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서호건,김명환,최성호,김정석,장경영 한국비파괴검사학회 2012 한국비파괴검사학회지 Vol.32 No.4
Using a single-line pulsed laser beam is well known as a useful noncontact method to generate a directional surface acoustic wave. In this method, different laser beam energy profiles produce different waveforms and frequency characteristics. In this paper, we considered two typical kinds of laser beam energy profiles, Gaussian and square-like, to find out a difference in the frequency characteristics. To achieve this, mathematical models were proposed first for Gaussian laser beam profile and square-like respectively, both of which depended on the laser beam width. To verify the theoretical models, experimental setups with a cylindrical lens and a line-slit mask were respectively designed to produce a line laser beam with Gaussian spatial energy profile and square-like. The frequency responses of the theoretical models showed good agreement with experimental results in terms of the existence of harmonic frequency components and the shift of the first peak frequencies to low.
Improvement of Crack Sizing Performance by using Nonlinear Ultrasonic Technique
서호건,장경영,Kyung-Cho Kim,홍동표 한국정밀공학회 2014 International Journal of Precision Engineering and Vol.15 No.11
The nonlinear ultrasonic technique (NUT) based on the contact acoustic nonlinearity (CAN) has been considered as a promisingmethod for the closed crack detection. However, most of the previous studies were limited to the modeling of the second-orderharmonic wave generation at contacted interfaces and its verification by testing artificially contacted interfaces in the throughtransmissionmethod. In this study, we investigated experimentally the contact acoustic nonlinearity at a real crack by using themeasurement system constructed in the pitch-catch method that permits the transducers to access the only single side of a teststructure. Results showed that the magnitude of the second-order harmonic wave represented the existence of the closed area clearlyand that the crack sizing performance was greatly improved by the combination of the linear and nonlinear ultrasonic techniques.
서호건,Jin-Gyum Kim,윤성희,장경영 한국정밀공학회 2015 International Journal of Precision Engineering and Vol.16 No.13
The laser ultrasonic technique in the ablation regime was studied for the effective excitation of ultrasound. First, the optimal laser beam intensity to maximize the amplitude of ultrasound was obtained. This is useful because the amplitude of ultrasound does not always increase as the laser beam intensity increases due to the plasma shielding effect. When the laser energy is fixed, for a steel specimen, the maximum ultrasonic amplitude is obtained at the laser beam intensity in the range of 10-20 GW/cm2. In addition, an inline method to obtain the optimal laser beam intensity was proposed based on the phenomena that the amplitude of the air-borne sound induced by plasma is inversely proportional to the amplitude of the generated ultrasound. Experimental results verified the usefulness of the proposed method by showing that the amplitude of ultrasound reached its maximum when the plasma-induced airborne sound disappeared.
서호건,송동기,장경영 한국비파괴검사학회 2016 한국비파괴검사학회지 Vol.36 No.2
Measurement of elastic constants is crucial for engineering aspects of predicting the behavior of materials under load as well as structural health monitoring of material degradation. Ultrasonic velocity measurement for material properties has been broadly used as a nondestructive evaluation method for material characterization. In particular, pulse-echo method has been extensively utilized as it is not only simple but also effective when only one side of the inspected objects is accessible. However, the conventional technique in this approach measures longitudinal and shear waves individually to obtain their velocities. This produces a set of two data for each measurement. This paper proposes a simultaneous sensing system of longitudinal waves and shear waves for elastic constant measurement. The proposed system senses both these waves simultaneously as a single overlapped signal, which is then analyzed to calculate both the ultrasonic velocities for obtaining elastic constants. Therefore, this system requires just half the number of data to obtain elastic constants compared to the conventional individual measurement. The results of the proposed simultaneous measurement had smaller standard deviations than those in the individual measurement. These results validate that the proposed approach improves the efficiency and reliability of ultrasonic elastic constant measurement by reducing the complexity of the measurement system, its operating procedures, and the number of data
포인트 클라우드 및 투영 이미지를 이용한 다중 모달 형상 분류
서호건(Hogeon Seo) 한국비파괴검사학회 2022 한국비파괴검사학회지 Vol.42 No.1
포인트 클라우드의 형상을 분류하기 위해 심층신경망을 활용할 때, 점들의 좌푯값만을 활용하거나 연산 부담이 큰 3차원 렌더링을 통해 생성한 이미지를 활용하여 형상 분류를 수행한다. 본 연구에서는 좌푯 값과 해당 좌표를 활용해 생성한 투영 이미지를 함께 다중 모달로 심층신경망의 입력으로 활용하는 포인트 클라우드의 형상 분류 기법을 제안한다. 성능 향상 여부를 확인하고자, 좌푯값 기반으로 형상을 분류하는 PointNet과 이미지 기반 분류 모델인 ResNet-18을 조합하여 다중 모달 모델을 구성하고 ModelNet40 데이터셋에 대해서 투영 이미지의 여부 및 방향(등각면, 정면, 측면, 상면)에 따른 성능 평가를 수행하였다. 그 결과 측면 투영 이미지가 함께 고려될 때 가장 성능이 좋았으며, 정면 투영 이미지의 경우가 두 번째로 우수한 성능을 보였다. 이는 포인트 클라우드의 형상 분류에 있어서 좌푯값과 더불어 투영 이미지를 함께 입력으로 활용하는 것이 형상 분류에 효과적임을 뒷받침한다. A deep neural network is used to classify the shape of the point cloud using only the coordinate values of the points or the rendered image. In this study, we proposed a point cloud shape classification technique using the coordinate values and the projection image generated using the coordinates as an input to a multi-modal deep neural network. To verify the performance improvement, the multi-modal model built using PointNet, which classifies the shapes based on coordinate values, and ResNet-18, an image-based classification model, was evaluated for the shape classification performance on a ModelNet40 dataset. The result showed that the performance was the best when the side projection image was additionally considered and the second best when the front projection image was considered. This supports the idea that the shape classification performance of the point cloud can be improved using the coordinate values and its projection image as the input to the deep neural network in a multi-modal manner.
펄스 와전류 시계열 데이터 딥러닝을 통한 배관 두께 추정
서호건(Hogeon Seo),전지현(Jihyun Jun),신정우(Jeong Woo Shin),박덕근(Duck-Gun Park) 한국비파괴검사학회 2021 한국비파괴검사학회지 Vol.41 No.3
와전류 탐상법은 전도성이 있는 검사체에 대한 비파괴평가에 효과적이지만, 신호를 판독함에 있어 검사자의 높은 숙련도를 요구한다. 본 연구는 펄스 와전류 탐상으로 획득한 시계열 데이터 기반 딥러닝을 통해 배관 두께를 추정함으로써 검사자의 의존성이 보완될 수 있음을 확인하였다. 이를 위해, 9단계의 배관 두께에 대해 각 배관 두께별로 8개 동경 지점에서 10회씩 측정하여 총 720개 데이터를 수집하였다. 이를 9:1 비율로 분리해 각각 학습과 평가에 사용하였다. 심층신경망은 2차원 이미지 데이터 분류에 활용되는 인셉션 모델을 1차원 시계열 데이터를 입력 받아 연산하도록 구성했다. 평균 절대 오차를 평가지표로 삼았고, 샘플링 길이와 이동 평균 적용 여부, 학습 시의 배치 크기에 따른 평균 절대 오차를 비교했다. 이로부터 시계열 데이터 기반 딥러닝을 통해 펄스 와전류 신호로부터 배관 두께를 추정할 수 있음을 확인했다. Eddy current testing is effective in the nondestructive evaluation of conductive specimens; however, high proficiency of an inspector is required in signal interpretation. This study confirmed that the dependence of the inspector can be complemented by estimating the pipe thickness via deep learning based on time series data acquired by pulse eddy current detection. In this study, a total of 720 data were collected by measuring ten times at eight longitude points for each of nine pipe thicknesses. They were separated by a 9:1 ratio and used for learning and evaluation, respectively. A deep neural network was built by modifying the Inception model used for classifying two-dimensional image data to input and operate one-dimensional time series data. Mean absolute error (MAE) was used as an evaluation index, and MAE values were compared according to the sampling length, moving averaging, and batch size in deep learning. Consequently, it was confirmed that the pipe thickness could be estimated from the pulsed eddy current signal by deep learning based on time series data.