• Journal of the Korean Ceramic Society
• /
• v.27 no.8
• /
• pp.991-1003
• /
• 1990

A precipitation method, one of the most effective liquid phase reaction methods, was adopted in order to prepare high-tech Al2O3/ZrO2 composite ceramics. Al2(SO4)3.18H2O, ZrOCl2.8H2O and YCl3.6H2O were used as starting materials and NH4OH as a precipitation agent, various types of metal hydroxides were obtained by single precipitation(series A) and co-precipitation(series B) method at the pH condition between 7 and 11. Fine Al2O3-ZrO2 powders were prepared at optimum calcination condition and the effects of ZrO2 on microstructures and mechanical properties of Al2O3 were investigated. The composition of Al2O3/ZrO2 composites wax fixed as Al2O3-15 v/o ZrO2(＋3m/o Y2O3). ZrO2 limited the grain growth of Al2O3 and increased grain size homogeneity of Al2O3 more effectively than MgO.Flexural strength values in Al2O3 and Al2O3/ZrO2 composites were 340-430 MPa and 540-820 MPa, respectively, and the effect of strength improvement showed 20-50% by adding ZrO2 to Al2O3. Fracture toughness of Al2O3/ZrO2 composites was improved by stress-induced phase transformation of tetragonal ZrO2 and toughening effect by microcrack was not observed. Also, ZrO2 particles located at Al2O3 grain junction contributed to toughening, while spherical ZrO2 particles located within Al2O3 grain did not contribute to toughening. Weibull moduli of Al2O3 ceramics and Al2O3/ZrO2 composites of series A and series B were 4.34, 5.17 and 9.06, respectively. Above 0.5 of failure probability, strength values in Al2O3 ceramics and Al2O3/ZrO3 composites of series A and series B were above 400 MPa, 700 MPa and 650 MPa, respectively.

• Journal of the Korean Ceramic Society
• /
• v.55 no.5
• /
• pp.492-497
• /
• 2018

This paper reports the efficacy of tape casting using an Ag-10 wt% CuO filler for the successful joining of a sintered $Ce_{0.9}Gd_{0.1}O_{2-{\delta}}-La_{0.7}Sr_{0.3}MnO_{3{\pm}{\delta}}$ (GDC-LSM) ceramic with a SUS 460 FC metal alloy by reactive air brazing. The as-prepared green tape was highly flexible without drying cracks, and the handling was easy when used as a filler material for reactive air brazing. Heat treatment for the GDC-LSM/SUS 460 FC joint was performed at $1050^{\circ}C$ for 30 min in air. Microstructural observations indicated a reliable and compact joining. The room temperature mechanical shear strength of the as-brazed joints was $60{\pm}8MPa$ with a cohesive failure. The flexural strength of joints was measured from room temperature up to $850^{\circ}C$, where the strength retention revealed to be almost 100% at $500^{\circ}C$. However, the joints showed a degradation in strengths at 800 and $850^{\circ}C$, exhibiting strength retentions of 57% and 37%, respectively.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.5 no.1
• /
• pp.42-48
• /
• 2001

Over two decades, many researchers have performed studies on strength size effects in composite laminates under tensile and flexural loads. It is well known that there is a tendency for the strength of fibre-reinforced composites to decrease with increasing specimen size. Under compressive load, however, little work has been done on the effect of specimen size to failure strength. This is due to the fact that compressive testing of composite is very difficult. In this paper, the effect of the test specimen size on the compressive strength of composites containing open hole was considered using T300/924C, $>[45/-45/0/90]_{3S}$. For sizing test specimens, the in-plane scaling method is used i.e., the change of two- dimensional specimen area in specimen width and gauge length. The results clearly show that there is a hole size effect in the finite width plates. In addition, the specimens which have the same a/W(hole diameter/specimen width) exhibit a tendency of size effect. In contrast, test results of the unnotched specimens did not show a clear strength size effect.

• Journal of the Korea Institute of Building Construction
• /
• v.18 no.1
• /
• pp.9-15
• /
• 2018

Radon gas, which is present on the earth, is a primary carcinogen released from rocks, soil, building materials, etc., and exists as a unique gas phase. In order to solve the risk of radon gas, we evaluated the basic performance which can be used as indoor finishing materials in addition to the radon gas reduction properties of the matrix using anthracite. An anthracite used as a conventional filter material was used to produce a matrix, and a test was conducted to replace the gypsum board, which is one of the building materials used in the existing room. As the anthracite replacement ratio increases, flexural failure load strength increases and thermal conductivity tends to decrease. Depending on the thickness of the board, the reduction performance of radon gas shows a slight difference.

• Journal of the Korean Society of Safety
• /
• v.31 no.3
• /
• pp.75-81
• /
• 2016

Tendon corrosion reliability in KICT-ultra high performance concrete (K-UHPC) bridges is assessed and predicted considering uncertainties in flexural bending capacity and corrosion occurrence. In post-tensioning bridge systems, corrosion is a one of most critical failure mechanisms due to strength reduction by it. During the entire service life, those bridges may experience lifetime corrosion deterioration initiated and propagated in tendons which are embedded not only in normal concrete but also in K-UHPC. For this reason, the time-variant corrosion performance has to be assessed. In the absence of in-depth researches associated with K-UHPC tendon corrosion, a reliability-based prediction model is developed to evaluate lifetime corrosion performance of tendon in K-UHPC bridges. In 2015, KICT built a K-UHPC pilot bridge at 168/5~168/6 milestone on Yangon-Mandalay Expressway in Myanmar, by using locally produced tendons which post-tensioned in longitudinal and lateral ways of K-UHPC girders. For an illustrative purpose, this K-UHPC bridge is used to identify the time-variant corrosion performance.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.21 no.3
• /
• pp.35-44
• /
• 2017

In this study, steel fiber-reinforced concrete beams with ordinary steel reinforcements, that are below minimum steel reinforcement amount specified in domestic concrete structure design code, were tested in flexure until failure. Steel reinforcement ratio considered were 44%, 66%, 78% and 100% of the minimum steel reinforcement. Considered steel fiber volume fractions were 0.25%, 0.50%, 0.75% and 1.00%. In results, it is confirmed that steel fibers greatly improve crack performance. Also, the steel fibers contributed to increment in yield load not in ultimate load. But the increment was not greater than the reduction by steel reinforcement reduction. The use of steel fibers in RC beams lightly reinforced below the minimum reinforcement ratio specified design code reduced ductility greatly. Consequently, steel reinforcement ratio in steel fiber-reinforced beams lightly reinforced below the minimum steel reinforcement should be increased in order to enhance proper ductility.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.10 no.2
• /
• pp.175-186
• /
• 2006

Strength method for determining nominal moment capacity of reinforced concrete members is also assumed to be suitable for strengthened members with FRP system. If the internal tensile forces of the strengthened member from steel and FRP is insufficient, the FRP system strain might become greater than its ultimate tensile strain which makes the strength method a contradiction and unapplicable. The experimental results of 27 strengthened beams with carbon fiber sheets which have relatively lower tensile forces from steel and FRP show that not only concrete compressive strain is lower than 0.003 but also measured ultimate moment was lower than nominal moment using the strength method.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.25 no.5
• /
• pp.182-189
• /
• 2021

VPS(Void Plywood Slab System, VPS) has optimized the shape of the hollow material. In addition, it has a function to prevent the floating of the hollow material and the separation due to the working load. In this study, the punching shear capacity of flat plate was performed using Void Plywood Slab System with form work panel proposed in the previous study. As a result of the test, the strength of the VSPS specimen in which the hollow material was placed beyond 2.0 times the column width from the loading point was reduced by 9.4% compared to the reference specimen. However, the strength value was about 1.57 times higher than the design value suggested by KBC 2016. It was found that there was no change in stiffness compared to the reference specimen until shear failure occurred in the VSPS specimen in which the hollow material was placed. It can be seen that this experiment is being destroyed by shear as the flexural reinforcing bars are sufficiently reinforced.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.31 no.6
• /
• pp.283-291
• /
• 2018

This study intends to develop an analytical model of unreinforced masonry(URM) walls for the nonlinear static analysis which has been generally used to evaluate the seismic performance of a building employing URM walls as seismic force-resisting members. The developed model consists of fiber elements used to capture the flexural behavior of an URM wall and a shear spring element implemented to predict its shear response. This paper first explains the configuration of the proposed model and describes how to determine the modeling parameters of fiber and shear spring elements based on the stress-strain curves obtained from existing experimental results of masonry prisms. The proposed model is then verified throughout the comparison of its nonlinear static analysis results with the experimental results of URM walls carried out by other researchers. The proposed model well captures the maximum strength, the initial stiffness, and their resulting load - displacement curves of the URM walls with reasonable resolution. Also, it is demonstrated that the analysis model is capable of predicting the failure modes of the URM walls.

• Journal of the Korea Institute of Building Construction
• /
• v.19 no.2
• /
• pp.139-147
• /
• 2019

In this study, an experiment was conducted to evaluate the properties of cement matrix using diatomite and silica gel as adsorbents of radon. The adsorption properties of diatomite of a natural adsorbent and silica gel of an artificial sorbent were examined to confirm the reduction of radon gas concentration of the removal of radon gas in the indoor environment of the human body. We conducted a performance evaluation for the study. The fluidity, air content, density, absorption, flexural failure load, thermal conductivity and radon gas concentration of the specimen using diatomite and silica gel were measured. the fluidity and the air content of the adsorbed matrix with diatomite were decreased as the diatomite replacement ratio increased. Which seems to affect the subsequent matrix by the absorption of the compounding water of diatomite. As the replacement rate of silica gel increased, the fluidity decreased and the air content increased up to constant replacement rate. It is judged that the surface of the silica gel has a critical point at which it can react with moisture.