The aerospace and power generation industries have an increasing demand for high-temperature, highstrength materials. However, conventional materials typically lack sufficient fracture toughness and oxidation resistance at high temperatures. This study aims to enhance the high-temperature properties of Nb-Si-Ti alloys through ball milling. To analyze the effects of milling time, the progression of alloying is evaluated on the basis of XRD patterns and the microstructure of alloy powders. Spark plasma sintering (SPS) is employed to produce compacts, with thermodynamic modeling assisting in predicting phase fractions and sintering temperature ranges. The changes in the microstructure and variation in the mechanical properties due to the adjustment of the sintering temperature provide insights into the influence of Nb solid solution, Nb5Si3, and crystallite size within the compacts. By investigating the changes in the mechanical properties through strengthening mechanisms, such as precipitation strengthening, solid solution strengthening, and crystallite refinement, this study aims to verify the applicability of Nb-Si-Ti alloys in advanced material systems.

The torque shear high strength bolt is clamped normally at the break of pin-tail specified. However, the clamping forces on slip critical connections do not often meet the required tension, as it considerably fluctuates due to torque coefficient dependent on lubricant affected temperature. In this study, the clamping tests of torque shear bolts were conducted independently at indoor conditions and at construction site conditions. During last six years, temperature of candidated site conditions was recorded from -11℃ to 34℃. The indoor temperature condition was ranged from -1 0℃ to 50℃ at each 10℃ interval. As for site conditions, the clamping force was reached in the range from 159 to 210 kN and the torque value was from 405 to 556 Nㆍm. The range of torque coefficient at indoor conditions was analyzed from 0.126 to 0.158 while tensions were indicated from 179 to 192 kN. The torque coefficient at site conditions was ranged from 0.118 to 0.152. Based on this test, the variable trends of torque coefficient, tension subjected temperature can be taken by statistic regressive analysis. The variable of torque coefficient under the indoor conditions is 0.13%/℃ while it reaches 2.73%/℃ at actual site conditions. When the indoor trends and site conditions is combined, the modified variable of torque coefficient can be expected as 0.2% /℃. and the modified variable of tension can be determined as 0.18%/℃.

High temperature plasma coating technology has been applied to recover damaged aluminum dies from wear by spraying pure aluminum and alumina powder. However, the coated mixed powder layer composed of aluminum and alumina often undergoes a detachment from the substrate, making the coated substrate die unable to maintain its expected life span. In this study, in order to increase the bonding strength between the substrate and the coating layer, a pure aluminum layer was applied as an intermediate bond layer. In order to prepare the specimen with variable bond coating conditions, the bond coat layers with a various gun speed from 10 cm/sec to 30 cm/sec were prepared with coating cycle variations ranging from three to nine cycles. The specimen with a bond coat layer coated with a gun speed of 20 cm/sec and three coating cycles exhibited ~13MPa of adhesion strength, while the specimen without a bond coat layer showed ~6 MPa of adhesion strength. The adhesion strength with a variation of bond coat layer thickness is discussed in terms of coating parameters.

The clamping of torque shear high strength bolt is induced when the pin-tail is broken. Sometimes the clamping forces on slip critical connections do not meet the required tension due to torque coefficient dependent on conditions such as outdoor temperature, moisture and dust. From this study, the clamping forces of torque shear bolts at indoor conditions were compared with actual data investigated from 24 construction sites last five years. Similarly to foreign literature results, torque shear bolts showed that torque coefficients were fluctuated greatly by temperature conditions. The range of torque coefficient at indoor conditions was analyzed from 0.126 to 0.158 while tensions were indicated from 179 to 192 Nㆍm. Instead, the range of torque coefficient at site conditions was analyzed from 0.118 to 0.152. Based on this test, the variable trends of torque coefficient subjected temperature can be traced via statistic regressive analysis. In case of indoor conditions, it was showed that the variable of torque coefficient was 0.13% per 1℃, while the variable at actual site conditions reached 2.73% per 1℃.

Effects of Cu and B on effective grain size and low-temperature toughness of thermo-mechanically processed high-strength bainitic steels were investigated in this study. The microstructure of the steel specimens was analyzed using optical, scanning, and transmission electron microscopy; their effective grain size was also characterized by electron back-scattered diffraction. To evaluate the strength and low-temperature toughness, tensile and Charpy impact tests were carried out. The specimens were composed of various low-temperature transformation products such as granular bainite (GB), degenerated upper bainite (DUB), lower bainite (LB), and lath marteniste (LM), dependent on the addition of Cu and B. The addition of Cu slightly increased the yield and tensile strength, but substantially deteriorated the low-temperature toughness because of the higher volume fraction of DUB with a large effective grain size. The specimen containing both Cu and B had the highest strength, but showed worse low-temperature toughness of higher ductile-brittle transition temperature (DBTT) and lower absorbed energy because it mostly consisted of LB and LM. In the B-added specimen, on the other hand, it was possible to obtain the best combination of high strength and good low-temperature toughness by decreasing the overall effective grain size via the appropriate formation of different low-temperature transformation products containing GB, DUB, and LB/LM.

TiC-21mol% Mo solid solution (δ-phase) and TiC-99 mol% Mo solid solution (β-phase), and TiC-(80~90)mol%Mo hypo-eutectic composite were deformed by compression in a temperature range from room to 2300 K and in a strain raterange from 4.9×10−5 to 6.9×10−3/s. The deformation behaviors of the composites were analyzed from the strengths of theδ- and β-phases. It was found that the high strength of the eutectic composite is due primarily to solution hardening of TiCby Mo, and that the δ-phase undergoes an appreciable plastic deformation at and above 1420 K even at 0.2% plastic strainof the composite. The yield strength of the three kinds of phase up to 1420 K is quantitatively explained by the rule of mixture,where internal stresses introduced by plastic deformation are taken into account. Above 1420 K, however, the calculated yieldstrength was considerably larger than the measured strength. The yield stress of β-phase was much larger than that of pure TiC.A good linear relationship was held between the yield stress and the plastic strain rate in a double-logarithmic plot. Thedeformation behavior in δ-phase was different among the three temperature ranges tested, i.e., low, intermediate and high. Atan intermediate temperature, no yield drop occurred, and from the beginning the work hardening level was high. At the testedtemperature, a good linear relationship was held in the double logarithmic plot of the yield stress against the plastic strain rate.The strain rate dependence of the yield stress was very weak up to 1273 K in the hypo-eutectic composite, but it becamestronger as the temperature rose.

In this study, prediction of later-age compressive strength of ultra-high strength concrete, based on the accelerated strength of concrete cured in hot water was investigated. Comparing other acceleration method, hot water curing method is relatively easy and intuitive to use in the real construction site. The amount of time for evaluation of the concrete strength using the hot water curing method in KS and JIS is too long to predict the strength of the ultra-high strength concrete that are used in the tall building structure. For that reason, curing temperature of 40, 50, 60˚c 3 levels were examined to shorten the amount of time for the evaluation of the strength. As a result, the feasibility of the three days hot water curing method was confirmed.

As architectures have recently become high-risers and mega-structured, stable high strength products have been ensured. Accordingly, use of precast concrete accouplement has been increased in order to facilitate air compression and rationalize construction. Since external rising by the steam heating and internal rising by the accumulation of cement hydration heat for the temperature of members, precast concrete members with large cross-section used for high-rise mega-structure's columns and beams may exhibit different temperature history compared to the precast concrete members for wall and sub-floor with relatively small cross-sections. Therefore, this study aims to elucidate the characteristics of temperature history of mass concrete members cast with high-strength concrete for precast concrete application. In this study, large cross-sectional precast concrete mock-up, unit cement quantity, and temperature histories in manufacturing precast concrete member under different curing condition were inclusively investigated.

This study is basic experiment for estimating influence of strength by curing temperature of concrete's heat of hydration and estimate relationship of compressivε strength development by initial curing temperature factor, and then asume temperature factor which influence compressive strength development and for showing basic document of quality control. According to the result of cement mortar by the curing temperature factor high-curing temperature shows high strength on 3 day compare with low curing-temperature, shows higher strength than the piece of high curing temperature.

초합금의 진공정밀주조시에 진공하에서 용해한 합금을 1000~1700˚C로 가열한 세라믹 주형에 주입하고 난 후, 용탕이 장시간 주형안에 노출됨으로써, 주형의 고온강도가 높아야 하므로 고품위의 주형재를 사용하여 왔으나, 저품위의 값싼 소재를 사용하여 고품위의 주형과 동등한 효과를 갖게 하고자 주형내의 Silica 함량을 조절하였다. 그 결과 SiO2 첨가량이 7.7wt.%일 때, 다른 시험편에 비해 소성강도와 고온강도가 10-55%가량 증가 하였다. 따라서 일반적으로 정밀주조 주형으로 사용하는 용융알루미나와 colloidal silica의 혼합비를 제어하여 단결절 주조용 주형을 개발하였다.

Microstructure and mechanical properties were examined on rapidly solidified Al-8wt%Fe alloy. High temperature strength test was also undertaken, and it is shown that the refinement in microstructure resulting from extremely rapid cooling rates gives rise to improved high temperature strength, but the elongation to fracture of this material decreases with increasing temperature, particularly in the temperature range up to 30. Specimens heat-treated for 100 hrs were analyzed with TEM micrographs to understand the thermal stability of this material.

본 연구에서는 플라이애시가 90 %만큼 다량치환된 모르타르에 알칼리 활성화를 통한 강도증진을 동일한 양생온도 조건 에서 양생방법 및 유지시간 변화에 따라 비교 분석하고자 하였다. 연구 결과로 플래이애시를 90 % 치환한 경우, 소생재 도 포 후 40℃로 24시간 기중양생시 가장 높은 압축강도를 발휘하였지만, OPC의 압축강도까지 발휘하는 것은 어려울 것으로 분석되었다.

본 연구에서는 플라이애시가 다량치환된 모르타르에 알칼리 활성화를 통한 강도증진을 양생온도 및 양생방법 변화에 따라 비교 분석하였다. 연구 결과로 플라이애시를 60 % 과다치환한 경우 소생재 도포 후 40℃의 온도로 48시간 기중양생을 실시한다면 OPC의 28일 압축강도에 근접하게 발휘되는 것이 얻어졌다.

본 연구에서는 양생조건이 다른 압축강도 90 MPa 수준의 고강도 콘크리트 부재의 휨거동 실험을 수행하였다. 실험변수는 정상 및 저온 양생 조건, 인장 철근량 및 콘크리트 압축강도 수준 등을 고려하였다. 8개의 보 부재를 제작하여 휨 실험을 수행하였으며 균열 간격, 하중-처짐 관계, 하중-변형률 관계 및 연성지수를 파악하였다. 실험결과는 철근량이 증가함에 따라 균열 개수는 증가하고 균열간격은 감소하는 경향 을 나타내며, 콘크리트 강도가 높을수록 균열개수가 줄어들기는 하지만 그 효과는 철근량보다는 상당히 작은 것을 알 수 있었다. 설계기준에서 제안된 평균 균열 간격 식과 비교한 결과, 실험결과가 제안식의 결과보다 약간 크게 나타났으나, 제안식은 콘크리트 강도 및 양생조건을 반영 하지 못하는 문제점이 있다. 정상 양생된 부재들의 연성지수는 3.36~6.74이며, 저온 양생된 부재들의 연성지수는 1.51~2.82으로 나타나, 저온 양생된 부재들의 거동은 정상 양생된 부재들에 비해서 연성도지수가 저감됨을 확인하였으며, 본 연구와 기존 연구의 연성지수를 비교한 결과, 고강도 콘크리트 부재의 연성지수는 선행연구의 보통강도 콘크리트의 연성지수 보다 크게 나타났으나, 더 구체적인 결과를 파악하기 위해서 는 추가연구가 필요하다고 판단된다.

This study examined the effect of outside temperature on the properties of high-strength concrete to determine conditions for four-season construction. With 20 ℃ as the reference temperature, 20, 30, and 40 ℃ were set as hot weather conditions, and 5, -10, and –20 ℃ as cold weather conditions. Properties as the effect of outside temperature on compressive strength of high-strength concrete was studied.

This study examined the effect of outside temperature on the properties of high-strength concrete to determine conditions for four-season construction. With 20 ℃ as the reference temperature, 20, 30, and 40 ℃ were set as hot weather conditions, and 5, -10, and –20 ℃ as cold weather conditions. Properties as the effect of outside temperature on compressive strength of high-strength concrete was studied.