• 대한기계학회논문집A
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• 제25권7호
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• pp.1147-1154
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• 2001

One of major problems in analyzing failure mechanism of real components is the accurate measurement of crack size and shape. The DCPD(Direct Current Potential Drop) method has been widely used for the crack measurement of a structure and finite element analysis has been used for the derivation of calibration equations, which relates the potential drop with the crack depth. In this paper, finite element analyses were performed for semi-elliptical surface cracks with various crack shapes(a/c) and crack depths(a/t). As a result, a calibration equation has been derived for the measurement of a semi-elliptical surface crack in wide plates. Analytical results are compared with experimental results to evaluate the validity and the applicability of the derived equation. The proposed method is expected to provide efficient and accurate measurement of a surface crack during crack growth.

• 한국가스학회지
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• 제4권1호
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• pp.49-54
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• 2000

구조물에 존재하는 균열을 측정하기 위해 DCPD법이 널리 사용되고 있다. 본 연구를 통해 표면균열의 성장뿐만 아니라 형상변화도 측정 가능한 다채널 DCPD 시스템을 개발하였고, 이를 위한 전용 소프트웨어를 개발하였다. DCPD 시스템을 CT 시편에 적용하여 그 효용성을 검증한 후, 넓은 평판에 존재하는 표면균열 측정실험에 시스템을 적용하였다. 표면균열 성장시 개발한 다채널 DCPD 시스템을 적응함으로써 효과적이고 정확한 균열측정이 가능하다는 결론을 얻었다.

• 한국음향학회지
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• 제16권3호
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• pp.51-56
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• 1997

본 논문에서는 SLAM(Scanning Laser Acoustic Microscope) 시스템을 이용하여 고체 내부에 발생한 크랙의 깊이를 측정하는 방법을 연구하였다. SLAM 시스템은 초음파의 투과계수에 따른 그림자 영상을 재생시키므로 크랙 깊이에 대한 정확한 측정방법이 요구된다. 이를 위하여 시료에 초음파를 사각으로 입사시켜 얻은 SLAM 영상의 그림자영역과 시료내의 초음파 모드 변환에 대한 기하학적 구조를 이용하여 크랙의 깊이를 측정할 수 있는 방법을 제안하고 실험을 통하여 확인하였다. 실험을 위하여 알루미늄에 서로 다른 깊이로 수직형 라인-크랙의 결함을 갖는 시료를 가공하였고 시료에 초음파를 사각 입사시키기 위하여 20$^{\circ}$ 각도로 웨지를 제작하였다. 실험 결과, 크랙의 깊이가 증가할수록 SLAM 영상의 그림자 영역이 비례적으로 증가함을 보였고, 결함의 깊이에 대한 측정치와 실제치를 비교한 결과 약 6% 이하의 측정오차를 보였다.

• 대한금속재료학회지
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• 제46권6호
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• pp.345-350
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• 2008

The magnitude of constraint effect $A_2$ value was experimentally estimated by using crack tip opening displacement(CTOD) between elastic and plastic regions near crack tip front for CT specimen with $25.4t{\ss}{\AE}$ SS400 steel. The constraint effect, $A_2$ was dependent on specimen configuration and on the measured positions of CTOD near crack front. $A_2$ should be estimated using the opening displacement calculated within crack front plastic region. If not, it's not reliable to evaluate of constraint effect at crack growth initiation in this paper.

• 한국구조물진단유지관리공학회 논문집
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• 제19권3호
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• pp.104-112
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• 2015

본 논문에서는 비접촉 표면파 측정을 이용하여 콘크리트 슬래브에 발생한 표면균열의 깊이를 측정하기 위한 비파괴 검사법을 연구하였다. 이를 위하여 표면파 측정, 해석 및 균열 깊이 평가의 과정을 포함한 새로운 측정모델을 제안하였다. 먼저, 3차원 유한요소해석 모델을 이용하여 표면파의 에너지와 콘크리트 균열의 깊이의 상관관계를 표현하는 표면파 전달함수를 구하였다. 제안된 측정모델은 실험을 통하여 증명하였다. 한 쌍의 비접촉 센서를 이용하여 깊이 0~100mm의 10개의 표면균열을 포함한 콘크리트 슬래브 ($1500{\times}1500{\times}180mm^3$)을 통과하여 전달되는 표면파를 측정하였다. 측정모델은 콘크리트 균열 깊이에 대하여 약 최대 10%의 오차를 보이며 실제 깊이를 예측하는 것으로 나타났다. 비접촉 표면파 측정을 통하여 얻은 결과는 기존의 TOFD에 바탕을 둔 초음파법에서 얻은 결과보다 향상된 정확도를 보이는 것으로 나타났다. 특히 비접촉 센서의 특성상 매우 향상된 측정 속도 및 측정값의 일관성을 얻을 수 있었다. 본 연구에서는 모델의 실제구조물에 적용성에 관한 토의를 포함하고 있다.

• Steel and Composite Structures
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• 제18권4호
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• pp.1045-1062
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• 2015

Concrete-filled circular t steel tubular joints (CFSTJs) in practice are frequently subjected to fluctuated loadings caused by wind, earthquake and so on. As fatigue crack is sensitive to such cyclic loadings, assessment on performance of CFSTJs with crack-like defect attracts more concerns because both high stress concentration at the brace/chord intersection and welding residual stresses along weld toe cause the materials in the region around the intersection to be more brittle. Once crack initiates and propagates along the weld toe, tri-axial stresses in high gradient around the crack front exist, which may bring brittle fracture failure. Additionally, the stiffness and the load carrying capacity of the CFSTJs with crack may decrease due to the weakened connection at the intersection. To study the behaviour of CFSTJs with initial crack, experimental tests have been carried out on three full-scale CFCST T-joints with same configuration. The three specimens include one uncracked joint and two corresponding cracked joints. Load-displacement and load-deformation curves, failure mode and crack propagation are obtained from the experiment measurement. According to the experimental results, it can be found that he load carrying capacity of the cracked joints is decreased by more than 10% compared with the uncracked joint. The effect of crack depth on the load carrying capacity of CFCST T-joints seems to be slight. The failure mode of the cracked CFCST T-joints represents as plastic yielding rather than brittle fracture through experimental observation.

• 대한기계학회논문집A
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• 제28권5호
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• pp.640-647
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• 2004

In order to perform elastic-plastic fracture mechanical analyses, fracture resistance curves for concerned materials are required. The unloading compliance method and the DCPD(Direct Current Potential Drop) method have been widely used for measuring the crack length and the extension for a standard specimen fracture resistance curve test. However it is difficult to apply the unloading compliance method to a real pipe fracture resistance curve test. The objective of this paper is to propose the calibration equation between the normalized crack length and the normalized electric potential, and to apply to pipe fracture experiments. For these, finite element analyses were performed with various current input locations and crack front configurations. Also the 4-point bending jig was manufactured for a pipe test and the DCPD method was used to measure crack extensions and crack lengths for a pipe test. The calculated crack length by the DCPD method agreed with the measured crack length within 5% error.

• Journal of Welding and Joining
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• 제30권6호
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• pp.74-79
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• 2012

It depends on the joint configuration, dimensions and constraints of the joint whether the residual stress at the root of single-sided butt weld is tensile or not. Therefore, recommendation is generally made that high R ratio should be used in the fatigue test of welded joint in order to prevent excessively long life caused by compressive residual stress. In this research, the residual stress profile in butt weld joint was obtained through compliance method, using successive extension of a slot and measurement of the variation of strain during the slot extension. The residual stress profile was firstly assumed to be the linear summation of Legendre polynomials up to 9th order excluding 0th and 1st order. Strain variation on the surface was measured while the slot was being extended by cutting to find out the 8 unknown coefficients of each polynomial term. The cut was made by the electric discharge machine. It was concluded that the residual stress near the surface is positive valued, however, it turned into the negative value as soon as it passed through 2 or 3 mm of the depth.

• 한국공작기계학회논문집
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• 제10권2호
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• pp.29-37
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• 2001

The application of laser in pipelines was started from the base of using laser in nuclear facilities industries and power plants. Because laser can be delivered to a remote area without any difficulties, the application of laser in many industries can solve many difficulties from limitation of access in danger area and reduced the risks of workers. Therefore, we developed a new experimental technique to measure internal defects of pressure vessels with a combination of shearog-raphy and image processing technique. Conventional NDT methods have been taken relatively much time, money and manpower because of performing as the method of contact with objects to be inspected. But digital shearography is laser-based optical method which allows full-field observation of surface displacement derivatives. This method has many advantages in practical use, such as low sensitivity to environmental noise, simple optical configuration and real time mea-surement. In this paper, we find the optimum shearing magnitude with EFM and experiment and measured internal crack length of the pressure vessels at a real time and estimated the error of the results.

• Composites Research
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• 제17권2호
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• pp.28-33
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• 2004

광섬유 브래그 격자센서를 이용하여 인장하중을 받는 직교적출 복합해로 법에서 발생하는 파손신호를 계측하였다. 민감도와 주파수 대역성능을 향상시키기 위하여 수동 마흐_젠더 간섭계 방식의 복조기를 제안하였다. 제안한 FBG 센서시스템은 위상변조기와 같은 능동소자가 없기 때문에 구성이 간단하고 쉽게 구현할 수 있으며 충격이나 파손 신호와 같이 작은 진폭의 고주파 진동 측정이 가능하다. FBG 센서로 측정한 복합재료의 파손신호는 복합재료 시편의 90도 층에서 모재 균열이 진전하면서 발생하는 급격한 변형률 변이에 해당하는 초기 오프？ 값과 최대 수백 킬로헤르쯔에 이르는 주파수로 진동하고 었었다.