David A. Kaminski

A model for hysteretic magnetic properties under the application of noncoaxial stress and field

Although descriptions of the effect of stress on spontaneous magnetization within a single domain already exist, there remains no adequate mathematical model for the effects of noncoaxial magnetic field and stress on bulk magnetization in a multidomained specimen. This article addresses the problem and provides a phenomenological theory that applies to the case of bulk isotropic materials. The magnetomechanical hysteresis model of Sablik and Jiles is thus extended to treat magnetic properties in the case of noncoaxial stress and magnetic field in an isotropic, polycrystalline medium. In the modeling, noncollinearity between magnetization and magnetic field is taken into account. The effect of roll‐axis anisotropy is also considered. Both magnetic and magnetostrictive hysteresis are describable by the extended model. Emphasis in this article is on describing properties like coercivity, remanence, hysteresis loss, maximum flux density, and maximum differential permeability as a function of stress for variou...

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.

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.

Finite element simulation of magnetic detection of creep damage at seam welds

Using appropriately modified magnetization curves for each element of creep-damaged material, a finite element calculation has been carried out to simulate magnetic detection of non-uniform creep damage around a seam weld in a 2.25 Cr 1 Mo steam pipe. The magnetization curves for the creep-damaged elements were obtained from an earlier model for the magnetic effect of a uniformly creep-damaged material as given by Chen, et al. In the finite clement calculation, a magnetic C-core with primary and secondary coils was placed with its pole pieces flush against the specimen in the vicinity of the weld. The secondary emf was shown to be reduced when creep damage was present inside the pipe wall at the cusp of weld and in the vicinity of the cusp. The C-core detected the creep damage best if it completely spanned the weld seam width. Also, the current in the primary needed to be such that the C-core was not magnetically saturated.

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. >

Magnescope: Applications in nondestructive evaluation

This paper describes recent results obtained with the Magnescope, which has been used on location in industrial environments and has successfully detected impending fatigue failure, creep damage, applied stress, and microstructural differences. It is concluded that the device provides a useful nondestructive method for evaluating the mechanical properties of materials through the measurement of their structure sensitive magnetic properties.

Application of Barkhausen effect measurements for detection of near surface stress (abstract)

Barkhausen effect emissions have been shown to be dependent on stress in previous work. This paper shows how the technique has been applied to evaluation of stress in automobile crankshafts. These components often fail prematurely due to residual tensile stress in the surface of the components, which leads to cracking and sudden fatigue failure. The measurements were made using an excitation frequency of 8 or 128 Hz with signal detection bandwidths of 3–15, 20–70, and 70–200 kHz. These corresponded to nominal magnetic field penetrations of 0.2, 0.07, and 0.02 mm. Results showed that measurements from the near surface (0.02 mm) were most sensitive to stress. A number of calibration experiments on different materials confirmed this result. In addition, it was found that low values of the root‐mean‐square Barkhausen voltage correlated with low tensile strength of the material. This measurement could, therefore, be used for detection of components that were likely to encounter early failure. A calibration of ...

Stress Detection in Steels Through Variations in Magnetic Properties

The evaluation of stress in large scale constructed facilities, such as pipelines in power plants, is important for assessing the condition of the structure. The development of viable NDE techniques for high precision evaluation of stress in steels has remained a long standing problem. Some success has been achieved in recent years in the quantitative measurement of stress along a single axis coaxial with applied magnetic field. This paper reports on subsequent work aimed at the evaluation of the stress in two dimensions. This is clearly a problem of direct relevance to the estimation of stress in the surface of a pipeline.In operating nuclear reactors, the steel pressure vessel is exposed to neutron irradiation. The high energy part (>1 MeV) of this irradiation, over a long period, makes the steel brittle and susceptible to rupture.

Magnetic property variation in carbon steel and chrome‐molybdenum steel as a function of uniaxial stress noncoaxial with the magnetic field (abstract)

Magnescope1 magnetic measurements were made on carbon steel specimens ranging from 0.1–0.8 wt %C and on chrome‐molybdenum steel specimens cut from electric power plant pipes previously in service. The carbon steel specimens were heat‐treated using three procedures: (1) spheroidization, (2) quenching, and (3) quench and tempering. The specimens were subjected to uniaxial tension up to 40 ksi. The inspection head was aligned so that the magnetic field was oriented at different angles with respect to the stress axis. Magnetic properties (such as coercivity and maximum differential permeability) were extracted from digitized magnetic hysteresis loop measurements. Magnetic properties were studied as a function of stress at each angle of stress‐field orientation. To our knowledge, such a comprehensive study of noncoaxial stress and field effects has never been accomplished before for such a wide variety of steel specimens. Results for the various materials are presented for different orientation angles and comp...