• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.4C
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• pp.213-220
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• 2008

A study on cemented sand reinforced with short fibers was carried out to improve its unconfined compressive strength and brittle behavior. Nak-dong River sand was mixed with Portland cement and polyvinyl alcohol (PVA) fibers. A PVA fiber widely used for concrete reinforcement is randomly distributed into cemented sand. Nak-dong River sand, cement and fibers with optimum water content were compacted in 5 layers and then cured for 7 days. The effect of fiber reinforcement rather than cementation was emphasized by using a small amount of cement. Weakly cemented sand with a cement/sand ratio less than 8% was fiber-reinforced with different fiber ratios and tested for unconfined compression tests. The effect of fiber ratio and cement ratio on unconfined compressive strength was investigated. Fiber-reinforced cemented sand with 2% cement ratio showed up to six times strength to non-reinforced cemented sand. Because of ductile behavior of fiber-reinforced specimens, an axial strain at peak stress of specimens with 2% cement ratio increases up to 7% as a fiber ratio increases. The effect of 1% fiber addition into 2% cemented sand on friction angle and cohesion was analyzed separately. When the fiber reinforcement is related to friction angle increase, the 8% of applied stress transferred to 1% fibers within specimens.

• Journal of the Korean Society for Nondestructive Testing
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• v.18 no.5
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• pp.357-364
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• 1998

To investigate accuracies between intensity method and count method for measurement of the fiber orientation distribution, fiber orientation function is derived by drawing simulation figure for the fiber orientation as varying fiber aspect ratio, fiber area ratio, and fiber orientation state, respectively. The values of fiber orientation function measured by intensity method and count method are compared with the calculated ones from simulation figures. The results show that measurement accuracy of fiber orientation angle distribution obtained by count method is by 4% higher than that by intensity method.

• Computers and Concrete
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• v.7 no.5
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• pp.455-468
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• 2010

This paper presents a prediction and simulation method of tensile stress-strain curves of Engineered Cementitious Composites (ECC). For this purpose, the bridging stress and crack opening relations were obtained by the fiber bridging constitutive law which is quantitatively able to consider the fiber distribution characteristics. And then, a multi-linear model is employed for a simplification of the bridging stress and crack opening relation. In addition, to account the variability of material properties, randomly distributed properties drawn from a normal distribution with 95% confidence are assigned to each element which is determined on the basis of crack spacing. To consider the variation of crack spacing, randomly distributed crack spacing is drawn from the probability density function of fiber inclined angle calculated based on sectional image analysis. An equation for calculation of the crack spacing that takes into quantitative consideration the dimensions and fiber distribution was also derived. Subsequently, a series of simulations of ECC tensile stress-strain curves was performed. The simulation results exhibit obvious strain hardening behavior associated with multiple cracking, which correspond well with test results.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.407-412
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• 2001

In this research, damage behavior of singly oriented ply (SOP) fiber metal laminate (FML) subject to concentrated load was studied. The static indentation tests were conducted to study fiber orientation effect on damage behavior of FML. During the static indentation tests, Acoustic Emission technique (AE) was adopted to study damage characteristics of FML. AE signals were obtained by using AE sensor with 150kHz resonance frequency and the signals were compared with indentation curves of FML. As fiber orientation angle increases, the crack initiation load of SOP FML increases because the stiffness induced by fiber orientation is increased. The penetration load of SOP FML is influenced by the deformation tendency and boundary conditions. Cumulative AE counts were well predicted crack initiation and crack propagation and AE amplitude were useful for prediction of damage failure mode. During the matrix cracking, fiber debonding and fiber breakage, AE amplitude has $45{\sim}60dB,\;60{\sim}80dB\;and\;90{\sim}100dB$, respectively.

• Proceedings of the KIEE Conference
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• 1999.07d
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• pp.1627-1629
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• 1999

In order to analysis the degradation process of epoxy/glass fiber for outdoor condition, FRP laminate was exposed to high temperature and water. Then the degradation process was evaluated by comparing contact angle, surface potential decay, and surface resistivity. For the change of wettability, the contact angle of thermal-treated specimen with the high temperature of $200^{\circ}C$ increased. But that of water-treated specimen decreased. The characteristic of surface potential decay shows the tendency of the remarkable decrease on water-treated specimens, but no difference on thermal-treated specimen compared with untreated one. Also, for the surface resistivity, it shows the same trend compared with the change of contact angle.

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.6
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• pp.566-576
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• 2003

Carbon-fiber reinforced plastics (CFRP) composite laminates often possess strong in-plane elastic anisotropy attributable to the fiber orientation and layup sequence. The layup orientation thus greatly influences its properties in a composite laminate. It could result in the part being rejected or discarded if the layup orientation of a ply is misaligned. A nondestructive technique would be very beneficial, which could be used to test the part after curing and to require less time than the optical test. In this paper, ultrasonic scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. The motorized scanner was built first for making transmission measurements using a pair of normal-incidence shear wave transducers. Another scanner was then built fer the acousto-ultrasonic configuration using contact transducers. A ply-by-ply vector decomposition model has been developed, simplified, and implemented for composite laminates fabricated from unidirectional plies. We have compared the test results with model data. It is found that strong agreement are shown between tests and the model developed in characterizing cured layups of the laminates.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.21 no.12
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• pp.3277-3285
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• 1996

In this paper, w designed an opical power distribution device for application to an optical switching and an optical subscriber loop. We fabricated PSG thin film by LPCVD. Based on the measured index of fabricted thin film, rib-type waveguide was transformed to two-dimension by the effective index method and we simulated dispersion property to find asingle-mode condition. We found that the optimum design parameters of rib-type waveguide are:cladding layer of 3.mu.m, core layer of 3.mu.m, buffer layer of 10.mu.m, and core width of 4.mu.m. Each side of the guiding region was etched down to 4.mu.m to shape the core. We used these optimum parameters of the rib-type waveguide with branching angle of 0.5.deg. and simulted the Y-branch waveguide by the BPM simulation. Numerical loss in branching area was claculated to be 0.1581dB and equal to the total loss of the Y-branch. The loss of the fabricated Y-branch waveguide on PSG film ws 1.6dB at .lambda.=1.3.mu.m before annealing but was 1.2dB after annealing at 1000.deg. C for 10 minutes. Consequently, the loss of branching area from 3000.mu.m to 6000.mu.m in the z-direction was 0.8dB, and single-mode propagation was confirmed by measuring the near field pattern. For coupling the fabricated Y-branch waveguide with an optical fiber, we fabricated V-groove which was used as the upholder of optical fiber. An etching angle was 54.deg. and the width and depth of guiding groove was 150.mu.m, 70.mu.m, respectively. The optical fiber is inserted onto V-groove. Both the Y-branch and V-groove were connected through the index matching oil. Coupling loss after connecting Y-branch and the optical fiber on V-groove was 0.34dB and that after injecting index mateching oil was 0.14dB.

• Composites Research
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• v.31 no.5
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• pp.276-281
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• 2018

In this paper, elastic behavior of woven fabric composites with various fiber yarn structure were predicted through a theoretical calculation model. A representative volume elements (RVE) that can represent the mechanical properties of the woven composites were selected and crimp angle of the weave yarn was defined by several sinusoidal functions. The effective material properties of the woven composite such as young's modulus, shear modulus and poisson's ratio was predicted by classical laminate theory (CLT). The fiber volume fractions were calculated according to the shape and pattern (plain, twill weave) of the fiber yarn, and the elastic behavior of each woven composite was obtained through a theoretical calculation model. Also, to verify the theoretical predictions, woven composite specimens of plain and twill weave were fabricated by vacuum assisted resin transfer molding (VARTM) process and then mechanical test was conducted. As a results, a good correlation between theoretical and experimental results for the elastic behavior of woven composites could be achieved.

• Membrane Journal
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• v.29 no.6
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• pp.362-376
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• 2019

In this study, hydrophilic coating characteristics of PVDF [poly(vinylidene fluoride)] hollow fiber membranes with flower type cross-section prepared by thermally induced phase separation were studied. The hollow fiber used in this study was provided from PureEnvitech Co. Ltd., and the hydrophilic coating experiment was performed with different concentration and number of coating of PEBAX 1657, 2533 and 3533 block copolymer solution using a dip coating method. The hydrophilic coated hollow fiber membrane was characterized to scanning microscope and contact angle measurements to determine the degree of hydrophilization. As a result of SEM characterization, it was confirmed that the thickness of the coating layer increased as the coating concentration increased and the number of coatings increased. Contact angle of surface of hollow fibers decreased as the concentration of the coating solution increased and the number of coatings increased. Gas permeance of oxygen gas was measured for the application of the hydrophilized hollow fiber to Membrane Areated Biofilm Reactor. As a result of gas permeation test, it was confirmed that gas permeance decreased with increasing coating concentration and number of coatings, and the more hydrophilized hollow fiber coated with PEBAX 1657 showed lower gas permeance than those coated with PEBAX 2533 and 3533.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.127.1-127.1
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• 2005