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Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes/issues, but are citable by Digital Object Identifier (DOI).
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, Available online  , doi: 10.21656/1000-0887.430115
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Compared with fixed wings, flapping wing has a significant aerodynamic performance advantage at low speed and low Reynolds number, which draws more and more attentions. Due ti the most studies focusing on rigid airfoil, the aerodynamic performance of flexible airfoil of flapping wing is still unclear. In this paper, a fluid-solid coupling model for a flexible elliptical airfoil is developed to analyze the flow field around the airfoil, deformation of airfoil, as well as the aerodynamic characteristics of airfoils, at different wind speed and attack angle. Compared with the rigid airfoil, the flexible airfoil can delay the shedding time of the wake vortex and reduce the oscillation frequency of the disturbance of lift force. The flexible airfoil significantly suppresses the disturbance of wake flow and reduces the oscillation amplitude of disturbance. Even, the disturbance oscillation can be completely eliminated at the appropriate Young's modulus airfoil. These results provide theoretical guidance for the design of soft aircraft.
, Available online  , doi: 10.21656/1000-0887.420206
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Vibration suppression performance of a damping composite structure depends on the material layout and the properties of damping material. This paper proposes a topology optimization method for damping material microstructure with varied volume constraints, which aims to obtain the damping material microstructure with the desired property under the smallest material consumption. Based on the homogenization method, a three-dimensional finite element model of the damping material is established, and the effective elastic matrix of damping material is formulated. The Hashin-Shrilman bounds theory is used inversely to estimate the volume fraction bound of the damping material corresponding to the desired effective modulus, and a movement criterion of volume constraint bounds for damping material is constructed. Then the optimization problem aiming to the desired property of damping material with microstructure is transformed into another problem of maximizing the desired modulus under volume constraints, and topology optimization model of the damping material microstructure is established. Optimality criteria method is employed to update the design variables, and the optimized topology configurations of damping material microstructure is obtained. The feasibility and effectiveness of the proposed method are verified by several numerical examples, and the influence of the initial configurations, mesh density and Young's modulus on the microstructure configurations of damping material are also discussed.
, Available online  , doi: 10.21656/1000-0887.420318
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The computation consumption of finite element analysis for structural optimization design of holding pole is large, and it is difficult to determine the feasible region. The response surface method (RSM) was used to simulate the real response of the holding pole, and an improved arithmetic optimization algorithm (IAOA) was proposed to optimize the holding pole. Fractional-order calculus was introduced into arithmetic optimization algorithm (AOA) to improve the exploitation ability of AOA. Latin hypercube sampling was applied to select the test samples of each member of the holding pole, and the least square method was employed to analyze the sample points. Then, the second-order response surface surrogate model of the stress and displacement of the holding pole on the cross-sectional size of each member was established. An optimization model was constructed with the minimum mass as the optimization objective and the allowable stress and displacement as constraints, and the IAOA was implemented to solve the model. The results show that the second-order response surface model can accurately predict the response value of the holding pole. The solution accuracy of the IAOA is significantly improved. The surrogate model can greatly decrease the calculation cost of finite element analysis. The mass of the holding pole is reduced 8.2% after optimization. The RSM and the IAOA can be combined to solve the optimization design problem of large spatial truss structures effectively.
, Available online  , doi: 10.21656/1000-0887.430104
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Traditional lattice structures usually maintain their mechanical properties throughout their lifetime. Designing and manufacturing intelligent materials with environmental adaptability, programmable sense and responses to external changes (such as light, pressure, solution, temperature, electromagnetic field and electrochemical reaction), shape transformation, mode conversion and performance regulation in space and time, are still important scientific challenges in the field of artificial materials. In this paper, multimaterial lattice structures with thermally programmable mechanical responses were proposed by means of polymer materials with disparate glass transition temperatures and temperature dependencies, and through reasonable design of the spatial distribution of the materials. By theoretical analysis combined with finite element simulation, the effects of the relative stiffnesses of constitute materials on Poisson's ratios, deformation modes and structural stability of the multimaterial lattice structures, were studied. The elastic constants, crushing responses and structural stability of multimaterial lattice structures were regulated by temperature control, consequently the multimaterial lattice structures were endowed with giant thermal deformation, hyperelasticity and shape memory effects. This paper opens up new avenues for the design and manufacture of adaptive protection equipment, biomedical devices, aerospace morphing structures, flexible electronic devices, self-assembly structures and reconfigurable soft robots.
, Available online  , doi: 10.21656/1000-0887.420367
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Underwater explosion poses a serious threat to underwater structures. Flexible coatings or sandwich plates can reduce the underwater explosion impact responses of underwater structures, and make a research hotspot. Previous studies focused on the protective mechanism of the coating against shock waves, which is suitable for underwater explosion at a long distance. Besides the shock wave, the high-speed water jet towards the structure produced by explosion bubble collapse, is more deadly in the short-distance underwater explosion. In view of this situation, based on the dimensional principle, the reduced-scale similarity relationship was deduced. Through the reduced-scale-model underwater explosion test, it is found that, the cavitation micro-bubble group on the surface of the foam coating interferes with the formation process of the explosion bubble collapse high-speed water jet. The protection mechanism of the foam coating against the underwater explosion bubble collapse water jet for coated steel plates was put forward.
, Available online  , doi: 10.21656/1000-0887.430103
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A new formation mechanism of the low-frequency broadband within gapped nonlinear local resonance structures was revealed based on the nonlinear chaos theory, and a novel concept for designing nonlinear local resonant structures with small gaps was further proposed. Due to the small gaps, the nonlinear chaos phenomenon occurs in the local resonance system, which can change the spectrum structure in vibration noise successfully, and the linear spectral energy greatly weakens and a continuous broad spectrum forms after chaotic motion, to effectively isolate the low-frequency spectrum. Most importantly, the finite element results show that, the nonlinearity of the small gap indeed leads to the low-frequency band-gap within the nonlinear local resonance. Therefore, the new idea for designing the nonlinear local resonance structure makes a new way to the development of local resonant elastic metamaterials, and the formation mechanism of low-frequency band-gap based on the nonlinear chaos theory lays a very important theoretical basis for vibration and noise reduction.
, Available online  , doi: 10.21656/1000-0887.420202
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The anti-plane problem of asymmetric collinear interface cracks emanating from a circular hole in one-dimensional hexagonal quasicrystal bi-material is studied. Using Stroh formula and complex function method, the complex potential functions under the coupling action of phonon field and phason field are obtained. The analytical expressions of stress intensity factor (SIF) and energy release rate (ERR) at the crack tip are given. The effects of the radius of the circular hole and the length of the crack on the SIF, and the effects of the coupling coefficient, the phonon field and the phason field stresses on the ERR are discussed. The results show that the SIF tends to be stable with the increase of the length of the right crack when the radius of the circular hole is constant. When the phason field stress takes a certain value, the ERR reaches the minimum, which indicates that the specific phason field stress can inhibit the crack growth.
, Available online  , doi: 10.21656/1000-0887.420328
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The complex internal structures and moisture states of porous materials are of great significance to heat and mass transfer, and their coupling heat and mass transfer processes widely exist in energy development and engineering heat insulation. Beyond the unilateral analysis of heat and mass transfer characteristics of porous materials under ideal conditions, the distribution parameters of porous channels, rough surface, wet states and phase-change were considered, and the fractal theory was used to deduce the expressions of the seepage coefficient and the coupling equivalent thermal conductivity of porous materials with wet phase-change rough surface. The results show that, the seepage coefficient is positively correlated with the area fractal dimension and the moisture saturation, and negatively correlated with the relative roughness and the tortuous fractal dimension. The coupling equivalent thermal conductivity is positively correlated with the seepage coefficient and the phase variable, but negatively correlated with the relative roughness. In addition, the phase variable and the gas expansion pressure difference caused by phase-change also have important effects on the coupling heat and mass transfer.
, Available online  , doi: 10.21656/1000-0887.430065
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Aimed at vortex-induced vibration (VIV) of the submarine reconnaissance telescope lifting above the water surface, a theoretical model for VIV of a cantilever cylinder under the actions of 2 different cross flows, i.e. gas and liquid, was established. The effects of parameters such as the distribution ratio and the density ratio for these 2 fluids on VIV responses of the cylinder were studied. Based on the Galerkin technique and the Runge-Kutta algorithm, numerical results of the cylinder vibration responses were obtained. The results show that, the increase of the distribution ratio can widen the lock-in range of the cylinder. The peak amplitude of the cylinder increases first and then decreases with the distribution ratio. The amplitude reaches the maximum value with a distribution ratio of 0.5, and this maximum value will increase with the decrease of the density ratio. In addition, single-period and multi-period motions will occur with the change of the fluid distribution ratio. The present research provides a theoretical guidance for the design and analysis of the submarine reconnaissance telescope.
, Available online  , doi: 10.21656/1000-0887.420295
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The radial basis function partition of unity method (RBF-PU) is applied to obtain the numerical solution of two-dimensional nonlocal diffusion and peridynamic problems. The main idea is partitioning the original domain into several patches and using the RBF approximation on each local domain, and then weight to obtain the global approximation of the unknown function. The radial basis function method based on the strong form of the equation has many advantages, such as avoiding an additional layer of integral calculation, no need to deal with intersections of neighborhoods with the mesh, and it is easy to implement. The numerical results show that this method can solve nonlocal diffusion equations and peridynamic equations accurately and efficiently.
, Available online  , doi: 10.21656/1000-0887.420326
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The circular membrane solar array has attracted extensive attention due to its high storage ratio and strong power supply capability. In order to adjust the tension of large film structure, a tension adjusting device composed of rope and spring is usually introduced. Its mechanical characteristics are highly nonlinear. But its influence has not been studied yet. In this paper, In order to study the influence of tension, a mechanism model is proposed in this paper. The nonlinear dynamics equation of the system with two-degree-of-freedom is established by using Lagrange energy method. Taking an engineering prototype as an example, the response of a tension mechanism with unsymmetrical ribs under resonance excitation is studied. The results show that the change of excitation amplitude has an important influence on the characteristics of the beat response of the system. It makes the response of the system appear chaotic, almost periodic and multifold periodic phenomena. These results have an important reference to the parameter design of tension mechanism.
, Available online  , doi: 10.21656/1000-0887.420109
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In this paper, we consider a macroscopic nonlinear Keller-Segel model for chemotactic cell migration, where the existence region of the model is bounded convex on $\varOmega\subset\mathbb{R}^N(N\geqslant2)$. The global existence of the solution on $\varOmega\subset\mathbb{R}^3$ is obtained by using the energy estimates method. The lower bounds of the blow-up time are proved when $N=3$ and $N=2$.
, Available online  , doi: 10.21656/1000-0887.430009
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Based on the deterministic network infectious disease model, a stochastic network infectious disease model under the influence of white noise is established, and the existence and uniqueness of the global solution of the model is proved. Using the theory of stochastic differential equation, sufficient conditions for stochastic extinction and persistence of infectious diseases are obtained. The results show that white noise has a great impact on the transmission dynamics of infectious diseases on the network. White noise can effectively suppress the spread of infectious diseases, and large white noise can even make the original persistent infectious diseases become extinct. Finally, the theoretical results are verified by numerical simulations.
, Available online  , doi: 10.21656/1000-0887.420177
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The grazing-induced non-smooth dynamical behaviors of a single-degree-of-freedom cantilever beam system with bilateral elastic constraints are studied. Firstly, based on the dynamical equations of cantilever beam with soft impact and the definition of grazing points, the existence condition of bilateral grazing periodic motion is analyzed. Secondly, the zero-velocity Poincaré section is selected to derive the high-order discontinuous mapping with parameters near bilateral grazing orbits. Then a new composite piecewise normal form mapping is established by combining smooth flow mapping and high-order discontinuous mapping.Finally,the validity of the high-order mapping is verified by comparing the bifurcation diagram of the low-order mapping with the high-order mapping, and the grazing dynamics of the cantilever beam with soft impact are further revealed through numerical simulation.
, Available online  , doi: 10.21656/1000-0887.420353
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Superconducting thin film is a kind of multilayer structure prepared by chemical coating. As a conductive functional structure material with excellent performance, its structural integrity is directly related to current-carrying capacity. During the preparation of superconducting thin films, it is hard to avoid the interface cracks between the superconducting layer and the metal substrate. In this case, along with the current-carrying operation, the strength of the interface crack in an external magnetic becomes a key problem. Therefore, based on the theory of flux through thin film and linear elastic fracture, an analytical model of the strength of the interface crack between the superconducting film and the substrate is established. The influence of viscous flux flow on the stress field and the energy release rate of the crack tip is obtained. The results show that the higher the flux flow velocity is, the greater the stress and energy release rate at the crack tip of the interface will be, which will lead to crack propagation at the interface. The results obtained in this paper are helpful for the analysis of interface cracks mentioned above.
, Available online  , doi: 10.21656/1000-0887.420248
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This paper is devoted to the investigation of a class of multi-objective fractional semi-infinite optimization problems with uncertain data. Firstly, a robust counterpart for the uncertain multi-objective optimization problem is introduced in terms of robust optimization method. And following the idea due to Dinkelbach, we associate the robust counterpart with a multi-objective optimization problem. Then, by using a scalarization method, its corresponding scalarization optimization problem is established. The relationship between robust solutions of the multi-objective optimization problem and that of its corresponding scalarization optimization problem is described. Finally, by using a robust type sub-differential constraint qualification, the robust optimality condition for approximate quasi-efficient solutions of the multi-objective fractional optimization problem is established.
, Available online  , doi: 10.21656/1000-0887.430082
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Based on classical laminated plate theory and cohesive zone model, a theoretical model of general delamination cracked laminates is established for crack propagation of pure mode Ⅱ ENF specimen. Compared with the conventional beam theory, the proposed model fully consideres the softening process of the cohesive zone and introduced the nonlinear behavior of ENF specimens before failure. The predicted failure load is smaller than the beam theory and closer to the experimental data in literature. Compared with the beam theory with only fracture toughness considered, the proposed model can simultaneously analyze the influence of interface strength, fracture toughness and initial interface stiffness on the load-displacement curves in ENF tests. The results show that the interface strength mainly affects the mechanical behavior of specimens before failure, but has no influence on crack propagation. The fracture toughness is the main parameter affecting crack propagation, and the initial interface stiffness only affects the linear elastic loading section. The length of cohesive zone increases with the increase of fracture toughness and decreases with the increase of interface strength. The effect of interface strength on the length of cohesive zone is more obvious than fracture toughness. When the tip of the adhesive zone reaches the half-strength of the specimen, the length of the adhesive zone decreases to a certain extent.
, Available online  , doi: 10.21656/1000-0887.420190
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Considering the phenomenon of competing water resources between young vegetation and adult vegetation in arid and semi-arid areas, a vegetation-soil water dynamic model with intraspecies competition delay is established. We analyze the conditions for the existence of an unique vegetation survival equilibrium and the local stability of the vegetation extinction equilibrium. The generating conditions of Hopf bifurcating periodic solutions for non-spatial and spatial systems are given, respectively. The periodic oscillation pattern of vegetation evolutes with time is shown by numerical simulations. Through the parameter sensitivity analysis, it is found that the rainfall and vegetation growth rate have significant influences on generation, amplitude and period of this pattern, but the effect of evaporation is the least significant. These results indicate that rainfall and vegetation properties have profound effects on the evolution and development of vegetation in arid and semi-arid areas. It is found that the introduction of spatial diffusion inhibits the occurrence of this pattern, but doesn't affects on amplitude and period. The results obtained explain the phenomenon of vegetation periodic oscillation widely observed in nature, which provide theoretical support for the sustainable development of vegetation system.
, Available online  , doi: 10.21656/1000-0887.430062
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As a newly emerged light porous material, carbon aerogels are a class of carbonaceous solid material with high porosity, low density and excellent environmental stability. With the combination of high elasticity, high energy absorption, as well as special properties such as shock absorption, sound absorption and electromagnetic shielding, carbon aerogels are both functional and structural widely applied in the fields of flexible sensors, energy equipment, acoustic equipment and environmental protection. However, the existing general conflicts between mechanical robustness and intrinsic sparse network in porous aerogel materials have been a common challenge faced by researchers ranging from fields of material science, solid mechanics, design application and so on. Good robustness could ensure the structural integrity and performance stability of aerogels in the application process, while sparse network is the prerequisite to ensure the lightweight and porous structure of aerogels. Here, we discuss the recently emerged strategies for optimizing the mechanical robustness, including cell-wall strengthening, cell-wall orientation, pore topology controlling and joint reinforcement. Specially, we summarize the advanced design principles to realize the tensile elasticity in ultra-light all-carbon aerogels without intrinsic stretchable elastomers. In addition, we briefly overview the recent applications of robust carbon aerogels and outlook the problems to be solved in this field.
, Available online  , doi: 10.21656/1000-0887.420314
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Thermal dispersion coefficient is an important parameter to characterize heat and mass transfer in porous media, which is related to the physical properties of fluid and the structure of porous media. The pore-throat structure model of fractal porous media was established, and the local head loss and velocity dispersion effect of fluid changing from turbulent state to laminar state at pore-throat structure were studied. The relationship of thermal dispersion coefficient was derived considering the influence of micropore-throat structure and velocity dispersion effect. The results showed that the thermal dispersion coefficient is directly proportional to the pore-throat ratio, the number of pore-throat structures and tortuous fractal dimension, and inversely proportional to the porosity and surface integral dimension. Further study showed that the pore throat ratio has a significant influence on velocity dispersion effect in the range of 1~150, and the fluid has local head loss at the pore throat structure, which leads to the enhancement of velocity dispersion effect and the increase of thermal dispersion coefficient.
, Available online  , doi: 10.21656/1000-0887.420405
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Aiming at the active thermal protection with lattice sandwich structure, an unsteady heat transfer theoretical model which couples facesheet and core heat conduction and coolant convection in the sandwich structure was established, the model equations were discretized by finite volume method and solved iteratively in Matlab; the constriction thermal resistance between the facesheet and the lattice strut was considered for the first time in the model, and the approximate analytical solution of the constriction thermal resistance was obtained using the method of separation of variables; based on the unit-cell model and periodic boundary conditions, the heat transfer coefficients hb and hfin required by the model were first obtained by numerical simulation. Finally, a case study with a multi-cells structure was carried out to compare the numerical and theoretical results, and the influence of constriction thermal resistance on the prediction accuracy was discussed. The results show that the theoretical model can accurately predict the temperature variation of the sandwich structure and the internal fluid, and the maximum deviation between theory and simulation is less than 1%. As the external heat flux increases, the error of theoretical prediction increases when the constriction resistance is ignored. Compared with the numerical simulation, the theoretical model can significantly reduce the calculation time and save calculation resources, thus it is especially suitable for actively cooled lattice sandwich structure subjected to complex, non-uniform and unsteady thermal load.
, Available online  , doi: 10.21656/1000-0887.430061
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In order to analyze the load-bearing capacity and failure modes of carbon fiber reinforced polymer composite honeycomb sandwich structures with curved wall under three-point bending load, theoretical prediction, numerical simulation and test were carried out for structures with different core height and face panel thickness. According to the main failure modes of sandwich structure, different theoretical prediction formulas and failure mechanism diagrams were firstly made. Then, the numerical simulation model of the CFRP sandwich structure with honeycomb core was established to simulate its failure behavior under three-point bending load. Finally, different sizes of CFRP sandwich structures were fabricated by molding process, and the experimental results were compared with theoretical and simulation results. The results show that the bearing capacity of sandwich structure is positively correlated with the height of core and thickness of face panel, and the stiffness of core and face panel decreases with the decrease of size, which results in the structural failure mode changing from debonding between core and face panel to face crushing.