Satoshi Nagai

Laser generation of antisymmetric Lamb waves in thin plates

Abstract A convenient method is described for optically exciting and monitoring Lamb waves in thin plates. The interference pattern of two high power laser beams on a sample surface produces periodic heating and thus coherent antisymmetric Lamb waves are excited. By varying the interface fringe spacing, the acoustic frequency is easily and continuously tunable from 2.5 to 23 MHz with the present experimental apparatus. The ultrasound velocities as a function of frequency are measured to determine a dispersion curve, from which the Youngs modulus is deduced. Furthermore, the temperature dependence is estimated from the velocity changes. Experimental results are presented for several kinds of brittle plate and thin film materials.

Crack Measurements by Laser Ultrasonic at High Temperatures

A noncontact optical system for generation and detection of ultrasound is described. A confocal Fabry-Perot interferometer combined with a multimode optical fiber is used to reconstruct ultrasound images on a sample with a rough surface. The optical fiber is automatically adjusted to optimal positions for effective collection of scattered light from the sample. The confocal Fabry-Perot interferometer provides sensitive detection independent of surface roughness. The system is applied to a fatigue test sample, with which a conventional interferometer has poor sensitivity because of irregularities on the sample surface. The 2D profiles of the fatigue crack are successfully obtained at temperatures up to 1000°C. The crack length estimated from the profile agrees well with that measured using an optical method.

Optical Detection of Transient Lamb Waves on Rough Surfaces by a Phase-Conjugate Method

A convenient method is described for optically detecting Lamb waves on rough surfaces. A pulsed YAG laser excites Lamb waves, which are detected in a noncontact manner by a Michelson interferometer with a self-pumped phase-conjugate mirror. The present method automatically corrects the wavefront distortion caused by rough surfaces and also shows a flat spectral response over large bandwidths necessary for the analysis of the dispersion relationship. To illustrate the validity of the method, experimental results are presented for two kinds of thin plates.

Optical detection of ultrasound on rough surfaces by a phase-conjugate method

We describe an optical method to detect ultrasonic waves on materials with rough surfaces. Scattered beams from the rough surfaces are changed to the spatially coherent ones by a phase conjugate technique. A Michelson interferometer with a flat frequency response is adopted for faithful detection of the waveforms. The present method is applied to plasma-sprayed materials with the roughness of 10 μm. Longitudinal and surface waves are fairly detected, and the surface waves are then analysed in the frequency domain to deduce the dispersion curves. Experimental results on two alumina coatings prepared with different spray distances are demonstrated.

Acoustic power measurement using an optical heterodyne method

Abstract An optical heterodyne method is presented for measuring the Raman-Nath parameter of ultrasound. A light beam passes through a phase grating induced by ultrasound and then is recombined with another beam of light at a shifted frequency. The interference distribution of the two light beams is measured by a photodetector placed in the Fresnel region. The Raman-Nath parameter is determined from the amplitudes of frequency components in the beat signal and is independent of observation position. A theoretical analysis is given which includes pulsed ultrasound. Acoustic power determined using the technique is shown to agree fairly well with the radiation force method. Acoustic fields from a linear array transducer are also investigated. A problem involved in focused ultrasound is discussed.

Ultrasonic velocity measurements in molten materials with the use of laser-generated ultrasound

A novel approach for measurements of sound velocity at high temperatures in solid and molten materials is presented. The sample is contained by a transparent fused quartz cell, through which the laser-generated ultrasound is probed by an optical method. A Michelson-type interferometer is adopted for the faithful detection of the ultrasonic waveforms. The interferometer is combined with a phase conjugator, which converts a scattered beam from the sample into a well collimated beam, thus the ultrasound on a diffusing surface is easily detected at a distance. The temperature dependences of the longitudinal velocity in Sn and Ge have been obtained. The uncertainties in the velocity measurement for the Sn sample have been estimated to be 0.34% and 0.48% in the solid and molten phases, respectively.

Ultrasound detection by using a confocal Fabry-Perot interferometer with phase-modulated light

Abstract The optical detection of ultrasound with a radio-frequency (RF) sideband type of optical discriminator is described. The sidebands are produced on a laser beam by an optical phase modulator. A confocal Fabry–Perot interferometer with a lens at the mid-point in the cavity has been used to provide frequency discriminator functions. The cavity is tuned to the maximum transmission point, and then the RF beat signal in the back-reflection beam is detected. The ultrasound signals are obtained from the phase changes between the detected RF signal and the modulation source. Several experimental results are presented.

A New Interpretation of Critical Ultrasonic Absorption in the Nematic Phase of Liquid Crystals

The critical ultrasonic absorption in the nematic phase near the nematic-isotropic transition is interpreted in terms of three parts; the fluctuations of the director n and the scalar order parameter S and the critical slowing down of S itself. The fluctuations of n and S are analyzed, extending Imura and Okanos theory for the isotropic phase to the nematic phase. These processes merge into the fluctuations of the tensor order parameter Qαβ in the isotropic phase. The relaxation of S itself exists only in the nematic phase. The combinations of these three processes account quantitatively for the previously published data on 5CB.

Numerical study of neutral beam probe spectroscopy for edge fluctuation measurements

A thermal neutral beam probing combined with a spectroscopic technique is numerically demonstrated to be available to the measurement of local electron‐density fluctuations in edge plasmas, although the spatial attenuation of the beam due to the electron‐impact ionization is not negligible. It is found that the fluctuations are accurately measured in the region where the density is lower than (4–6) × 1012 cm−3, and that this critical density decreases slightly for the fluctuations which propagate in the beam direction with phase velocities comparable to a thermal speed of the beam. Some results of the beam probe mounted on the TEXTOR tokamak are also described and good agreement is found between numerical and experimental results.

Microcomputer-assisted system for measurement of ultrasonic velocity

Abstract A versatile control technique is described for the automatic measurement of ultrasonic velocities. This technique is compatible with a commercially available pulsed ultrasonic system and can continuously record transit time changes between a pair of echoes with gate circuits. A microcomputer is used for controlling the gate width and position. The resolution about ±0.1 ns is realized for frequencies up to 15 MHz. Some experimental results are reported for the elastic moduli determination of ceramics at elevated temperatures.