• Composites Research
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• v.15 no.2
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• pp.11-17
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• 2002

The piezoelectric thin film sensor can be used to interpret variations in structural and material properties, e.g. for structural integrity monitoring and assessment. To illustrate one of this potential benefit, PVDF (polyvinylidene fluoride) film sensors are used for monitoring impact damage in Gr/Ep composite panels. Both PVDF film sensors and strain gages are attached to the surface of Gr/Ep specimens. A series of impact tests at various impact energy by changing impact mass the height are performed on the instrumented drop weight impact tester. The sensor responses are carefully examined to predict the onset of impact damage such as indentation, matrix cracking, and delamination, etc. Test results show that the particular waveforms of sensor signals implying the damage initiation and development are detected above the damage initiation impact energy. As expected, the PVDF film sensor is found to be more sensitive to impact damage initiation event than the strain gage.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.7
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• pp.492-498
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• 2016

It is well known that since abrupt faults in induction motors tend to lead to subsequent faults and deterioration of the drive apparatus, motor faults may lead to several operating restrictions, such as security problems and economic loss. A lot of research has been done in the area of diagnosis to detect machine faults and to prevent catastrophic hazards in the motor drive system. This paper presents a new method of motor current signature analysis in which the DC-link current of the inverter-driven induction motor system, where a single current sensor is employed instead of three AC current sensors, is measured, and fast Fourier transform analysis is performed. This proposed method makes it possible to easily discern and clearly separate the motor fault current signature from the normal operation current flowing through the stator and rotor windings.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.8
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• pp.721-724
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• 2016

Because of the inherent uncertainties caused by the manufacturing process variations, future loading conditions, and incomplete damage models, the lifetimes of mechanical structures under field conditions are significantly different from the results obtained in the laboratories. In this study, a dual sensor was developed to prognosticate the fatigue failure of structures under these uncertain conditions, and its effectiveness was demonstrated on a rectangular columnar structure under repeated uni-axial loading. The dual sensor is a slightly weaker structure embedded in the target structure, so that failure occurs in the sensor earlier than in the target structure. From the signal differences in the strain gauges in the embedded dual sensor, it is possible to differentiate between the normal status and warning status, even under variable loads.

• Composites Research
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• v.30 no.2
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• pp.126-131
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• 2017

For structural health monitoring of a complex shaped structure a new sensor that can compensate for the drawbacks of the current sensors such as brittleness is needed and the sensor should be highly flexible and durable. In this study a textile sensor made of polyvinylidene fluoride trifluoroethylene (PVDF-TrFE) which is a type of electroactive polymer was fabricated. And the textile sensors were applied to a complex shaped structure (an egg-box panel made of carbon/epoxy composite) for checking their feasibility of structural health monitoring. To correlate the collapse response with failure mechanisms of the structure the multiply-interrupted compressive test was carried out. During the test, the textile sensors succeeded to prove their applicability for damage detection (crack initiation) by generating electric voltages (0.05 V-0.25 V) in the real time.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.10
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• pp.1075-1081
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• 2011

A leak-detection method has been developed by measuring the pressure variation between the inner and outer heattransfer tubes of a double-wall tube steam generator. An experiment was carried out to measure the leak rate in the gap between two surfaces pressed with a hydraulic press in order to simulate the phenomena, and a correlation was determined for the leak rate in a micro gap. However, in the correlation, the gap width and friction coefficient were coupled with the surface roughness, which affects the two parameters. The two parameters were separated using a surface-contact model to develop a correlation for the friction coefficient. The correlation was compared with the existing correlations used for crack analysis. Although the applied ranges of Reynolds numbers were different, the developed correlation for Reynolds numbers of 0.1.0.35 showed similar tendencies to existing correlations used for higher Reynolds numbers.

• Journal of the Korea Institute of Building Construction
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• v.24 no.2
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• pp.203-213
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• 2024

This study delves into the mechanical properties of fiber-reinforced cement composites(FRCC) concerning the dispersion method of multi-walled carbon nanotubes(MWCNTs). MWCNTs find utility in industrial applications, particularly in magnetic sensing and crack detection, owing to their diverse properties including heat resistance and chemical stability. However, current research endeavors are increasingly directed towards leveraging the electrical properties of MWCNTs for self-sensing and smart sensor development. Notably, achieving uniform dispersion of MWCNTs poses a challenge due to variations in researchers' skills and equipment, with excessive dispersion potentially leading to deterioration in mechanical performance. To address these challenges, this study employs ultrasonic dispersion for a defined duration along with PCE surfactant, known for its efficacy in dispersion. Test specimens of FRCC are prepared and subjected to strength, drawing, and direct tensile tests to evaluate their mechanical properties. Additionally, the influence of MWCNT dispersion efficiency on the enhancement of FRCC mechanical performance is scrutinized across different dispersion methods.

• The Journal of Engineering Geology
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• v.19 no.1
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• pp.63-70
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• 2009

The optical fiber cable acting as a sensor was embedded in the underground research tunnel and portal area in order to monitor their stability and the spatial temperature variation. This system includes two types of sensing function to monitor the distributed strain and temperature along the line, where sensor cable is installed, not a point sensing. According to the results of one year monitoring around the KURT, there is no significant displacement or movement at the tunnel wall and portal slope. However, it would be able to aware of some phenomena as an advance notice at the tunnel wall which indicates the fracturing in rockmass and shotcrete fragmentation before rock falls accidently as well as movement of earth slope. The measurement resolution for rock mass displacement is 1 mm per 1 m and it covers 30 km length with every 1m interval in minimum. In temperature, the cable measures the range of $-160{\sim}600^{\circ}C$ with $0.01^{\circ}C$ resolution according to the cable types. This means that it would be applicable to monitoring system for the safe operation of various kinds of facilities having static and/or dynamic characteristics, such as chemical plant, pipeline, rail, huge building, long and slim structures, bridge, subway and marine vessel. etc.

• 한국방재학회:학술대회논문집
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• 2011.02a
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• pp.89-89
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• 2011

도로의 가드레일 지주 근입깊이의 부족에 의한 자동차의 전락사고 이 후, 일본의 국토교통성 등의 관계자들이 그 대책 세우기에 부심해 왔으나, 기설 지주의 근입깊이를 측정할 수 있는 방법은 아직까지 알려져 있지 않으며, 현재로서는 작업의 전 과정을 비디오로 촬영하여 그 기록을 남기도록 되어있다. 그러나 그것은 상당히 비효율적인 작업으로 엄밀한 감시기능을 다하지 못하고 있으며, 감독자와 시공자의 양자로부터 계측 도구의 개발이 절실히 요구되고 있다. 일부의 초음파 측정기 업자가 가드레일 지주의 근입깊이를 측정할 수 있다고 주장하고 있으나, 시장에는 아직 나타나지 않고 있으며, 그 측정시스템의 측정여부와 성능의 검증이 이루어지지 않고 있는 상황이다. 지금까지 충격탄성파법 또는 초음파법을 이용하여, 매설된 가드레일 지주의 근입깊이를 측정한 성공사례가 정식으로 보고된 바는 없으며, 같은 강관주인 눈사태 방지책의 지주 파이프에 대한 근입깊이의 측정은 본 연구그룹의 의해 행하여진 바가 있다. 검사봉이나 해머 등으로 대상물을 두드려서 탄성파를 발생시키고, 그것을 가속도계 또는 속도계의 진동센서로 감지하여 그 파형을 분석함으로써 대상물의 치수 등을 측정하는 충격탄성파법은, 특히 콘크리트를 대상으로 공동 및 매설물 등의 탐사, 균열깊이의 측정 등에 폭 넓게 사용되고 있다. 하지만 이 측정방법을 가드레일의 지주의 근입깊이 측정에 적용할 경우, 일반적으로 행하여지는 방법, 즉 진동센서를 대상물의 상단부(캡)에 설치하는 방법으로는 접합부에 의한 탄성파의 손실과 캡의 휨 진동에 의한 노이즈 등을 해결하기가 곤란해진다. 또한 지반의 존재로 인한 진동 모드의 변화와 진동에너지의 감소 등의 문제점을 해결하지 않으면 안 된다. 본 연구는 충격탄성파법을 이용하여 지반에 설치된 눈사태 방지책이나 가드레일의 지주와 같은 강관 구조물의 근입깊이를 측정하고자 하는 연구이다. 이를 위해 진동센서를 캡이 아닌 측면부에 취부장치를 이용하여 설치함으로써 길이방향의 탄성파를 측정할 수 있도록 하고, 실제 구조물에 대해 측정을 실시하여 그 측정시스템의 성능과 유용성을 검토하고자 한다. 또한 다양한 길이의 실험용 강관 파이프를 매설하고 측정실험을 실시하여 측정시스템의 적용성에 대해서도 검토하였다. 본 연구를 통하여, 수신센서를 파이프의 측면에 선접촉하게 함으로서 종파를 감지하여 근입깊이를 포함한 파이프의 전 길이를 측정하는 본 측정시스템은 매설된 강관 구조물의 길이 측정에 기본적으로 적용 가능함을 확인할 수 있었다. 특히 오거 굴착으로 시공된 경우에는 높은 정도의 측정성능을 보여주었다. 또한 항타관입 파이프에 대해서는 지반의 영향을 고려함으로써 길이의 측정이 가능하다는 것을 확인할 수 있었다. 즉, 오거 굴착 또는 항타 관입 등 시공방법에 따라 측정결과에 대한 지반의 영향 정도가 달라지며 파형 분석 및 길이 산정시 그 영향을 고려하여야 함을 확인하였다.

• Composites Research
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• v.24 no.1
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• pp.45-50
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• 2011

Transparent and conductive carbon nanotube on polyethylene terephthalate (PET) were prepared by dip-coating method for self-sensing multi-functional nanocomposites. The changes in the electrical and optical properties of CNT coating mainly depended on the number of dip-coating, concentration of CNT solution. Consequently, the surface resistance and transmittance of CNT coating were sensitively controlled by the processing parameters. Surface resistance of CNT coating was measured using four-point method, and surface resistance of coated CNT could be better calculated by using the dual configuration method. Optical transmittance of PET film with CNT coating was evaluated using UV spectrum. Surface properties of coated CNT investigated by wettability test via static and dynamic contact angle measurement were consistent with each other. As dip-coating number increased, surface resistance of coated CNT decreased seriously, whereas the transmittance exhibited little lower due to the thicker CNT networks layer. Interfacial microfailure properties were investigated for CNT and indium tin oxide (ITO) coatings on PET substrates by electrical resistance measurement under cyclic loading fatigue test. CNT with high aspect ratio exhibited no change in surface resistance up to 2000 cyclic loading, whereas ITO with brittle nature showed a linear increase of surface resistance up to 1000 cyclic loading and then exhibited the level-off due to reduced electrical contact points based on occurrence of many micro-cracks.

• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.4
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• pp.351-359
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• 2011

In a structure, damage can occur at several scales from micro-cracking to corrosion or loose bolts. This makes the identification of damage difficult with one mode of sensing. Hence, a multi-mode actuated sensing system is proposed based on a self-sensing circuit using a piezoelectric sensor. In the self sensing-based multi-mode actuated sensing, one mode provides a wide frequency-band structural response from the self-sensed impedance measurement and the other mode provides a specific frequency-induced structural wavelet response from the self-sensed guided wave measurement. In this study, an experimental study on the pipeline system is carried out to verify the effectiveness and the robustness of the proposed structural health monitoring approach. Different types of structural damage are artificially inflicted on the pipeline system. To classify the multiple types of structural damage, a supervised learning-based statistical pattern recognition is implemented by composing a two-dimensional space using the damage indices extracted from the impedance and guided wave features. For more systematic damage classification, several control parameters to determine an optimal decision boundary for the supervised learning-based pattern recognition are optimized. Finally, further research issues will be discussed for real-world implementation of the proposed approach.