• 콘크리트학회논문집
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• 제28권2호
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• pp.223-234
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• 2016

이 연구에서는 노치 도입 인장시편을 사용하여 직접인장강도 실험을 통해 UHPC의 파괴거동을 살펴보고, 강섬유 혼입률에 따른 UHPC의 초기균열강도와 인장강도를 제안하였다. 실험결과 UHPC와 초기균열강도와 인장강도, 그리고 파괴에너지 등은 강섬유 혼입률이 증가할수록 증가하는 것으로 나타났다. 균열선단에서의 응집응력은 Barenblatt의 가정을 사용하여 결정되었으며, 이를 토대로 변형경화 현상이 발생하는 강섬유 혼입률이 1% 이상인 UHPC의 최대응집응력을 예측할 수 있는 간편식을 제안하였다. 인장강도는 강섬유 혼입률과 압축강도의 함수로 제안되었으며, 파괴에너지는 인장강도의 함수로 제안되었다. 제안된 간편식들은 실험값과 비교적 잘 일치하였으며, 향후 압축강도가 140~170 MPa이고, 강섬유 혼입률이 2% 이하인 UHPC에 적용가능 할 것으로 판단된다.

• E2M - 전기 전자와 첨단 소재
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• 제10권4호
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• pp.327-333
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• 1997

Experiments on mechanical fatigue were conducted using the specimens which were cut from hydrogen cooled generator(rated 22kV and 50OMW) stator windings. We have investigated the aged mechanism of mica/epoxy insulation systems under air or hydrogen by both the tensile and compressive loadings. The fracture of generator stator windings is generally affected by mechanical stress. Thus, the tensile strength test were conducted. In this case, the maximum strength and strain are quite different between sound and aged specimens. It is observed that low bonded interface parts of tapes generally have lower strength than those of normal tapes which causes stress. In order to estimate the effects of cyclic load by the electromagnetic forces while the generator starts/stops, the mechanical fatigue test was also conducted. It is confirmed that the equation of expected life depends on stress amplitude and number of cycles. Though the stress amplitude and number of cycles are very tiny, the tensile fatigue of aged specimens under hydrogen atmosphere is bigger than those under air. In the case of hydrogen atmosphere, the tensile stress gives bigger effect than the compressive one.

• 한국농공학회지
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• 제37권3_4호
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• pp.90-98
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• 1995

The recycled concrete, average compressive strength of which was 2l0kg/cm$^2$ or higher with slump range of 14~18cm, was prepared by replacing 25% and 50% by weight of coarse aggregate with recycled aggregate from waste concrete. Mix design method for crushed aggregates was used and all specimens were cured by normal moisture curing method. A plasticiser and a fly ash were added to the mix to improve performance of recycled concrete. Flexural strength, stress- strain relationship and fracture toughness were evaluated by comparing with those of normal concretes. Recycled concrete showed, in general, lower flexural strength and fracture toughness, and higher strain under the same stress level. Fly ash in the concrete had an effect of reducing the strength and fracture toughness on both normal and recycled concretes. Since fly ash is known to improve many properties of concrete, while reducing strength properties, decision for using fly ash should be made carefully depending on the intended usage of the recycled concrete.

• Journal of Ceramic Processing Research
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• 제19권5호
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• pp.419-423
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• 2018

A newly conceived geopolymer composite was fabricated by a combination of the geopolymer and polyurethane sponge. The density and porosity of hardened geopolymer composite, corresponded to different pore sizes of polyurethane sponge, exhibited no significant differences from each other. However, the mechanical behavior, the compressive strength and flexural strength, showed slight differences accordingly. Fracture of the geopolymer composite exposed to high compressive load was not observed from all specimens containing polyurethane sponge. The toughness enhancement of the geopolymer composite, due to spontaneous elasticity of polyurethane sponge, crack spread, and crack diffraction, was identified through the stress-strain curve and microstructure of fracture surface. The newly designed geopolymer composite having a 3-dimensional sponge skeleton showed relatively higher flexural strength of 8.0 MPa than other conventional geopolymer composites.

• Advances in concrete construction
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• 제1권4호
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• pp.341-358
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• 2013

Due to fast growth in urbanisation, a highly developed infrastructure is essential for economic growth and prosperity. One of the major problems is to preserve, maintain, and retrofit these structures. To meet the requirements of construction industry, the basic information on all the mechanical properties of various concretes is essential. This paper presents the details of development of various concretes, namely, normal strength concrete (around 50 MPa), high strength concrete (around 85 MPa) and ultra high strength concrete (UHSC) (around 120 MPa) including their mechanical properties. The various mechanical properties such as compressive strength, split tensile strength, modulus of elasticity, fracture energy and tensile stress vs crack width have been obtained from the respective test results. It is observed from the studies that a higher value of compressive strength, split tensile strength and fracture energy is achieved in the case of UHSC, which can be attributed to the contribution at different scales viz., at the meso scale due to the fibers and at the micro scale due to the close packing of grains which is on account of good grading of the particles. Micro structure of UHSC mix has been examined for various magnifications to identify the pores if any present in the mix. Brief note on characteristic length and brittleness number has been given.

• Structural Engineering and Mechanics
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• 제46권2호
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• pp.295-322
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• 2013

Steel fiber reinforced self-compacting concrete (SFRSCC) is a relatively new composite material which congregates the benefits of self-compacting concrete (SCC) technology with the profits derived from the fiber addition to a brittle cementitious matrix. Steel fibers improve many of the properties of SCC elements including tensile strength, toughness, energy absorption capacity and fracture toughness. Modification in the mix design of SCC may have a significant influence on the SFRSCC mechanical properties. Therefore, it is vital to investigate whether all of the assumed hypotheses for steel fiber reinforced concrete (SFRC) are also valid for SFRSCC structures. Although available research regarding the influence of steel fibers on the properties of SFRSCC is limited, this paper investigates material's mechanical properties. The present study includes: a) evaluation and comparison of the current analytical models used for estimating the mechanical properties of SFRSCC and SFRC, b) proposing new relationships for SFRSCC mixtures mechanical properties. The investigated mechanical properties are based on the available experimental results and include: compressive strength, modulus of elasticity, strain at peak compressive strength, tensile strength, and compressive and tensile stress-strain curves.

• 한국기계가공학회지
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• 제2권1호
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• pp.22-31
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• 2003

The compressive residual stress, which is induced by shot peening process, seems to be an Important factor in increasing the fatigue strength. And then it was showed that residual stress was disappearenced at the high temperature. The fatigue charateristic investigation of a SUP9 spring steel processed shot peening is performed by considering the high temperature service conditions in the range of room temperature through $180^{\circ}C$ in the range of stress ratio of 0.3 by means of opening mode displacement. The fatigue resistance characteristics and fracture strength at high temperature is considerable lower than that of room temperature in the early stage and stable of fatigue crack growth region.

• 한국공작기계학회논문집
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• 제17권3호
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• pp.115-121
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• 2008

A knowloedge of the residual stress distribution at circumferential welds can increase the prediction accuracy of a fracture assessment in pipe lines. In this study, in order to predict the residual stress distribution in the circumferential butt-welded pipes were measured, using the hole-drilling strain gauge method. Their practical applications were performed in to two kinds of pipes. As the results, the following characteristics were found. On the inner surface of pipes, the circumferential and axial residual stresses were both tensile near the center line of welding and both of them changed from tensile to compressive as the distance from the center line increased. On the outer surface, however, the circumferential residual stress was shown to be tensile wile the axial residual stress was compressive near the center line of welding, and later they were revered at the region far away from the centerline.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2002년도 추계학술대회 논문집
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• pp.347-353
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• 2002

The compressive residual stress, which is induced by shot peening process, seems to be an important factor of increasing the fatigue strength. And then it was showed that residual stress was disappearenced at the high temperature. The fatigue characteristic study of a SUP9 spring steel processed shot peening is performed by considering the high temperature service conditions in the range of room temperature through $180^{\circ}C$ in the range of stress ratio of 0.3 by means of opening mode displacement. The fatigue resistance characteristics and fracture strength at high temperature is considerable lower than that of room temperature in the early stage and stable of fatigue crack growth region.

• 대한기계학회논문집A
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• 제26권2호
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• pp.275-282
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• 2002

Tensile residual stress happen by difference of coefficients of thermal expansion between fiber and matrix is one of the serious problems in metal matrix composite(MMC). In this study, TiNi fiber was used to solve the tensile residual stress as the reinforced material. TiNi fiber improves the tensile strength of composite by occurring compressive residual stress in matrix using shape memory effect of it. Pre-strain was added to generate compressive residual stress inside TiNi/A16061 composite. It was also evaluated the effect of compressive residual stress corresponding to pre-strains variation. AE technique was used to clarify the microscopic damage behavior at high temperature and the effect of pre-strain difference of TiNi/A16061 shape memory alloy composite.