• Korean Journal of Materials Research
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• v.23 no.10
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• pp.592-598
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• 2013

Graphene oxide powders prepared by two different drying processes, freeze drying and spray drying, were studied to compare the effect of the drying method on the physical properties of graphene oxide powder. The graphene oxide dispersion was prepared from graphite by chemical delamination with the aid of sulfuric acid and permanganic acid, and the dispersion was further washed and re-dispersed in a mixed solvent of water and isopropyl alcohol. A freeze drying method can feasibly minimize damage to the sample, but it requires a long process time. In contrast, spray drying is able to remove a solvent in a relatively short time, though this process requires exposure to a high temperature for a rapid evaporation of the solvent. The powders prepared by freeze drying and spray drying were characterized and compared by Raman spectroscopy, X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and by an elemental analysis. The graphene oxide powders showed similar chemical compositions; however, the morphologies of the powders differed in that the graphene oxide prepared by spray drying had a winkled morphology and a higher apparent density compared to the powder prepared by freeze drying. The graphene oxide powders were reduced at $900^{\circ}C$ in an atmosphere of $N_2$. The effect of the drying process on the properties of the reduced graphene oxide was examined by SEM, TEM and Raman spectroscopy.

• Advances in aircraft and spacecraft science
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• v.3 no.2
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• pp.171-195
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• 2016

The present paper is a review of the main activities carried out within the context of the COMPTINN' program, a joint research project founded by a FUI program (Fonds $Unifi{\acute{e}}s$ $Interminist{\acute{e}}riels$) in which four research teams focused on the thermo-oxidation behaviour of HTS-TACTIX carbon-epoxy composite at 'high' temperatures ($120^{\circ}C-180^{\circ}C$). The scientific aim of the COMPTINN' program was to better identify, with a multi-scale approach, the link between the physico-chemical mechanisms involved in thermo-oxidation phenomena, and to provide theoretical and numerical tools for predicting the mechanical behaviour of aged composite materials including damage onset and development.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.939-941
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• 2017

The use of composites is increasing for lightweight aerospace structures. Among these structures, the ring frame and the parts of the projectile body are mainly made of a fiber reinforced composite material which is less susceptible such as delamination to structural damage. As the use of fiber reinforced composites increases, interest in modeling efficient methods of stiffness and strength is increasing. This paper predict the mechanical strength according to the repeating unit cell(RUC) of the 2-D triaxial braided composites of fiber reinforced composites. Yarn slice definition, incremental approach and stiffness reduction model were used as strength prediction. Finally, the results of strength prediction are verified by comparing with experimental data of 2-D triaxial braided composites specimens.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.1
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• pp.39-45
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• 2001

Fourier transform has been one of the most common tools to study the frequency characteristics of signals. With the Fourier transform alone, however, it is difficult to tell whether signal's frequency contents evolve in time or not. Except for a few special cases, the frequency contents of most signals encountered in the real world change with time. Time-frequency analysis methods are developed recently to overcome the drawbacks of Fourier transform, which can represent the information of signals in time and frequency at the same time. In this study, damage process of a cross-ply carbon fiber reinforced plastic (CFRP) under monotonic tensile loading was characterized by acoustic emission. Different kinds of CFRP specimens were used to determine the characteristics of AE signals. Time-frequency analysis methods were employed for the analysis of fracture mechanisms in CFRP such as mix cracking, debonding, fiber fracture and delamination.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.6
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• pp.545-554
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• 2000

Compton X-ray backscatter technique has been used to quantitatively assess the impact damage in quasi-isotropic laminated composites and to obtain a cross-sectional profile of impact-damaged laminated composites from the density variation of the cross section. An adaptive filter is applied to the Compton backscattering data for the reconstruction and noise reduction from many sources including quantum noise, especially when the SNR(signal-to-noise ratio) of the image is relatively low. A nonlinear reconstruction model is also proposed to overcome distortion of the Compton backscatter image due to attenuation effects, beam hardening, and irregular distributions of the fibers and the matrix in composites. Delaminations masked or distorted by the first few delaminations near the front surface are detected and characterized both in width and location, by application of an error minimization algorithm.

• International Journal of Ocean System Engineering
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• v.2 no.2
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• pp.106-115
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• 2012

In the present study, TiN and CrN films were coated by arc ion plating equipment onto aluminum alloy substrate, A2024. The film thickness was about 4.65 ${\mu}m$. TiN and CrN films were analyzed by X-ray diffraction and energy dispersive X-ray equipments. The Young's modulus and the micro-Vickers hardness of aluminum substrate were modified by the ceramic film coatings. The difference in Young's modulus between substrate and coating film would affect on the wear resistance. The critical load, Lc, was 75.8 N for TiN and 85.5 N for CrN. It indicated from the observation of optical micrographs for TiN and CrN films that lots of cracks widely propagated toward the both sides of scratch track in the early stage of MODE I. TiN film began to delaminate completely at MODE II stage. The substrate was finally glittered at MODE III stage. For CrN film, a few crack can be observed at MODE I stage. The delamination of film was not still occurred at MODE II and then was happened at MODE III. This agrees with critical load measurement which the adhesive strength was greater for CrN film than for TiN film. Consequently, it was difficult for CrN to delaminate because the adhesive strength was excellent against Al substrate. The wear process, which the film adheres and the ball transfers, could be enhanced because of the increase in loading. The wear weight of ball was less for CrN than for TiN. This means that the wear damage of ball was greater for TiN than for CrN film. It is also obvious that it was difficult to delaminate because the CrN coating film has high toughness. The coefficient of friction was less for CrN coating film than for TiN film.

• The Journal of the Convergence on Culture Technology
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• v.8 no.6
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• pp.727-732
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• 2022

Applying the calculated cross-sectional reduction due to the corroded rebar investigated in the field to the numerical analysis model, the damage pattern and delamination of concrete in the field showed a tendency relatively similar to the numerical analysis results. It was analyzed that when the expansion pressure due to corrosion of the reinforcing bar is greater than the tensile stress of the concrete, cracks are generated and the concrete cover can be fracture. As a result of this study, the correlation between the corrosion rate of reinforcing bars and the crack occurrence of the concrete cover of the subway box structure was verified based on the numerical analysis and field test results. To prevent rebar corrosion, the corrosion rate can be reduced by applying rust prevention to the reinforcing bar and changing the material. In the case of exposed to a corrosive environment, the tensile strength of the concrete is improved by adjusting the concrete compressive strength to secure durability against the expansion pressure caused by the corroded rebar.

• Journal of the Korea Concrete Institute
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• v.15 no.2
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• pp.234-245
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• 2003

Most of the concrete bridge structures are exposed to damage due to the excessive traffic loading and the aging of the structure. The damage of concrete causes the further deterioration of the function in the concrete structure due to corrosion of the reinforced bars and decohesion between the concrete and the reinforced bar. The quick rehabilitation of the damaged concrete structures has become of great importance in the concrete structural system in order to avoid the further deterioration of the structures. Recently fiber sheets are used for strengthening the damaged concrete structures due to its many advantages such as its durability, non-corrosive nature, low weight, ease of application, cost saving, control of crack propagation, strength to thickness ratio, high tensile strength, serviceability and aesthetic. However, the lack of analytical procedures for assessing the nominal moment capacity by the fiber sheet reinforcement leads to difficulties in the effective process of decisions of the factors in the strengthening procedure. In this work, flexural strengthening effects by fiber sheets bonded on bottom face of the member are studied for the reinforced concrete T beam. In addition, auxiliary flexural strengthening effects by U-type fiber sheets bonded on bottom and side faces of the member to prevent delamination of the bottom fiber sheet are theoretically investigated. The analytical solutions are compared with experimental results of several references to verify the proposed approach. It is shown that the good agreements between the predicted results and experimental data are obtained.

• Journal of the Korea Concrete Institute
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• v.17 no.2 s.86
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• pp.263-271
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• 2005

HPFRCC(High Performance Fiber Reinforced Cementitious Composite) is a class of FRCCs(Fiber Reinforced Cementitious Composites) that exhibit multiple cracking. Multiple cracking leads to improvement in properties such as ductility, toughness, fracture energy, strain hardening, strain capacity, and deformation capacity under tension, compression, and bending. These improved properties of HPFRCCs have triggered unique and versatile structural applications, including damage reduction, damage tolerance, energy absorption, crack distribution, deformation compatibility, and delamination resistance. These mechanical properties of HPFRCCs become different from the kinds and shapes of used fiber, and it is known that the effective size of fiber in macro crack is different from that in micro crack. This paper reports an experimental findings on the mechanical properties of HPFRCCs reinforced with the micro fiber(PP50, PVA100 and PVA200) and macro fiber(PVA660, SF500). Uniaxial compressive tests and three point bending tests are carried out in order to compare with the mechanical properties of HPFRCCs reinforced with micro fibers or hybrid fibers such as compressive strength, ultimate bending stress, toughness, deformation capacity and crack pattern under bending, etc.,

• Composites Research
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• v.15 no.4
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• pp.23-31
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• 2002

The two dimensional size effect of specimen gauge section ($length{\;}{\times}{\;}width$) was investigated on the compressive behavior of a T300/924 $\textrm{[}45/-45/0/90\textrm{]}_{3s}$, carbon fiber-epoxy laminate. A modified ICSTM compression test fixture was used together with an anti-buckling device to test 3mm thick specimens with a $30mm{\;}{\times}{\;}30mm,{\;}50mm{\;}{\times}{\;}50mm,{\;}70mm{\;}{\times}{\;}70mm{\;}and{\;}90mm{\;}{\times}{\;}90mm$ gauge length by width section. In all cases failure was sudden and occurred mainly within the gauge length. Post failure examination suggests that $0^{\circ}$ fiber microbuckling is the critical damage mechanism that causes final failure. This is the matrix dominated failure mode and its triggering depends very much on initial fiber waviness. It is suggested that manufacturing process and quality may play a significant role in determining the compressive strength. When the anti-buckling device was used on specimens, it was showed that the compressive strength with the device was slightly greater than that without the device due to surface friction between the specimen and the device by pretoque in bolts of the device. In the analysis result on influence of the anti-buckling device using the finite element method, it was found that the compressive strength with the anti-buckling device by loaded bolts was about 7% higher than actual compressive strength. Additionally, compressive tests on specimen with an open hole were performed. The local stress concentration arising from the hole dominates the strength of the laminate rather than the stresses in the bulk of the material. It is observed that the remote failure stress decreases with increasing hole size and specimen width but is generally well above the value one might predict from the elastic stress concentration factor. This suggests that the material is not ideally brittle and some stress relief occurs around the hole. X-ray radiography reveals that damage in the form of fiber microbuckling and delamination initiates at the edge of the hole at approximately 80% of the failure load and extends stably under increasing load before becoming unstable at a critical length of 2-3mm (depends on specimen geometry). This damage growth and failure are analysed by a linear cohesive zone model. Using the independently measured laminate parameters of unnotched compressive strength and in-plane fracture toughness the model predicts successfully the notched strength as a function of hole size and width.