The Design, Fabrication and Mechanical Characterization of Three-dimensional Periodic Auxetic Cellular Structures

Author:Wang Xin Tao

Supervisor:ma li

Database:Doctor

Degree Year:2018

Download:57

Pages:141

Size:9519K

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As promising metamaterials,auxetic materials and structures have attracted great interest.In recent years,3D periodic auxetic cellular structures(PACSs)have attracted considerable interest and became a focus of auxetic material and structure research.However,due to their omplicate geometries,less progress has been made on the fabrication of 3D PACSs.In addition,auxetic materials are manily of low stiff.Therefore,the study of the fabrication methods and how to improve their structural stiffness are the key issues in the study of the 3D PACSs.Using high-performance fibre reinforced polymer(FRP)composite to fabricate the auxetic structures can significantly improve their weight specific stiffness and strength.However,there are few reports on FRP composites 3D PACSs.In this thesis,an interlocking assembly method for preparing 3D re-entrant auxetic cellular structures and a stacking assembly method for preparing 3D double-arrow-head auxetic structures are proposed.Polymer,metal and FRP composite 3D re-entrant auxetic cellular structures as well as FRP composite 3D double-arrow-head auxetic structures have been successfully manufactured.Then,the deformation mechanism,Poisson’s ratio and compressive modulus of the structures are studied by means of experimental characterization,numerical simulation and theoretical analysis.This work aims at providing references for the design,fabrication and application of 3D PACSs.The main contents are as follows:Firstly,an interlocking assembly method for manufacturing 3D re-entrant auxetic cellular structures is proposed,which makes the manufacturing of 3D PACSs with non-additive manufacturing process realized.Based on the interlocking assembly method,a series of test specimens made of metal(304 stainless steel)are successfully fabricated.The Poisson’s ratio and equivalent compressive modulus of the specimen were studied by uniaxial compression testing.Then,the influence of the geometric parameters of the structure(including the cross-sectional dimension of the struts as well as the ratio of the length of the vertical strut to the length of the oblique strut)on the mechanical properties of the 3D re-entrant auxetic cellular structures are studied by finite element simulation.The results show that the Poisson’s ratio of the 3D re-entrant auxetic cellular structures parabolically dependent on the re-entrant angle.With the thickening of the struts the compression modulus of the structure monotonously increases and the Poisson’s ratio of the structure will gradually changes from negative to positive.The vertical strut length to oblique strut length ratio plays fewer roles on themechanical properties comparedwith the re-entrant angle and the strut thickness.Secondly,the theoretical analysis model of 3D re-entrant auxetic cellular structures is established,and the Poisson’s ratios and equivalent compression stiffness in each main direction are studied.In order to make the theoretical model suitable for the structure with relatively stubby struts,the effects of the overlap between the vertical strut and the oblique strut on their effective deformation length as well as axial extension or compression of the oblique strut(mostly neglected in former studies)are considered in the model.In the model,four deformation mechanisms including the bending,shearing,axial loading of the oblique strut as well as the axial loading of the vertical strut are considered.The theoretical analysis model is verified by finite element results and experimental results.Then,the deformation mechanism and its variation with the slenderness ratio and the re-entrant angle under uniaxial compressive loading are discussed.Subsequently,the influence of geometry parameters on the Poisson’s ratios and the equivalent compressive modulus of the structure in each main direction are analyzed based on the verified theoretical analysis model.The results show that when the struts are slender enough,the bending of the struts play decisive role on the deformation of the structure and other mechanisms can be ignored;while when the struts become relative stubby,all the mechanisms including bending,shearing and axial loading need to be considered;The often-ignored axial extension or compression term may even play decisive role on determining the lateral Poisson’s ratio of the structure when the struts are relative stubby.Then,the interlocking assembly method is extended to the manufacture of composite 3D auxetic cellular structures.Carbon fiber-reinforced polymer(CFRP)composites 3D re-entrant auxetic cellular structures are successfully manufactured.Uniaxial compression tests are performed on the test specimens,through which the auxetic nature of the CFRP composites 3D re-entrant auxetic cellular structures are confirmed.In addition,the influence of the variation of the re-entrant angle on the Poisson’s ratio and the equivalent compressive modulus of the composites 3D re-entrant auxetic cellular structures are studied by finite element simulations.The results show that the auxetic character of the composite 3D re-entrant structure is more significant than the steel structure.The specific stiffness of the composite 3D auxetic structure is comparable to(when the re-entrant angle is relative large)or higher than(when the reentrant angle is relative small)the metal(steel)structure.Finally,a stacking assembly method for fabricating 3D double-arrow-head auxetic structures is proposed.Based on the stacking assembly method,a series of composite 3D double-arrow-head auxetic structure samples are successfully manufactured.The mechanical performances of the structures are studied by uniaxial compression test.In order to further study the mechanical properties of the structure,finite element simulations and theoretical analysis are also conducted.The influence of structural geometric parameters on the Poisson’s ratios and the equivalent compressive modulus of the structure are given.The results show that the structures display nonlinearity in their elastic range,with the increase of compression strain,the structures become more auxetic and their effective compression modulus also increase.The specific stiffness of composite 3D DAH structures made from CFRP are much higher than that of the metal 3D DAH structures.