• Composites Research
• /
• 제32권6호
• /
• pp.382-387
• /
• 2019

상대습도와 온도가 PET 필름의 점탄성 특성에 미치는 영향을 조사하기 위해 RH-DMA를 이용하여 single frequency strain mode 시험, stress relaxation mode 시험, creep 시험을 수행하였다. 상대습도는 10%, 30%, 50%, 70%, 90%를 적용하고 온도는 single frequency strain mode 시험의 경우 30~95℃, stress relaxation mode 시험의 경우 30℃ 와 70℃, creep 시험의 경우 5~95℃를 고려하였다. 연구결과에 따르면 상대습도가 높아지면 저장탄성계수와 손실탄성계수는 낮아지며 손실탄성계수의 최대값은 상대습도의 변화에 큰 영향을 받지 않고 거의 일정해진다. 이완탄성계수는 초기에 급격히 감소하다가 일정한 값을 가지며 높은 온도에서는 상대습도의 변화에 민감해진다. 변형률 회복는 초기에 급격히 증가하며 온도가 높아지면 이완 탄성계수와 마찬가지로 상대습도에 민감하게 변한다. 크리프 컴플라이언스의 증가 정도는 온도가 높아지면 커지며 유리전이온도보다 온도가 높아지면 증가 정도는 더욱 커진다. 시간-온도 중첩법을 통해 구해지는 마스터 선도를 이용하면 상대습도와 온도 등의 운용 조건에서의 장기 성능을 예측할 수 있는 정보를 얻을 수 있다.

• 대한기계학회논문집A
• /
• 제33권12호
• /
• pp.1455-1463
• /
• 2009

Theoretical mechanics analysis and finite element simulation were performed to investigate creep deformation behavior at the crack tip of transversely isotropic materials under small scale creep (SCC) conditions. Mechanical behavior of material was assumed as an elastic-$2^{nd}$ creep, which elastic modulus ( E ), Poisson's ratio ( ${\nu}$ ) and creep stress exponent ( n ) were isotropic and creep coefficient was only transversely isotropic. Based on the mechanics analysis for material behavior, a constitutive equation for transversely isotropic creep behavior was formulated and an equivalent creep coefficient was proposed under plain strain conditions. Creep deformation behavior at the crack tip was investigated through the finite element analysis. The results of the finite element analysis showed that creep deformation in transversely isotropic materials is dominant at the rear of the crack-tip. This result was more obvious when a load was applied to principal axis of anisotropy. Based on the results of the mechanics analysis and the finite element simulation, a corrected estimation scheme of the creep zone size was proposed in order to evaluate the creep deformation behavior at the crack tip of transversely isotropic creeping materials.

• 한국농공학회논문집
• /
• 제52권3호
• /
• pp.73-79
• /
• 2010

In the present study, the loading frequency dependencies of cyclic shear strength and elastic shear modulus of reconstituted clay were examined by performing undrained cyclic triaxial tests and undrained cyclic triaxial tests to determine deformation properties. The result of undrained cyclic triaxial test of reconstituted and saturated clay shows that a faster frequency leads to higher stress amplitude ratio, but when the frequency becomes fast up to a certain point, the stress amplitude ratio will reach its maximum limit and the frequency dependence becomes insignificant. And also, the result of undrained cyclic triaxial deformation test shows a fact that a faster loading frequency leads to higher equivalent shear modules and smaller hysteresis damping ratio, and confirms the frequency dependence of cohesive soil. Meanwhile, the result of the creep test shows that continuing creep is created in the undrained cyclic triaxial test with slow loading frequency rate, and since loading rate becomes slower at the vicinity of the maximum and the minimum deviator stress due to sine wave loading, the vicinity of the maximum and the minimum deviator stress shall be more influenced by creep.

• Geomechanics and Engineering
• /
• 제20권6호
• /
• pp.517-525
• /
• 2020

The long-term stability of rock engineering is significantly affected by the time-dependent deformation behavior of rock, which is an important mechanical property of rock for engineering design. Although the hard rocks show small creep deformation, it cannot be ignored under high-stress condition during deep excavation. The inner mechanism of creep is complicated, therefore, it is necessary to investigate the relationship between microscopic creep mechanism and the macro creep behavior of rock. Microscopic numerical modeling of sandstone creep was performed in the investigation. A numerical sandstone sample was generated and Parallel Bond contact and Burger's contact model were assigned to the contacts between particles in DEM simulation. Sensitivity analysis of the microscopic creep parameters was conducted to explore how microscopic parameters affect the macroscopic creep deformation. The results show that the microscopic creep parameters have linear correlations with the corresponding macroscopic creep parameters, whereas the friction coefficient shows power function with peak strength and Young's modulus, respectively. Moreover, the microscopic parameters were calibrated. The creep modeling curve is in good agreement with the verification test result. Finally, the creep curves under one-step loading and multi-step loading were compared. This investigation can act as a helpful reference for modeling rock creep behavior from a microscopic mechanism perspective.

• 한국지반공학회지:지반
• /
• 제9권4호
• /
• pp.93-102
• /
• 1993

연약 지반위에 축조된 토질 구조물은 제체의 자중으로 인하여 오랜 기간 동안 침하가 계속되는데 이는 시간표존적 거동인 압밀과 크리프(creep)가 동시에 작용하여 발생된다. 본 연구에서는 안산의 해성점토에 대하여 크리프거동 해석에서 필요한 정수를 구하고 수치해석시 중요시되고 있는 구속응력과 응력수준에 따른 크리프정수의 영향 특성에 대하여 연구하였다. 그 결과 탄성계수 E1과 크리프정수 n는 적용응력 수준에 영향을 받는 것으로 나타나 응력수준에 따른 크리프정수 결정식을 제시하였다.

• 콘크리트학회논문집
• /
• 제11권3호
• /
• pp.23-34
• /
• 1999

In designing PSC box girder bridge, the dead load, prestressing force, creep and shrinkage of concrete are the main factors which influence the camber and deflection of segmental concrete structure under construction. Among these factors the creep and shrinkage are the functions of the time-dependent property which, therefore, must considered with time. The prediction model for estimating creep and shrinkage of concrete has been suggested by ACI, CEB/FIP, JSCE and KSCE design code. In this study the creep and shrinkage test were carried out for four curing ages of concrete which was applied to the pretressed concrete box-girder bridge at a construction site, and the results of test were compared to the values of prediction by the design code. Shrinkage test shows that the test results are similar to KSCE-96 and JSCE-96 but very higher than other prediction model and creep test results are generally similar to ACI-209 and DSCE-96 but lower than other prediction models in contrast to shrinkage test.

• 산업기술연구
• /
• 제33권A호
• /
• pp.109-116
• /
• 2013

In this study the compressive strength data were collected from ready­mix concrete plants, and the analysis result showed that when using A­D test the concrete of 24MPa is suitable than that of 21MPa for normal distribution. The prediction formula for average compressive strength were proposed to $f_{cu}=f_{ck}+4(MPa)$. When comparing the proposed equations and existing relationship, the estimation variations of elastic modulus and creep modulus were not significant. The proposed equation confirmed that there was no effect to the influence function for modulus of elasticity and creep. Therefore, it was concluded that the proposed equation could replace the exiting interaction formula.

• International Journal of Concrete Structures and Materials
• /
• 제9권3호
• /
• pp.337-343
• /
• 2015

Previous studies have shown that the drying shrinkage of recycled aggregate concrete (RAC) is greater than that of natural aggregate concrete (NAC). Drying shrinkage is the fundamental reason for the cracking of concrete, and tensile creep caused by the restraint of drying shrinkage plays a significant role in the cracking because it can relieve the tensile stress and results in the delay of cracking occurrence. However, up till now, all research has been focusing on the compressive creep of RAC. Therefore, in this study, a uniaxial restrained shrinkage cracking test was executed to investigate the tensile creep properties caused by the restraint of drying shrinkage of RAC. The mechanical properties, such as compressive strength, tensile splitting strength, and Young's modulus of RAC were also investigated in this study. The results confirmed that the tensile creep of RAC caused by the restraint of shrinkage was about 20-30 % larger than that of NAC.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2000년도 춘계학술대회논문집
• /
• pp.187-191
• /
• 2000

High temperature creep behaviour of Al-5 wt% Mg alloy reinforced with 7.5% BN flakes was studied. The composite specimens showed two main creep characteristics : (1) the value of the apparent stress exponent of the composite was high and varied with applied stress (2) the apparent activation energy for creep was much larger than that for self-diffusion in aluminum The true stress exponent of the composite was set equal to 5. Temperature dependence of the threshold stress of the composite was very strong. Which could not be rationalized by allowing for the temperature dependence of the elastic modulus change. AIN particles which were incorporated into the Al matrix during fabrication of the composite by the PRIMEXTM method were found to be effective barriers to dislocation motion and to give rise the threshold stress during creep of the composite

• Nuclear Engineering and Technology
• /
• 제53권9호
• /
• pp.2953-2959
• /
• 2021

A new practical modeling of the Norton's power law creep is proposed and implemented to analyze the high temperature behaviors of Alloy 690 SG tube material. In the model, both the stress exponent n and the rate constant B are simply treated as the temperature dependent parameters. Based on the two-step optimization procedure, the temperature function of the rate constant B(T) was determined for the data set of each B value after fixing the stress exponent n value by using the prior optimized function at each temperature. This procedure could significantly reduce the numerical errors when using the power law creep equations. Based on the better description of the steady-state creep rates, the experimental rupture times could also be well predicted by using the Monkman-Grant relationship. Furthermore, the difference in tensile strengths at high temperatures could be very well estimated by assuming the imaginary creep stress related to the given strain rate after correcting the temperature effects on the elastic modulus.