Tsuyoshi Mihara

Evaluation of Closed Cracks by Model Analysis of Subharmonic Ultrasound

Cracks in solids can be detected by ultrasound if they are open. However, their detection is not easy when they are closed with a closure stress, and thus it is a fundamental problem in ultrasonic testing. Subharmonics with half the input frequency is potentially useful in the detection and evaluation of such cracks, although quantitative analysis has not been established. In this work, we develop analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, for the first time. We proved their validity by comparison with experiments on a well-defined fatigue crack in aluminum alloy, finding reasonable agreements. Based on these theories, it will be possible to estimate important parameters of partially closed cracks by fitting measured waveforms to theoretical predictions, which solves the fundamental problem in ultrasonic testing of cracks.

Nanoscale elasticity measurement with in situ tip shape estimation in atomic force microscopy

For a quantitative evaluation of nanoscale elasticity, atomic force microscopy, and related methods measure the contact stiffness (or force gradient) between the tip and sample surface. In these methods the key parameter is the contact radius, since the contact stiffness is changed not only by the elasticity of the sample but also by the contact radius. However, the contact radius is very uncertain and it makes the precision of measurements questionable. In this work, we propose a novel in situ method to estimate the tip shape and the contact radius at the nanoscale contact of the tip and sample. Because the measured resonance frequency sometimes does not depend so sensitively on the contact force as expected from the parabolic tip model, we introduced a more general model of an axial symmetric body and derived an equation for the contact stiffness. Then, the parameters in the model are unambiguously determined from a contact force dependence of the cantilever resonance frequency. We verified that this me...

Ultramultiple roundtrips of surface acoustic wave on sphere realizing innovation of gas sensors

A thin beam of wave usually diverges due to diffraction, which is a limitation of any device using such waves. However, a surface acoustic wave (SAW) on a sphere with an appropriate aperture does not diverge but is naturally collimated, realizing ultramultiple roundtrips along an equator of the sphere. This effect is caused by the balance between diffraction and focusing on a spherical surface, and it enables realization of high-performance ball SAW sensors. The advantage of ball SAW is most fully appreciated when applied to a very thin sensitive film for which the multiple-roundtrip enhances the sensitivity, but the attenuation loss is not very large. It is exemplified in a hydrogen gas sensor that realizes a wide sensing range of 10 ppm to 100% for the first time, and realizes relatively fast response time of 20 s without heating the sensitive film.

Ultrasonic Measurement of Closed Stress Corrosion Crack Depth Using Subharmonic Phased Array

We formed a stress corrosion crack (SCC) more than 10 mm deep to simulate those generated in atomic power plants. We precisely imaged the formed SCC by a novel imaging method, namely, the subharmonic phased array for crack evaluation (SPACE), and found that it had complex branches. Subsequently, we cut the specimen for the optical observation of the cross sections, and discussed the origin of the SCC extension on the basis of the optical microscopic observation of the cross sections. To examine the open and closed parts of crack in the optical images, we superposed the crack extracted from the optical images onto the SPACE images. We compared the optically and SPACE-measured crack depths, and demonstrated that SPACE is useful in reducing the underestimation of closed-crack depths.

Stress dependence of leaky surface wave on PMMA by line-focus-beam acoustic microscope

The authors discuss a line-focus-beam (LFB) acoustic microscope as a system for evaluating local stresses in polymeric materials. The goal of this paper is to reveal the stress-dependence of the velocity of the leaky surface skimming-compressional wave (LSSCW), which is excited by the LFB acoustic microscope and propagates on the boundary of the water/specimen surface.

Surface acoustic waves on a sphere with divergent, focusing, and collimating beam shapes excited by an interdigital transducer

The surface acoustic wave (SAW) device on a ball is expected to provide high-performance sensors. Since we must deal with three-dimensional wave fields for precise design of ball SAW devices, numerical methods, such as the finite element method, will require too many elements. Therefore, we applied an analytical solution to a practical calculation of the full-field acoustic waves. For an IDT aperture of 6° and 80° with a wave number parameter of 45, the SAW has a divergent beam and a focusing beam, respectively. For an intermediate aperture of 30°, the SAW forms a collimating beam. This calculation confirmed the possibility of an extremely sensitive sensor with diffraction-free SAWs without disturbance by spurious bulk waves.

Ball SAW device for hydrogen gas sensor

We propose a novel gas sensor using surface acoustic waves (SAW) on a sphere, with a typical velocity change of 10 ppm, and sensitivity of 10 ppb. In the first implementation for hydrogen sensor, a long propagation path of 1.3 m at 41th turns of 45 MHz SAW around a 10-mm-/spl phi/ quartz ball realized the propagation time as long as 403 /spl mu/s. Consequently, a sampling period of 25 ps realized relative temporal resolution of 0.06 ppm. By virtue of this high resolution, when sensing film was as thin as 20 nm, 7 ppm velocity change when exposed to 3% hydrogen in Ar gas was precisely measured. The response time was 60s, shortest among reported SAW hydrogen sensors, due to the small Pd film thickness. We also found a SAW attenuation increase which may be used to enhance the gas selectivity.

Evaluation of Response Time in Ball Surface-Acoustic-Wave Hydrogen Sensor using Digital Quadrature Detector

Hydrogen leak detection sensors must have high sensitivity and a short response time of 1 s or less. A ball surface-acoustic-wave (SAW) hydrogen sensor has a high sensitivity and can detect hydrogen in a very wide concentration range of 10 ppm to 100%. Moreover, a fast response can be expected because of the very thin sensitive film used. In this study, we developed a digital quadrature detector (DQD) to measure responses of less than 1 s, and measure phases in 1 ms intervals with excellent sensitivity. We evaluated the response time of the ball SAW hydrogen sensor where the signal was averaged 256 times in 0.256 s using the DQD. As a result, the response time was found to be 1 s or less for 3.0 vol % hydrogen gas in nitrogen.

Analysis of ball surface acoustic wave sensor response to wide variety of gases using gas chromatography

It is necessary to detect a wide variety of dangerous gases for environment assessment and security. Gas chromatography using a gas separation column has been used for this purpose; however, a compact and high-performance system has not yet been established. Thus, we propose a new approach to measure gases using a ball surface acoustic wave (SAW) sensor, after separating them using gas separation columns. In this work, amplitude responses were observed for mixed alcohol gases. With water–ethanol mixed gas, both delay time and amplitude responses were observed, and different response and recovery times were found for these responses. Also, an amplitude response reflecting the leaky loss of SAW was observed and an approximate equation of leaky loss was derived to quantitatively evaluate the roles of leaky loss and other parameters observed in the ball SAW sensor. These results suggest that the proposed approach is promising for developing compact and high-performance multigas sensors.

Measurement of Surface Acoustic Wave on a Quartz Ball with Proximate Electrodes to Improve Performance of Ball Surface Acoustic Wave Device

In order to improve the performance of ball surface acoustic wave (SAW) devices, we have developed an orientation control apparatus to find the Z axis of a quartz crystal ball by an optical birefringence measurement, and to find the X and Y axes by measuring the propagation characteristics of a SAW using an interdigital transducer (IDT), which is moved close to the ball using a translation stage. The soft spring of the vacuum chuck of the ball manipulator prevented damage to the ball or IDT during the contact. The concave shape of the ball holder of the chuck enabled repeatable alignment of the ball center and the manipulator axis. As a preliminary result, we confirmed that the SAW velocity variation had 60° periodicity and succeeded in identifying the -Y axis-equivalent direction on the quartz ball.