Abstract
In this study, a modified imaging algorithm was implemented to improve the imaging accuracy for defects located on a structure. Based on analysis of the Lamb wave mode, a guided ultrasonic wave inspection technique was applied, which was able to illustrate images of defects in a 6 mm steel plate simulating containment liner plate (CLP) in nuclear power plants. The dominant Lamb wave mode was determined through short-time Fourier transform waveform analysis and imaging verification. Following tomography verification, limitations of the antisymmetric mode in the thick steel plate were identified. In addition, a modified shape factor, based on the energy distribution factor according to the beam pattern and beam width, was suggested for field applications and improved imaging accuracy. Results of the analysis revealed a beam skewing phenomenon for the Lamb wave mode. In the case of S0 2.7 MHz·mm, skewing as well as distortion effects are not observed in the experiment, while the S0 modes at 2.64 and 2.74 MHz·mm show either of them. Considering skewing width, the size of the shape function was modified. Application of the modified shape function allows us to obtain more accurate image to actual defect shape.
Highlights
From the viewpoint of mechanical and structural engineering, effective assessment of huge facilities and instruments has already been conducted to verify safety factors
RAPID technique was supplemented where the correct defect area was not visualized for RAPID technique was supplemented where the correct defect area was not visualized for some defects
Previous studies have attempted to increase the accuracy of the corresponding defects through the application of a variable shape factor, but this method has a limitation in that it is difficult to apply because of the environmental factors of the structure being imaged
Summary
From the viewpoint of mechanical and structural engineering, effective assessment of huge facilities and instruments has already been conducted to verify safety factors. Much research has aimed at developing the resolution of the imaging process and the applicability of SHM and real-time based assessment systems Despite these engineering enhancements, in the case of application of the RAPID algorithm, an imaging revision technique is required to illustrate defects with good reconstruction. Advanced RAPID tomographic techniques are required to improve the resolution and accuracy of images with respect to the operating surroundings To explain this effectively, abbreviations used are short-time Fourier transforming (STFT), nondestructive evaluation (NDE), nondestructive test (NDT), structural health monitoring (SHM), radioactive test (RT), eddy current test (ECT), computed tomography (CT), Lamb wave synthetic aperture focusing technique (L-SAFT), reconstruction algorithm for probabilistic inspection of damage (RAPID), meridional ultrasound tomography (MUT), helical ultrasonic tomography (HUT), time of flight (TOF), and signal difference coefficient (SDC). By analyzing the beam pattern and energy level, the modified shape factor was applied to the reconstruction of images
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