• Composites Research
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• v.28 no.4
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• pp.191-196
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• 2015

In this study, the static analysis and experiment were carried out on DCB specimens manufactured with aluminum foam in order to investigate the fracture toughness at the adhesive joint of the structure bonded with adhesive. In case of static analysis, all specimen models were shown to have the maximum reaction force when the forced displacement proceeded as much as 5 mm. The maximum reaction forces became 0.25 kN, 0.28 kN and 0.5 kN respectively in cases of specimen thicknesses of 35 mm, 45 mm and 55 mm. Two specimens in case of static experiment were selected to verify these analysis results. The maximum reaction forces were shown when the forced displacement proceeded as much as 5 to 6 mm. The maximum reaction forces became 0.22 kN and 0.3 kN respectively in cases of specimen thicknesses of 35 mm and 45 mm. By comparing the derived results, it could be shown that there was not much difference between the data of analyses and experiments. Therefore, It is inferred that the study data can be secured with only analysis by no extra experimental procedure. It is thought that the mechanical properties at the structure bonded of DCB with the type of mode III can be analyzed systematically.

• Journal of the Korea Concrete Institute
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• v.14 no.4
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• pp.449-456
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• 2002

Recently, the application of high strength and high performance concrete has been gradually increased as an important construction material for high rise and huge scaled construction. However, high performance concrete has undesirable characteristics of spalling subjected to high temperature due to its dense microstructure content. A spalling by fire brings surface failure and falling off concrete member. It is considered that spalling by fire should be taken into account for the safety of the concrete structure under fire. Therefore, in this paper, tests are carried out using high performance concrete containing polypropylene(PP) fiber in order to improve the fire resistance performance. PP fiber contents and member sizes are varied. According to experimental results, as for the influence of PP fiber contents, all the test specimens without PP fiber show entire failure in W/C of 35％, while they show nearly sound shape except some kinds of surface fracture in W/C of 55％. When PP fiber is contained more than 0.07％, favorable prevention effects of spatting by fire are obtained. As for the effects of test specimens size, it tends to increase the possibilities of spatting by fire as test specimens become larger. And spatting by fire at the edge of test specimens occurs more frequently than at the surface of test specimens. Residual compressive and tensile strength shows 45∼65 ％ of its original strength at W/C of 35％, and 30∼40％ at W/C of 55 ％.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.13 no.4
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• pp.176-181
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• 2003

Rapid densification of a SiC powder with additive 0.5 wt% $B_4$C was conducted by spark plasma sintering (SPS). The unique features of the process are the possibilities of using very fast heating rate and short holding time to obtain fully dense materials. The heating rate and applied pressure were kept to be $100^{\circ}C$/min and 40 MPa, while sintering temperature and soaking time varied to 1800, 1850, 1900 and $1950^{\circ}C$ and 10, 20 and 30 min, respectively. All of the SPS-sintered specimens at $1950^{\circ}C$ reached near-theoretical density. The XRD found that 3C-to-6H transformation at $1850^{\circ}C$. The microstructures of the rapidly densified SiC ceramics consisted of duplex microstructure with ultrafine equiaxed grains under 2 $\mu\textrm{m}$ and elongated grains of 0.5∼2 $\mu\textrm{m}$ wide, length 3∼10 $\mu\textrm{m}$. The biaxial strength increased with the increase of sintering time. Strength of 392.7 MPa was obtained with the fully densified specimen sintered at $1950^{\circ}C$ for 30 min, in agreement with the general tendency that strength increases with decreases pore. On the other hand, the fracture toughness shows the value of 2.17∼2.34 MPa$.$$m^{1/2}$ which might be due to the transgranular fracture mode.

• Korean Journal of Materials Research
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• v.9 no.12
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• pp.1170-1175
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• 1999

The microstructures and mechanical properties of $Si_3N_4$ ceramics produced by nitridation and post-sintering using semiconductor-waste-Si sludge were investigated. Lots of microcracks were observed in the waste-Si powders which contained some amounts of amorphous $SiO_2$. The nitridation rate of waste-Si compacts showed lower value than that of commercial Si powder compacts. The nitridation rate was increased with increasing nitridation temperature and then the percent of nitridation at 1470$^{\circ}C$ showed 98%. The phases of $Si_3N_4$ in the reaction-bonded bodies were mixed with ${\alpha}$ and ${\beta}$-type, and small amounts of $Si_2N_2O$ phase while those after post-sintering were ${\beta}$-$Si_3N_4$ and ${\alpha}$-Sialon. The sample post-sintered at 1950$^{\circ}C$ showed the fracture toughness of 5.6 $^MPa{\cdot}m^{1/2}$ and the fracture strength of 497 MPa which were lower than those of sintered body using commercial Si powder possibly due to the formation of ${\alpha}$-Sialon phase.

• Journal of the Korea institute for structural maintenance and inspection
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• v.27 no.1
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• pp.78-85
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• 2023

This study evaluates the performance of reinforced concrete columns using hybrid fiber sheets for structural behavior. The purpose of this method is to improve the load-bearing capacity of the reinforced structure by impregnating a hybrid fiber sheet, which is woven by arranging aramid and glass fibers uniaxially and attached to an aged concrete structure requiring reinforcement with epoxy. In particular, not only the weight reduction of the material obtained by using a fiber lighter than the steel material, but also the low-strength, high-toughness fiber element among the fibers used delays the brittle fracture of the high-strength, low-toughness fiber element. The low-strength, high-toughness fiber element among the fibers used delays the brittle fracture of the high-strength, low-toughness fiber element, resulting in weight reduction compared to steel. The study conducted structural tests on four specimens, with the hybrid reinforcement method and failure mode as main variables. Specimen size and loading conditions were chosen to be comparable with previous studies. The structural performance of the specimen was evaluated using energy dissipation capacity and ductility. Analysis shows that excellent results can be obtained with the hybrid fiber sheet reinforcement.

• Composites Research
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• v.36 no.4
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• pp.281-287
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• 2023

Environmental pollution from waste and the climate crisis, due to rising global average temperatures, are reaching critical levels threatening human survival. Research is ongoing across various fields to solve this problem, with a key focus on developing eco-friendly, carbon-neutral materials. Our study aimed to integrate natural fibers, known for their environmentally friendly properties and lower carbon emissions, with carbon fibers. In general, combining high-strength and low-strength materials results in intermediate properties. However, we found that certain properties in our study exceeded those of typical carbon fiber composite materials. To validate this, we produced both carbon fiber composite materials and carbon fiber/natural fiber hybrid composite materials. We then compared their mechanical properties using a range of specific tests. Our results revealed that the hybrid composite material exhibited superior bending strength and fracture toughness compared to the carbon fiber composite material. We also identified the underlying mechanisms contributing to this strength enhancement. This breakthrough suggests that the use of hybrid composite materials may allow the production of stronger structures. Moreover, this can play a significant role in mitigating environmental pollution and the climate crisis by reducing carbon emissions, a major contributing factor to these global challenges.

• Journal of Korea Technical Association of The Pulp and Paper Industry
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• v.41 no.2
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• pp.13-19
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• 2009

In order to produce high quality paper at the lowest cost in high speed, typically various polyelectrolytes as retention aids were used. Retention systems such as single polymer system, dual polymer system, and microparticle system were used. The objective of this study was to analyze the changes of retention, drainage, formation and fracture toughness depending on types of retention system, molecular weight of C-PAM and dosage sequences of agents. When single polymer system was applied, retention was increased with poor formation and drainage. When common microparticle system(C-PAM/bentonite) was used, high molecular weight PAM gave high retention and fast drainage, but poor formation. When the microparticle system with reverse dosage sequence(bentonite/C-PAM) was used, low molecular weight PAM gave high retention, fast drainage and good formation. When various retention agents were applied, fracture toughness was increased than that of blank. When using high molecular weight PAM and consequently causing excessive flocculation, fracture toughness was decreased.

• Polymer(Korea)
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• v.39 no.3
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• pp.426-432
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• 2015

Poly(ether sulfone) (PES) embedded diglycidylether of bisphenol-A (DGEBA) epoxy composites were fabricated for improving its mechanical properties and thermal stability. The mechanical properties such as tensile, flexural and impact strength of the composites changed significantly with the introduction of PES. The value of the fracture toughness of this composite also was increased remarkably about 24%. Thermal stability of PES/epoxy composites also improved 12%, which was calculated with integral procedural decomposition temperature (IPDT). From the differential scanning calorimeter (DSC) result, the curing temperature and curing heat decreased according to the increase of PES contents. These were attributed to the good distribution and the formation of the semi-interpenetrating polymer networks (semi-IPNs) composed of the epoxy network and linear PES.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.3
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• pp.121-125
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• 2018

In this study, finite element analyses of crack propagation along a sinusoidal interface are performed by using cohesive elements. BK law is used for cohesive zone to consider mixed mode traction-separation relation at the crack tip on a sinusoidal interface of a double cantilever beam specimen. The shape of a sinusoidal interface crack and the cohesive strength and the cohesive energies in mixed mode cohesive laws are varied in numerical experiments, and load-displacement curves at the ends of a double cantilever beam specimen are obtained to investigate the crack propagation behavior along a sinusoidal interface.

• Journal of Industrial Technology
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• v.20 no.A
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• pp.247-256
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• 2000

Precursor gels with the composition of $xZrO_2{\cdot}(100-x)SiO_2$ systems (x=10, 20 and 30 mol%) were prepared by the sol-gel method. Kinetic parameters, such as activation energy, Avrami's exponent, n, and dimensionality crystal growth value, m, have been simultaneously calculated from the DTA data using Kissinger and Matusita equations. The crystallite size dependence on tetragonal to monoclinic transformation of $ZrO_2$ was investigated using XRD, in relation to the fracture toughness. The crystallization of tetragonal $ZrO_2$ occurred through 3-dimensional diffusion controlled growth(n=m=2) and the activation energy for crystallization was calculated using Kissinger and Matusita equations, as about $310{\sim}325{\pm}10kJ/mol$. The growth of $t-ZrO_2$, in proportion to the cube of radius, increased with increasing heating temperature and heat-treatment time. It was suggested that the diffusion of Zr4+ions by Ostwald ripening was rate-limiting process for the growth of $t-ZrO_2$ crystallite size. The fracture toughness of $xZrO_2{\cdot}(100-x)SiO_2$ systems glass ceramics increased with increasing crystallite size of $t-ZrO_2$. The fracture toughness of $30ZrO_2{\cdot}70SiO_2$ system glass ceramics heated at $1,100^{\circ}C$ for 5 h was $4.84Mpam^{1/2}$ at a critical crystaliite size of 40 nm.