In the tensor analysis, symmetry breakages exist in concepts——there is differential of tensor, but there is no variational of tensor; in theories——there is differential theory of tensor, but there is no variational theory of tensor. However, the symmetry breakages can be compensated, as shown in this paper. The developing history of the concept of the virtual particle time derivative will be reviewed, as well as the process of abstracting the concept of the local variational of tensor and the procedure of constructing the variational theory of tensor. The symmetry between the variational theory and the differential theory for tensor will thus be revealed.
The fracture toughness test of low constraint specimens is of great significance for the safe operation of oil and gas pipelines. In this paper, the existing fracture toughness testing methods and the development process of low constraint specimens are reviewed, and the common fracture toughness characterization parameters such as crack tip opening displacement (CTOD) and J-integral are introduced. The key problems such as the stress intensity factor, the J-integral plasticity factor, the J-integral and CTOD conversion factor, the crack size measurement method, and the digital image correlation method, in the fracture toughness test, are analyzed, and the problems that need to be further studied are summarized, to provide a necessary basis for the development of fracture toughness test of low constraint specimens.
The aircraft dynamics and control has achieved a significant progress, but also faces a series of problems that need to be dealt with. Deep learning provides a new solution for these problems, and it is good in many aspects, such as the working model of the experience storage, the intelligent accumulation and the off-line training. In this study, around the subject of the autonomy and the intelligence enhancement for the flight control, the applications of the deep learning in aircraft dynamics and control are reviewed in three aspects: (1) applications of deep learning in dynamic modeling to improve the computational efficiency and accuracy of modeling, or to solve the problem of inverse dynamics; (2) applications of deep learning in optimal control to improve the speed of trajectory planning or the real-time performance and autonomy of flight control; (3) applications of deep learning in mission design to improve optimization speed and decision-making intelligence. Furthermore, the advantages and the disadvantages are analyzed and representative papers are introduced. Finally, four suggestions to apply the deep learning in aircraft dynamics and control are given.
After the evolution of hundreds of millions of years, most turtles have developed excellent shells with high specific stiffness and toughness. Obviously, the multi-scale structural characteristics and the biological material mechanical behavior of these turtle shells can be used for promoting the bio-inspired design of safety protection structures. This paper reviews the research progress of the turtle shell relevant to its protective functions, including the macro and micro features of the carapace and the corresponding mechanical properties. In addition, some carapace-inspired designs that could be adopted in protection structures are summarized. Finally, the existing problems and the focus of the future research are discussed, to provide some food for thought in the bio-inspired design of safety protection systems in future.
This paper discusses a basic problem: Is the coordinate transformation coefficient the component of a tensor? The conventional viewpoint is that the coordinate transformation coefficient is not the component of a tensor. To reveal the essence of the coordinate transformation coefficient, the basic concept of the tensor is revisited. A new concept, i.e. the hybrid tensor, is defined. On the basis of the new concept, the conventional viewpoint is negated: the coordinate transformation coefficient is indeed the component of a tensor. That is to say, it is the hybrid component of a metric tensor. Thus, the concept of tensors is extented.
The asteroid is of great significance for studying the formation of planets, the origin of life on the Earth, the defense against the collision of asteroids to the Earth and the mining on asteroids. Due to the insufficient capacity of the existing propulsion technology, the velocity increment needed to optimize the asteroid capture is the key for a successful capture mission. This paper reviews the capture orbit optimization method for asteroid and the extended capture period with the impulse thrust and lowthrust propulsions, including the strategy to capture asteroids by the gravity assisted and resonant orbital techniques, the continuous low thrust and temporary asteroid capture, and the scheme of extending the capture period.
With its great national strategic importance and socio-economic benefits, the high-speed vehicle always plays a specific role in the field of aerospace. In this paper, the structural optimization technique and its application in the aircraft design are reviewed firstly. Then, the specific applications of structural optimization techniques in the high-speed vehicle design are then discussed in the context of the typical service environment and the design requirement. We show typical cases subsequently with applications in the concept design of the high-speed vehicle and in the innovative and improved design of their parts over the last years. In view of the solid theoretical foundations as well as the numerous successful engineering practices of the structural optimization, it not only provides an effective tool, but also brings about revolutionary changes for the design of high-speed vehicle structures. It is anticipated that the structural optimization supplemented by the empirical design will surely be a standard procedure for the aircraft design, and the practice-oriented research will undoubtedly enhance the core competitiveness of the aerospace industry in China
This is the question asked in class: is it possible to obtain the covariant derivative of natural base vectors? To answer this question, the idea of axiomazitaion is introduced, the concepts of the generalized component and the generalized covariant derivative are defined. Based on these new concepts, the classical covariance is developed into the generalized covariance, and the classical covariant differentiation is developed into the generalized covariant differentiation. This paper summarizes the main difficulties and the important points of the above explorations, and shows the abstractions of the generalized covariant derivatives and the advances of the generalized covaraibilities.
The influence of the locked-in stress on the rock mechanical properties cannot be ignored. Since Chen Zongji proposed the definition of the locked-in stress in rocks, there were very few related studies. Starting from the definition, combined with the current development of the rock mechanics, this paper expands the concept of the locked-in stress，in a new definition of the locked-in stress in rocks. The forms of the stress in accordance with the new definition of the locked-in stress in the rock mechanics research are classified. The variation law of various kinds of locked-in stress and its empirical formula are summarized. The correlation between the locked-in stress and the excavation of the underground engineering and its influence are discussed. This paper provides a new way to explain the special phenomenon of deep rock mechanics.
The flexible stress sensitive conductive polymer composites (CPCs) have a wide application prospect in the fields of the wearable human machine interaction, the medical monitoring, the portable sports equipment and the bionic robot, and others. The conductive percolation theory, the response mechanisms and the sensitivity of the stress sensitive CPCs are discussed in this paper. The response characteristics of different types of stress sensitive CPCs are reviewed, including the numerical predications. Finally, the future development is commented.
The compression boundary layer transition and separation caused by the interference of the shock and the boundary layer directly affect the drag, surface thermal protection and the flight performance of flying vehicles. This paper first reviews the past work on the interference between the shock wave and the boundary layer. And then, the effects of the forward shock, the oblique shock and the head shock on the laminar flow and the turbulent boundary layer of supersonic and transonic flows are studied and compared. The shock waves of different intensities have different effects on the boundary layer.A strong interference is more likely to cause the separation and the airfoil stall.
Some fundamental concepts and problems in the research field of the structural topology optimization are introduced briefly. The essential characteristics of the truss-like and its discretization are analyzed. The classical analytical solutions as the benchmarks are listed in the references. The optimization strategies and the characteristics of various numerical methods for the structural topology optimization are reviewed. The numerical instabilities, such as the mesh-dependencies, the singular optima, the checkerboards and the local constraints, commonly in the structural topology optimization, are explained. The fundamental principles of the optimality criteria, the sequential programming and the heuristic methods are discussed.
The techniques for the drag reduction of large transport aircraft are reviewed in this paper. The techniques intended to reduce the friction drag, the induced drag and the wave drag are reviewed specifically. Techniques like the suction control for delay boundary layer transition, the riblets for reducing turbulent friction drag, and the winglet are relatively mature for practical application. Distributed roughness, plasma actuators and the contour bump are most like to be used in the newly designed large transport aircraft. These newly developed techniques might be useful in further improving the drag characteristic of the next generation aircraft.
Since the hypersonic vehicle is acted by a serious aerodynamic thermal load caused by the flight through the atmosphere, it is necessary to consider the effect of the high temperature in the ground test of the vehicle structures. But the structural modal experiment in a high temperature environment is far more complex than the normal modal experiment with many technical difficulties to be solved. In this paper, the advance of the thermal modal test technology is reviewed. The characteristics and the applicability of the excitation and measurement techniques to be used in the high temperature environment are discussed in detail. The development direction and the research focus of the structural modal test technology in the high temperature environment are suggested.
The Trefftz finite element method (TFEM) is an efficient numerical approach with many joint advantages of the conventinal finite and boundary element methods. Based on the mutual independent interpolation modes, the finite element formulation involving the boundary integrations only is derived by incorporating the hybrid functional and the Gaussian divergence theorem. The research advances in the internal interpolation function, the treatment of the source term, the special-purpose element and the nonisotropic material during the past decade (2007-2016) are reviewed and several directions are pointed out for the future development.
The astrodynamics software is a bridge between the astrodynamics theory and the engineering application, which can evidently improve the e-ciency and the ability of the space mission design and analysis. There are many mature software systems in the world, but many of them are not available in our country. In recent years, great progress has been made in the fleld of the national astrodynamics research and the engineering applications, but the mature astrodynamics software remains almost a blank area. The development of the international astrodynamics software is surveyed and sorted into two kinds:the space mission analysis and the spacecraft trajectory optimization. The representative open-source and commercial software systems are introduced, including JAT, STK, Freeflyer, POST and ASTOS. Additionally, the development of the Chinese astrodynamics software is also reviewed. At last, the technical characteristics of difierent astrodynamics softwares are summarized, and preliminary suggestions for the development of a mature astrodynamics software in China are given.
In the development of mechanics, the geometrization of theoretical systems of mechanics, is introduced. Dynaminc systems of mechanics reduced to Riemann geometry and Symplectic geometry are presented, the concepts of duel space, symmetry, transform, invariant, and there general applications in mechanics are introducd.
The high-resolution capability is an important development direction for satellite in our country. In this respect, a key technique is the satellite micro-vibration control. This paper reviews the developments of the parallel platform for solving the problem of micro-vibration control in China and abroad, with a comparison of the platform manufacture and the performance conditions. It is shown that the level of the micro-vibration control technology based on the parallel platform in China is still some way behind those of other advanced countries. This paper reviews the typical configuration design, the dynamics modeling, the control system design, the platform optimization and the ground test of the parallel platform. The further research directions for the micro-vibration control technology in our country are suggested.
The thermal vibration is the inherent vibration of nanostructures, which plays an important role in the dynamics of nanostructures. The quantum effects, the boundary effects and the van der Waals interaction have a significant influence on the thermal vibration of nanostructures. The carbon nanotubes and the graphene were extensively studied for their novel electronic properties and superior mechanical strength. This paper reviews some research methods for the thermal vibration of nanostructures, the thermal vibration of the carbon nanotube and the graphene in low temperatures and the nonlinear thermal vibration of the carbon nanotube.
The transfer orbit for the human lunar mission means that between the trans-lunar orbit and the trans-earth orbit, and it has the characteristics of long flight time, complex dynamics model, and strong nonlinear and variable coe-cients. The controllability of the inevitable deviation afiects the result of the task execution, even the success or the failure of the engineering task directly. This paper reviews the studies of the transfer orbit of human lunar mission, the analysis method of the deviation propagation, the mid-course correction optimal strategy and the mid-course correction aiming algorithm. At last, some suggestions are made or the mid-course correction of the Chinese human lunar mission in the future.
Millions of asteroids are in various shapes, which show unique dynamical phenomena, including their own revolutions and the vicinal surrounding objects. This paper reviews the recent development of studies of asteroids. Significant effects on the long-term evolution of asteroids' attitude and orbit are introduced, i.e., the Yarkovsky effect and the YORP effect. Modeling methods of their irregular gravitational fields are presented as well as naturally periodic orbits in complex universal curves. Some basic problems and potential research directions are suggested.
Tubular structures such as circular tubes and square tubes under axial compression, are widely used as structural components in engineering applications. Considering the difference of tube geometries, boundary conditions and material properties, we classify the failure of tubes under axial compression into 5 different mechanisms: progressive buckling, global buckling, inversion, expansion and splitting. In this paper, the theoretical, experimental and numerical studies about the different failure modes of tubes under axial compression are reviewed, and the mechanical responses and energy absorption properties are compared and discussed.
The studies on discrete Boltzmann modeling and simulation of phase separation are reviewed. According to the system component, the studies cover the single-component two-phase separation and the multi-component phase separation. According to the physical modeling, the studies cover the LBM (lattice Boltzmann method) simulations based on traditional hydrodynamic modeling and discrete kinetic modeling of the phase separation system. According to the main focus, the topic covers method/model studies and physical behavior investigations. The discrete kinetic modeling has brought some deeper insights into the phase separation process. The features of non-equilibriums obtained from discrete Boltzmann simulations can serve as some simple and effective physical criteria for dividing the two stages of phase separation, and can be used to discriminate and track various interfaces.
Sloshing of liquid propellant in microgravity and its influence on the attitude control system have been studied for several decades. Accurate modeling is necessary, especially, for modern spacecrafts carrying large amounts of liquid propellant. Research progress on this issue is reviewed, including a summary of analytical methods for both linear and nonlinear sloshing, an introduction of numerical techniques such as modal analysis and CFD (computational fluid dynamics) methods, and also a description of experimental approaches and advances. Finally, some critical problems concerning liquid sloshing in microgravity are proposed and discussed.
Cables are widely used as tension structure members in engineering as they are light, flexible and of high strength. On one hand, the external excitations induce cable's complex dynamic responses with large amplitudes, which might cause structure's failure. On the other hand, the cable structure is a typical nonlinear system with both quadratic and cubic nonlinearities, which means extensive nonlinear dynamic behaviors. Therefore, the cable dynamics is an interesting research issue in both engineering and applied mechanics, and our knowledge about the cable dynamics is much enriched in the past few decades. This paper reviews the progress of cable dynamics, including cable's dynamic models, internally resonant dynamics, moving support induced cable dynamics, and complex excitation induced cable dynamics. And we also discuss the limitations of the present cable dynamics.
In the numerical simulation of hydraulic fracturing in a shale gas reservoir, both the shale rock's properties and the multistage hydro-fracking technique of the horizontal well should be considered, which is a difficult problem of mechanics. In this paper, the mechanical characteristics of the shale rock and the multistage fracking technology of the horizontal well in the shale gas reservoir are discussed, as well as the applications of the extended finite element method, the boundary element method and the discrete element method in hydraulic fracturing and their advantages and limitations. It is indicated that the three dimensional displacement discontinuity method of the boundary element method is an effective approach to model the propagation of multistage fractures.
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.
This paper firstly reviews the studies of the effective stress of unsaturated soils and points out that two stress variables are preferred to be chosen to establish a constitutive model at present. The paper also explains the definition of the matric suction, which involves the effects of the capillary component and the adsorptive component. Secondly the paper discusses the strength of unsaturated soils. The shear strength is usually formulated by the extended Mohr-Coulomb Criterion. The tensile strength also affects the soil behavior due to the capillary effect and thus cannot be ignored. Thirdly, the paper reviews the studies of the constitutive models for unsaturated soils, including the models based on the net stress and the matric suction and that based on the Bishop effective stress and the matric suction. The double pore structure model is also discussed. Finally, the paper addresses the applications of the thermodynamic method and the porous media theory in unsaturated soils. It is shown that these two methods-using the thermodynamic principles to establish the constitutive framework, and using the porous media theory to study the complicated coupling effect of unsaturated soils are subjects very worthy to be developed.
The rockburst (rock burst) is a peculiar, abnormal, localized, isolated, delayed, sudden and damaging phenomenon associated with the underground rock excavation. Several hours after the completion of the excavation, some parts of the in-situ intact rock wall might be actively, suddenly and vigorously ejected, thrown and blown off from the original wall into the excavated open space, with high initial ejecting velocity and kinetic energy. It often causes fatality, injury and damage because its occurrence is usually unforeseeable and unpredictable. The rockburst is still a worldwide tough problem that puzzles the international communities of rock mechanics in spite of tremendous research efforts over the past 50 years. This paper puts forward a hypothesis of micro-fluid inclusions as the cause and mechanism of the rockburst. The fluid inclusions in the micro-voids of intact rocks may have a high initial pressure equaling to the average of the three in-situ principal stresses. The fluid inclusions with a high initial pressure can have a high expansion power. They can flow, migrate and turn from the liquid phase into the gas phase after the rock excavation. Their physical and chemical expansion power can cause the rockburst of the excavated intact rock wall. This hypothesis offers effective solutions for the rockburst problem to reduce and eliminate its damage.
The drill string fatigue failure caused by drill string vibration is increasingly an issue related with the development of ultra-deep drilling operations. The research methods of drill string vibration can be classified into two types, the theoretical analysis or numerical simulation, and the the measurement technology. Owing to various complicated nonlinear mechanical problems, especially, the impact between moving drilling tools and hole wall, it is dificult to study the drill string vibration in many cases using the theoretical analysis or numerical simulation method, and therefore, the down hole vibration measurement technology becomes more and more important. This paper gives a comprehensive and systematic review of the status and progress of drill string vibration measurement technology, followed by a detailed discussion of the working mechanism, analytical approach and application technology for several overseas measurement tools which have been widely used around the world. The results would provide important supports to develop the down hole vibration measurement technology in our country.