Mechanical Properties Of Sandwich Structure Based On New Shear Stiffness Model

Sandwich structure is widely used in many fields,including astronautics,aeronautics and automotive manufacturing,due to its high specific stiffness and specific strength.With the developing of technology,lattice core with better performance has been designed.The properties and the designability of the lattice core can be increased by introducing the concept of bionic gradient.As a result of better designability,structural design plays a more important role in the performance of structure.That leads to the requirement of better equivalent models.In this paper,shear stiffness models considering the deformation of the skins are proposed and verified by experiments and simulations.Meanwhile,the vibration,failure and design of the graded lattice structures are studied.The main research contents are organized as follows.In Chapter 2,the shear stiffness of the corrugated core sandwich structure is studied.Firstly,an analytical model of the shear stiffness is verified.This model separates the corrugated structure into three parts,including the bonding area,the skins and the core.The core is seen as hinged on the skins with torsional stiffness.The deformation coordination equation is rebuilt.The analytical model is verified by FEM simulation.The result proves that the new model can predict the shear stiffness of the structure with bonding area accurately.The underestimation of the shear stiffness of thick panel structures is overcome.The variation tendency of the shear stiffness with the increasing of bonding area is revealed,and the location of the valley point is predicted by the new model accurately.Secondly,according to the boundary conditions of corrugated plate shear deformation,a new experimental scheme for shear stiffness measurement of corrugated plates is designed.Comparing with the standard test method for shear properties of sandwich core materials ASTM C273,the new experiment can accurately measure the torsional stiffness between the panel and the core,and measure the range of shear stiffness of the structure with bending area more accurately than the standard experiment.The experiment also proves the accuracy of the analytical model.In Chapter 3,the shear stiffness of the lattice core sandwich structure is studied.The analytical model of the shear stiffness takes the deformation of the skins into consideration.The deformation of the skins is analyzed by Levy process,and the shear stiffness of the lattice structure is calculated by substituting the deformation into the coordination equation.The analytical model is verified by FEM simulation.The result proves that the new model can accurately predict the shear stiffness of the structure with more than one connect point.The connections between the core and the skins affect significantly on the shear stiffness of the structure.The clamped connection can greatly increase the shear stiffness of the structure.In Chapter 4,the shear stiffness of the corrugated lattice core sandwich structure is studied.Firstly,the longitudinal shear stiffness is derived.The skins are separated into two rectangular plate with constant thickness from the connect point.Then the deformation of the skins is analyzed by Levy process,and the shear stiffness of the lattice structure is calculated by substituting the deformation into the coordination equation.Secondly,the transverse shear stiffness is derived.The skins are seen as variable thickness beams,and the deformation of the skins is analyzed.Then the shear stiffness of the lattice structure is calculated by substituting the deformation into the coordination equation.The analytical model is verified by FEM simulation.The result proves that the new model can predict the shear stiffness of the structure with more than one connect point accurately.In Chapter 5,the bending failure of graded lattice structures is studied.A parametric modeling method(PM method)of graded lattice structure is proposed.The continuum damage model is used to evaluate the failure load of the structures.Two types of graded lattice structure are built.The cell-size-changed graded lattice structure(the densities of the structures are constant)and the Bar-width-changed graded lattice structure(the masses of the structures are constant).The failure loads of the structures under 3-point bending load are investigated.The result proves that the gradient design can significantly improve the properties of the lattice structure.In Chapter 6,the vibration of the lattice structure of the truss is studied.Firstly,the equivalent properties of the lattice core are analyzed by using energy method.The shear stiffness of the structure is analyzed by using the results in Chapter 3 and Chapter 4.Then the equivalent properties of the lattice structure are analyzed.The FEM models of the lattice cantilever beam are built by PM method which is proposed in Chapter 5.The equivalent FEM models of the lattice cantilever beam are built by combining the equivalent properties and the PM method.The result proves that the new model can avoid the error increasing with the increase of the vibration order.In Chapter 7,a two layer graded lattice structure is designed.The height of the middle plane of the cell varies under the control of the gradient parameters.The designed lattice structure is prepared by Fused Deposition Molding(FDM)process and tested by three-point bending experiment.The experiment of the colour changing PLA structure shows the residual thermal stress of the 3D printing process,and the stress concentration on the structure under three point bending.Experiments with high-resistance resin graded lattice structure show that reasonable gradient design can significantly enhance the energy absorption capacity of the structure.