• 대한토목학회논문집
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• 제26권5D호
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• pp.831-838
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• 2006

본 연구에서는 차량의 주행속도, 차축하중 그리고 타이어 압력변화에 따른 아스팔트 포장의 주요 응답 특성을 분석하고자 하였다. 시험도로 아스팔트 포장 중 기층의 두께가 서로 다른 A5(180mm)와 A8(280mm)단면을 선정하여 표준 3축 덤프트럭에 의한 아스팔트의 변형률과 수직응력의 변화를 계측하였다. 모든 주행시험은 각 포장 단면의 주행차로에서 진행되었으며 실제 주행속도와 이동경로는 레이저 원더링 시스템을 적용하여 실시간으로 관찰, 기록하였다. 아스팔트 포장의 변형률은 차량의 주행속도가 증가할수록 그 크기가 감소하는 일반적인 점탄성 거동을 보였다. 특히 수직응력은 차축하중 뿐만 아니라 주행속도에도 영향을 받는 것으로 보아 속도별 차량의 운동특성이 각 차축으로 전달되는 연직하중의 크기에 많은 영향을 주고 있음을 알 수 있었다. 일반적으로 타이어 공기압이 증가하고 차축하중이 증가할수록 아스팔트 하부의 최대 인장변형률은 증가하였다. 두 아스팔트 포장 단면에서 다층탄성해석을 수행한 결과 변형률은 계측된 결과보다 크게 예측되었으며 수직응력은 수치해석결과가 계측결과보다 작게 평가되었다.

• 콘크리트학회지
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• 제9권3호
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• pp.127-135
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• 1997

본 연구에서는 콘크리트의 피로균열 성장거동을 구명하기 위하여 쐐기쪼갬실험( WST)을 수행하였다. 연구의 주안점인 피로균열 성장거동의 주요영향인자는 콘크리트의 강도로서 28,60,118 MPa 등 3가지의 강도를 변수로 택하였다. 한편, 응력비를 6,13%의 2가지로 변화시켜 그 영향을 관찰하였다. 소정의 응력비을 주기 위하여 최고피로하중수준을 75~85%, 최저응력수준을 5~10%로 각각 유지하였다. 피로실험전에 균열개구변위( CMOD)컴플라이언스 보정 실험을 수행한 후, 그 결과인 균열길이와 컴플라이언스의 관계를 피로실험 중에 균열길이를 예측하는데에 이용하였다. 또한 CMOD컴플라이언스 보정법의 타당성을 검증하기 위하여 선형탄성 파괴역학( LEFM) 및 염색법에 의하여 예측된 균열길이와 비교하였다. 실험결과에 의하여 선형탄성 파괴역학에 근거한 피로균열 성장속도 모델(da/dN- K1 관계)을 제시하였고 콘크리트의 강도가 증가함에 따라 피로균열의 성장속도가 빨라지는 것으로 평가되었다. 또한 응력비가 피로균열 성장속도에 영향을 미치는 것으로 나타났는데 강도가 증가함에 따라 그 정도가 감소하는 경향을 보였다. LEFM 과 염색법 의하여 예측된 균열길이와 비교하여 본 결과 CMOD컴플라이언스 보정법이 쐐기쪼갬실험(WST)에 적용될 수 있음이 검증되었다.

• Geomechanics and Engineering
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• 제14권6호
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• pp.581-588
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• 2018

In order to explore the comprehensive evaluation means of the extent of hydraulic fracturing region in coal seams, we analyzed the feasibility of detecting the response of coal rock direct current (DC) apparent resistivity to hydraulic-fracturing using Archie's theory, and conducted experimental researches on the response of DC resistivity in the hydraulic fracturing process using small-scale coal rock samples. The results show that porosity and water saturation are the two factors affecting the apparent resistivity of coal rock while hydraulic fracturing. Water has a dominant effect on the apparent resistivity of coal rock samples. The apparent resistivity in the area where water flows through is reduced more than 50%, which can be considered as a core affect region of hydraulic fracturing. Stress indirectly impacts the apparent resistivity by changing porosity. Before hydraulic fracturing, the greater axial load applied, the more serious the rupture in the samples, resulting in the greater apparent resistivity. Apparent resistivity testing is a potential regional method to evaluate the influence range of hydraulic fracturing in coal seams.

• 한국건축시공학회지
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• 제2권2호
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• pp.129-134
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• 2002

The strength and durability of concrete are affected by various factors such as the quality of material, mixing ratio, construction, the method of cure, time elapsed. the condition of test and etc., it is very difficult to pre-estimate the strength of concrete with the use of experimental specimen. The domestic standard of specimen cylindrical type and its sizes are both l0cm$\times$20cm and 15cm$\times$30cm, which are prescribed in KS F2405, and the loading speed is prescribed to test with 2~3kgf/$\textrm{cm}^2$ per second. The loading speed should have great effect on the compressive strength, but in reality in the construction site sometimes the loading speed is applied so dubiously that the value of the compressive strength can be unreliable. And the cross sectional area of a specimen should be level and smooth, otherwise it can be broken at a lower stress than the real strength through the eccentric or intensive working of the load. Capping should be carried out in order to measure the strength correctly. And used for capping are various materials such as capping compound, cement glue, plaster, mechanical grinding and etc. In this study, therefore, I have carried out an experiment on the relationship among the loading speed, the ratio of height to diameter of specimen, the method of capping, and the compressive strength, for the efficient quality control of concrete structures. So this study has been purposed to provide some basic data that can be used effectively at construction sites.

• 한국강구조학회 논문집
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• 제17권6호통권79호
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• pp.689-697
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• 2005

보 플랜지 절취형 (Reduced Beam Section, RBS) 내진 철골모멘트접합부의 최근 실험결과를 살펴보면, 보 웨브를 볼트 접합한 시험체는 보 웨브를 용접한 시험체에 비해 조기에 스캘럽에서 취성파단이 발생하는 열등한 내진성능을 나타냈다. 과거 여러 연구자들이 수행한 실험 결과 및 본 연구의 수치해석 결과를 종합해 볼 때, 이러한 접합부의 조기 취성파괴는 고전 휨이론과 매우 다른 응력전달 메카니즘에서 기인하는 웨브 볼트의 슬립, 그리고 재료의 인성이 가장 낮은 스캘럽 부근의 응력집중과 밀접한 관련이 있는 것으로 분석된다. 본 연구에서는 실험 및 해석결과를 바탕으로 RBS 접합부의 실제 응력전달경로에 부합되는 새로운 보 웨브 볼트 설계법 및 개선된 상세를 제시하고 실물대 실험을 통하여 방안의 타당성을 입증하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.43-48
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• 2000

In this study, a small punch creep(SP-Creep) test using miniaturized specimen$(10{\times}10{\times}0.5mm)$ has been described for the development of the newly semi-destructive creep test method for high temperature structural components such as headers and tubes of boiler turbine casino and rotor and reactor vessel. The SP-Creep testing technique has been applied to 2.25Cr-1Mo(STBA24) steel used widely as boiler tube material and the creep test temperature are varied at $550^{\circ}C{\sim}600^{\circ}C$. The overall deformations of SP-Creep curves are definitely depended with applied load and creep test temperature and show the creep behaviors of three steps like conventional uniaxial creep curves. The steady state creep rate${\delta}_{ss}$ of SP-Creep curve for miniaturized specimen increases with increasing creep temperature, but the exponential value with creep loading is decreased. The activation energy$(Q_{spc})$ during SP-Creep deformation with various test temperatures shows 605.7kJ/mol that is g.eater than 467.4kJ/mol reported in uniaxial creep test. This may be caused by the difference of stress states during creep deformation In two creep test. But from the experimental results, e.g. SP-Creep curve behaviors, the steady state creep rate${\delta}_{ss}$ with creep temperature, and the exponential value(n) with creep loading, it can be summarized that the SP-Creep test may be a useful test method to evaluate the creep properties of the heat resisting material.

• 한국주조공학회지
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• 제25권1호
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• pp.16-22
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• 2005

This study aims to investigate the effects of the microstructures and nodule type on the fatigue characteristics of cast iron. Fatigue tests were carried out in tension-tension mode using a servo-hydraulic testing machine with load control mode operating at a frequency of 15 Hz. The tests were conducted at stress ratio R=Kmin/Kmax, of 0.1. Initial crack ${\Dalta}K$ values were highly performed with increase in tensile strength of DCI fatigue specimens. ${\Dalta}K_{th}$ region, fatigue crack propagation was primarily advanced through cell boundary and in periphery of near nodule. Fatigue crack propagation rate of D2 consisted with 2Phase(Ferrite+Pearlite) was slow due to crack closure enhanced by crack deflection and occurred crack branching. The generation of crack branch was occurred due to interaction of crack-nodule. At Threshold and Paris zone, the fractographs of the fatigue fracture surface for DCI show typical striations of a ductile fracture and isolated cleavage planes near graphite. The effect of microstructure on fatigue crack propagation of GC strongly depends on the type of flake. The generation of crack branch occurred due to interaction of crack-nodule. The fractographs of the fatigue fracture surface for GC show cleavage plane along the flake graphite.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.561-566
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• 2001

A long bar impact test to alumina plates(AD 85 and AD 90) was carried out by using fabricated impact testing apparatus. The apparatus adopting a long bar of 2.1m in length made it possible to measure directly the applied impact force to the specimen during bar impact. The dimension of specimens was $33{\times}33mm$ and thickness was 3.4mm. Confinement of D2=18mm outer diameter and D1=10.5mm inner diameter was used to provide contact pressure to the specimen. Contact pressure of p=100 or 200MPa was applied to specimen before impact test. Damage caused in those cases were compared with the case of without contact pressure. The damage of specimen was different depending upon the pressure level of confinement. The existence of confinement had suppressed the development of radial cracks from the bottom of specimen and reduced the extent of damage as compared with cases without contact pressure(p=0MPa). Because the application of contact pressure to the specimen increased the apparent flexural stiffness of specimen during bar impact, it had produced the change of developed damage in the specimen; from the radial cracks to the local contact stress dominant damage. It would contribute to the improvement of the ballistic property in ceramic plates.

• The Journal of Advanced Prosthodontics
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• 제12권4호
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• pp.218-224
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• 2020

PURPOSE. The objectives of this study were to evaluate the fracture strength and fracture patterns of provisional crowns fabricated from different materials and techniques after receiving stress from a simulated oral condition. MATERIALS AND METHODS. A monomethacrylate-based resin (Unifast Trad) and a bis-acryl-based (Protemp 4) resin were used to fabricate provisional crowns using conventional direct technique. A milled monomethacrylate resin (Brylic Solid) and a 3D-printed bis-acrylate resin (Freeprint Temp) were chosen to fabricate provisional crowns using the CAD/CAM process. All cemented provisional crowns (n=10/group) were subjected to thermal cycling (5,000 cycles at 5°-55℃) and cyclic occlusal load (100 N at 4 Hz for 100,000 cycles). Maximum force at fracture was tested using a universal testing machine. RESULTS. Maximum force at fracture (mean ± SD, N) of each group was 657.87 ± 82.84 for Unifast Trad, 1125.94 ± 168.07 for Protemp4, 953.60 ± 58.88 for Brylic Solid, and 1004.19 ± 122.18 for Freeprint Temp. One-way ANOVA with Tamhane post hoc test showed that the fracture strength of Unifast Trad was statistically significantly lower than others (P<.01). No statistically significant difference was noted among other groups. For failure pattern analysis, Unifast Trad and Brylic Solid showed less damage than Protemp 4 and Freeprint Temp groups. CONCLUSION. Provisional crowns fabricated using the CAD/CAM process and the conventionally fabricated bis-acryl resins exhibited significant higher fracture strength compared to conventionally fabricated monomethacrylate resins after the aging regimen. Therefore, CAD/CAM milling and 3D printing of provisional restorations may be good alternatives for long term provisionalization.

• International Journal of Concrete Structures and Materials
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• 제11권2호
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• pp.343-363
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• 2017

The objective of this work was finding out the most advisable testing conditions for an effective and robust characterization of the tensile strength (TS) of concrete disks. The independent variables were the loading geometry, the angle subtended by the contact area, disk diameter and thickness, maximum aggregate size, and the sample compression strength (CS). The effect of the independent variables was studied in a three groups of experiments using a factorial design with two levels and four factors. The likeliest location where failure beginning was calculated using the equations that account for the stress-strain field developed within the disk. The theoretical outcome shows that for failure beginning at the geometric center of the sample, it is necessary for the contact angle in the loading setup to be larger than or equal to a threshold value. Nevertheless, the measured indirect tensile strength must be adjusted to get a close estimate of the uniaxial TS of the material. The correction depends on the loading geometry, and we got their mathematical expression and cross-validated them with the reported in the literature. The experimental results show that a loading geometry with a curved contact area, uniform load distribution over the contact area, loads projected parallel to one another within the disk, and a contact angle bigger of $12^{\circ}$ is the most advisable and robust setup for implementation of BT on concrete disks. This work provides a description of the BT carries on concrete disks and put forward a characterization technique to study costly samples of cement based material that have been enabled to display new and improved properties with nanomaterials.