Hegeon Kwun

A model for the effect of stress on the low‐frequency harmonic content of the magnetic induction in ferromagnetic materials

A simple model used previously by the authors to explain stress variation of magnetic hysteresis is now employed to explain the effect of stress on the amplitudes of the first‐ and third‐order harmonics of the magnetic induction signal resulting from application of an ac magnetic field of low frequency to a steel specimen. An improved expression for the effective field contribution Hσ due to stress has been derived from thermodynamic considerations.

Model for the effect of tensile and compressive stress on ferromagnetic hysteresis

A model is presented for the stress‐dependent effective field, which when used in conjunction with the Jiles–Atherton theory, qualitatively accounts for (1) the change in slope and shape of the hysteresis curves with uniaxial stress and (2) the convexity of the curves depicting remanent and peak magnetization as a function of stress. Also, the model can produce the Villari reversal if parameters are selected appropriately.

Effects of tensile loading on the properties of elastic-wave propagation in a strand

Effects of tensile loading on the properties of longitudinal-mode elastic-wave propagation in a 1.52-cm-diam, seven-wire strand used for prestressed concrete structures were investigated experimentally. In an unloaded state, the wave propagation properties in strand matched those seen in individual wires comprising the strand, namely, straight center wire and helical outer wires. In the strand, however, extraneous signals were found to be produced from the propagating wave due to physical interactions between the adjacent wires. Under tensile loading, it was observed that a certain portion of the frequency components of the wave became highly attenuative and, thus, absent in the frequency spectrum of the wave. The center frequency of this missing portion, called notch frequency, was found to increase linearly with log N, where N is the applied tensile load. In addition, on both sides of the notch frequency, the wave exhibited a large dispersion in a manner similar to the behavior near a cutoff frequency. ...

Effects of grain size, hardness, and stress on the magnetic hysteresis loops of ferromagnetic steels

Effects of grain size, hardness, and stress on the magnetic hysteresis loops of AISI 410 stainless steel and SAE 4340 steel specimens were investigated experimentally. It was observed that both hardness and stress significantly influenced the hysteresis loops, while the grain size had a minimal effect. For each material, the mechanically harder specimen was more difficult to magnetize. Upon application of uniaxial stress, the magnetic induction increased under tension and decreased under compression, with the sides of the hysteresis loops becoming inclined more toward the vertical axis under tension and the horizontal axis under compression. For each material, the effects of stress on the hysteresis loops were greater for the mechanically softer specimen and exhibited an inverse relationship to the hardness. The effects of stress were not dependent on grain size.

Feasibility of under-lagging corrosion detection in steel pipe using the magnetostrictive sensor technique

The feasibility of inspecting under-lagging steel pipes without lagging removal using the magnetostrictive sensor (MsS) technique is described. With the MsS technique, elastic waves are launched along the length of a pipe, and reflected echoes from defects such as cracks or corroded areas are detected using noncontacting sensors. Experimental results obtained from 6-m long, 3.8-cm diameter steel pipe samples before and after inducing simulated defects in the pipewall are presented, and the potential capabilities of the MsS technique are discussed.

Dispersion of longitudinal waves propagating in liquid-filled cylindrical shells

The dispersion of the first two longitudinal wave modes, L(0,1) and L(0,2), was experimentally investigated for a cylindrical shell (such as a pipe or tube) that was completely filled with a liquid. It was observed that the presence of a liquid inside the cylinder dramatically alters the dispersion curve for the L(0,2) mode by dividing (or branching) the curve into approximately equally spaced regions separated by cutoff-type behavior. This branching was attributed to coupling between the unperturbed L(0,2) mode in the shell and the unperturbed longitudinal modes in a liquid cylinder with rigid boundaries, LL(0,2N), where N is an integer. The physical mechanism for the mode coupling was determined to be radial resonances in the combined liquid/pipe system. In time domain, the liquid effects on the dispersion are manifested as a long-duration signal or a series of short-duration pulses, depending on the pulse length of the transmitted signal relative to the reciprocal of the frequency interval between bran...

Micromagnetic model for biaxial stress effects on magnetic properties

Abstract A micromagnetic formulation has been developed for modeling the effect of biaxial stress on magnetoelastic processes in polycrystalline steels. The formulation uses a modified version of the Kashiwaya model for the effect of biaxial stress on magnetic properties and combines it with the Schneider-Cannell-Watts model for magnetoelastic processes in steels. In particular, the model involves use of an effective stress equal to one of the deviatoric (i.e. distortional) normal stress components, depending on whether the field is parallel to a tensile or compressive axis or to the third axis perpendicular to the plane of biaxial stress. Computer results are compared to experimental results on the effects of biaxial stress on magnetic properties in mild steel and in SAE-4130 steel. Good qualitative agreement is found in almost all cases, in that in going from one biaxial stress case to the next, the same kinds of changes are seen magnetically.

Long-range guided wave inspection of pipe using magnetostrictive sensor technology: the feasibility of defect characterization

The feasibility of characterizing a defect in pipe from the defect signal detected by using the magnetostrictive sensor (MsS) technique, which utilizes longitudinal guided waves for long-range inspection of pipe, was investigated. Signals from notches of various cross-sectional areas showed that the reflection coefficient of the wave from these planar defects is insensitive to the wave frequency and increases monotonically with the increasing cross-sectional area of the defect. Signals from simulated corrosion pits, on the other hand, showed that the reflection coefficient from these volumetric defects is dependent on both the cross-sectional area and the axial extent of the defect as well as on the wave frequency. The results indicated that it is feasible to differentiate planar from volumetric defects and to determine the defect size -- namely, its cross-sectional area and axial extent if it is volumetric.

Magnetostrictive sensor for active health monitoring in structures

A flat magnetostrictive sensor for active health monitoring of a large area of a structure has been developed. The sensor consists of a thin nickel foil and a coil placed over the nickel and, much like a strain gauge, is permanently bonded to the surface of a structure under monitoring. When activated, the sensor generates guided waves in the structure for interrogation and detects signals that are reflected back from the structural geometries and defects in the structure. Since guided waves can travel a long distance in the structure, a large area of the structure can be interrogated and monitored by using the sensor. By periodically acquiring the data and comparing it with the baseline data established at the time of sensor installation, structural changes occurred over that time can be quickly determined for suitable structural management decision. In addition to the ability to actively inspect and monitor a large area of the structure, the sensor is also rugged and inexpensive and therefore has high potential for practical use. As an example of its applicability to aircraft structure, data showing the monitoring of defect growth in fastener holes in a wing structure are presented.

DETECTION OF AXIAL CRACKS IN TUBE AND PIPE USING TORSIONAL GUIDED WAVES

Guided‐waves are now widely used for long‐range inspection of piping and tubing for detection of corrosion metal loss areas and circumferential cracks. The reflection coefficient of guided‐waves from a defect is proportional to the circumferential cross‐sectional area of the defect. Since axial cracks have negligibly small circumferential cross‐sectional area, they are usually undetectable. However, when the depth of axial crack reaches about 70‐percent of wall thickness, the interaction mechanism between the torsional wave and the axial crack changes and the crack begins to produce detectable signals accompanied with characteristic tailing signals. Experimental data from various sized pipes including a seam‐welded pipe with lack of fusion are presented and potential interaction mechanisms are discussed.