Yoshikazu Ohara 교수

The authors constructed a novel apparatus based on subharmonic ultrasound for the accurate imaging of closed cracks. Linear and nonlinear responses not only from the tip but also from other parts of cracks were observed in fundamental and subharmonic images, which were changed with varying closure stress. The subharmonic images always gave an accurate length of partially closed cracks, in contrast to the fundamental images in which the crack length was underestimated. Significant similarities in

We developed a novel imaging method, subharmonic phased array for crack evaluation (SPACE) based on subharmonic waves and a phased array algorithm, to measure closed-crack depth in the thickness direction. This implementation of SPACE used a LiNbO3 single-crystal transmitter to generate the intense ultrasound required for subharmonic generation and a phased array sensor as a receiver for focusing using delay laws. We applied SPACE to closed fatigue and stress corrosion cracks and found that the

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

A 79-year-old woman presented with chest pain. Her symptoms, combined with the results of an electrocardiogram, echocardiogram and laboratory investigations were compatible with an extensive acute anterior myocardial infarction. However, emergency coronary angiography showed no stenotic lesion in any coronary artery, but left ventriculography revealed apical ballooning akinesis and basal hyperkinesis and she was diagnosed as having transient left ventricular apical ballooning. After 7 days, she

A nonlinear resonant ultrasonic method using water immersion has been developed to detect micro defects in solids. This method allows us to reduce the strong nonlinearity of water and amplify the weak second harmonics generated at internal micro defects. In this method, both the fundamental and second harmonic frequencies are selected to match the resonant frequencies based on the resonant spectra of a sample. The detection of the micro defects in SiC f /Ti composite has been demonstrated.

Subharmonic waves realize a high selectivity for closed cracks. However, when a short-burst wave is used to achieve a high temporal resolution, not only closed cracks but also linear scatterers appear in the subharmonic image owing to leakage in frequency filtering. They are ghosts that degrade the selectivity for closed crack in the subharmonic image. Here, we propose an amplitude difference phased array (ADPA), where the ghosts are eliminated by subtracting a subharmonic image at a small input

The nondestructive evaluation of closed cracks is a challenging subject in ultrasonic testing. Recently, nonlinear ultrasonic phased array with fixed-voltage fundamental wave amplitude difference (fixed-voltage FAD) has been proposed as a practical approach. In this study, the maximum incident wave amplitude, which is one of the most critical parameters in closed-crack imaging, was investigated. First, a theoretical model was formulated to explicitly show the essence of the fundamental principle

Objective: To investigate the relation between thrombolysis in myocardial infarction (TIMI) frame count (TFC) and coronary blood flow velocity (CBFV) parameters reflecting the degree of microvascular injury in patients with acute myocardial infarction. Results: TFC and CBFV were measured after primary coronary angioplasty in 103 consecutive patients with their first anterior wall acute myocardial infarction. TFC correlated inversely with the averaged peak velocity ( r = −0.43, p < 0.0001). Ho

To accurately measure crack lengths, we developed a real-time surface imaging method (SAW PA) combining an ultrasonic phased array (PA) with a surface acoustic wave (SAW). SAW PA using a Rayleigh wave with a high sensitivity to surface defects was implemented for contact testing using a wedge with the third critical angle that allows the Rayleigh wave to be generated. Here, to realize high sensitivity imaging, SAW PA was optimized in terms of the wedge and the imaging area. The improved SAW PA w

The authors developed a practical method based on the thermal stress dependence of cracks for the accurate imaging of closed cracks with high selectivity. A tightly closed fatigue crack was visualized in linear phased array (PA) images by opening the crack with the tensile thermal stress induced by global preheating and local cooling (GPLC), although the conventional method of using only local cooling could not fully open the crack. Furthermore, by the subtraction of the PA images before and aft

Abstract Nondestructive evaluation of closed cracks is one of the most challenging issues in ultrasonic testing. Thus far, we have developed a closed crack imaging method, subharmonic phased array for crack evaluation (SPACE) of using only longitudinal (LL) subharmonic waves. In this study, we investigated the imaging capabilities of SPACE with LL and mode-converted shear (LS) subharmonic waves. After formulating the imaging algorithm, this method was applied to a complexly branched stress corro