Structurally And Thermally Integrated Metal Lattice Multifunctional Structure

For hypersonic vehicles and reusable launch vehicles,the heat protection structure integrating heat insulation,bearing and dimension maintenance is an important system to ensure safe flight of an aircraft.To improve the maneuverability and achieve a large thrust-weight ratio and long voyage range of the vehicles,design of lightweight structures with thermal protection is particularly important,while many thermal and mechanical properties related to the novel lightweight structures need to be further explored.Due to the superior bending performance of hourglass lattice truss,panel buckling resistance and blast resistance,hourglass lattice truss sandwich structures have become one of the recent research hotpots gradually.The internal cavity in the structure opens the development space for the multifunctionalization and integration of the structure.In this thesis,the out-of-plane compression,in-plane shear,heat transfer and thermal deformation of hourglass truss-lattice sandwich panels are explored.At the same time,the mechanical and thermal properties of the multifunctional structure with grid-hourglass truss-lattice sandwich structure as its supporting framework are also studied.This multifunctional structure provides a guidance for the design of new multifunctional integrated structure.The main contents of the thesis are as follows:A novel method for manufacturing the lattice truss sandwich structures made from aluminum alloy was developed through a combination of topologicalreinforcement and material-strengthening.The method could overcome the bottleneck problem in fabricating aluminum alloy thin panel lattice structure.Making use of wire electrical discharge machining(WEDM)and vacuum brazing method,three dimensional aluminum alloy hourglass truss-lattice sandwich structures were fabricated under two heat treatments.The theoretical formulas were derived for predicting the out-of-plane compression and in-plane shear strength of the lattice truss sandwich structures.The out-of-plane compression and shear properties of the lattice truss sandwich structures under three different relative densities and two heat treatments were experimentally investigated,and the results were well agreement with those from theoretical prediction,which verified the theoretical prediction model.The study particularly showed that the strengthening treatment can effectively improve the mechanical properties of the aluminum alloy hourglass lattice structure.Compared to the as-brazed hourglass truss-lattice sandwich structures,the out-ofplane compressive strength and in-plane shear strength of the age-hardened ones with relative densities of 1.72%,2.92% and 4.59% were increased by 1.1?2.06 times and0.81?1.3 times,respectively.The heat transfer performance of hourglass lattice sandwich structure under constrant temperature loading was studied,while a series of theoretical formulas were also derived,which included the equivalent heat conductivity outside the surface and the equivalent heat conductivity inside the surface.The equivalent thermal conductivity of the hourglass lattice-truss sandwich structure was analyzed by finite element simulation and theory prediction,and the results were compared with that from typical sandwich structures.The effects of the truss thickness,the inclination angle of truss and the thickness of the core on the thermal deformation resistance of the hourglass lattice-truss sandwich structure were studied.Compared to the pyramid truss lattice sandwich structure and grid-pyramid truss-lattice sandwich structure,hourglass lattice-truss sandwich structure shows better thermal deformation resistance.In addition,the thermal deformation resistance of the grid-stiffened lattice-truss sandwich structure can be significantly improved.The grid-hourglass lattice sandwich structures with three relative densities were designed and prepared,and their mechical behavior and energy absorption characteristics under compression loading were studied.The effect of the inclination angle,thickness of lattice truss and grid-stiffener on the thermal deformation performance of hourglass lattice-truss sandwich structure under the condition of nonuniform temperature rise were also studied.The mechanical and thermal equivalent models of multi-layer grid-hourglass lattice sandwich structure were built.The formulas for core relative density,elasticity modulus,and equivalent thermal conductivity in different directions were derived,and the finite element calculation of equivalent thermal conductivity was carried out.The result shows that the gridhourglass lattice sandwich structure has the best out-of-plane compressive strength among the three structures.Meanwhile,its peak load of in-plane compression is 1.14 times and 5.3 times than that of the hourglass lattice truss sandwich structure and the pyramid lattice truss sandwich structure,respectively.With the increase of the truss inclination angle,the thickness of the lattice core and the thickness of the stiffened bar of the grid-hourglass truss lattice core,the thermal deformation of this sandwich structure decreases.The strongest resistance to thermal deformation of grid-hourglass lattice-truss sandwich structure is in Y direction.Based on the basic mechanical properties and heat transfer properties of the gridhourglass lattice sandwich structure,a multi-functional integrated design was conducted for the fuel storage tank,a typical component in an aircraft.To meet the needs of multi-layer temperature control for reducing input heat flow,the layered conceptual design of the structure was introduced.According to the requirements of service environment,the influence of the shape,size and spacing of the pipe on heat transfer performance were studied,and the thermal control performance of multifunctional structure was evaluated.The mechanical strength,deformation and thermal properties of the multi-functional structure under mechanical and thermal loads were studied by simulation analysis.The convection heat transfer coefficient meeting the upper limit temperature(140?)of the aluminum alloy structure is closely related to the inner diameter of the pipe,while the outer diameter of the pipe has little influence on it.When the distance between pipelines is below 70 mm,the temperature difference of the multi-functional structure could be controlled to be less than 10?,which is close to 1/4 of the temperature difference in traditional structures.