• 열처리공학회지
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• 제24권6호
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• pp.311-317
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• 2011

The effect of strain rate on the yield stress of an Al-Li alloy has been investigated at temperatures between 77 and 523 K and over the strain rate range from $1.77{\times}10^{-4}s^{-1}$ to $1.77{\times}10^{-2}s^{-1}$. At testing temperatures below 373 K, the yield stress is almost independent of strain rate at any aging stage. At testing temperatures above 373 K, the yield stress increases linearly with the logarithm of strain rate, and the strain rate dependence increases with increasing testing temperature. The yield stresses of under-aged alloy at temperatures between 373 and 473 K at high strain rates are greater than the yield stress at 77 K. For the alloy under-aged or aged nearly to its peak strength, the temperature range within which the positive temperature dependence of yield stress appears expands to the higher temperature side with increasing strain rate. The strain rate dependence of the yield stress is slightly negative at this aging stage. The yield stress of the over-aged alloy decreases monotonically with decreasing strain rate and with increasing testing temperature above 373 K. The modulus normalized yield stress is nearly constant at testing temperatures below 373 K at any strain rate investigated. And, strength depends largely both on the aging conditions and on the testing temperature. The peak positions in strength vs. aging time curves shift to the side of shorter aging time with increasing testing temperature. For the specimens aged nearly to the peak strength, the positive temperature dependence of yield stress is observed in the temperature range. The shift of peak positions in the aging curves are explained in terms of the positive temperature dependence of cutting stress and the negative temperature dependence of by-passing stress.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2000년도 춘계학술대회논문집
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• pp.183-186
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• 2000

The high temperature deformation behavior of spray-formed Al-19wt%Si-1.87wt%Mg-0.085wt.%Fe alloy was studied by torsion testing in the strain rate range of 0.001-1 sec-1 and in the temperature range of 300-500 $^{\circ}C$. The relationship between stress temperature and strain rate is expressed using the Power law. the behavior of dynamic recrystallization is showed in 300-35$0^{\circ}C$, 1-0.1sec-1 and the behavior of dynamic recovery is showed in 450-50$0^{\circ}C$, 0.01-0.001sec-1 The size of Si particles is mall when the temperature is low and the strain rate is high. The strain rate sensitivity(m) and the apparent activation energy(Q) indicate the dependence on strain rate and temperature for flow stress respectively. The hot ductility is high when m is high and Q is low. The maps of strain rate sensitivity and apparent activation energy suggest the optimum processing conditions.

• Asian-Australasian Journal of Animal Sciences
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• 제13권5호
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• pp.694-697
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• 2000

Laying hens kept in different light and dark periods of the day at high ambient temperature of maximum $35^{\circ}C$ were challenged to $38.5{\pm}0.5^{\circ}C$ acute heat 3 hours daily for 7 consecutive days. They were found to have a significant (p<0.01) acclimatization response (rectal temperature) to heat stress during the dark period compared to those exposed to the same temperature during the light period. The blood pH was not significantly different. The partial pressure of carbon dioxide ($PCO_2$) was significantly high (p<0.01) except in day 4. Similarly the blood bicarbonate ($HCO_3$) concentration was significantly high (p<0.05) except day three and day four. Acute heat exposure in the first day increased the body temperature in both groups (Light and Dark) reaching $44^{\circ}C$, followed by gradual reduction in body temperature. The dark treated birds showed rapid reduction in body temperature ($42.88^{\circ}C$) and adaptation to high temperature during days 2-4 but that this was lost to some extent in days 6-8. However this was not obvious in the light treated birds. It is concluded that darkness reduce hyperthermia and enhance acclimatization responses during acute heat stress.

• Advances in concrete construction
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• 제9권5호
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• pp.459-467
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• 2020

Mechanical and thermal properties of composite sandwich wall panels are affected by changes in their external environment. Humidity and temperature changes induce stress on wall panels and their core connectors. Under the action of ambient temperature, temperature on the outer layer of the wall panel changes greatly, while that on the inner layer only changes slightly. As a result, stress concentration exists at the intersection of the connector and the wall blade. In this paper, temperature field and stress field distribution of UHPC-RW-RC (Ultra-High Performance Concrete - Rock Wool - Reinforced Concrete) wall panel under high temperature-sprinkling and heating-freezing conditions were investigated by using the general finite element software ABAQUS. Additionally, design of the connection between the wall panel and the main structure is proposed. Findings may serve as a scientific reference for design of high performance composite sandwich wall panels.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2008년도 추계학술대회 논문집
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• pp.63-64
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• 2008

• 대한기계학회논문집A
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• 제27권5호
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• pp.750-757
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• 2003

In order to determine creep stress intensity limit of high-temperature components, the usefulness of the creep work and time equation, defined as W$\_$c/t$\^$p/ = B(where W$\_$c/ = $\sigma$$\varepsilon$ is the total creep work done during creep, and p and B are constants), was investigated using the experimental data. For this Purpose, the creep tests for generating 1.0% strain for commercial type i16 stainless steel were conducted with different stresses; 160 MPa, 150 MPa, 145 MPa, 140 MPa and 135 MPa at 593$^{\circ}C$. The plots of log W$\_$c/ - log t showed a good linear relation up to 10$\^$5/ hr, and the results of the creep work-time relation for p, B and stress intensity values showed good agreement to those of isochronous stress-strain curves (ISSC) presented in ASME BPV NH. The relation can be simply obtained with only several short-term 1% strain data without ISSC which can be obtained by long-term creep data. Particularly, this relation is useful in estimating stress intensity limit for new and emerging class of high-temperature creeping materials.

• 한국생산제조학회지
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• 제4권4호
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• pp.33-41
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• 1995

This study includes thermal plasticity analyses for a turbine rotor with the simple geometry and the boundary conditions. When centrifugal or thermal stress are applied at the high temperature material of engine blade, stress distributions I material ($\sigma$${\gamma}$${\gamma}$, $\sigma$$\theta$$\theta$, $\tau$${\gamma}$$\theta$, Mises stress) are analyzed by computer simulation(ABQUS) as followings; 1. The maximum stress at the radial direction() is applied at the upper middle part of spline hole. 2. The maximum stress at the tangential direction() is applied at the upper right boundary of spline hole. 3. The maximum shear stress () in () direction is applied at the upper middle part of spline hole. 4. The maximum Mises stress is applied at the upper right boundary of spline hole. This stress is due to the critical stress by which rotor can be fractured according to elapsed time.

• International Journal of Automotive Technology
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• 제7권5호
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• pp.571-577
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• 2006

This paper deals with dynamic tensile characteristics for the polypropylene used in an IP(Instrument Panel). The polypropylene is adopted in the dash board of a car, especially PAB(Passenger Air Bag) module. Its dynamic tensile characteristics are important because the PAB module undergoes high speed deformation during the airbag expansion. Since the operating temperature of a car varies from $-40^{\circ}C$ to $90^{\circ}C$ according to the specification, the dynamic tensile tests are performed at a low temperature($-30^{\circ}C$), the room temperature($21^{\circ}C$) and a high temperature($85^{\circ}C$). The tensile tests are carried out at strain rates of six intervals ranged from 0.001/sec to 100/sec in order to obtain the strain rate sensitivity. The flow stress decreases at the high temperature while the strain rate sensitivity increases. Tensile tests of polymers are rather tricky since polymer does not elongate uniformly right after the onset of yielding unlike the conventional steel. A new method is suggested to obtain the stress-strain curve accurately. A true stress-strain curve was estimated from modification of the nominal stress-strain curves obtained from the experiment. The modification was carried out with the help of an optimization scheme accompanied with finite element analysis of the tensile test with a special specimen. The optimization method provided excellent true stress-strain curves by enforcing the load response coincident with the experimental result. The material properties obtained from this paper will be useful to simulate the airbag expansion at the normal and harsh operating conditions.

• 한국안전학회지
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• 제1권1호
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• pp.41-49
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• 1986

Modern technological progress demands the use of materials at high temperature and high pressure. One of the most critical factors in considering such applications-perhaps the most critical one-is creep behavior. In this study the activation energy for the creep rupture (Qf) and the stress dependence of rupture time (n') have been determined during creep of Al 7075 alloy eve, the temporature range of $200^{\circ}C to 500^{\circ}C$ and stress range of 0.64 kgf/$\textrm{mm}^2$ to 9.55 kgf/$\textrm{mm}^2$, respectively, in order to investigate the creep-rupture property. Constant load creep tests were carried out in the enperiment At around the temperature $210^{\circ}C~390^{\circ}C$ and the stress level 1.53~9.55(kgf/$\textrm{mm}^2$), the stress dependence of rupture time(n') had the value of 6.6~6.78 but at 50$0^{\circ}C$, the value of 1.3. Besides at around the temperature of $200^{\circ}C~500^{\circ}C$ and under the stress level of 0.89~8.51 (kgf/$\textrm{mm}^2$), the activation energy for the creepprupture (Qf) was nearly equal to that of the volume self diffusion of pure aluminum (34Kca1/mo1e)

• 한국유체기계학회 논문집
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• 제8권6호
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• pp.40-46
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• 2005

Because of design and manufacturing costs, it is important to predict an expected life of bellows with component stresses of bellows as its design factors and material characteristics. In this study, numerical analyses are carried out to elucidate the thermal and flow characteristics of the bellows-seal gate and globe valves for high temperature (max. $600^{\circ}C$) and for high pressure (max. $104 kgf/cm^2$) conditions. Using commercial codes, FLUENT, which uses FVM and SIMPLE algorithm, and ANSYS, which uses FEM, the pressure and temperature fields are graphically depicted. In addition, when bellows have an axial displacement, thermal stress affecting bellows life is studied. The pressure and temperature values obtained from the flow analyses are adopted as the boundary conditions for thermal stress analyses. As the result of this study, we got the reasonable coefficients for valve and thermal stress for bellows, compared with existing coefficients and calculated values.