Riichi Murayama

Driving mechanism on magnetostrictive type electromagnetic acoustic transducer for symmetrical vertical-mode Lamb wave and for shear horizontal-mode plate wave

Abstract The properties of electromagnetic acoustic transducers (EMATs) for using the symmetrical vertical mode-(S 0 ) Lamb wave and the shear horizontal fundamental mode-(SH 0 ) plate wave were experimentally studied. The EMAT, consisting of a meandering coil and an electromagnet, can effectively transmit and receive the S 0 mode Lamb wave and the SH 0 -mode plate wave in a thin metal plate. For ferromagnetic materials, such as steels, the magnetostriction is important for the transduction. The wave amplitude is not proportional to the static field strength, but is dependent on the magnetostrictive properties of the material. In this study, the amplitude and the phase of the S 0 mode Lamb wave and SH 0 mode plate wave generated by the EMATs are measured while changing the static field strength to the sheets. The amplitude of the S 0 -mode Lamb wave in a low carbon steel sheet initially decreases with a static field to the minimum value, then increases again accompanying the phase inversion by 180 ° and finally decreases without any phase inversion. The amplitude of the SH 0 mode plate wave in a low carbon steel sheet initially increases to the maximum value with the static field, then decreases to the minumum value and increases again accompanied by a phase inversion of about 90 °. These data were discussed on the basis of the magnetostrictive curve to the magnetizing current of an electromagnet in low carbon steel sheets.

Development of an on-line evaluation system of formability in cold-rolled steel sheets using electromagnetic acoustic transducers (EMATs)

An on-line evaluation system of r-values in cold-rolled steel sheet with EMATs has been developed. First the principle of nondestructive r-value measurement is described. Next the characteristics of a Lorentz and a magnetostrictive type EMAT are evaluated to permit an optimum sensor for on-line measurement. Based on the results, a system with a magnetostrictive type EMAT was fabricated. The capability for measuring r-values was evaluated using many samples and products in a continuous annealing line. The system was confirmed to continuously measure r-values in the production line with an accuracy of 1 σ = 0.07.

Study of driving mechanism on electromagnetic acoustic transducer for Lamb wave using magnetostrictive effect and application in drawability evaluation of thin steel sheets

The mechanism of an electromagnetic acoustic transducer (EMAT) for the fundamental symmetrical Lamb wave using the magnetostrictive effect in thin steels sheet was studied. The EMAT was constructed from a magnet for loading the field parallel to the steel sheets surface and the meandering coil for carrying the transmitting current. First, the field dependence of the received signal amplitude and the phase for the transmitter and the receiver EMAT was studied for two different positions of the meandering coil. Next, the driving mechanism was discussed on the basis of the experimental results. It was then shown that the EMAT separately generates a Lamb wave using the time and the spatial change of the synthesized magnetic field and generates a Lamb wave using a different driving mechanism under the lead lines and between the lead lines of the meandering coil. Finally, it is indicated that it would be effective to independently drive under the lead lines or between the lead lines of the meandering coil for the exact evaluation of the drawability in thin steel sheets.

Characterization of formability in cold-rolled steel sheets using electromagnetic acoustic tranducers

An ultrasonic technique for evaluating the formability of cold-rolled steel sheets is developed. The technique is based on the generalized dispersion relation, which correlates the velocity anisotropy of ultrasonic plate modes guided in the sheet plane to the texture defined by orientation distribution coefficients. Electromagnetic acoustic transducers (EMAT’s), constructed of permanent magnets and meander-line coils, allow the accurate and easy measurement of transit times of theSo (fundamental symmetric) mode. The planar average of the transit times shows a close correlation with both the planar average of plastic strain ratios (r value), obtained through tensile tests and the pole intensities, measured with the X-ray diffraction method. These favorable comparisons with the destructive tests indicate a good possibility of texture monitoring with the noncontacting EMAT’s.

On-line measurement of steel sheetr-value using magnetostrictive-type EMAT

An ultrasonic on-line system to measurer values in cold rolled steel sheets has been developed with electromagnetic acoustic transducers (EMATs). These EMATs are composed of meanderline coils and electromagnets and operate with the magnetostrictive mechanism. The EMAT instrument measures the propagation times of the fundamental symmetrical Lamb (So) wave at a low frequency and relates them to ther values through a calibrated regression curve. Preliminary tests indicate that the on-line monitoring ofr is quite feasible with a standard deviation of 0.07 for various low carbon steel sheets; the thicknesses range from 0.5 mm to 2.5 mm. The measuring time is 20 msec per data. The liftoff is allowed to 5 mm with 2 mm thick sheet. Ther evaluation is independent of the line speed up to 325 m/min. These promising results promote installation of ultrasonicr-value measurement systems in steel production lines.

DEVELOPMENT OF EMAT MONITORING SYSTEM OF FORMABILITY IN COLD-ROLLED SHEETS

Abstract A nondestructive Lankford value (r-value) evaluation system applicable to a production line was developed. The system measures ultrasonic velocity changes induced by texture with S0 Lamb wave Electromagnetic Acoustic Transducers (EMATs) The system is composed of EMATs, driving circuits, receiving circuits and the frame bridge. Three pairs of EMATs were placed at three different angles (0°, 45°, 90°) to the sheet rolling direction. The propagation time measured by the EMATs was analyzed and the averaged r-value was evaluated continuously Basic experiments with permanent-magnet and electromagnet type EMATs for S0 mode Lamb wave were made to investigate characteristics such as sensitivity, gap effect, magnetic effect, production line speed effect, etc. Finally, the electromagnet-type EMATs making use of the magnetostrictive effect were adopted, since they had a high sensitivity at 5mm air gap from a steel sheet in the production line. By using the system with this type of EMATs, we confirmed that th...

Non-destructive evaluation of formability of zinc-coated steel sheets using electromagnetic acoustic transducer

Abstract An ultrasonic technique for evaluation of the formability of zinc-coated steel sheets has been developed. The technique is based on the dispersion relation, which correlates the velocity anisotropy of the S 0 -mode Lamb wave to the texture defined by orientation distribution coefficients. In cold rolled steel sheets, we have already developed the evaluation system and installed it on two continuous annealing production lines. In this paper, we describe how to remove the effect of the zinc-coated layer from the measured velocity and the evaluation results of the formability for zinc-coated steel sheets. The coating induced a decrease in the S 0 -mode Lamb wave velocity. The change in ultrasonic velocity due to the zinc-coated layer has been shown to be compensated by using a simple correction curve, which enables the correction for the effect of zinc coating from the known coating/substrate thickness ratio. The test results indicate that the monitoring of the formability for zinc-coated steel sheets as well as for cold rolled steel sheets is quite feasible.

Development of On-Line Lamb Wave Testing Technique using Real Time Digital Signal Processing

Lamb wave testing technique has been used for on-line defect inspection of steel strips. However, conventional Lamb wave testing technique has the following two large problems. One problem is the existence of the wide dead zones which appear in near and far field. Another one is the misdetection due to parasitic echo or electrical noise. We have developed the method which solves these problems by using the signal processing technique. We designed and constructed the new system which collected the received signals and change it into the two-dimensional signal. It distinguish defect signals from the other signals by the two dimensional signal characteristics and the if-then rule logic. This method does not require the inspection gate. As a result, the dead zone and the misdetection can be reduced. This paper describes the measurement principle, the algorithm of the signal processing and the distinction logic. The results showed that the dead zone was reduced to 100mm from the strip edge.

Nondestructive evaluation of formability in thin steel sheets using fundamental Lamb wave

This paper describes the nondestructive evaluation method of the Lankford value (r-value) in thin steel sheets using the transit time of the fundamental Lamb wave. The (r-value) in thin steel sheet depends on the texture, that is, the (r-value) increases as the (111) crystal plane contained in the rolling plane increases, while it decreases as the (100) crystal plane sheet increases. The (r-value) evaluation method using the averaged fundamental Lamb wave velocity in the rolling plane has already been developed by many researchers. However, this method needs different calibration curve for steel sheets produced by different manufacturing processes, because some sheets have the same (r-value) but different texture, leading to a different velocity distribution. This paper describes why different Lamb wave velocities would be observed for thin steel sheets having the same (r-value) and how to evaluate the (r-value) by utilizing a combination of the averaged fundamental Lamb wave transit time and the characteristics of the transit time distribution. This result provides a more flexible measurement than the conventional methods and simplifies the evaluation procedures in industrial application.