Design And Mechanics Behavior Of Carbon/Epoxy Composite Sandwich Beams With Graded Lattice Core

Advanced lightweight structures are constantly being developed for the purpose of reducing weight and increasing the payload of aircrafts.Composite lattice structures which have ultra-lightweight and multifunctional characteristics is recognized as one of the most promising new materials in the aerospace field.But traditional lattice structures are homogeneous structures composed of periodic cells,which can't increase the payload under non-uniform load.Actually,aircrafts are subjected to concentrated or non-uniform load in the flying scene.In order to increase the payload of aircrafts,a new composite sandwich structure with graded lattice core was presented.The core is non-uniform which is based on bionics principle.Based on theoretical analysis,finite element simulation and experimental measurement,the bending and vibration performance of composite sandwich beams with graded lattice core are systematically studied.The main contents are as follows:1.The concept of graded lattice structure is proposed for the first time.Two different types of composite sandwich beams with axially graded lattice core are proposed and designed by changing the size of unit cell and the width of the strut.The theoretical model of graded lattice beam is established and the theoretical formula for predicting the mechanical properties of the garded structure under bending load is derived through improving Allen's classical theory.The traditional fabricating method is improved and an integrated process of automatic cutting and hot pressing is proposed to fabricate the graded lattice beams.Bending text of graded lattice core sandwich structure in thickness direction was investigated to verify the correctness of the theory.Bending performance is studied for sandwich beams with axially graded lattice core through changing geometric parameters and the gradient distribution.The results show that the graded sandwich beam has higher flexural strength than the homogeneous sandwich beam under the same mass.The relationship between the geometric size and failure modes of the graded sandwich beam is revealed by investigating the failure mechanism maps.The optimal design of graded lattice sandwich beam is carried out by using complex method.2.The properties of core are equivalent to the continuous function of x to establish the vibration model of the axially graded lattice sandwich beam.Using the method of Rayleigh-Ritz,the natural frequency of the axial graded lattice cantilever beam is calculated and compared with results of numerical simulation to verification the theory.The relationship between geometric size and natural frequency is studied by using the method of theoretical analysis,finite element simulation and experimental test.Study show that the natural frequency of the structure can be increased or decreased by adjusting the gradient distribution without increasing the quality.3.The traditional lattice structures are made of the equal struts.In this paper,sandwich beam with thickness-direction lattice core is designed and fabricated by using trapezoid core replacing traditional uniform strut.Because of the special structure,the traditional lattice theory is not applicable.Based on mesoscopic model of graded unit cell,the theoretical formula for predicting the mechanical properties of the structure under compression and bending by integration method and Bessel function.Compress and bending performance of sandwich structures graded lattice core with thickness-direction graded lattice core were investigated by experiment and theory.Study show that the failure mode can be transformed by changing the graded parameter of sandwich structures graded lattice core with thickness-direction graded lattice core.Based on the analysis of failure mechanism,the internal relationship between the graded parameter and failure mode is described.4.Physical model of composite sandwich beams with thickness-direction graded lattice core was established by equivalence theory of the unit cell.The governing equations of motion were derived from Hamilton's principle and solved though eigenvectors method.The natural frequency of composite sandwich beams with graded lattice core calculated by variational method was compared with Rayleigh-Ritz method.The results show that the relative error is less than 1%.The finite element software is used to establish the numerical model of the graded sandwich beam and the relation between geometric parameters and natural frequency of the structure is studied.The vibration performance of sandwish beam with graeded lattice core is studied by modal experiments.