Yohei Shintaku

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.

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.

Ultrasonic imaging of stress corrosion crack formed in high temperature pressurized water using subharmonic phased array

Closed stress corrosion cracks (SCCs) have been generated in high temperature pressurized water in weld parts of important structure of atomic power plants. The depth of closed SCCs is often underestimated or overlooked, since the ultrasound is transparent through the closed cracks. This leads to catastrophic accidents. To solve this problem, we have developed a novel imaging method, namely, the subharmonic phased array for crack evaluation (SPACE), on the basis of subharmonic waves and a phased array algorithm. In this study, we formed an SCC in heat affected zone of weld part of austenitic stainless steel in high temperature pressurized water (280 deg C) to simulate those formed in actual atomic power plants. Then we imaged the SCC using SPACE with LN array transmitter. As a result, the subharmonic images visualized the crack tip which was deeper than in the fundamental images corresponding to linear phased-array image. Thus, we demonstrated that SPACE with LN array transmitter is useful in reducing the...

Two dimensional model for subharmonic generation at closed cracks with damped double nodes

We describe the first two dimensional (2D) model to reproduce subharmonic generation at closed cracks using damped double nodes (DDN). Numerical simulation using finite difference time domain (FDTD) method was performed using DDN. We verify transparency of low amplitude wave to closed cracks and typical stress divergence at crack tip produced by large amplitude wave.

Evaluation of Acoustic Anisotropy to Image Defects in Weld Metal by Ultrasonic Phased Array

An ultrasonic phased array (PA) does not work well in some weld metals owing to the strong acoustic anisotropy. However, the analysis of anisotropy for the compensation of the shift of the defect image in the PA has not been performed. In this study, we compared the results of imaging a slit in a Ni-based weld metal, commercially available as Inconel alloy 600, in an experiment and in simulations using the analytical solution of group velocity or the finite-difference time-domain (FDTD) method. As a result, we succeeded in reproducing the shift and broadening of the defect image obtained in an experiment by the simulations.

Formation and evaluation of closed stress corrosion cracks in Ni-based alloy weld metal for nuclear power plants

Closed stress corrosion cracks (SCCs) have been generated in nuclear power plants, resulting in the underestimation and nondetection. To solve this problem, we have developed closed-crack imaging method, the subharmonic phased array for crack evaluation (SPACE), on the basis of subharmonic waves and phased array technique. Here, after verifying the SPACE in a realistic SCC specimen, we present a two-step method for forming deep closed SCC for a reasonable amount of time. The SCC closure was verified by SPACE. This significantly contributes to the improvement of nondestructive evaluation methods and training/educating of inspection engineers.