• 한국재료학회지
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• 제26권5호
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• pp.281-286
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• 2016

In the present study, the tensile properties and dynamic strain aging of an Fe-24.5Mn-4Cr-0.45C alloy were investigated in terms of strain rate. During tensile testing at room temperature, all the stress-strain curves exhibited serrated plastic flows related to dynamic strain aging, regardless of the strain rate. Serration appeared right after yield stress at lower strain rates, while it was hardly observed at high strain rates. On the other hand, strain-rate sensitivity, indicating a general relationship between flow stress and strain rate at constant strain and temperature, changed from positive to negative as the strain increased. The negative strain-rate sensitivity can be explained by the Portevin Le Chatelier effect, which is associated with dynamic strain aging and is dependent on the strain rate because it is very likely that the dynamic strain aging phenomenon in high-manganese steels is involved in the interaction between moving dislocations and point-defect complexes.

• 한국자동차공학회논문집
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• 제12권3호
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• pp.204-210
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• 2004

Most auto-body members fabricated by the sheet metal forming process. During this process the thickness and material properties of the sheet metal are changed with the residual stress and plastic strain. This paper deals with the material properties of the sheet metal at the high strain rate considering the pre-strain effect. Specimens are selected from sheet metals for outer panels and inner members, such as SPCEN, SPRC45E, SPRC35R and EZNCD. The specimens are prepared with the pre-strain of 2, 5 and 10 % by tensile elongation in Instron 5583, which could be equivalent to the plastic strain in sheet metal forming. High speed tensile tests are then carried out with the pre-stained specimens at the strain rate of 1 to 100/sec. The experimental result informs that the material properties are noticeably influenced by the pre-strain when the yield stress of the specimens is moderate as SPCEN, SPRC35R and EZNCD. The result also demonstrates that the ultimate tensile strength as well as the yield stress is increased as the amount of the pre-strain is increased.

• 소성∙가공
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• 제10권2호
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• pp.137-143
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• 2001

The change of flow stress due to dynamic recrystallization during hot forming process is investigated. A series of mechanical tests has been conducted at various temperatures, and constitutive relations and recrystallization kinetics were formulated from the test results. The effect of dynamic recrystallization to the flow stress was implemented to a commercial FEM code by conditioned remapping of state variables. The datum strain of stress compensation was optimized to minimize the overestimation of forming loads. Suggested datum was formulated as an exclusive function of critical strain for recrystallizalion and validated by mechanical tests and microstructural observations.

• 한국표면공학회지
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• 제41권3호
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• pp.121-128
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• 2008

High strength 7xxx aluminum alloys have been applied to automotive bump back beam of the some limited model for light weight vehicle. The aluminum bump back beam is manufactured through extrusion, bending and welding. The residual stress given on these processes combines with the corrosive atmosphere on the road spreaded with corrosive chemicals to melt snow to occur the stress corrosion cracking. The composition of commercial 7xxx aluminum has Zn/Mg ratio about 3 and Cu over 2 wt% for better strength and stress corrosion cracking resistivity. But this composition isn't adequate for appling to the automotive bump back beam with high resistance to extrusion and bad weldability. In this study the composition of 7xxx aluminum alloy was modified to high Zn/Mg ratio and low Cu content for better extrusion and weldability. To estimate the resistivity against stress corrosion cracking of this aluminum alloy by slow strain rate test, the corrosion atmosphere and strain rate separate the stress corrosion cracking from conventional corrosion must be investigated. Using 0.6 Mol NaCl solution on slow strain rate test the stress corrosion cracking induced fracture was not observed. By adding 0.3% $H_2O_2$ and 0.6M $Na_2SO_4$ to 1M NaCl solution, the corrosion potential and current density of polarization curve moved to active potential and larger current density, and on the slow strain rate test the fracture energy in solution was lower than that in pre-exposure. These mean the stress corrosion cracking induced fracture can be estimated in this 1M NaCl + 0.3% $H_2O_2$ + 0.6M $Na_2SO_4$ solution. When the strain rate was below $2{\times}10^{-6}$, the stress corrosion cracking induced fracture start to be observed.

• Advances in concrete construction
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• 제2권3호
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• pp.177-192
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• 2014

One of the main applications of FRP composites is confining concrete columns. Hence identifying the cyclic and monotonic stress-strain behavior of confined concrete columns and the parameters influencing this behavior is inevitable. Two significant parameters affecting the stress-strain behavior are aspect ratio and corner radius. The present study aims to scrutinize the effects of corner radius and aspect ratio on different aspects of stress-strain behavior of FRP confined concrete specimens (rectangular, square and circular). Hence 44 FRP confined concrete specimens were tested and the results of the tests were investigated. The findings indicated that for specimens with different aspect ratios, the relationship between the ultimate stress and the corner radius is linear and the variations of the ultimate stress versus the corner radius decreases as a result of an increase in aspect ratio. It was also observed that increase of the corner radius results in increase of the compressive strength and ultimate axial strain and increase of the aspect ratio causes an increase of the ultimate axial strain but a decrease of the compressive strength. Investigation of the ultimate condition showed that the FRP hoop rupture strain is smaller in comparison with the one obtained from the tensile coupon test and also the ultimate axial strain and confined concrete strength are smaller when a prism is under monotonic loading. Other important results of this study were, an increase in the axial strain during the early stage of unloading paths and increase of the confining effect of FRP jacket with the increase and decrease of the corner radius and aspect ratio respectively, a decrease in the slope of reloading branches with cycle repetitions and the independence of this trend from the variations of the aspect ratio and corner radius and also quadric relationship between the number of each cycle and the plastic strain of the same cycle as well as the independence of this relationship from the aspect ratio and corner radius.

• Structural Engineering and Mechanics
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• 제46권1호
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• pp.111-135
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• 2013

An experimental study has been carried out on square plain concrete (PC) and reinforced concrete (RC) columns strengthened with carbon fiber-reinforced polymer (CFRP) sheets. A total of 78 specimens were loaded to failure in axial compression and investigated in both axial and transverse directions. Slenderness of the columns, number of wrap layers and concrete strength were the test parameters. Compressive stress, axial and hoop strains were recorded to evaluate the stress-strain relationship, ultimate strength and ductility of the specimens. Results clearly demonstrate that composite wrapping can enhance the structural performance of square columns in terms of both maximum strength and ductility. On the basis of the effective lateral confining pressure of composite jacket and the effective FRP strain coefficient, new peak stress equations were proposed to predict the axial strength and corresponding strain of FRP-confined square concrete columns. This model incorporates the effect of the effective circumferential FRP failure strain and the effect of the effective lateral confining pressure. The results show that the predictions of the model agree well with the test data.

• 콘크리트학회논문집
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• 제13권5호
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• pp.447-456
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• 2001

그 동안 많은 연구자들은 콘크리트의 응력-변형률 관계의 비선형 거동을 적절한 수식으로 나타내기 위해 많은 노력을 해 왔다. 그러나 이 응력-변형률 관계에 대한 대부분의 경험식은 경화된 콘크리트에 촛점을 맞추어 왔으며, 초기재령에서의 콘크리트의 거동을 잘 나타내지 못하였다. 여기서 초기재령에서 경화시까지 걸친 전 콘크리트의 재령에 대한 폭 넓은 이해는 콘크리트구조물의 내구성과 잔존수명을 평가하는데 있어서 매우 중요하다. 본 논문에서는 5가지의 강도수준과 12시간에서 28일까지의 재령에 대하여 응력-변형률 관계를 검토하였으며, 20$\pm$3$^{\circ}C$ 에서 수중양생된 ø100$\times$200mm의 원주공시체에 대하여 1축압축강도실험을 수행했다. 실험결과에 대한 회귀분석을 수행하여 강도와 재령에 따른 응력-변형률 관계의 모델식을 제시하였으며, 제시된 모델식의 검증을 위하여 실험결과와 기존의 실험결과와 모델식에 대한 해석적 검토도 수행하였다. 해석결과, 제시된 모델식이 실험결과와 잘 맞으며 응력-변형률 관계에 강도와 재령이 미치는 영향을 잘 나타내고 있음을 알 수 있었다.

• 대한기계학회논문집A
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• 제26권9호
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• pp.1787-1794
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• 2002

TiNi alloy fiber was used to solve the problem of the tensile residual stress as the reinforced material. TiNi alloy fiber improves the tensile strength of composite by occurring compressive residual stress in the matrix using shape memory effect. In order to generate compressive residual stress in TiNi/Al6061 shape memory alloy(SMA) composite, 1, 3 and 5% pre-strains were applied to the composite in advance. It was also evaluated the effect of compressive residual stress corresponding to the pre-strain variation and the volume fraction of TiNi alloy. AE technique was used to clarify the microscopic damage behavior at high temperature and the effect of pre-strain in TiNi/Al6061 SMA composite. The results of the microscopic damage evaluation of TiNi/Al6061 SMA composite under various pre-strain using AE technique can be divided into three stage corresponding to the AE signals. AE counts and events were useful parameters to evaluate the fracture mechanism according to the variation of pre-strain. In addition, two dimensional AE source location technique was applied for monitoring the crack initiation and propagation in composite.

• Nuclear Engineering and Technology
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• 제9권3호
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• pp.117-123
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• 1977

소입된 질칼로이-4의 가공시효(strain ageing) 현상에 미치는 시효시간 및 온도의 효과를 523-588 狂 온도 구간에서 1~52초의 짧은 시효기간에 조사하였다. 이 실험 조건하에서, 가공시효응력은 5.55$\times$$10^{-4}$ sec$^{-1}$ 신연속도(strain rate) 경우 시효시간 및 시효온도가 증가할수록 증가하였다. 소입된 질칼로이-4의 응력부하 하의 가공시효 현상이 두 한계로 나타나는 것이 확인되었는데 첫단계는 0.39ev의 활성화 에너지 값의 구간으로 이는 전위 (dislocation) 주위의 응력상태로 인한 침입형 산소원자들의 Snoek타입의 배열 (ordering)로 인한 것으로 고려되었으며 두번째 단계는 0.6ev 활성화에너지 값을 갖는 구간으로 이는 주로 산소원자들의 장범위확산에 기인하는 것으로 해석되었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.138-142
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• 2000

In LNG Tank, it is very important to measure the strain in Membrane by theoretical and experimental stress analysis. In this paper, perform the test about strain gage, thermal sensor and lead wire to make clear the properies. The test results conclude that stress measurement by strain gage must consider the effect of many factors to measure strain acculately. The corrections should be made on apparent strain, lead wire length and Membrane shape. It is also important to measure the temperatures accurately at the strain gage location