• The Journal of Korean Academy of Prosthodontics
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• v.42 no.2
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• pp.142-153
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• 2004

Statement of problem: Increasing demand of esthetic restorations made lots of kinds of ceramic materials. Among them, zirconia has been being focused by many dentists. But, mechanical properties of zirconia were still unclear. Purpose : The purposes of this study were to analyze the flexural strength of various zirconia ceramics which had been currently used for clinic i.e., In-Ceram Zirconia(Vita Zahnfabrik, Bad $S\"{a}ckingen$, Germany), Celay Zirconia(Vita Zahnfabrik, Bad $S\"{a}ckingen$, Germany) and CAD/CAM Zirconia (Adens Zi-Ceram. Seoul , Korea). Material and methods: The four point bending test(ASTM Cl161) was used to measure the flexural strength of a specimen before and after circular heat treatment and fatigue loading. Results : 1. The average value of flexural strengths of CAD/CAM Zirconia, Celay Zirconia, In-Ceram Zirconia in dry condition were 806.5 MPa, 669.9 MPa, 605.6 MPa, respectively. There was a statistically significant difference in strength among the types (P<0.05). 2. After thermocycling, the average flexural strengths of CAD/CAM Zirconia, Celay Zirconia, In-Ceram Zirconia were 791.2 MPa, 604.2 MPa, 605.4 MPa, respectively. CAD/CAM Zirconia showed statistically significant higher strength(P<0.05). The others showed no significant difference after thermocycling(P>0.05). 3. After fatigue loading in wet condition. the average flexural strengths of CAD/CAM Zirconia, Celay Zirconia, In-Ceram Zirconia were 806.0 MPa, 674.9 MPa, 601.7 MPa, respectively. There was a significant difference in strength among the types(P<0.05). 4. There was no statistically significant difference in strength of the specimens according to experimental methods except for before and after thermocycling in Celay Zirconia(P>0.05). Conclusion: Besides high esthetic quality, zirconia had sufficiently high mechanical strength.

• The Journal of Korean Academy of Prosthodontics
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• v.38 no.5
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• pp.575-582
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• 2000

This investigation was designed to determine whether heat-pressing and/or simulated heat treatments affected the flexure strength and the microstructure of the lithium disilicate glass-ceramic in the IPS Empress 2 system. Four groups of the specimens were prepared as follows: group 1 - as-received material, group 2 - heat-pressed material; group 3 - heat-pressed and simulated initial heat-treated material; group 4 - heat-pressed and the simulated heat-treated material with full firings for a final restoration. The three-point bending test and the scanning elec-tron microscope (SEM) analysis was conducted for the purpose of this study. The flexure strength of group 2 was significantly higher than that of group 1. However, there were no significant differences in strength among group 2, 3, and 4, and between group 1 and 4. The SEM micrographs of the lithium disilicate glass-ceramic showed the closely packed, multi-directionally interlocking microstructure of numerous lithium disilicate crystals protruding from the glass matrix. The crystals of the heat-pressed materials (group 2, 3, and 4) were a little denser and about two times bigger than those of the as-received material (group 1). This change of microstructure is more obviously exhibited particularly between group 1 and 2. However, there was no a marked difference among group 2, 3, and 4 after the heat-pressing procedure. Although there were significant increase of the strength and some changes of the microstructure after the heat-pressing operation, the combination of the heat-pressing and the simulated subsequent heat treatments did not produce the increase of strength of IPS Empress 2 glass-ceramic.

• International Journal of Highway Engineering
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• v.3 no.1 s.7
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• pp.185-197
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• 2001

This paper dealt with modification of effective crack length model (ECM) by adding Poisson's ratio term to evaluate fracture toughness of asphalt concrete which varies its material property by temperature. The original ECM model was developed for solid materials, such as cement concrete, and Poisson's ratio of materials was not considered. However, since asphalt concrete is sensitive to temperature variation and changes its Poisson's ratio by temperature, it should be taken into consideration to know exact fracture property under various temperatures. Four binders, including 3 polymer-modified asphalt (PMA) binders, were used to make a dense-grade asphalt mixture and 3-point bending test was peformed on notched beam at low temperatures, from -5oC to 35oC. Elastic modulus, flexural strength and fracture toughness were obtained from the test. The results showed that, since Poisson's ratio was considered, the more accurate test values could be obtained using modified ECM equation than original ECM. PMA mixture showed higher stiffness and fracture toughness than normal asphalt mixture under very low temperatures.

• Korean Journal of Human Ecology
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• v.24 no.1
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• pp.97-109
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• 2015

Recently, because the market for compression wear now includes all consumers, not just professionals, various items for recovery after exercising or for enhanced effects from exercise have been introduced. In this research, a systematic and stepwise design process was proposed to develop compression garment that has both functional area and appropriate pressure to protect the knee when exercising. The U-V format functional area that wraps underneath the knee was selected by considering the shape and change in the skin length when bending the knee. After the selection of the functional area, a total of seven knee design areas, including the existing product, were designed to determine the appropriate pressure. After various movements, the compression garment was ranked in terms of support of the knee, level of pressure, discomfort of seam line, and comfort of popliteal; the preferred design was selected using the quad method. Four compression wear garments were produced using two selected preferred designs; the wear evaluation was performed using a seven-point Likert scale. As a result, the optimal reduction rate of the pattern was calculated based on Ziegert and Keil's method. The applied percentage of the fabric stretch at the upper part of the crotch was 66% for the width and 50% for the length; for the lower part of the crotch, only 66% for the width was applied. Moreover, it was determined that the design of the U-V knee protection part was preferred when a 7 mm square was placed at a 1 mm distance because this not only supports the knee but also allows the fabric to accommodate various skin deformations.

• Korean Chemical Engineering Research
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• v.46 no.2
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• pp.389-396
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• 2008

The effect of thermal cycling on bond strength and residual stress at the interface between benzocyclobutene (BCB) and plasma enhanced chemical vapor deposited (PECVD) silicon dioxide ($SiO_2$) coated silicon wafers were evaluated by four point bending and wafer curvature techniques. Wafers were bonded using a pre-established baseline process. Thermal cycling was done between room temperature and a maximum peak temperature. In thermal cycling performed with 350 and $400^{\circ}C$ peak temperature, the bond strength increased substantially during the first thermal cycle. The increase in bond strength is attributed to the relaxation in residual stress by the condensation reaction of the PECVD $SiO_2$: this relaxation leads to increases in deformation energy due to residual stress and bond strength.

• The korean journal of orthodontics
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• v.48 no.5
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• pp.316-325
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• 2018

Objective: The aim of this systematic multiscale analysis was to evaluate the effects of thermoforming on the physical and mechanical properties of thermoplastic materials used to fabricate transparent orthodontic aligners (TOAs). Methods: Specimens were fabricated using four types of thermoplastic materials with different thicknesses under a thermal vacuum. Transparency, water absorption and solubility, surface hardness, and the results of three-point bending and tensile tests were evaluated before and after thermoforming. Data were analyzed using one-way analysis of variance and Student's t-test. Results: After thermoforming, the transparency of Duran and Essix A+ decreased, while the water absorption ability of all materials; the water solubility of Duran, Essix A+, and Essix ACE; and the surface hardness of Duran and Essix A+ increased. The flexure modulus for the 0.5-mm-thick Duran, Essix A+, and eCligner specimens increased, whereas that for the 0.75-/1.0-mm-thick Duran and eClginer specimens decreased. In addition, the elastic modulus increased for the 0.5-mm-thick Essix A+ specimens and decreased for the 0.75-mm-thick Duran and Essix ACE and the 1.0-mm-thick Essix ACE specimens. Conclusions: Our findings suggest that the physical and mechanical properties of thermoplastic materials used for the fabrication of TOAs should be evaluated after thermoforming in order to characterize their properties for clinical application.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.2
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• pp.137-146
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• 2007

Numerical analyses are performed to show the effect of stiffening facade riggers on the behavior of the structure and to investigate the optimum locations of facade riggers. Optimum locations of the facade riggers to minimize the drift at the top of the structure are obtained by maximizing the drift reduction caused by the facade riggers and are significantly influenced by the bending and shear stiffnesses of the braced frame and facade riggers. Three standard load cases of uniformal and triangularly distributed lateral loads as well as a lateral point load at the top of the structure are considered in this paper Optimum locations of facade riggers are plotted as functions of nondimensional relative stiffness parameters ${\omega}$ and ${\beta}$ for structures with one to four riggers. Although the analysis presented herein is based on certain simplifying assumptions, it is believed that the results do provide sufficiently accurate information for determining the optimum locations of facade riggers in highrise structures.

• Journal of the Society of Naval Architects of Korea
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• v.57 no.2
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• pp.80-87
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• 2020

As the Arctic sea ice decreases due to various reasons such as global warming, the demand for ships and offshore structures operating in the Arctic region is steadily increasing. In the case of sea ice, the anisotropy is caused by the uncertainty inside the material. For most of the research, nevertheless, estimating the ice load has been treated deterministically. With regard to this, in this paper, a four-point bending strength analysis of an ice specimen was attempted using a stochastic finite element method. First, spatial distribution of the material properties used in the yield criterion was assumed to be a multivariate Gaussian random field. After that, a direct method, which is a sort of stochastic finite element method, and a sensitivity method using the sensitivity of response for random variables were proposed for calculating the probabilistic distribution of ice specimen strength. A parametric study was conducted with different mean vectors and correlation lengths for each material property used in the above procedure. The calculation time was about ten seconds for the direct method and about three minutes for the sensitivity methods. As the cohesion and correlation length increased, the mean value of the critical load and the standard deviation increased. On the contrary, they decreased as the friction angle increased. Also, in all cases, the direct and sensitivity methods yielded very similar results.

• Computers and Concrete
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• v.22 no.5
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• pp.481-492
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• 2018

In this paper, a new hybrid fiber system (NHFS) is investigated for the application of slab. The steel fiber, polyvinyl alcohol (PVA) fiber and calcium carbonate ($CaCO_3$) whisker is added to form NHFS. The four-point bending test is carried out on the flexural properties of slab with plain, steel fiber, traditional hybrid fiber system (THFS) and NHFS reinforced cementitious composites. The flexural behavior is evaluated by ASTM C1018-97, JCI-SF4 and post-crack strength (PCS) technique. The evaluation parameters of flexural toughness such as toughness index (TI), equivalent flexural strength (EFS) and PCS are determined. The size of slab specimens is $15mm(thickness){\times}50mm(width){\times}200mm(length)$. The results show that adding $CaCO_3$ whisker to THFS can significantly improve the flexural strength, TI, EFS, PCS of the slab. The empirical relation between reinforcing index ($RI_v$) and flexural parameters show that flexural parameters of slabs increase first and then decrease; which indicates that optimum $RI_v$ values can be helpful in the considering the mix design of steel-PVA fibers-$CaCO_3$ whisker composites for achieving the desired flexural-related properties. The scanning electron microscopy is performed to observe the micro-morphological characteristics of the fracture surface, which proved the positive hybrid effect among the different fibers in cementitious composites. The NHFS can arrest the generation and propagation of the crack from micro to macro level.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.1
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• pp.30-39
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• 2017

This paper concerns flexural strength of reinforced concrete beams containing recycled coarse aggregate (RCA) with compressive strength ranging from 31 to 38 MPa. The experimental parameters were replacement ratio of RCA and rebar ratio. Replacement ratio of RCA was 0, 30, 50 and 100%, and rebar ratio was 0.50, 0.79 and 1.14%. The RCA concrete beams were tested by using four-point bending test, and experimental results were discussed regarding crack and failure patterns, load-deflection relationship. Crack pattern of concrete beams with RCA was similar to that of concrete beams with natural coarse aggregate (NCA) but overall crack spacing of concrete beams with RCA was smaller than that of concrete beams with NCA. The crack width of RCA and NCA concrete beams was similar to each other. In addition, the test results of flexural strength were compared to the design code predictions. The design code predictions for flexural strength underestimated the experimental results. Therefore, the design code predictions for flexural strength of RCA concrete beams would offer conservative design.