• Restorative Dentistry and Endodontics
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• v.45 no.3
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• pp.32.1-32.12
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• 2020

Objectives: To evaluate the polymerization efficiency of a matrix-modified bulk-fill composite, and compare it to a conventional composite which has a similar filler system. The degree of conversion (DC%) and monomer elution were measured over different storage periods. Additionally, fillers' content was examined. Materials and Methods: Cylindrical specimens were prepared, in bulk and incrementally, from Filtek Bulk Fill (B) and Filtek Supreme XTE (S) composites using a Teflon mold, for each test (n = 6). Using attenuated total reflection method of Fourier transformation infrared spectroscopy, DC% was measured after 24 hours, 7 days, and 30 days. Using high-performance liquid chromatography, elution of hydroxyethyl methacrylate, triethylene glycol dimethacrylate, urethane dimethacrylate, and bisphenol-A glycidyl dimethacrylate was measured after 24 hours, 7 days and 30 days. Filler content was examined by scanning electron microscopy (SEM). Data were analyzed using 2-way mixed-model analysis of variance (α = 0.05). Results: There was no significant difference in DC% over different storage periods between B-bulk and S-incremental. Higher monomer elution was detected significantly from S than B. The elution quantity and rate varied significantly over storage periods and between different monomers. SEM images showed differences in fillers' sizes and agglomeration between both materials. Conclusions: Matrix-modified bulk-fill composites could be packed and cured in bulk with polymerization efficiency similar to conventional composites.

• Composites Research
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• v.36 no.6
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• pp.429-434
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• 2023

In this study, aimed at improving the existing acoustic emission sensor for real time monitoring, a macro-fiber composite (MFC) transducer was employed as the acoustic emission sensor in the gas leak detection system. Prior to implementation, structural analysis was conducted to optimize the MFC's design. Consequently, the flexibility of the MFC facilitated excellent adherence to curved pipes, enabling the reception of acoustic emission (AE) signals without complications. Analysis of AE signals revealed substantial variations in parameter values for both high-pressure and low-pressure leaks. Notably, in the parameters of the Fast Fourier Transform (FFT) graph, the change amounted to 120% to 626% for high-pressure leaks compared to the case without leaks, and approximately 9% to 22% for low-pressure leaks. Furthermore, depending on the distance from the leak site, the magnitude of change in parameters tended to decrease as the distance increased. As the results, in the future, not only will it be possible to detect a leak by detecting the amount of parameter change in the future, but it will also be possible to identify the location of the leak from the amount of change.

• Smart Structures and Systems
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• v.15 no.6
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• pp.1463-1479
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• 2015

Lamb waves have been a promising candidate for quantitative damage identification for various engineering structures, taking advantage of their superb capabilities of traveling for long distances with fast propagation and low attenuation. However, the application of Lamb waves in damage identification so far has been hampered by the fact that the characteristic signals associated with defects are generally weaker compared with those arising from boundary reflections, mode conversions and environmental noises, making it a tough task to achieve satisfactory damage identification from the time series. With awareness of this challenge, this paper proposes a time reversal-based technique to enhance the strength of damage-scattered signals, which has been previously applied to bulk wave-based damage detection successfully. The investigation includes (i) an analysis of Lamb wave propagation in a plate, generated by PZT patches mounted on the structure; (ii) an introduction of the time reversal theory dedicated for waveform reconstruction with a narrow-band input; (iii) a process of enhancing damage-scattered signals based on time reversal focalization; and (iv) the experimental investigation of the proposed approach to enhance the damage identification on a composite plate. The results have demonstrated that signals scattered by delamination in the composite plate can be enhanced remarkably with the assistance of the proposed process, benefiting from which the damage in the plate is identified with ease and high precision.

• Journal of the Korean GEO-environmental Society
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• v.24 no.11
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• pp.11-17
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• 2023

In recent construction research, the focus has primarily been on developing 3D printers and construction-specific materials. 3D printing technology in construction is growing rapidly due to its potential benefits. However, there's a notable lack of research on the fire performance of 3D Printed Concrete (3DPC) walls. This study addresses this gap by investigating how 3DPC walls respond to controlled heating conditions in a simplified test. The research aims to provide crucial insights into the behavior of 3D-printed mortar composite walls when exposed to fire. The findings have the potential to enhance safety and reliability in 3D printing technology within the construction industry. Furthermore, it could contribute to improving the fire safety standards of architectural structures and expand the use of 3D printing in future construction projects.

• Composites Research
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• v.30 no.1
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• pp.15-20
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• 2017

In this study, LIPCA-S2 actuator with a piezoelectric single crystal layer and a carbon/epoxy layer was designed and evaluated to increase actuation performance of piezo-composite unimorph actuator. A curvature change model generated by the induced strain of a piezoelectric layer was used to predict the tip displacement of the piezo-composite unimorph cantilever. However, we found that there was big difference between the predicted and the measured tip displacement of LIPCA-S2 cantilever actuator when we used the previous linear prediction model. A new prediction model considering the change of piezoelectric strain coefficient and elastic modulus for the compression stress variation of the PMN-29PT single crystal layer was used and it was found that the difference between the predicted and the measured tip displacement reduced considerably.

• Composites Research
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• v.16 no.3
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• pp.58-67
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• 2003

To perform the realtime strain and fracture monitoring of the smart composite structures, two optical fiber sensor systems are proposed. The two types of the coherent sources were used for fracture signal detection - EDFA with FBG and EDFA with Fabry-Perot filter. These sources were coupled to EFPI sensors imbedded in composite specimens. To understand the characteristics of matrix crack signals, at first, we performed tensile tests using surface attached PZT sensors by changing the thickness and width of the specimens. This paper describes the implementation of time-frequency analysis such as short time Fourier transform (STFT) and wavelet transform (WT) for the quantitative evaluation of fracture signals. The experimental result shows the distinctive signal features in frequency domain due to the different specimen shapes. And, from the test of tensile load monitoring using optical fiber sensor systems, measured strain agreed with the value of electric strain gage and the fracture detection system could detect the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signal.

• Composites Research
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• v.19 no.3
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• pp.43-49
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• 2006

In this paper, a long gauge Fiber Bragg Grating (FBG) sensor system is described and long gauge FBGs are well-suited for measuring the upper parts of the bridge piers under the extremely severe movement conditions. In the experiments, we used more than 30m long FBG sensors to measure the movement of top part of the bridge piers which are separated from the main bridge by cutting the decks. With the actuator, the deck and girders were pushed and released. We checked the movement of the top of the pier while releasing the pressure of the actuator with the long gauge fiber sensor. In order to measure the movement of the upper part of the pier, the reference point must be outside of the pier. Using the optical fiber sensors, one end of the sensor is attached to the top of the pier and the other end is attached to the bottom of the next pier. The fiber sensors showed good response to the release loading and we could calculate the movement of the top part of the pear.

• Smart Structures and Systems
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• v.7 no.3
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• pp.213-222
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• 2011

The native extracellular matrix (ECM) consists of an integrated fibrous protein network and proteoglycan-based ground (hydrogel) substance. We designed a novel electrospinning technique to engineer a three dimensional fiber-hydrogel composite that mimics the native ECM structure, is injectable, and has practical macroscale dimensions for clinically relevant tissue defects. In a model system of articular cartilage tissue engineering, the fiber-hydrogel composites enhanced the biological response of adult stem cells, with dynamic mechanical stimulation resulting in near native levels of extracellular matrix. This technology platform was expanded through structural and biochemical modification of the fibers including hydrophilic fibers containing chondroitin sulfate, a significant component of endogenous tissues, and hydrophobic fibers containing ECM microparticles.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.2 s.257
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• pp.152-159
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• 2007

A study on the control of free vibration and stability characteristics of rotating hollow circular shafts subjected to compressive axial forces is presented in this paper. Both passive structural tailoring technique and active control scheme via collocated piezoelectric sensing and actuation are used in the study Gyroscopic and centrifugal forces combined with the compressive axial force contribute to the occurrence of divergence and flutter instabilities of the rotating shaft. The dual methodology based on the passive and active control schemes shows a high degree of efficiency toward postponement of these instabilities and expansion of the domain of stability of the system. The structural model of the shaft is based on an advanced thin-walled beam structure that includes the non-classical effects of transverse shear, anisotropy of constituent materials and rotatory inertia.

• Computers and Concrete
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• v.32 no.5
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• pp.439-454
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• 2023

In this article, the surface stress effects on the buckling analysis of the annular sandwich plate is developed. The proposed plate is composed of two face layers made of carbon nanotubes (CNT) reinforced composite with assuming of fully bonded to functionally graded porous core. The generalized rule of the mixture is employed to predict the mechanical properties of the microcomposite sandwich plate. The derived potentials energy based on higher order shear deformation theory (HSDT) and modified couple stress theory (MCST) is solved by employing the Ritz method. An exact analytical solution is presented to calculate the critical buckling loads of the annular sandwich plate. The predicted results are validated by carrying out the comparison studies for the buckling analysis of annular plates with those obtained by other analytical and finite element methods. The effects of various parameters such as material length scale parameter, core thickness to total thickness ratio (hc/h), surface elastic constants based on surface stress effect, various boundary condition and porosity distributions, size of the internal pores (e0), Skempton coefficient and elastic foundation on the critical buckling load have been studied. The results can be served as benchmark data for future works and also in the design of materials science, injunction high-pressure micropipe connections, nanotechnology, and smart systems.