2022 Vol. 43, No. 5

Display Method:
A Speical Issue on Foundational Theories and Applications of Lightweight Porous Materials and Structures
Forwards
LU Tianjian, LIU Shaobao
2022, 43(5): 1-2.
Abstract(356) PDF(79)
Abstract:
A Calculation Model for Temperature Responses of Active Cooling Lattice Sandwich Structures for Thermal Protection
PENG Shibin, GUO Rui, FENG Shangsheng, JIN Feng
2022, 43(5): 477-489. doi: 10.21656/1000-0887.420405
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Abstract:

Aimed at the active thermal protection with lattice sandwich structures, an unsteady heat transfer theoretical model coupling facesheet and core heat conduction with coolant convection in the sandwich structure, was established. The model equations were discretized with the finite volume method and solved iteratively in MATLAB; the constriction thermal resistance between the facesheet and the lattice struts was considered for the first time in the model, and the approximate analytical solution of the constriction thermal resistance was obtained with the variable separation method; based on the unit-cell model and periodic boundary conditions, heat transfer coefficients hb and hfin required by the model were first obtained through numerical simulation. Finally, a case study with a multi-cell structure was carried out to compare the numerical and theoretical results, and the influence of the constriction thermal resistance on the prediction accuracy was discussed. The results show that, the theoretical model can accurately predict the temperature variations 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 rises with the constriction thermal resistance 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 active cooling lattice sandwich structures subjected to complex, non-uniform and unsteady thermal loads.

Three-Point Bending Properties of Carbon Fiber Reinforced Polymer Composite Honeycomb Sandwich Structures With Curved Wall
LIU Xin, WU Qianqian, YU Guocai, WU Linzhi
2022, 43(5): 490-498. doi: 10.21656/1000-0887.430061
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Abstract:

In order to analyze the load-bearing capacities and failure modes of carbon fiber reinforced polymer composite honeycomb sandwich structures with curved wall under 3-point bending loads, theoretical prediction, numerical simulation and tests were carried out for structures with different core heights and facesheet thicknesses. According to the main failure modes of sandwich structures, different theoretical prediction formulas and failure mechanism diagrams were firstly made. Then, the numerical simulation model for the CFRP sandwich structure with a honeycomb core was established to simulate its failure behavior under the 3-point bending load. Finally, different-size CFRP sandwich structures were fabricated by a molding process, and the experimental results were compared with theoretical and simulation results. The results show that, the bearing capacity of the sandwich structure is positively correlated with the core height and the facesheet thickness, and the core and facesheet stiffness decrease with the structure size, which results in the structural failure modes changing from core-facesheet debonding to face crushing.

Enhancement Strategies for Mechanical Robustness of Carbon Aerogels and Their Applications
GUO Fan, YANG Cao, GUO Rui, JIANG Wei
2022, 43(5): 499-514. doi: 10.21656/1000-0887.430062
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Abstract:

As a newly emerging light porous material, carbon aerogels are a class of carbonaceous solid materials 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, and widely applied in the fields of flexible sensors, energy equipment, acoustic equipment and environmental protection. However, the existing general conflicts between the mechanical robustness and the intrinsic sparse network in porous aerogel materials have been a common challenge faced by researchers in 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 the sparse network is the prerequisite to ensure the lightweight and porous structure of aerogels. Here, the recent enhancement strategies for the mechanical robustness, including cell-wall strengthening, cell-wall orientation, pore topology controlling and joint reinforcement, were discussed. Specially, the advanced design principles to realize the tensile elasticity in ultra-light all-carbon aerogels without intrinsic stretchable elastomers, were summarized. In addition, the recent applications of robust carbon aerogels were reviewed and the problems to be solved in this field were listed.

Interlaminar Crack Propagation Analysis of ENF Specimens Based on the Cohesive Zone Model
DENG Jian, XIAO Pengcheng, WANG Zengxian, SHAO Guangran, LU Tianjian
2022, 43(5): 515-523. doi: 10.21656/1000-0887.430082
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Abstract:

Based on the classical laminated plate theory and the cohesive zone model, a theoretical model for general delamination cracked laminates was established for crack propagation of pure mode Ⅱ ENF specimens. Compared with the conventional beam theory, the proposed model fully considered the softening process of the cohesive zone and introduced the nonlinear behavior of ENF specimens before failure. The predicted failure load is smaller than that under the beam theory and closer to the experimental data in literatures. Compared with the beam theory with only fracture toughness considered, the proposed model can simultaneously analyze the influences of the interface strength, the fracture toughness and the 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 stage. The cohesive zone length increases with the fracture toughness and decreases with the interface strength. The effect of the interface strength on the cohesive zone length is more obvious than that of the fracture toughness. When the adhesive zone tip reaches the half length of the specimen, the adhesive zone length will decrease to a certain extent.

The Low-Frequency Broadband Mechanism of Nonlinear Elastic Metamaterials With Gaps
LIN Zhen, WU Jiuhui
2022, 43(5): 524-533. doi: 10.21656/1000-0887.430103
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Abstract:

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.

Multimaterial Lattice Structures With Thermally Programmable Mechanical Behaviors
YANG Hang, MA Li
2022, 43(5): 534-552. doi: 10.21656/1000-0887.430104
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Abstract:

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.

A Fractal Model for Thermal Dispersion Coefficients of Porous Media
ZHANG Jie, ZHANG Sai, GAO Weiye, HU Shiwang, WANG Zhenyi
2022, 43(5): 553-560. doi: 10.21656/1000-0887.420314
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Abstract:

The 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 for fractal porous media was established, and the local head loss and the velocity dispersion effect were studied for the fluid changing from the turbulent state to the laminar state around the pore-throat structure. The thermal dispersion coefficient formula was derived under the influences of the micropore-throat structure and the velocity dispersion effect. The results show that, the thermal dispersion coefficient is directly proportional to the pore-throat ratio, the number of pore-throat structures and the tortuous fractal dimension, and is inversely proportional to the porosity and the area fractal dimension. Furthermore, in the range of 1~150, the pore-throat ratio has a significant influence on the velocity dispersion effect, and the fluid has a local head loss around the pore-throat structure, which leads to an enhancement of the velocity dispersion effect and an increase of the thermal dispersion coefficient.

Thermo-Mass Coupling Fractal Study of Wet Phase-Change Rough Porous Materials
GAO Weiye, ZHANG Sai, ZHANG Jie, HU Shiwang, WANG Zhenyi
2022, 43(5): 561-568. doi: 10.21656/1000-0887.420328
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Abstract:

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.

Reduced-Scale Experiment Study on the Protective Mechanism of Foam Coating Against Underwater Explosion Bubble Jet
DU Zhipeng, ZHANG Lei, CHEN Yong, HUA Hongxing
2022, 43(5): 569-576. doi: 10.21656/1000-0887.420367
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Abstract:

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.

A Study on the Vortex-Induced Vibration Mechanism of Cantilever Cylinders Under Gas-Liquid Cross Flows
YAN Hao, DAI Huliang, WANG Lin, NI Qiao
2022, 43(5): 577-585. doi: 10.21656/1000-0887.430065
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Abstract:

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 two 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 two 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.

Simulation of Aerodynamic Performances of Flexible Flapping Wing Airfoils
WANG Qi, ZHU Yinxin, NIU Peixing, LIU Shaobao
2022, 43(5): 586-596. doi: 10.21656/1000-0887.430155
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Abstract:

Compared with fixed wings, the flapping wing has a significant aerodynamic performance advantage at low speeds and low Reynolds numbers, which draws more and more attentions. But most previous studies focus on rigid flapping airfoils, the aerodynamic performances of flexible flapping airfoils are still unclear. A fluid-solid coupling model for the flexible elliptical airfoils was developed to analyze the flow field around the airfoil, the airfoil deformation and the aerodynamic characteristics of airfoils, at different wind speeds and attack angles. 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 on the lift force. The flexible airfoil significantly suppresses the disturbance of the wake flow and reduces the oscillation amplitude of disturbance. Even, the airfoil disturbance oscillation can be completely eliminated at an appropriate Young’s modulus of the airfoil. These results provide a theoretical guidance for the design of soft aircraft.