• 열처리공학회지
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• 제31권4호
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• pp.180-186
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• 2018

High temperature flow behaviors of Fe-Cr-Mo-V-W-C tool steel were investigated using isothermal compression tests on a Gleeble simulator. The compressive test temperature was varied from 850 to $1,150^{\circ}C$ with the strain rate ranges of 0.05 and $10s^{-1}$. The maximum height reduction was 45%. The dynamic softening related to the dynamic recrystallization was observed during hot deformation. The constitutive model based on Arrhenius-typed equation with the Zener-Hollomon parameter was proposed to simulate the hot deformation behavior of Fe-Cr-Mo-V-W-C steel. An artificial neural network (ANN) model was also developed to compare with the constitutive model. It was concluded that the ANN model showed more accurate prediction compared with the constitutive model for describing the hot compressive behavior of Fe-Cr-Mo-V-W-C steel.

• Geomechanics and Engineering
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• 제7권2호
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• pp.213-231
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• 2014

An elastoplastic model for structured clays, which is formulated based on the fact that the difference in mechanical behavior of structured and reconstituted clays is caused by the change of fabric in the post-yield deformation range, is present in this paper. This model is developed from an elastoplastic model for overconsolidated reconstituted clays, by considering that the variation in the yield surface of structured clays is similar to that of overconsolidated reconstituted clays. However, in order to describe the mechanical behavior of structured clays with precision, the model takes the bonding and parabolic strength envelope into consideration. Compared with the Cam-clay model, only two new parameters are required in the model for structured clays, which can be determined from isotropic compression and triaxial shear tests at different confining pressures. The comparison of model predictions and results of drained and undrained triaxial shear tests on four different marine clays shows that the model can capture reasonable well the strength and deformation characteristics of structured clays, including negative and positive dilatancy, strain-hardening and softening during shearing.

• Structural Engineering and Mechanics
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• 제72권3호
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• pp.313-327
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• 2019

Damage evolution in the form of void nucleation, propagation and coalescence is the primary cause that is responsible for the ductile failure of microalloyed steels. The Gurson-Tvergaard-Needleman (GTN) damage model has proven to be extremely robust for characterizing the microscopic damage behavior of ductile metals. Nonetheless, successful applications of the model on a given metal type are limited by the correct identification of damage parameters as well as the validation of the calculated void growth rate. The purpose of this study is two-fold. First, we aim to identify the damage parameters of the GTN model for Q345 steel (Chinese code), due to its extensive application in mechanical and civil industries in China. The identification of damage parameters is facilitated by the well-suited response surface methodology, followed by a complete analysis of variance for evaluating the statistical significance of the identified model. Second, taking notched Q345 cylinders as an example, finite element simulations implemented with the identified GTN model are performed in order to analyze their microscopic damage behavior. In particular, the void growth rate predicted from the simulations is successfully correlated with experimentally measured acoustic emissions. The quantitative correlation suggests that during the yielding stage the void growth rate increases linearly with the acoustic emissions, while in the strain-hardening and softening period the dependence becomes an exponential function. The combined experimental and finite element approach provides a means for validating simulated void growth rate against experimental measurements of acoustic emissions in microalloyed steels.

• 대한금속재료학회지
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• 제47권11호
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• pp.687-693
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• 2009

Experiments have demonstrated that a moderate amount of Be added to $Zr_{57.5}Cu_{38.3}Al_{4.2}$ amorphous alloy enhances the plasticity of the alloy. In particular, $Zr_{54}Cu_{36}Al_{4}Be_{6}$ alloy exhibited 19% of strain to fracture along with a strength exceeding 2 GPa. Energy dispersive x-ray spectroscopy conducted on the $Zr_{54}Cu_{36}Al_{4}Be_{6}$ alloy exhibited the presence of compositional modulation, indicating that nm-scale phase separation had occurred at local regions. In this study, the role played by the nm-scale phase separation on the plasticity was investigated in terms of structural disordering, structural softening and shear localization in order to better understand the structural origin of the enhanced plasticity shown by the developed alloy.

• Geomechanics and Engineering
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• 제29권6호
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• pp.627-643
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• 2022

Water-rock interactions have a significant influence on the mechanical behavior of rocks. In this study, uniaxial compression and tension tests on different water-treated sandstone samples were conducted. Acoustic emission (AE) monitoring and micro-pore structure detection were carried out. Water-rock interactions and their effects on rock mechanical behavior were discussed. The results indicate that water content significantly weakens rock mechanical strength. The sensitivity of the mechanical parameters to water treatment, from high to low, are Poisson ratio (𝜇), uniaxial tensile strength (UTS), uniaxial compressive strength (UCS), elastic modulus (E), and peak strain (𝜀). After water treatment, AE activities and the shear crack percentage are reduced, the angles between macro fractures and loading direction are minimized, the dynamic phenomenon during loading is weakened, and the failure mode changes from a mixed tensile-shear type to a tensile one. Due to the softening, lubrication, and water wedge effects in water-rock interactions, water content increases pore size, promotes crack development, and weakens micro-pore structures. Further damage of rocks in fractured and caved zones due to the water-rock interactions leads to an extra load on the adjoining coal and rock masses, which will increase the risk of dynamic disasters.

• Steel and Composite Structures
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• 제48권3호
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• pp.335-351
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• 2023

This research studies the primary resonance and nonlinear vibratory responses of multilayer functionally graded shallow (MFGS) shells under external excitations. The shells considered with functionally graded porous (FGP) core and resting on two types of nonlinear viscoelastic foundations (NVEF) governed by either a linear model with two parameters of Winkler and Pasternak foundations or a nonlinear model of hardening/softening cubic stiffness augmented by a Kelvin-Voigt viscoelastic model. The shells considered have three layers, sandwiched by functionally graded (FG), FGP, and FG materials. To investigate the influence of various porosity distributions, two types of FGP middle layer cores are considered. With the first-order shear deformation theory (FSDT), Hooke's law, and von-Kármán equation, the stress-strain relations for the MFGS shells with FGP core are developed. The governing equations of the shells are consequently derived. For the sake of higher accuracy and reliability, the P-T method is implemented in numerically analyzing the vibration, and the method of multiple scales (MMS) as one of the perturbation methods is used to investigate the primary resonance. The results of the present research are verified with the results available in the literature. The analytical results are compared with the P-T method. The influences of material, geometry, and nonlinear viscoelastic foundation parameters on the responses of the shells are illustrated.

• 한국지반공학회논문집
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• 제24권2호
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• pp.5-14
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• 2008

국내 석산에서 생산되는 골재를 사용하여 대형삼축시험과 대형직접전단시험을 동시에 수행한 후 전단특성을 비교하였다. 비교를 위하여 시험조건을 가능한 일치시켰으며, 상대밀도를 50%, 70%, 90%의 세 가지로 변화시켜 시험을 수행하였다. 시험결과 응력-변형률 거동은 두 가지 시험에서 동일하게 나타나며, 전단강도는 상대밀도의 크기에 따라 시험방법별로 추세가 달라진다. 즉, 낮은 상대밀도에서는 대형직접전단시험의 내부마찰각이 대형삼축압축시험 결과 값에 비하여 작게 나타나고, 높은 상대밀도에서는 이 현상이 역전됨을 확인하였다.

• Steel and Composite Structures
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• 제47권3호
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• pp.423-436
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• 2023

In this research, we tested 10 simply supported concrete-encased composite columns under monotonic eccentric loads and investigated their shear behaviour. The specimens tested were two reinforced concrete specimens, three steel-reinforced concrete (SRC) specimens with an H-shaped steel section (also called a beam section), and five SRC specimens with a cruciform-shaped steel section (also called a column section). The experimental variables included the transverse steel shape's depth and the longitudinal steel flange's width. Experimental observations indicated the following. (1) The ultimate load-carrying capacity was controlled by web compression failure, defined as a situation where the concrete within the diagonal strut's upper end was crushed. (2) The composite effect was strong before the crushing of the concrete outside the steel shape. (3) We adjusted the softened strut-and-tie SRC (SST-SRC) model to yield more accurate strength predictions than those obtained using the strength superposition method. (4) The MSST-SRC model can more reasonably predict shear strength at an initial concrete softening load point. The rationality of the MSST-SRC model was inferred by experimentally observing shear behaviour, including concrete crushing and the point of sharp variation in the shear strain.

• 로봇학회논문지
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• 제19권2호
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• pp.221-228
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• 2024

As the importance of cardiovascular health is highlighted, research on its correlation with blood pressure, the most important indicator, is being actively conducted. Therefore, extensive clinical data is essential, but the measurement of the central arterial blood pressure waveform must be performed invasively within the artery, so the quantity and quality are limited. This study suggested a mock circulatory robot and artificial aorta to reproduce the blood pressure waveform generated by the overlap of forward and reflected waves. The artificial aorta was fabricated with biomimetic silicone to mimic the physiological structure and vascular stiffness of the human. A pressurizing chamber was implemented to prevent distortion of the blood pressure waveform due to the strain-softening of biomimetic silicone. The reproduced central arterial blood pressure waveforms have similar magnitude, shape, and propagation characteristics to humans. In addition, changes in blood pressure waveform due to aging were also reproduced by replacing an artificial aorta with various stiffness. It can be expanded to construct a biosignal database and health sensor testing platform, a core technology for cardiovascular health-related research.

• 한국구조물진단유지관리공학회 논문집
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• 제12권2호
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• pp.193-200
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• 2008

탄-소성론에 근거한 콘크리트나 토질과 같은 재료의 파괴 포락선은 주응력을 축으로 하는 공간 좌표계상에서 인장의 등압(hydrostatic stress)축을 향해 기울어진 형태를 가지며 소성흐름이 상관소성흐름 법칙(associated flow rule)에 따라 결정될 경우 콘크리트의 거동 예측시에 과도한 체적 팽창률을 나타내게 된다. 본 논문에서는 콘크리트의 다축응력 하에서의 거동을 예측하기 위하여 비균일 경화(nonuniform hardening)를 적용한 5계수 파괴 포락선과 등압축 방향 성분의 소성 흐름을 수정하는 비상관 소성흐름 법칙(non-associated flow rule)을 사용하여 비선형 유한요소해석 프로그램을 개발하였으며 신뢰성 있는 연구자의 다축응력 실험결과와 유한요소해석 프로그램의 해석결과를 비교하였다.