S. B. Biner

Nondestructive evaluation of creep damage in power-plant steam generators and piping by magnetic measurements

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.

Angular dependence of the magnetic properties of polycrystalline iron under the action of uniaxial stress

Abstract In previous work we have shown that when the magnetic field is changed under constant uniaxial stress there is a stress contribution to the anisotropy which increases or decreases the permeability depending on the sign of the stress and its direction relative to the field axis. In this paper we will give results on the effects of stress on magnetic properties and discuss how the anisotropy model can be extended when the field and stress are not coaxial. This leads to a more general model in which the permeabilities along different directions change in different ways under the action of the stress.

Assessment of creep damage of ferromagnetic material using magnetic 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. >

Monitoring fatigue damage in materials using magnetic measurement techniques

Measurements of hysteresis and Barkhausen effect (BE) have been made on 0.1 wt % C Fe–C alloys subjected to strain-controlled fatigue at various strain amplitudes. A relationship between the fatigue lifetime and strain amplitude was observed. The hysteresis properties of the samples cycled at different strain amplitudes were found to vary systematically with expended fatigue life. These properties showed significant changes in the initial and final stages of fatigue, while between these stages they remained stabilized. In the stable stage the remanence was found to decrease, whereas the coercivity increased with increasing strain amplitude. Variations in BE signal during fatigue were found to be closely related to the microstructural changes observed on the sample surface. These results are interpreted in the context of the changes in microstructure caused by fatigue damage, and the effects of the formation and propagation of fatigue cracks on the field distribution and domain structure in the vicinity of...

Evaluation of fatigue damage in steel structural components by magnetoelastic Barkhausen signal analysis

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.

Effects of fatigue-induced changes in microstructure and stress on domain structure and magnetic properties of Fe-C alloys

A study of the effects of microstructural changes on domain structure and magnetic properties as a result of fatigue has been made on Fe–C alloys subjected to either cold work, stress-relief annealing, or heat treatment that produced a ferritic/pearlitic structure. The magnetic properties varied with stress cycling depending on the initial condition of the samples. Variations in coercivity in the initial stage of fatigue were closely related to the changes in dislocation structure. In the intermediate stage of fatigue the observed refinement of domain structures was related to the development of dislocation cell structures and formation of slip bands. In the final stage of fatigue the remanence and maximum permeability decreased dramatically, and this rate of decrease was dependent on the crack propagation rate.

Evaluation of fatigue damage using a magnetic measurement technique

A series of strain-controlled fatigue tests were performed on 0.1 wt% C Fe-C alloy samples under different strain amplitudes. The measured coercivity of these samples was found to behave differently in the initial and final stages of fatigue. In the intermediate stage of fatigue it was found that coercivity tended to increase, while initial and maximum permeabilities tended to decrease. This could be related to the changes in the surface structure which were observed by SEM. The present results indicate that hysteresis parameters are related to the accumulation of fatigue damage, and therefore it is possible to develop a magnetic measurement technique for monitoring fatigue damage in steel components.

Magnetic property variations in nickel caused by non-magnetic inclusions

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.

Variation of coercivity of ferromagnetic material during cyclic stressing

The relationship between coercivity and the number of stress cycles is reported in this paper. The linear relation between coercivity and the logarithm of the number of stress cycles was confirmed in load controlled fatigue tests, for both pre-strained and unprestrained specimens. It is believed that the dislocation activity is the main factor changing the magnetic properties of a ferromagnetic material under fatigue, rather than other microstructures and internal stress. As a result, a theoretical model was developed to explain the correlation between the coercivity and the number of stress cycles. The model is based on domain wall dislocation interaction and the strain-dislocation relation. We predict the relation between magnetic properties and the stage of fatigue life as Hc-Hc/sub 0/=b ln(N), where Hc-Hc/sub 0/ is the change in coercivity, N is the number of stress cycles and b is a constant. >

Evaluation of Low-Cylce Fatigue Damage in Steel Structural Components by a Magnetic Measurement Technique

Fatigue is one of the leading causes of failure in structural components. The development of a viable NDE technique which can detect fatigue damage in its early stages of development, monitor its progress and be capable of predicting the onset of catastrophic failure is very essential. If the damage can be detected at an early stage, corrective measures can be taken either in the form of repairs to, or replacement of, the damaged part.