity, remanence and hysteresis loss are sensitive to the creep damage. Previous metallographic studies have shown that creep changes the microstructure of the material by introducing voids, dislocations, and grain boundary cavities. As cavities develop, dislocations and voids move out to the grain boundaries; therefore the total pinning sources for domain wall motion are reduced. This, together with the introduction of demagnetization field due to the cavities, results in the decrease of both coercivity and remanence. Numerical computations with a modified Jiles-Atherton model are presented which are consistent with the proposed mechanisms

This project consisted of research into the use of magnetic inspection methods for the estimation of fatigue life of nuclear pressure vessel steel. Estimating the mechanical and magnetic properties of ferromagnetic materials are closely interrelated, therefore, measurements of magnetic properties could be used to monitor the evolution of fatigue damage in specimens subjected to cyclic loading. Results have shown that is possible to monitor the fatigue damage nondestructively by magnetic techniques. For example, in load-controlled high-cycle fatigue tests, it has been found that the plastic strain and coercivity accumulate logarithmically during the fatigue process. Thus a quantitative relationship between coercivity and the number of fatigue cycles could be established based on two empirical coefficients, which can be determined from the test conditions and material properties. Also it was found that prediction of the onset of fatigue failure in steels was possible under certain conditions. In strain-controlled low cycle fatigue, critical changes in Barkhausen emissions, coercivity and hysteresis loss occurred in the last ten to twenty percent of fatigue life.

In nuclear power plants, neutron embrittlement of pressure vessel steels has been one of the main concerns. The use of micromagnetic Barkhausen emissions is a promising method to monitor the variations in microstructural and subsurface stress states due to their influence on these emissions. Measurements of these emissions can reveal neutron irradiation degradation in nuclear power plant components. Samples which were irradiated at different neutron fluences and annealed at different temperatures were obtained from three reactor surveillance programs. The results of different neutron fluences and annealing procedures showed noticeable fractional changes in the magnetic Barkhausen effect signal parameter, ΔMBE/MBE, and in the mechanical properties of these specimens. For example, increased intensity of neutron fluence decreased the ΔMBE/MBE as well as impact energy and upper-shelf energy, but increased Rockwell hardness and yield strength. Typical changes in this parameter were in the range from −20% to −45% for fluences of up to 25×1018 n cm−2.

This paper is concerned with using a magnetic technique for the evaluation of fatigue damage in steel structural components. It is shown that Barkhausen effect measurements can be used to indicate impending failure due to fatigue under certain conditions. The Barkhausen signal amplitude is known to be highly sensitive to changes in density and distribution of dislocations in materials. The sensitivity of Barkhausen signal amplitude to fatigue damage has been studied in the low-cycle fatigue regime using smooth tensile specimens of a medium strength steel. The Barkhausen measurements were taken at depths of penetration of 0.02, 0.07, and 0.2 mm. It was found that changes in magnetic properties are sensitive to microstructural changes taking place at the surface of the material throughout the fatigue life. The changes in the Barkhausen signals have been attributed to distribution of dislocations in stage I and stage II of fatigue life and the formation of a macrocrack in the final stage of fatigue.