• 비파괴검사학회지
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• 제28권1호
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• pp.46-53
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• 2008

복합재료의 내부 결함 평가를 위해 일반적으로 적용되어온 초음파 C-스캔 기법은 섬유자동 배열을 통한 정밀 성형 공정에 적용하기에는 많은 어려움이 있다. 따라서 본 연구는 복합재료의 정밀 성형 공정 중에 발생되는 각종 내부 결함들을 비파괴적, 비접촉으로 평가하기 위한 새로운 하이브리드 초음파 평가 기법을 확립하는데 목적이 있다. 이를 위하여 본 연구에서는 초음파 산란 반사(scattering reflection) 방식을 토대로 한 새로운 이중 피치-캐치(dual pitch-catch) 기법을 확립하여 기존의 결함평가를 위해 시험편의 스캔에 소요되는 시간을 줄이면서 우수한 결함 영상을 얻을 수 있는 새로운 하이브리드 기법을 개발하였다. 즉 두 가지 종류의 열경화성 및 열가소성 복합재료(carbon/epoxy, carbon/PPS) 적층판의 내부 박리(delamination) 결함의 영상화를 위하여 레이저를 이용한 유도 초음파의 발생 및 이중 피치-캐치(pitch-catch)방식을 토대로 한 비접촉식 공기 정합 트랜스듀서(air-coupled transducer)를 이용하여 결함 영상을 얻기 위한 핵심 알고리즘을 확립하였다.

• 비파괴검사학회지
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• 제22권1호
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• pp.53-58
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• 2002

계자코일에 흐르는 교류전류로 전자기장을 유도하고, 결함에 의하여 누설되는 자속을 간극을 갖는 헤드로 측정하여 결함을 검출하는 새로운 비파괴탐상장치를 구성하였다. 이 탑상장치는 비접촉으로 강자성체의 구조물 요소에 존재하는 표면결항을 검출하고, 그 크기를 평가하는데 적용할 수 있다. 본 논문은 강자성체에 도입한 2차원 표면결함에 적용하여 결함의 평가를 실시하였다. 또한 측정주파수와 lift-off가 이들 평가에 미치는 영향을 검토하였다. 결함은 전압 분포에서 최대치로 검출할 수 있고, 이 최대전압의 변화는 결함의 깊이가 증가함에 따라 증가한다. 결함의 최대전압은 주파수와 lift-off 의 영향을 받지만, 작은 깊이의 결함은 깊이와 전압이$(V_0/V_{ave})$의 관계로부터 선형적으로 예측할 수 있다.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2002년도 춘계학술발표대회 논문집
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• pp.199-202
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• 2002

Interfacial properties and electrical sensing for fiber fracture in carbon and SiC fibers/epoxy composites were investigated by the electrical resistance measurement and fragmentation test. As fiber-embedded angle increased, interfacial shear strength (IFSS) of two-type fiber composites decreased, and the elapsed time was long to the infinity in electrical resistivity. The initial slope of electrical resistivity increased rapidly to the infinity at higher angle, whereas electrical resistivity increased gradually at small angle. Furthermore, both fiber composites with small embedded angle showed a fully-developed stress whitening pattern, whereas both composites with higher embedded angle exhibited a less developed stress whitening pattern. As embedded angle decreased, the gap between the fragments increased and the debonded length was wider for both fiber composites. Electro-micromechanical technique can be a feasible nondestructive evaluation to measure interfacial sensing properties depending on the fiber-embedded angle in conductive fiber reinforced composites.

• 비파괴검사학회지
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• 제27권3호
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• pp.246-254
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• 2007

The scanning laser source (SLS) technique is a promising new laser ultrasonic tool for the detection of small surface-breaking defects. The SLS approach is based on monitoring the changes in laser-generated ultrasound as a laser source is scanned over a defect. Changes in amplitude and frequency content are observed for ultrasound generated by the laser over uniform and defective areas. The SLS technique uses a point or a short line-focused high-power laser beam which is swept across the test specimen surface and passes over surface-breaking or subsurface flaws. The ultrasonic signal that arrives at the Rayleigh wave speed is monitored as the SLS is scanned. It is found that the amplitude and frequency of the measured ultrasonic signal have specific variations when the laser source approaches, passes over and moves behind the defect. In this paper, the setup for SLS experiments with full B-scan capability is described and SLS signatures from small surface-breaking and subsurface flaws are discussed using a point or short line focused laser source.

• 대한기계학회논문집A
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• 제26권4호
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• pp.745-753
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• 2002

It is required to evaluate nondestructively the crack depth of surface-breaking cracks for the assurance of safety of structure. Optical generation of ultrasound produces well defined pulses with a repeatable frequency content, that are free of any mechanical resonances; they are broad band and are ideal for the measurement of attenuation and scattering over a wide frequency range. Self-calibrating surface signal transmission measurement is very sensitive and practical tool for surface-breaking crack depth. In this paper, the self-calibrating technique by laser-based ultrasound is used to evaluate the depth of surface-breaking crack of material. It is suggested that the relationship between the signal transmission and crack depth can be used as a practical model for predicting the surface-breaking crack depths from the signal transmission measured in structure.

• 비파괴검사학회지
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• 제23권6호
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• pp.577-582
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• 2003

This paper describes a new technique for thickness measurement of a very thin layer less than one-quarter of the wavelength of ultrasonic wave used in the ultrasonic pulse-echo measurements. The technique determines the thickness of a thin layer in a tapered medium from constructive interference of multiple reflection waves. The interference characteristics are derived and investigated in theoretical and experimental approaches. Modified total reflection wave g(t) defined as difference between total and first reflection waves increases in amplitude as the interfacial layer thickness decreases down to zero. A layer thickness less than one-tenth of the ultrasonic wavelength is measured using the maximum amplitude of g(t) with a good accuracy and sensitivity. The method also requires no inversion process to extract the thickness information from the waveforms of reflected waves, so that it makes possible to have the on-line thickness measurement of a thin layer such as a lubricating oil film in thrust bearings and journal bearings during manufacturing process.

• 비파괴검사학회지
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• 제32권2호
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• pp.115-121
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• 2012

Exploration in microelectromechanical systems(MEMS) and nanotechnology requires evaluation techniques suitable for sub-micron length scale so that thermal and mechanical properties of novel materials can be investigated for optimal design of miro/nanostructures. The ultrafast optical pump-probe technique provides a contact-free and non-destructive way to characterize nanoscale thin-films, and its ultrahigh temporal resolution enables the study of heat-transport phenomena down to a sub-picosecond regime. This paper reviews the principle of optical pump-probe technique and introduces its application to the area of micro/nano-NDE.

• 비파괴검사학회지
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• 제27권3호
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• pp.217-223
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• 2007

This paper introduces an active sensing node using radio-frequency (RF) telemetry. This device has brought the traditional impedance-based structural health monitoring (SHM) technique to a new paradigm. The RF active sensing node consists of a miniaturized impedance measuring device (AD5933), a microcontroller (ATmega128L), and a radio frequency (RF) transmitter (XBee). A macro-fiber composite (MFC) patch interrogates a host structure by using a self-sensing technique of the miniaturized impedance measuring device. All the process including structural interrogation, data acquisition, signal processing, and damage diagnostic is being performed at the sensor location by the microcontroller. The RF transmitter is used to communicate the current status of the host structure. The feasibility of the proposed SHM strategy is verified through an experimental study inspecting loose bolts in a bolt-jointed aluminum structure.

• 동력기계공학회지
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• 제21권3호
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• pp.102-107
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• 2017

A tungsten inert gas (TIG) welding method was used for the bonding of stainless steel. TIG welding using inert gas (He or Ar gas) is a method to prevent oxidation and nitriding of materials and to combine non-ferrous metals. This method has the advantage of obtaining a smooth weld surface. In this study, the welding characteristics of 304 stainless steel welded by TIG welding method were analyzed by using nondestructive technique. Ultrasonic and Acoustic Emission (AE) was applied to evaluate the micro-damage of TIG welded 304 stainless steel. The velocity and damping coefficient of ultrasonic wave showed a slight difference in HAZ, which is the welding part of stainless steel. The AE parameters of average frequency, rise time and event were analyzed for the dynamic behavior of stainless steel during loading. Optimal AE parameters for evaluating the degree of damage to the specimen have been derived. Fractograph and metal structures of 304 stainless steel using SEM and optical microscope were discussed.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2001년도 ICCAS
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• pp.109.4-109
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• 2001

We have developed a laser ultrasonic system for visualization of elastic waves propagating on a solid surface, in order to visualize ultrasonic waves propagating on opaque media. This system can produce a series of successive images as an animation of wave propagation, because of scanning an optical heterodyne probe to measure surface transient displacements. Using this visualization technique, we observed the scattering and diffraction of ultrasonic waves around various shapes of artificial defects, and examined its application to nondestructive inspection. This imaging system provides various kinds of visualization images such as propagation image, amplitude image, arrival time image and velocity image. We have been confident that this technique is available for nondestructive inspection and materials ...