• Structural Monitoring and Maintenance
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• 제5권1호
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• pp.129-150
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• 2018

This paper presents the structural performance monitoring of an urban footbridge located in Hangzhou, China. The structural health monitoring (SHM) system is designed and implemented for the footbridge to monitor the structural responses of the footbridge and to ensure the structural safety during the period of operation. The monitoring data of stress and displacement measured by the fiber Bragg grating (FBG)-based sensors installed at the critical locations are used to analyze and assess the operation performance of the footbridge. A linear regression method is applied to separate the temperature effect from the stress monitoring data measured by the FBG-based strain sensors. In addition, the static vertical displacement of the footbridge measured by the FBG-based hydrostatic level gauges are presented and compared with the dynamic displacement remotely measured by a machine vision-based measurement system. Based on the examination of the monitored stress and displacement data, the structural safety evaluation is executed in combination with the defined condition index.

• Geomechanics and Engineering
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• 제25권3호
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• pp.207-219
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• 2021

Several additives are used to enhance the geotechnical properties (e.g., shear wave velocity, shear modulus) of soils to provide sustainable, economical and eco-friendly solutions in geotechnical and geo-environmental engineering. In this study, piezoelectric ring actuators are used to measure the shear wave velocity of unreinforced, fiber, cemented, and fiber reinforced cemented Toyoura sand. One dimensional oedometer tests are performed on medium dense specimens of Toyoura sand-cement-fiber-silica flour mixtures with different percentages of silica flour (0-42%), fiber and cement (e.g., 0-3%) additives. The experimental results indicate that behavior of the mixtures is significantly affected by the concentration of silica flour, fiber and cement additives. Results show that with the addition of 1-3% of PVA fibers, the shear wave velocity increases by only 1-3%. However, the addition of 1-4% of cement increases the shear wave velocity by 8-35%. 10.5-21% increase of silica flour reduces the shear wave velocity by 2-5% but adding 28-42% silica flour significantly reduces the shear wave velocity by 12-31%. In addition, the combined effect of cement and fibers was also found and with only 2% cement and 1% fiber, the shear wave velocity increase was found to be approximately 24% and with only 3% cement and 3% fibers this increased to 35%. The results from this study for the normalized shear modulus and normalized mean effective stress agree well with previous findings on pure Toyoura sand, Toyoura silty sand, fiber reinforced, fiber reinforced cemented Toyoura sand. Any variations are likely due to the difference in stress history (i.e., isotropic versus anisotropic consolidation) and the measurement method. In addition, these small discrepancies could be attributed to several other factors. The potential factors include the difference in specimen sizes, test devices, methods of analysis for the measurement of arrival time, the use of an appropriate Ko to convert the vertical stresses into mean effective stress, and sample preparation techniques. Lastly, it was investigated that there is a robust inverse relationship between α factor and 𝞫₀ exponent. It was found that less compressible soils exhibit higher 𝜶 factors and lower 𝞫₀ exponents.

• Composites Research
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• 제21권4호
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• pp.15-21
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• 2008

단일 탄소섬유에 부착된 반구형 미소본드 시험편을 제사하여 에폭시수지와 난소섬유사이의 계면전단강도를 평가하였다. 반구형 미소시험편의 경우, 드랍레트 미소접합시험편 및 역반구형 미소접합시험편과 비교하여 계면강도측정값들이 높은 회귀계수 및 삭은 편차를 보여주었다. 이는반구형 시험편의 메니스커스 부분이 다른 미소시험편보다 작으며 핀홀 부하장치의 선단과 접촉하고 있는 수지부분에서 응력집중이 감소했기 때인 것으로 사료된다. 이들 시험에 대한 유한요소해석결과, 반구형시험편에서는 수지/섬유의 계면부를 따라 전단응력분포가 응력모드의 전환이 없이 안정하였다. 또한 이들 계면강도 측정데이터는 미소 바이스의 선단과 핀홀 판의 선단과 같은 부하장치의 종류에 따라 달라졌음을 알았다.

• Composites Research
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• 제27권4호
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• pp.152-157
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• 2014

Non-destructive testing methods of composite materials are very important for improving material reliability and safety. AE measurement is based on the detection of microscopic surface movements from stress waves in a material during the fracture process. The examination of AE is a useful tool for the sensitive detection and location of active damage in polymer and composite materials. FBG (Fiber Bragg Grating) sensors have attracted much interest owing to the important advantages of optical fiber sensing. Compared to conventional electronic sensors, fiber-optical sensors are known for their high resolution and high accuracy. Furthermore, they offer important advantages such as immunity to electromagnetic interference, and electrically passive operation. In this paper, the crack detection capability of AE (Acoustic Emission) measurement was compared with that of an FBG sensor under tensile testing and buckling test of composite materials. The AE signals of the PVDF sensor were measured and an AE signal analyzer, which had a low pass filter and a resonance filter, was designed and fabricated. Also, the wavelength variation of the FBG sensor was measured and its strain was calculated. Calculated strains were compared with those determined by finite element analysis.

• 조명전기설비학회논문지
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• 제29권5호
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• pp.13-18
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• 2015

In this paper, we implemented a polarimetric vibration sensor using a Sagnac birefringence interferometer composed of polarization-maintaining photonic crystal fiber(PM-PCF). By changing the amplitude and frequency of vibration applied to PM-PCF employed as the sensor head of the proposed sensor, sensor responses to various types of vibration were investigated. First, the vibration characteristic of the sensor was explored for a single frequency in a frequency range from 1 to 3000Hz with a cylindrical piezoelectric transducer, and then the sensor response to naturally damped vibration was examined by utilizing a metal cantilever. It was experimentally observed that the sensor output signal was deteriorated by more than 3dB at ~1900Hz in the single frequency vibration measurement with a minimum detectable strain perturbation of ${\sim}1.34n{\varepsilon}/Hz^{1/2}$ at 1500Hz and the peak value of the sensor output signal was proportional to the strength of initially applied stress in the naturally damped vibration measurement.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2001년도 정보화시공 학술발표회
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• pp.61-78
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• 2001

Various kind of fiber optic gauge sensors are available that can be bonded to civil engineering structures such as bridges, dams, tunnels and pipelines. A new fiber optic long gauge has significant advantages over other fiber optic sensors. These gauges can vary in length from less than 10 cm up to 30 m and provide the structural engineer with accurate measurements of displacement. These displacements can be converted to strains by dividing the measurement by the long gauge length. Using new optical technology, the long gauge instrument developed by FOX-TEK can choose max. 30 meters of gauge length so as to measure the very early stress/strain correlation curve. And this gauge length to be extended up to 100 meter in 2002.

• 폴리머
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• 제27권3호
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• pp.189-194
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• 2003

Electro-micromechanical 시험법을 이용하여 탄소 섬유 강화 열경화성 수지 복합재료의 경화 방법에 따른 계면 평가와 손상 감지능 및 경화 모니터링에 대해 고찰하였다. 경화 후 잔류 응력은 전기 저항 측정을 통해 모니터링 하였으며, 경화 방법에 따라 상호 비교하였다. 기지 재료의 인장 강도, 탄성률 및 계면 전단 강도는 열 경화의 경우가 자외선 경화보다 더 크게 나타났으며, 열 경화에서 경화 수축은 열팽창 계수 차이에 의한 잔류 응력 및 기지 재료의 수축에 의해 자외선 경화와 비교하여 더 크게 나타났다. 열 경화 시의 경화 중 전기 저항은 자외선 경화보다 더 큰 범위에서 변하였으며, 기지 재료의 기계적 물성과 계면 접착력에 의해 다르게 나타나는 겉보기 탄성률 또한 더 컸고, 같은 응력까지 더 빠르게 도달하였다.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2002년도 추계학술발표대회 논문집
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• pp.261-264
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• 2002

Interfacial evaluation, damage sensing and cure monitoring of single carbon fiber/thermosetting composite with different curing processes was investigated using electro-micromechanical test. After curing, residual stress was monitored by measurement of electrical resistance (ER) and then it was compared to correlate with various curing processes. In thermal curing, curing shrinkage appeared significantly by matrix shrinkage and residual stress due to the difference in thermal expansion coefficient (TEC). The change in electrical resistance (ΔR) on thermal curing was higher than that on ultraviolet (UV) curing. For thermal curing, apparent modulus was the highest and reaching time until same strain was faster. So far thermal curing shows strong durability on the IFSS after boiling test.

• Journal of Welding and Joining
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• 제29권3호
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• pp.76-81
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• 2011

Recently, the structural failures due to fatigue occur frequently with the increase of size of ships and offshore structures. In this respect, the assessment of fatigue life and the residual strength are very important. Currently, the smart materials technology has demonstrated a variety of possibilities for a diagnosis of structural strength and structural health condition for large structures. The benefits and feature of the MFC sensor are more flexible, durable and reliable than conventional smart material. In this study, Micro Fiber Composite (MFC) sensor for the measurement of stress intensity factor (SIF) of two dimensional cracks induced in a structure is developed. Two MFC sensors are placed in the vicinity of the crack tip close to each other with the crack tip in between them. The SIFs of Mode I($K_I$) as well as of Mode II($K_{II}$) based on the piezoelectric constitutive law and fracture mechanics are calculated. In this study, the SIF values measured by MFC sensors are compared with the theoretical results and measured value.

• Computers and Concrete
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• 제20권4호
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• pp.449-459
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• 2017

Fresh and hardened behaviors of a new hybrid fiber (steel fiber, polyvinyl alcohol fiber and calcium carbonate whisker) reinforced cementitious composites (HyFRCC) with admixtures (fly ash, silica fume and water reducer) have been studied. Within the limitations of the equipment and testing program, it is illustrated that the rheological properties of the new HyFRCC conform to the modified Bingham model. The relations between flow spread and yield stress as well as flow rate and plastic viscosity both conform well with negative exponent correlation, justifying that slump flow and flow rate test can be applied to replace the other two as simple rheology measurement and control method in jobsite. In addition, for the new HyFRCC with fly ash and water reducer, the mathematical model between the rheological and mechanical properties conform well with the quadratic function, and these quadratic function curves are always concave upward. Based on mathematical analysis, an optimal range of rheology/ flowability can be identified to achieve ideal mechanical properties. In addition, this optimization method can be extended to PVA fiber reinforced cement-based composites.