Yoshikazu Ohara

Ultrasonic Evaluation of Closed Cracks Using Subharmonic Phased Array

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 measurement error of SPACE in measuring crack depths was approximately 1 mm, while that in the conventional method was 20 mm in an extreme case. To establish the basis of SPACE, we propose the concept of localized subharmonic resonances (LSRs) and explain SPACE images as accumulations of LSRs. As an example of LSR, the Rayleigh-mode resonance of a crack is described. The similarities and differences between subharmonic waves at closed cracks and at microbubbles are also discussed.

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.

Detection of Internal Micro Defects by Nonlinear Resonant Ultrasonic Method Using Water Immersion

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 SiCf/Ti composite has been demonstrated.

Improvement of Closed Crack Selectivity in Nonlinear Ultrasonic Imaging Using Fundamental Wave Amplitude Difference

To realize the high selectivity of closed cracks, we propose a fundamental wave amplitude difference (FAD) method based on the threshold behavior of fundamental waves caused by the contact vibration of closed cracks. This is realized by the subtraction of a fundamental array (FA) image at a small input amplitude multiplied by the amplification factor from that at a large input amplitude. The formulation clarified that FAD can selectively image closed cracks while eliminating other linear scatterers, which cannot be completely eliminated by the subharmonic wave amplitude difference (SAD) method. Furthermore, FAD was experimentally verified in a closed fatigue crack specimen using the closed-crack-imaging method of subharmonic phased array for crack evaluation (SPACE). Thus, we demonstrated that FAD is useful for achieving the higher selectivity of closed cracks against other linear scatterers than previous amplitude difference methods without filtering out the subharmonic or superharmonic waves.

Nonlinear ultrasonic imaging method for closed cracks using subtraction of responses at different external loads

To improve the selectivity of closed cracks for objects other than cracks in ultrasonic imaging, we propose an extension of a novel imaging method, namely, subharmonic phased array for crack evaluation (SPACE) as well as another approach using the subtraction of responses at different external loads. By applying external static or dynamic loads to closed cracks, the contact state in the cracks varies, resulting in an intensity change of responses at cracks. In contrast, objects other than cracks are independent of external load. Therefore, only cracks can be extracted by subtracting responses at different loads. In this study, we performed fundamental experiments on a closed fatigue crack formed in an aluminum alloy compact tension (CT) specimen using the proposed method. We examined the static load dependence of SPACE images and the dynamic load dependence of linear phased array (PA) images by simulating the external loads with a servohydraulic fatigue testing machine. By subtracting the images at different external loads, we show that this method is useful in extracting only the intensity change of responses related to closed cracks, while canceling the responses of objects other than cracks.

Enhancement of Selectivity in Nonlinear Ultrasonic Imaging of Closed Cracks Using Amplitude Difference Phased Array

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 multiplied by the input amplitude ratio from that at a large input. We verified the ADPA method by a two-dimensional simulation based on the finite difference time domain (FDTD) method with damped double nodes (DDNs) for subharmonic imaging of closed cracks. Furthermore, the ADPA method was experimentally verified in a closed-crack specimen.

Two-Dimensional Analyses of Subharmonic Generation at Closed Cracks in Nonlinear Ultrasonics

Nonlinear ultrasonics plays an important role in the detection of closed cracks in critical structures and machines, such as atomic power plants and airplanes. Although the design and operation of nonlinear measurement require realistic modeling and simulation in two dimensions (2D), only one-dimensional modeling has succeeded in reproducing subharmonic generation. In this study, we propose 2D analysis using a damped double nodes (DDNs) with compression residual stress in the finite-difference time-domain method. We then succeeded in reproducing the essential feature of observed subharmonic generation at closed stress corrosion cracks.

Closed-crack imaging and scattering behavior analysis using confocal subharmonic phased array

To solve the underestimation of closed-crack depth, we have developed an imaging method, subharmonic phased array for crack evaluation (SPACE). However, a single-array SPACE can image only the vicinity of a transmission focal point (TFP) when the TFP is fixed. In this study, we have developed a confocal SPACE that defines multiple TFPs for imaging closed cracks over a wide area. We demonstrated its usefulness by measuring a stress corrosion crack (SCC). Moreover, we proposed a radarlike display that shows single-focus images with a line indicating the incident direction. By applying it to the SCC specimen, a moving crack response (MCR) was observed with varying incident angles. To analyze this behavior, we performed a simulation using a finite-difference time-domain (FDTD) method with a damped double node (DDN) model. Furthermore, we examined the ratio of the subharmonic to fundamental responses depending on the stress ratio between input wave stress and crack closure stress (σc).

Evaluation of Closed Stress Corrosion Cracks in Ni-Based Alloy Weld Metal Using Subharmonic Phased Array

Closed stress corrosion cracks (SCCs) have been generated in Ni-based alloy weld metal in nuclear power plants. The ultrasonic inspection is difficult because of the crack closure. For the application of new inspection methods and training/educating of inspection engineers, realistic closed SCC specimens are required. However, there is no means for forming such SCC specimens in a reasonable amount of time. Here, we present a two-step method. The first step is to form an open SCC in chemical solution. The second step is to close the SCC by generating oxide films between the crack faces in high-temperature pressurized water (HTPW). To verify the crack closure, we used a closed-crack imaging apparatus, the subharmonic phased array for crack evaluation (SPACE). Consequently, we found that parts of the SCC after 1321 h immersion were closed in the HTPW. Thus, we verified the two-step method for forming realistic closed SCC specimens in a reasonable amount of time.

MONITORING GROWTH OF CLOSED FATIGUE CRACK USING SUBHARMONIC PHASED ARRAY

To ensure the safety and reliability of atomic power plants and airplanes, the technique of monitoring closed fatigue cracks is requisite. Here we monitored the distribution of the crack depths and closure behavior in the length direction after 48000 and 87000 fatigue cycles using subharmonic phased array for crack evaluation (SPACE). The crack depths in the subharmonic images were larger than those in the fundamental images. Specifically, the difference was larger at near the side surface than at the center. The percentage of the closed part varied with the crack growth in the specimen. In addition, we fabricated shoe for SPACE to facilitate mechanical scanning. Thus, it was demonstrated that SPACE is useful in monitoring closed fatigue crack growth.