M. K. Devine

Numerical determination of hysteresis parameters for the modeling of magnetic properties using the theory of ferromagnetic hysteresis

The authors describe how the various model parameters needed to describe hysteresis on the basis of the Jiles-Atherton theory can be calculated from experimental measurements of the coercivity, remanence, saturation magnetization, initial anhysteretic susceptibility, initial normal susceptibility, and maximum differential susceptibility. The determination of hysteresis parameters based on this limited set of magnetic properties is of the most practical use since these are the properties of magnetic materials that are most likely to be available. >

Magnescope: A poratble magnetic inspection system for evaluation of steel structures and components

The authors describe a portable magnetic inspection device developed for nondestructive evaluation of ferromagnetic steel structures and components. The device consists of an inspection head, instrumentation for measuring magnetic-field and flux density, and a computer together with the necessary control and data-analysis software. The device has been used for nondestructive evaluation of the mechanical condition of steels, particularly, detection of stress and monitoring the progress of fatigue damage. The most important application is in the detection of incipient failure, such as occurs with fatigue damage, temper embrittlement, and creep damage. Results which demonstrate the successful use of this instrument in monitoring fatigue are described. >

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.

The law of approach as a means of modelling the magnetomechanical effect

Abstract This study is concerned with the development of a model theory of the changes in magnetization that a ferromagnetic material undergoes when subjected to a varying external applied stress.

The magnetic detection of material properties

Because of the nature of ferromagnetism, the magnetic properties of a material can provide useful information about other material properties. Forexample, magnetic-hysteresis parameters and Barkhausen-effect measurements are affected by the microstructure of materials and by applied or residual stresses. Considerable work has shown that magnetic hysteresis and the Barkhausen effect can be used to investigate the condition of materials or components by nondestructive means. Such investigations can be carried out fairly easily under laboratory conditions; adapting the methods to in-service components is more problematic.

Recent developments in modeling of the stress derivative of magnetization in ferromagnetic materials

The effect of changing stress on the magnetization of ferromagnetic materials leads to behavior in which the magnetization may increase, or decrease, when exposed to the same stress under the same external conditions. A simple empirical law seems to govern the behavior when the magnetization begins from a major hysteresis loop. The application of the law of approach, in which the derivative of the magnetization with respect to the elastic energy supplied dM/dW is proportional to the magnetization displacement Man−M, is discussed.

Detection of fatigue in structural steels by magnetic property measurements

The magnetic properties of ferromagnetic materials are sensitive to the mechanical and microstructural condition of the material. Fatigue can affect the magnetic properties due to microstructural changes, primarily dislocation production. Magnetic hysteresis measurements have been used to monitor the changes in the parameters due to low cycle fatigue, with the overall objective of developing a new tool to enhance the present NDE techniques for detecting failure. The magnetic measurements were performed using the Magnescope, a portable magnetic inspection system. Materials for fatiguing included plain low carbon steel and samples of quenched and tempered AISI 4340. The coercivity and remanence of the low carbon steel samples increased during the early stages of fatigue, reflecting strain hardening. As cycling progressed, the magnetic parameters leveled off and dropped sharply shortly before failure. The coercivity and remanence of the 4340 samples decreased during the initial stages of cycling, reflecting fatigue softening. The parameters plateaued, then decreased shortly before failure. The amount of change in the magnetic parameters was found to depend on the strain amplitude of the cycling.

Effects of cyclic stress on the magnetic hysteresis parameters of polycrystalline iron

Abstract Experiments have been conducted to investigate the changes in the hysteresis curves that result from large strain amplitude (d l / l =0.005 and 0.003) cyclic stresses. It was found that the remanence of the material increased sharply during the first few stress cycles but subsequently the increase in remanence was less pronounced. Coercivity was found to decrease rapidly in the initial stages for the same reason. This is known as fatigue softening. The magnitudes of the changes in the magnetic properties were dependent on both the strain amplitude and the number of stress cycles experienced. It appears that the effects of stress on magnetization consist of two components, one due to the applied stress and the other due to the cumulative effects of previous stress history.

Magnetomechanical effect in nickel and cobalt

The change in magnetization as a result of applied uniaxial stress has been measured in nickel and cobalt. Both tensile and compressive stresses were applied up to 125 MPa. Magnetostriction and anhysteretic magnetization as a function of stress were also measured. The change in magnetization with stress depended on the applied stress and the displacement between the prevailing magnetization and anhysteretic. At the loop tips, nickel showed a +6 mT (compression) and −6 mT (tension) magnetization change while cobalt displayed a +15 mT (compression) and −15 mT (tension) magnetization change. At remanence, nickel decreased in magnetization by 45 mT under either sign of stress, while cobalt decreased by 20 mT also under either sign of stress. Magnetomechanical changes in magnetization near the loop tips were mostly reversible, while at remanence the magnetomechanical change was predominately irreversible. Cobalt generally displayed larger changes in magnetization with stress than nickel at locations close to t...

New procedures for in situ measurement of the magnetic properties of materials: applications of the magnescope

The authors describe the magnescope, an instrument which is capable of making in situ magnetic measurements and evaluating the condition of materials through their magnetic properties. Measurements can be made easily and rapidly through the use of expert system software containing precise algorithms for the various magnetic procedures. The various inspection procedures contained in the software make the instrument very flexible, and since they are in software, new procedures can be easily added, allowing evaluation of the instrument to meet new needs. The organization of this instrument is discussed, together with some of the procedures incorporated into the software allowing a wide range of magnetic parameters to be calculated from a few simple measurements. Specific magnetic parameters that are affected by applied stress, residual stress, fatigue damage, creep damage, temper embrittlement, and grain boundary segregation have been determined. >