Structure Design And Mechanical Behavior Of Programmable Mechanical Metamaterials

Metamaterials are artificial composite materials or composite structures that have extraordinary,abnormal,controllable,and programmable physical properties that natural materials do not possess.Their performance depends on reasonably designed novel microstructures and appropriately selected constitutive materials.The concept of metamaterials has opened a door to exploring the specific mechanical properties of materials,greatly expanding the design space of advanced materials,breaking the cognitive limitations of humans on the physical properties and apparent natural laws of traditional materials,and has important research significance for basic science,engineering applications,and even people’s daily life.In addition,the improvement of basic mechanical properties of structures such as stiffness and strength cannot meet the development of modern high-tech,and there is an urgent need for advanced materials that combine lightweight and other excellent properties to meet the requirements of multifunctional integration.Mechanical metamaterials,with their extraordinary,abnormal,designable,controllable,and programmable physical properties,have unparalleled engineering application potential compared to traditional materials,and are bound to become the focus of multifunctional materials research.Based on this,this paper realizes the controllability and programmability of single and multiple equivalent mechanical parameters such as Poisson’s ratio,stiffness,and coefficient of thermal expansion through microstructure design.The stimuli-responsive mechanisms are introduced and their basic mechanical properties are studied through theoretical analysis,numerical simulation,and experimental verification.A series of novel mechanical metamaterials are prepared by combining various additive manufacturing techniques and interlocking assembly methods,exploring their potential in multifunctional applications such as impact resistance and energy absorption,sensors,actuators,and soft robots.The main work includes the following:Firstly,based on the 2D concave double-V straight beam negative Poisson’s ratio structure,a 3D concave double-U curved beam negative Poisson’s ratio lattice was designed and fabricated with high-performance stainless steel.The Euler-Bernoulli beam theory,plastic hinge theory,and momentum conservation theorem were used to predict the elastic constants as well as the plateau stresses under quasi-static and dynamic crushing of the 3D lattice.The quasi-static mechanical properties were studied through uniaxial compression tests,and the quasi-static and dynamic crushing processes were numerically simulated using finite element analysis software.The effects of geometric parameters and number of unit cells on the equivalent Poisson’s ratio and Young’s modulus of the structure were studied.The effects of strain hardening of the constitutive material,external impact velocity,initial configuration of the structure(concave negative Poisson’s ratio configuration and convex positive Poisson’s ratio configuration),and gradient design(member thickness and structural density gradient)on the collapse response of 3D double-U lattices were analyzed.The impact resistance and energy absorption properties of cellular lattice structures with different Poisson’s ratios and geometric gradients under different impact velocities were compared.Secondly,based on a single-material 2D concave double-V negative Poisson’s ratio structure,a thermal stimulus-responsive mechanism was introduced,and two polymer materials with different glass transition temperatures and temperature dependencies were designed and fabricated to have a thermally programmable mechanical response capability.The effects of the relative stiffness of the constitutive materials on the equivalent Poisson’s ratio,Young’s modulus,deformation mode,and energy absorption performance of the 2D concave double-V bimaterial structure were studied.A design method for adjusting and programming the elastic constants,collapse response,and energy absorption performance of the bimaterial lattice structure through temperature change was proposed.In addition,based on multiple materials with different thermal expansion coefficients and elastic moduli,a series of bimaterial 3D double-V specimens were prepared using a combination of single-material 3D printing and interlocking assembly,achieving common control of the equivalent Poisson’s ratio and thermal expansion coefficient.In addition,the mode superposition method was used to theoretically analyze and compare the snapping mechanisms of the compressed buckled beam and the original curved beam,and a von Mises truss model with the same mechanical response was proposed and studied.Based on the original curved beam,single-material 2D,3D,and cylindrical double-U 1D multistable snapping structures and cylindrical star-shaped 2D self-recovering snapping structures were designed.The effects of geometric parameters and gradient design on the tensile properties of 2D double U snapping structures were studied through theoretical analysis,numerical simulation,and experimental methods.The effects of geometric parameters,topological configuration,number of circumferential unit cells,and number of layers on the mechanical properties of cylindrical snapping structures were analyzed.The influence of the series effect of discrete snapping elements on the stable configurations,snapping order,and energy absorption performance of self-recovering snapping and multistable snapping structures was presented.Then,based on the fully symmetric bistable snapping mechanism of the compressed buckled beam,single-material 3D printing and interlocking assembly method were combined to design and prepare the multimaterial 1D multistable zero Poisson’s ratio 2D and 3D structures,multimaterial 2D multistable zero Poisson’s ratio 2D structures,bimaterial 3D multistable zero Poisson’s ratio 3D structures,bimaterial bistable negative Poisson’s ratio structures and angle-dependent bimaterial snapping structures.The effects of material gradient,geometric gradient,and topological configuration on the snapping process of multistable structures were studied.The sequence and programmability of the snapping process of multistable structures were realized by combining geometric gradient and material gradient.A discrete chain model with gradient snapping elements that had the same mechanical response as multistable snapping structures was proposed and studied using the Arc length method.Based on the shape reconfiguration of bistable/multistable structures,the design of 1D,2D,and 3D adjustable thermal expansion coefficients was proposed.The range of adjustable positive/zero/negative thermal expansion coefficients of 1D,2D,and 3D multistable zero Poisson’s ratio structures and bistable negative Poisson’s ratio structures was studied through theoretical analysis and numerical simulation.Finally,based on the asymmetric bistable snapping mechanism of the original curved beam and bimaterial design,the thermal stimulus-responsive mechanism was introduced,and two polymer materials with different glass transition temperatures and temperature dependencies were used to design and prepare bimaterial lattice structures with robust and simplified shape memory effects.A discrete chain model with the same mechanical response as the bimaterial lattice structure was proposed,and the stability transition mechanism,stimulus-responsive mechanism,and performance regulation mechanism of this novel metamaterial were analyzed by using the Arc-length method.The programmable multistep shape reconfiguration,superelasticity,extreme large thermal deformation,and shape memory ability of the bimaterial lattice structures were studied by numerical simulation and experimental validation.A series of multifunctional intelligent lattice structures that can respond to temperature,light,humidity,and other external stimuli were proposed,and their potential applications in the fields of sensors,actuators,and flexible robots were explored.