Bridges and tunnels are good solutions to transportation problems in large cities separated by large rivers. In bridge construction great success has been achieved in China, but large-sized immersed tube tunnel constr...Bridges and tunnels are good solutions to transportation problems in large cities separated by large rivers. In bridge construction great success has been achieved in China, but large-sized immersed tube tunnel construction is still new. Element immersing is an important process of immersed tube tunnel construction. The accuracy of tunnel element positioning directly determines the quality of tunnel construction. In order to study the behavior of elements during its lowering to the sea bed, the experiments carried out in the State Key Laboratory of Ocean Engineering of Shanghai Jiaotong University. In consideration of the construction experience abroad and by reference to published papers on the Oresund tunnel in Norway-Sweden and Tokyo Bay tunnel in Japan, an element model to an appropriate scale is developed. A concise description of the model experiment wave environments is carried out, and the feasibility of two immersing strategies is studied.展开更多
The flow rule of Prandtl-Reuss was adopted and incremental elasto-plastic finite-element analysis formulation of Coulomb’s friction law combining the finite deformation theory was established, and Lagrangian formulat...The flow rule of Prandtl-Reuss was adopted and incremental elasto-plastic finite-element analysis formulation of Coulomb’s friction law combining the finite deformation theory was established, and Lagrangian formulation for simulating the squaring process of circular tube was updated. Incremental Coulomb’s friction law was used in the global stiffness matrix to solve the sliding-sticking state of friction at the boundary contact interface. During the squaring process, the linear factor rmin was adopted to solve the non-linear boundary problems of changing node contact and separation, elasto-plastic transient situation in an element and the non-linear constitutive behavior of material so as to make each reasonable increment of the punch meet the demand of calculation for linear increment. The squaring process of circular tube, load distribution and final shape of work piece after unloading were simulated by this mode and compared with research data. It is known that the circular tube with higher geometrical ratio (R/t) could be pressed into symmetric square tube without collapse. This result can provide reference for the analysis of this process and evaluation and improvement of product defects.展开更多
Objective Our objective is to build a model that explains the association between the exposure to trace elements in the soil and the risk of neural tube defects. Methods We built a function with different parameters t...Objective Our objective is to build a model that explains the association between the exposure to trace elements in the soil and the risk of neural tube defects. Methods We built a function with different parameters to describe the effects of trace elements on neural tube defects. The association between neural tube defects and trace element levels was transformed into an optimization problem using the maximum likelihood method. Results Tin, lead, nickel, iron, copper, and aluminum had typical layered effects （dosage effects） on the prevalence of neural tube defects. Arsenic, selenium, zinc, strontium, and vanadium had no effect, and molybdenum had one threshold value that affected the prevalence of birth defects. Conclusion As an exploratory research work, our model can be used to determine the direction of the effect of the trace element content of cultivated soil on the risk of neural tube defects, which shows the clues by the dosage effect of their toxicological characteristics. Based on our findings, future biogeochemical research should focus on the direct effects of trace elements on human health.展开更多
This research deals with the forward extrusion process of tubes. In this process, a piercing process was carried out on the billet to produce the tube, followed directly by a reduction in the wall thickness. A specifi...This research deals with the forward extrusion process of tubes. In this process, a piercing process was carried out on the billet to produce the tube, followed directly by a reduction in the wall thickness. A specific geometrical shape for the piercing zone and the wall thickness reduction zone were chosen and designed. The effects of the redundant shear strain and the magnitude of the extrusion load were investigated and simulated with the finite element method using Q Form software program. Lead was used as model materials since (if the experiments were carried out at room temperature) it has the similar behavior of the steel at high temperature. The results obtained have shown that at the piercing zone, the lowest values of the extrusion load, the redundant strain, the total strain and the finite element effective strain were when a piercing tool (mandrel) of (C = 1.1) was used. While, at the die zone, the lowest values of the extrusion load, the redundant strain, the total strain was when a die of (C = 0.9) was used.展开更多
In this paper, the boundary element method is applied to investigate the internal state of stress of autofretted tube with notch and the calculated results are important in the practical design.
文摘Bridges and tunnels are good solutions to transportation problems in large cities separated by large rivers. In bridge construction great success has been achieved in China, but large-sized immersed tube tunnel construction is still new. Element immersing is an important process of immersed tube tunnel construction. The accuracy of tunnel element positioning directly determines the quality of tunnel construction. In order to study the behavior of elements during its lowering to the sea bed, the experiments carried out in the State Key Laboratory of Ocean Engineering of Shanghai Jiaotong University. In consideration of the construction experience abroad and by reference to published papers on the Oresund tunnel in Norway-Sweden and Tokyo Bay tunnel in Japan, an element model to an appropriate scale is developed. A concise description of the model experiment wave environments is carried out, and the feasibility of two immersing strategies is studied.
基金Project(95-2221-E-129-001) supported by the National Science Council, Taiwan, China
文摘The flow rule of Prandtl-Reuss was adopted and incremental elasto-plastic finite-element analysis formulation of Coulomb’s friction law combining the finite deformation theory was established, and Lagrangian formulation for simulating the squaring process of circular tube was updated. Incremental Coulomb’s friction law was used in the global stiffness matrix to solve the sliding-sticking state of friction at the boundary contact interface. During the squaring process, the linear factor rmin was adopted to solve the non-linear boundary problems of changing node contact and separation, elasto-plastic transient situation in an element and the non-linear constitutive behavior of material so as to make each reasonable increment of the punch meet the demand of calculation for linear increment. The squaring process of circular tube, load distribution and final shape of work piece after unloading were simulated by this mode and compared with research data. It is known that the circular tube with higher geometrical ratio (R/t) could be pressed into symmetric square tube without collapse. This result can provide reference for the analysis of this process and evaluation and improvement of product defects.
基金supported by the National "973" project on Population and Health (No. 2007CB5119001)the National Yang‐Zi Scholar Program, 211 and 985 projects of Peking University (No. 20020903)
文摘Objective Our objective is to build a model that explains the association between the exposure to trace elements in the soil and the risk of neural tube defects. Methods We built a function with different parameters to describe the effects of trace elements on neural tube defects. The association between neural tube defects and trace element levels was transformed into an optimization problem using the maximum likelihood method. Results Tin, lead, nickel, iron, copper, and aluminum had typical layered effects （dosage effects） on the prevalence of neural tube defects. Arsenic, selenium, zinc, strontium, and vanadium had no effect, and molybdenum had one threshold value that affected the prevalence of birth defects. Conclusion As an exploratory research work, our model can be used to determine the direction of the effect of the trace element content of cultivated soil on the risk of neural tube defects, which shows the clues by the dosage effect of their toxicological characteristics. Based on our findings, future biogeochemical research should focus on the direct effects of trace elements on human health.
文摘This research deals with the forward extrusion process of tubes. In this process, a piercing process was carried out on the billet to produce the tube, followed directly by a reduction in the wall thickness. A specific geometrical shape for the piercing zone and the wall thickness reduction zone were chosen and designed. The effects of the redundant shear strain and the magnitude of the extrusion load were investigated and simulated with the finite element method using Q Form software program. Lead was used as model materials since (if the experiments were carried out at room temperature) it has the similar behavior of the steel at high temperature. The results obtained have shown that at the piercing zone, the lowest values of the extrusion load, the redundant strain, the total strain and the finite element effective strain were when a piercing tool (mandrel) of (C = 1.1) was used. While, at the die zone, the lowest values of the extrusion load, the redundant strain, the total strain was when a die of (C = 0.9) was used.
文摘In this paper, the boundary element method is applied to investigate the internal state of stress of autofretted tube with notch and the calculated results are important in the practical design.
