• 소성∙가공
• /
• 제15권8호
• /
• pp.550-555
• /
• 2006

In this study, effects of forming angle and friction coefficient on a initiation of the Mannesmann hole defect were analyzed by using a response surface method. The maximum effective plastic strain at center point of specimen is utilized for the prediction of the starting point of crack occurrence, which is suggested by the comparison of integrals of four different ductile fracture models between the histories of the effective plastic strain at center point. It was revealed that the principal stress at the center is the dominant element to the increase of the effective plastic strain. It was also verified by the simulation results from the comparison of experiment and simulation. It is provided that the forming angle of 25 degrees and the spreading angle of 1 degree can be a proper design condition without an occurrence of internal hole defect and an excessive slip.

• 수산해양기술연구
• /
• 제21권2호
• /
• pp.151-156
• /
• 1985

본 연구에서는 양쪽크랙 인장시험편을 이용한 평면응력 파괴인성치 J 하(1C)를 검토하기 위하여, 두께 4.5mm 일반구조용 압연강판(SS41)을 균질화 및 연화처리하고 각각의 경우에 대하여 크랙비가 0.55, 0.65, 0.75인 시험편을 준비하여 J 하(1C) 및 저항곡선의 거동을 고찰하였다. 얻어진 결과를 요약하면 다음과 같다. 1) 양쪽 크랙 인장시험편을 사용하여 J 하(1C)를 구한 값은 한계하중에 의한 방법이 Rice가 제안한 방법보다 크게 나타남을 알 수 있다. 2) Rice가 제안한 방법으로 J 하(1C)는 실험한 크랙비 범위 0.55-0.75에서 거의 일정한 값을 나타내고 저항곡선의 기울기는 크랙비에 따라 증가하였다. 3) Rice가 제안한 방법으로 [SS41의 J 하(1C)의 평균값은 모재는 22.8kgf/mm, 균질화 처리재는 24.7kgf/mm, 연화처리재는 26.9kgf/mm를 얻었다

• 한국건축시공학회지
• /
• 제18권3호
• /
• pp.251-257
• /
• 2018

여름철 콘크리트는 수분증발에 의한 소성 및 건조수축균열, 슬럼프 저하, 콜드조인트 발생 등 다양한 문제가 발생한다. 이에 국내에서는 수분증발을 방지하고, 단열성능을 확보하기 위해 표면피복 양생을 위한 다양한 형태의 버블시트를 제작하여 사용하고 있으나, 여름철에는 적용사례가 없었다. 따라서 본 연구에서는 여름철 시공중인 아파트 현장에 1중의 투명, 백색 및 알루미늄 증착 버블시트와 PE필름 및 표면노출의 5가지 표면피복 양생재를 적용하여 콘크리트의 온도 및 균열특성 등 품질에 미치는 영향을 분석하였다. 실험결과 알루미늄 증착 버블시트가 온도저감, 균열저감 면에서 가장 뛰어난 것으로 나타났지만, 알루미늄 재질의 특성상 빛이 반사되어 시공자의 눈부심에 따른 시각공해가 발생하기 때문에 백색 버블시트의 사용이 가장 효과적인 것으로 밝혀졌다.

• 대한기계학회논문집A
• /
• 제27권8호
• /
• pp.1398-1408
• /
• 2003

Tensile and LCF(low cycle fatigue) tests were carried out in air at wide temperature range 20$^{\circ}C$-750$^{\circ}C$ and strain rates of 1${\times}$10$\^$-4//s-1${\times}$10$\^$-2/ to ascertain the influence of strain rate on tensile and LCF properties of prior cold worked 316L stainless steel, especially focused on the DSA(dynamic strain aging) regime. Dynamic strain aging induced the change of tensile properties such as strength and ductility in the temperature region 250$^{\circ}C$-600$^{\circ}C$ and this temperature region well coincided with the negative strain rate sensitivity regime. Cyclic stress response at all test conditions was characterized by the initial hardening during a few cycles, followed by gradual softening until final failure. Temperature and strain rate dependence on cyclic softening behavior appears to result from the change of the cyclic plastic deformation mechanism and DSA effect. The DSA regimes between tensile and LCF loading conditions in terms of the negative strain rate sensitivity were well consistent with each other. The drastic reduction in fatigue resistance at elevated temperature was observed, and it was attributed to the effects of oxidation, creep and dynamic strain aging or interactions among them. Especially, in the DSA regime, dynamic strain aging accelerated the reduction of fatigue resistance by enhancing crack initiation and propagation.

• 한국건축시공학회:학술대회논문집
• /
• 한국건축시공학회 2006년도 춘계학술논문 발표대회 제6권1호
• /
• pp.15-18
• /
• 2006

The test result of mat concrete applying both hydration heat difference and insulation curing method on new construction of Cheongju university educational building are summarized as following. Both fresh concrete and compressive strength properties were satisfied In aimed value. Setting time of concrete incorporating 15% of fly ash(FA) retarded 1.2 hour than control concrete. Temperature history of mali concrete indicated that the highest temperature of center was exhibited at $126^{\circ}C$ after 51 hours while the highest temperature of upper section was $10.6^{\circ}C$ after 46 hours. Temperature Difference between center and surface was managed at less than $6^{\circ}C$ during whole curing period. In addition the temperature of upper section secured more than $3.3^{\circ}C$ while the temperature of outside was indicated at less than $-10^{\circ}C$. Maturity by parts of construction secured more than $30^{\circ}C$ DD higher than outside at 3 days. The more number of times, applying insulation curing method by double bubble sheets, increased, the higher economic effect was secured. Overall it was clear that applying both double bubble sheets and hydration heat difference method on this new construction can resist hydration heat crack, early frost demage and strength decrease. It also significantly contributed quality improvement of cold weather concreting

• 한국건축시공학회:학술대회논문집
• /
• 한국건축시공학회 2007년도 춘계학술논문 발표대회
• /
• pp.1-4
• /
• 2007

This study investigated the applicability of insulation curing method with double layer bubble sheets to the man concrete subjected to cold weather. Temperature history of the mass concrete indicated that the highest temperature of center section was exhibited at $34^{\circ}C$ while the that of surface section was $25^{\circ}C$. Difference between center and surface was shown to be less $10^{\circ}C$ when the temperature of center section was peak section, and thereby associated temperature cracking index was calculated 1.5, and occurrence probability of temperature cracking was 5%, so there was no temperature cracking caused by internal restraint. No temperature crack was observed by naked eye. It was clear that early frost demage and temperature cracking could be restrained due to reducing temperature difference between inner part and outside in the case double bubble sheets applied to insulation curing method in cold weather. The maturity of mass concrete is higher than outside about $72\sim89^{\circ}$ DD, so it was demonstrated to prevent early frost demage and ensure strength excellently. Insulation curing method with double bubble sheets was more economic than heating curing method.

• 소성∙가공
• /
• 제31권6호
• /
• pp.376-383
• /
• 2022

The potential damage region of a WC-Co cemented carbide die is investigated for cold forging process of a wheel-nut by numerical simulation with its chemical composition considered. Numerical simulation is utilized to calculate internal stress, especially for the WC-Co die, during the forging process. Finite element model is established, in which the elasto-plastic properties are applied to the work-piece of bulk steel, and elastic properties are considered for the lower die insert of the WC-Co alloy. This stress analysis enables to distinguish the potential damage regions of the WC-Co die. The regions from calculation are comparatively analyzed along with the crack area observed in the die after repetitive manufacturing. Effect of chemical composition of the WC-Co is also evaluated on characteristics of potential damage region of the die with variance of mechanical properties considered. Derived from Mohr-Coulomb fracture model, furthermore, a new stress index is presented and used for die stress analysis. This index inherently considers hydrostatic pressure and is then capable of deducing wide range of its distribution for representing stress state by modification of its parameter implying pressure sensitivity.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2008년도 추계학술대회 논문집
• /
• pp.230-233
• /
• 2008

Stainless steels are alloy steels with iron as the primary constituent and chromium, nickel, and manganese as principal alloying elements. In addition to automotive, construction, and transportation industries, stainless steels have a variety of applications in the food, chemical, and pharmaceutical industries. Some common products made from stainless steel are sinks, wash basins, kitchen vessels, and cutlery. Among ferritic stainless steels, type 409 can be cold-formed easily and are used for deep-drawn parts such as vessels for the chemical and food industries. In this study, forming analyses and experiments to prevent the occurrence of inferiority such as wrinkles, crack, and neck for automotive muffler part are carried out to save the optimal conditions during forming by modifying the blank size and shape, blank holding force etc.

• 한국콘크리트학회:학술대회논문집
• /
• 한국콘크리트학회 2001년도 봄 학술발표회 논문집
• /
• pp.469-474
• /
• 2001

Mass concrete is placed considering placing lifts in order to reduce thermal cracks by hydration heat. But they results in cold joint between placing lifts, which bring about the loss of strength, water tightness and undesirable appearance. Therefore, in this paper, mechanical and hydration heat of mass concrete using super retarding agent developed through previous study are investigated in order to reduce the hydration heat and place it without place lift. According to test results, placing lifts combined with normal concrete and concrete containing super retarding agent have positive effects on reducing hydration heat. Especially, the crack index by thermal stress of the concrete containing super retarding agent less than a quarter, compared to that of plain concrete without placing lifts, and less than a half, compared to that of plain concrete with placing lifts.

• 한국콘크리트학회:학술대회논문집
• /
• 한국콘크리트학회 2001년도 봄 학술발표회 논문집
• /
• pp.515-520
• /
• 2001

Mass concrete is placed considering placing lifts in order to reduce thermal cracks by hydration heat. But they results in cold joint between placing lifts, which bring about the loss of strength, water tightness and undesirable appearance. Therefore, in this paper, mechanical and hydration heat of mass concrete using super retarding agent developed through previous study are investigated in order to reduce the hydration heat and place it without place lift. According to test results, placing lifts combined with normal concrete and concrete containing super retarding agent have positive effects on reducing hydration heat. Especially, the crack index by thermal stress of the concrete containing super retarding agent less than a quarter, compared to that of plain concrete without placing lifts, and less than a half, compared to that of plain concrete with placing lifts.