The box girder's wing plates(including the roof, the cantilever plate and the bottom floor) are divided into parts of different generalized longitudinal displacements of shear lag. Taking a parabola as the transverse distribution form for each wing plate and introducing lateral position parameters 'η', the shear lag effect is analyzed for cases with varying lateral loading locations. A control differential equation is built for the shear lag effect of the box beam and it is solved for the simply supported beam and the cantilever beam under uniform load by using the energy variation principle. Examples show that the varying lateral loading locations affect the positive and negative shear lag characteristics of the loading wing plate and the magnitude of the stress in the non-load plate. The lateral framing effect makes much greater contribution than the shear deformation for the longitudinal stress. The result is consistent with the analytic result of the block finite element model, which shows that the algorithm can accurately analyze the shear lag effect of the box girder for cases with varying lateral loading locations.
The fatigue accumulative damage theories for metal materials can be divided into the following three categories:(1) linear cumulative damage theories of invariable damage;(2) linear cumulative damage theories of variable damage;(3) periodical linear cumulative damage theories of invariable damage. They are reviewed in this paper with respect to their theoretical bases, material constants, parameters involved, engineering applications and major typical models. Besides, several sets of variable loading tests of metal materials under a two-level spectrum, a multiple-level spectrum and a random spectrum are used to verify the major typical models. Finally, the applicable scopes and applications of the above three kinds of accumulative damage rules are compared and discussed according to their theoretical basis and the accuracy rate.
At the beginning of this century, the lunar exploration programs Constellation, Aurora and Chang'e are proposed by NASA(National Aeronautics and Space Administration), ESA(European Space Agency) and CAST(China Academy of Space Technology), respectively. They open a new page for human being returning to space, and pose new requirements for the manned lunar mission. These requirements include more diversified detection scope, more accurate trajectory design and more strict safety measures for human crew. These make the program more complicated and sophisticated. This paper reviews in detail the developments of manned lunar mission's trajectory, Earth——Moon transfer trajectory, autonomous rendezvous guidance and lunar global coverage design, and presents the relevant hot topics in these fields. Finally, this paper outlines the tendency of future researches on this direction.
An energy analysis is carried out for a snap-through oscillator to study its equilibrium and the bifurcation. The equilibrium equation is established based on the principle of the potential energy, and the stability of the equilibrium configurations can be determined via the extreme values of the potential energy. For different configurations of the spring, the equilibrium configurations and their bifurcation are numerically determined through the ratio of the spring stiffness to the mass changes. The results demonstrate that there are a stable equilibrium configuration and an unstable equilibrium one under the condition of a small control parameter. As the control parameter increases to a critical value, an additional stable equilibrium configuration and an additional unstable equilibrium bifurcation appear.
The electric resistance strain measuring technique is very important in the structure stress analysis. This paper analyzes the temperature strain compensation in the electric resistance strain measuring technique, including the forming principle of the structure temperature strain and the compensation principle of the structure temperature strain. The temperature strain in the testing structure and that in the compensation component have their relations and differences. A misconception of the temperature strain in the testing structure and that in the compensation component is pointed out and it is shown that it might lead to a wrong structure temperature strain compensation. Meanwhile, a correct condition is proposed to guarantee the objectivity and the reliability of the testing data during the engineering structure strain measurement.
This paper studies the influence of different meridian curves of a large hyperbolic cooling tower on the wind-induced response and the stability performance, by modeling in finite elements according to engineering situations, with considerations of the effect of the tower height, the wall thickness and the air intake height on the dynamic behavior of the structure and the wind-induced response. The influence mechanism for different curve types is analyzed and the wind-induced responses of the tower barrel and the pillar are compared under standard wind loads. Besides, the influence of the linear change on the cooling tower overall and local stability is also explored. It is shown that the optimization of the meridian curve is necessary. A little "shorter and fatter" tower type enjoys a higher structure fundamental frequency, but its wind load-carrying properties and stability are not as satisfactory as a "tall and slim" towel type. Furthermore, its displacement response is larger.
By combining instantaneous optimal control and iterative learning control(ILC), one new hybrid control strategy called instantaneous optimal iterative learning control is proposed. Linear system is chosen as the model for the new control strategy, and the quadratic performance function of the system is chosen as the objective function to be minimized. During the process of controlling responses of the system, the core idea of the iterative learning control is introduced in order to modify the control signals. By introducing the norms of matrices, the sufficient condition of convergence for the new control strategy is established in the paper. The model of a 20-floor building in the second generation benchmark vibration control is selected for numerical simulation. In the numerical simulation, the north-south component of the El wave is introduced as the excitation. Comparing to the instantaneous optimal control, results of the simulation show that instantaneous optimal iterative learning control improves the effectiveness.
The strain gradient field in the neighborhood of a Mode-I crack in an infinite plate under uniaxial tension is formulated based on the solution put forward by Irwin in the linear elastic fracture mechanics, and the relation between the strain gradient and the crack extension is established. Flexoelectric relationship between the electric polarization and the strain gradient is also presented, and a method of Mode-I crack monitoring to obtain the coordinates of the crack tip and the length of the crack extension using strain gradient sensors is proposed. This study provides some necessary theoretical preparations for implementing the strain gradient sensors, by which the crack extension in engineering structures can be monitored in a real-time manner. Flexoelectricity sensing technology has broad prospects in the field of structural health monitoring.
The single layer reticulated shell is a kind of imperfection sensitive structure. The initial geometric imperfections have a significant impact on the section optimization results of the shell. This paper studies the relationship among the rod section optimization value, the initial geometric imperfection distribution, the ultimate bearing capacity and the member buckling failure law of the structure. It is indicated that the most unfavorable stress appears early and keeps growing until the buckling failure in the place where the initial geometric imperfection is larger on the structure after the section optimization design. The section optimization results are directly related to the initial geometric imperfection distributions, which means that increasing the cross-section of the rod where the initial geometric imperfection is larger would effectively enhance the ultimate stability bearing capacity. Due to the random distribution of the initial geometric imperfection, the stable bearing capacity of the reticulated shell after the section optimization design must be checked under different defect distributions to ensure safety.
The buckling behavior of the rod string in a wellbore is one of the key issues in petroleum engineering. The slender rod string in a wellbore is studied in this paper. According to the sinusoidal and helical buckling geometries of the pressurized column, the post-buckling contact friction state between the rod string and the wellbore is considered. The Lagrange multiplier method is used to describe the sliding displacement boundary conditions for the wellbore and the gravitational potential energy and the friction dissipated energy are used in the energy method. The contact force and the friction resistance between the rod string and the wellbore are derived for vertical wells. The post buckling experimental apparatus is developed in this paper. The sinusoidal and helical buckling critical loads and the friction resistances are derived under different loads. The experimental results agree with the theoretical results. This paper provides an effective method for buckling columns, such as drilling, coiled tubing.
The vortex-induced vibration(VIV) will accelerate the fatigue failure of the cables and reduce the life and threaten the safety of the SFT(submerged floating tunnel). In this paper, the effect of the controlling VIV of cables with the fairing or the three control rods is analyzed by using the numerical method based on the fluid-structure interaction. The influence of different cable tilt angles and flow directions on the vibration of the cables is explored with regard to the vortex-induced vibration suppression of the marine riser. The results show that the two proposed methods have a good vibration control effect along the flow direction. The fairing has a more prominent effect and the vibration suppression effect of the three control rods will be greatly improved if the cables are tilted. The analysis also demonstrates that the three control rods may partly increase the VIV of the cables when the flow direction changes. In summary, the fairing has a better adaptability.