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Gang Chen,Yu Jin,Jing Wang,Cheng Zhang,Qiang Chen,Hongming Zhang,Xingjian Zhao,Zhiyong Li,Changhai Xie,Zhiming Du 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.9
Isothermal compression tests were carried out to study the hot deformation behavior of T2 copper under various conditions. The deformation parameters, such as temperature and strain rate, have strong influence on flow stress and microstructureevolution of the alloy. A unified dislocation density-based model considering dynamic recovery and recrystallization wasestablished. And material parameters of the developed model were optimized by genetic algorithm. Comparisons betweenthe experimental and model data demonstrates that the developed model can precisely describe the flow behavior at quitewide range of deformation conditions. Meanwhile, the designed iterative procedure allows the model to be applied in timevariantdeformation conditions. Processing map and microstructure examination were constructed to optimize the processingwindow of the studied alloy. According to the processing maps, flow instability mainly appeared at low temperatures of500–650 °C and strain rates higher than 0.1 s−1. The optimum deformation parameters of T2 copper was concluded as thetemperature range of 700–800 °C and the strain rate of 0.1–1 s−1.
Estimation of load and resistance factors based on the fourth moment method
Zhao-Hui Lu,Yan-gang Zhao,Alfredo H-S. Ang 국제구조공학회 2010 Structural Engineering and Mechanics, An Int'l Jou Vol.36 No.1
The load and resistance factors are generally obtained using the First Order Reliability Method (FORM), in which the design point should be determined and derivative-based iterations have to be used. In this paper, a simple method for estimating the load and resistance factors using the first four moments of the basic random variables is proposed and a simple formula for the target mean resistance is also proposed to avoid iteration computation. Unlike the currently used method, the load and resistance factors can be determined using the proposed method even when the probability density functions (PDFs) of the basic random variables are not available. Moreover, the proposed method does not need either the iterative computation of derivatives or any design points. Thus, the present method provides a more convenient and effective way to estimate the load and resistance factors in practical engineering. Numerical examples are presented to demonstrate the advantages of the proposed fourth moment method for determining the load and resistance factors.
Reliability-based assessment of American and European specifications for square CFT stub columns
Zhao-Hui Lu,Yan-gang Zhao,Zhi-wu Yu,Cheng Chen 국제구조공학회 2015 Steel and Composite Structures, An International J Vol.19 No.4
This paper presents a probabilistic investigation of American and European specifications (i.e., AISC and Eurocode 4) for square concrete-filled steel tubular (CFT) stub columns. The study is based on experimental results of 100 axially loaded square CFT stub columns from the literature. By comparing experimental results for ultimate loads with code-predicted column resistances, the uncertainty of resistance models is analyzed and it is found that the modeling uncertainty parameter can be described using random variables of lognormal distribution. Reliability analyses were then performed with/without considering the modeling uncertainty parameter and the safety level of the specifications is evaluated in terms of sufficient and uniform reliability criteria. Results show that: (1) The AISC design code provided slightly conservative results of square CFT stub columns with reliability indices larger than 3.25 and the uniformness of reliability indices is no better because of the quality of the resistance model; (2) The uniformness of reliability indices for the Eurocode 4 was better than that of AISC, but the reliability indices of columns designed following the Eurocode 4 were found to be quite below the target reliability level of Eurocode 4.
Quantum Cutting in SrAl12O19:Pr3+ Nanodisks under Vacuum-UV Excitation
Zhao-Gang Nie,Takayoshi Kobayashi,김기수 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.6
High-quality SrAl12O19:Pr3+ nanodisks, round in shape, with a mean diameter 60 nm and thicknesses between 5 and 10 nm were synthesized by a polymer complexation route. The spectroscopic properties of the SrAl12O19:Pr3+ nanodisks were investigated and compared with these of their bulk counterparts upon vacuum-UV excitations into the 4f5d states of Pr3+ ions and the conduction band of the host lattice. The characteristic quantum cutting process due to the 1S0- 1I6, followed by the 3P0 - 3H4, transitions is observed in the nano-crystalline systems, and the quantum efficiency is comparable to that of their bulk counterparts under 4f5d state excitations. Under excitations of the host lattice, the self-trapped-exciton- mediated energy transfer can result in the preferential population of the low lying 4f2 levels of Pr3+ ions, reducing the possibility of the quantum cutting process.
Zhao Gang,Zhang Ran,Wang Wei,Du Xiaoxiao 한국CDE학회 2022 Journal of computational design and engineering Vol.9 No.1
The simulation of large deformation contact problems has been a tough subject due to the existence of multiple nonlinearities, including geometric nonlinearity and contact interface nonlinearity. In this paper, we develop a novel method to compute the large deformation of 2D frictionless contact by employing Nitsche-based isogeometric analysis. The enforcement of contact constraints as one of the main issues in contact simulation is implemented by using Nitsche’s method, and the node-to-segment scheme is applied to the contact interface discretization. We detailedly derive the discrete formulations for 2D large deformation frictionless contact where NURBS is used for geometrical modeling and the Neo-Hookean hyperelastic materials are applied to describe the deformation of the model. The collocation method with Greville points is employed to integrate the contact interface and the classical Legendre–Gauss quadrature rule is used for contact bodies’ integration. The Lagrange multiplier method and penalty method are also implemented for comparison with Nitsche’s method. Several examples are investigated, and the obtained results are compared with that from commercial software ABAQUS to verify the effectiveness and accuracy of the Nitsche-based isogeometric analysis.