Katsuhiro Kawashima

Theory and numerical calculation of the acoustic field produced in metal by an electromagnetic ultrasonic transducer

The equations for calculating the acoustic field produced within a nonmagnetic metal by an interaction of eddy current with a static magnetic field were obtained on the assumptions that (1) an ultrasonic wave is generated by the electromagnetic force through classical and macroscopic phenomena; (2) the electric, magnetic, and elastic properties of the metal are linear, isotropic, and homogeneous throughout the metal, which occupies semi‐infinite space; (3) the whole system is axially symmetric; and (4) eddy currents and elastic waves show a steady‐state sinusoidal variation. The acoustic field produced by a specific electromagnetic ultrasonic transducer with axial symmetry was calculated numerically, and the results showed a well‐defined ultrasonic wave beam, which was narrower than had been expected from the size of the transducer.Subject Classification: [43]35.60; [43]85.48.

Nondestructive characterization of texture and plastic strain ratio of metal sheets with electromagnetic acoustic transducers

All the lowest‐order independent CODF (crystallite orientation distribution function) coefficients W400, W420, and W440 of rolled steel sheets, with thickness less than 1 mm, have been obtained nondestructively by using the known single‐crystal elastic constants and the relative ultrasonic velocities measured with EMATs (electromagnetic acoustic transducers). The ultrasonic velocities of all the waves, longitudinal waves, shear waves propagating in the through thickness direction, and SH0 (shear horizontal) plate waves, were measured by the EMATs. No acoustic coupling medium was necessary, making possible quick, accurate, and reproducible measurements. It is shown that the CODF coefficients can be obtained as the solutions of three linear equations in which the measured relative ultrasonic velocities are included. There was an overall correspondence between the ultrasonic pole figures drawn using the obtained CODF coefficients and x‐ray pole figures. All nine elastic constants of a steel sheet were also c...

Apparatus for generating and detecting an electromagnetic ultrasonic wave

An apparatus for generating and detecting an electromagnetic ultrasonic wave in which a magnetic field generator forms a magnetic field at the surface area of one side of a conductive material, and an eddy current and impulse of a magnetic field are produced in the surface area by supplying a high frequency current pulse to an ultrasonic wave generating coil, the interaction therebetween producing an ultrasonic wave which travels in the conductive material and further, the eddy current resulting from the interaction between the formed magnetic field and a reflected ultrasonic wave being reflected by another side of the conductive material or the internal defect contained therein is detected by a detecting coil which is electromagnetically separated from the ultrasonic wave generating coil by a decoupling means.

Experiments with two types of electromagnetic ultrasonic transducers

Two types of electromagnetic ultrasonic transducers were made and tested on mild steel, stainless steel, and aluminum. One of the transducers generates and detects a relatively large amount of longitudinal wave and simultaneously a small amount of radially polarized transverse wave in nonmagnetic metal. The other is for a large amount of radially polarized transverse wave and a small amount of longitudinal wave, also in nonmagnetic metal. Both transducers showed different characteristics for ferromagnetic metal, giving greater efficiency to the radially polarized transverse wave. It was also shown experimentally with an electromagnetic transducer that the mode conversion between longitudinal waves and radially polarized transverse waves occurs fairly efficiently at a vertical reflection.Subject Classification: [43]35.60, [43]35.80; [43]85.40, [43]85.48.

Nondestructive measurement of texture and nine elastic constants of rolled metal sheets by EMAT

All the lowest‐order independent coefficients W400, W420, and W440 for the crystallographic orientation distribution function were obtained nondestructively for thin rolled steel sheets by measuring ultrasonic wave speeds and making use of the known values of the single‐crystal elastic constants. All the ultrasonic wave speeds were measured by EMAT so that no acoustic coupling medium was necessary, and the measurements were done quickly and with very good reproducibility. Pole figures were calculated using the obtained coefficients W400, W420, and W440 and the results were quite similar to the pole figures obtained by the x‐ray diffraction method. It is well known that a rolled steel sheet can be modeled as an orthorhombic continuum having nine different elastic constants. It is also known that only six of the nine constants are independent when the sheet is a polycrystal of cubic crystallites and assuming that the macroscopic properties can be predicted by an averaging scheme. All these nine constants we...