• 한국지반공학회논문집
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• 제38권12호
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• pp.19-28
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• 2022

벽체 배면에 발생한 공동은 벽체의 안정성에 악영향을 줄 수 있다. 본 연구의 목적은 실내 실험을 통해 콘크리트 벽체 배면에 발생한 공동을 마이크로폰을 이용하여 평가하는 방법을 개발하는 것이다. 철근이 매설된 콘크리트 판과 건조토를 이용하여 콘크리트 벽체 구조물과 배면의 흙을 모사하였으며, 속이 빈 반구형 플라스틱 통을 콘크리트 판과 건조토 사이에 설치하여 벽체 배면의 공동을 모사하였다. 누설 탄성 표면파는 해머로 콘크리트 판을 타격하여 발생시켰으며, 마이크로폰을 이용하여 측정하였다. 획득한 마이크로폰 신호의 고유주파수를 분석하여 공동이 존재하는 구역을 평가하였다. 공동이 존재하는 구역에서는 콘크리트 판의 굽힘 진동 거동에 의하여 1차 고유주파수가 감소하였다. 또한 1차 고유주파수에 해당하는 진폭은 공동에서 멀어질수록 감소하였으며, 철근에 인접한 측점에서는 더 큰 감소폭을 보였다. 본 연구는 마이크로폰이 콘크리트 벽체 배면에 발생한 공동의 위치를 평가하기 위해 유용하게 이용될 수 있음을 보여준다.

• 비파괴검사학회지
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• 제21권2호
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• pp.204-212
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• 2001

주파수 응답함수 (FRF)를 이용하여 평판 구조물에 존재하는 손상을 규명하기 위한 새로운 구조손상 규명기법을 소개하였다. 본 연구의 구조손상 규명기법은 손상된 평판의 운동방정식으로부터 유도되었으며 손상 분포함수를 이용하여 손상의 특성을 나타내었다. 본 연구의 구조손상 규명기법의 주요 특징은 다음과 같다. 첫째, 손상된 구조물에 대한 모드 데이터를 요구하지 않으며, 둘째, 가진 주파수와 측정점을 적절히 다수 선택함으로써 손살규명에 요구되는 관계식을 확보하는데 널다 유리하다. 몇몇 수치모의실험을 통하여 본 연구에서 제안한 구조손상 규명기법의 적용가능성을 검증하였다.

• Smart Structures and Systems
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• 제11권1호
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• pp.19-34
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• 2013

The objective of this paper is to develop on-line system parameter estimation and damage detection technique from the response measurements through using the Recursive Covariance-Driven Stochastic Subspace identification (RSSI-COV) approach. To reduce the effect of noise on the results of identification, discussion on the pre-processing of data using recursive singular spectrum analysis (rSSA) is presented to remove the noise contaminant measurements so as to enhance the stability of data analysis. Through the application of rSSA-SSI-COV to the vibration measurement of bridge during scouring experiment, the ability of the proposed algorithm was proved to be robust to the noise perturbations and offers a very good online tracking capability. The accuracy and robustness offered by rSSA-SSI-COV provides a key to obtain the evidence of imminent bridge settlement and a very stable modal frequency tracking which makes it possible for early warning. The peak values of the identified $1^{st}$ mode shape slope ratio has shown to be a good indicator for damage location, meanwhile, the drastic movements of the peak of $2^{nd}$ mode slope ratio could be used as another feature to indicate imminent pier settlement.

• 대한조선학회논문집
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• 제39권1호
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• pp.1-7
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• 2002

기존의 소음계측 장치가 유동장 및 구조적 진동의 영향을 받을 수 있어 개선이 요구되었다. 새로이 설계된 2개의 Hydrophone Box에 수중청음기를 설치하여 성능을 검증하였으며, KRISO 캐비테이션 터널과 외국의 다른 캐비테이션 터널들과 배경소음 특성을 비교하였다. 현재의 실험결과로 미루어 프로펠러 캐비테이션 소음의 실선성능 예측이 가능하다고 사료되며, 저주파에서의 특성이 개선되었음을 알 수 있었다.

• 한국세라믹학회지
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• 제27권7호
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• pp.883-890
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• 1990

Although various theories have been presented on the mechanism of setting retardation of 3CaO·Al2O3, this phenomenon has not yet been defined. The present investigation was initiated in order to solve the mechanism from the view point of coordination chemistry. The solubility of Ca(OH)2 in aquous solution of soldium gluconate was abnormally high, and was proportional to the concentration of sodium gluconate. These phenomena were attributed to the soluble complex formation, that is, (1 : 1)Ca complex formation between calcium ion and gluconate ion. The author's proposal was further confirmed by the results of electrical conductivity measurement. The formation of calcium complex was also supported by IR spectra and DTA. When sodium gluconate was dissolved in 3CaO·Al2O3 suspension, calcium complex and aluminum complex were formed. As an experimental evidence, the asymmetric stretching vibration of carboxyl group in sodium gluconate was observed to be shifted to lower frequency from 1625cm-1 to 1585cm-1 characteristically. The characteristic exothermic peaks of the complexs at 430℃ and 700℃ observed in DTA curve also suggest the formation of the complexs between sodium gluconate and 3CaO·Al2O3.

• International Journal of Concrete Structures and Materials
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• 제2권2호
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• pp.91-98
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• 2008

Curved bridges are important components of a highway transportation network for connecting local roads and highways, but very few data have been collected in terms of their field performance. This paper presents two-years monitoring and system identification results of a curved concrete box-girder bridge, the West St. On-Ramp, under ambient traffic excitations. The authors permanently installed accelerometers on the bridge from the beginning of the bridge life. From the ambient vibration data sets collected over the two years, the element stiffness correction factors for the columns, the girder, and boundary springs were identified using the back-propagation neural network. The results showed that the element stiffness values were nearly 10% different from the initial design values. It was also observed that the traffic conditions heavily influence the dynamic characteristics of this curved bridge. Furthermore, a probability distribution model of the element stiffness was established for long-term monitoring and analysis of the bridge stiffness change.

• 대한토목학회논문집
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• 제9권1호
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• pp.71-78
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• 1989

본(本) 논문(論文)은 주거(住居) 및 상가구조물(商街構造物)이 밀집(密集)된 지역(地域)에서 구조물(構造物)의 말뚝기초공사(基礎工事)를 위한 항타작업시(抗打作業時) 야기되는 지반진동(地盤振動)의 영향(影響)을 평가(平價)하기 위해서 디젤해머로 PC콘크리트말뚝을 항타(抗打)하는 동안 발생(發生)되는 지반진동(地盤振動)을 항타지점(抗打地點)으로부터 진동측정거리(振動測定距離)를 변경시키면서 측정(測定) 분석(分析)하였다. 본(本) 논문(論文)의 측정(測定)과 분석(分析)을 통하여 항타진동(抗打振動)의 감소(減素) 및 주파수특성(周波數特性)이 파악되었으며, 이로부터 항타진동(抗打振動)의 크기를 사전(事前)에 예측(豫測)할 수 있는 진동추정식(振動推定式)의 유도 및 항타기(抗打機) 선정(選定)을 비롯한 제반문제에 대하여 고찰(考察)하였다.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2001년도 춘계학술대회 논문집
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• pp.938-941
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• 2001

A tool holder system has been designed and builted to measure cutting forces in diamond turning. This system design includes a 3-component piezo-electric tranducer. Initial experiments with tool holder system included verification of its predicted dynamic characteristics as well as a detailed study of cutting parameters. In this research, tool holder system is modeled by considering the element dividing, material properties, and boundary conditions using MSC/PATRAN. Mode and frequency analysis of structure is simulated by MSC/NASTRAN, for the purpose of developing the effective design. In addition, tool holder system is verified by vibration test using accelerometer. Many cutting experiments have been conducted on 6061-T6 aluminum. Tests have involved investigation of velocity effects, and the effects of depth and feedrate on tool force. Cutting velocity has been determined to have negligible effects between 4 and 21㎧.(6) Forces generally increase with increasing depth of cut. Increasing feedrate does not necessarily lead to higher forces. Results suggest that a sample model may not be sufficient to describe the forces produced in the diamond turning process.

• 한국공작기계학회논문집
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• 제16권5호
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• pp.108-114
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• 2007

A new cutting force estimating approach and machining state monitoring examples are presented which uses a cylindrical displacement sensor built into the spindle. To identify the tool-spindle system dynamics with frequency up to 2 kHz, a home-built electro-magnetic exciter is used. The result is used to build an algorithm to extract the dynamic cutting force signal from the spindle error motion; because the built-in spindle sensor signal contains both spindle-tool dynamics and tool-workpiece interactions. This sensor is very sensitive and can measure broadband signal without affecting the system dynamics. The main characteristic is that it is designed so that the measurement is irrelevant to the geometric errors by covering the entire circumferential area between the target and sensor. It is also very simple to be installed. Usually the spindle front cover part is copied and replaced with a new one with this sensor added. It gives valuable information about the operating condition of the spindle at any time. It can be used to monitor cutting force and chatter vibration, to predict roughness and to compensate the form error by overriding spindle speed or feed rate. This approach is particularly useful in monitoring a high speed machining process.

• Structural Monitoring and Maintenance
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• 제4권1호
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• pp.33-52
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• 2017

A cantilever beam with a breathing crack is studied to improve the breathing crack identification sensitivity by the parametric optimization of sample entropy and wavelet transformation. Crack breathing is a special bi-linear phenomenon experienced by fatigue cracks which are under dynamic loadings. Entropy is a measure, which can quantify the complexity or irregularity in system dynamics, and hence employed to quantify the bi-linearity/irregularity of the vibration response, which is induced by the breathing phenomenon of a fatigue crack. To improve the sensitivity of entropy measurement for crack identification, wavelet transformation is merged with entropy. The crack identification is studied under different sinusoidal excitation frequencies of the cantilever beam. It is found that, for the excitation frequencies close to the first modal frequency of the beam structure, the method is capable of detecting only 22% of the crack depth percentage ratio with respect to the thickness of the beam. Using parametric optimization of sample entropy and wavelet transformation, this crack identification sensitivity is improved up to 8%. The experimental studies are carried out, and experimental results successfully validate the numerical parametric optimization process.