Design,Fabrication And Mechanical Properties Of 4D Printing Mechanical Metamaterials

Mechanical metamaterials were complex artificial structures composed of fine designed periodic arrangement of cells,and exhibited mechanical properties that conventional materials did not have,such as auxetic(i.e.negative Poisson’s ratio),tension-torsion coupling deformation,negative thermal expansion,multi-stability,etc.The diversity of structural configurations and high designability of mechanical properties of mechanical metamaterials helped to develop advanced functional materials/devices and apply them in aerospace,biomedical devices,flexible robots and other fields.However,the extraordinary mechanical properties of mechanical metamaterials originated from the conformation/arrangement of the cells,which meant that the conformation and arrangement of the cells were fixed and cannot be changed after the metamaterials were fabricated.Therefore,the mechanical properties of metamaterials were fixed after preparation,and the configuration/mechanical properties lacked adaptive and reconfigurable characteristics.The emergence of smart materials with stimulus response properties(e.g.,shape memory polymers and their composites,hydrogels,liquid crystal elastomers,etc.)and the rapid rise of 4D printing technologies have led to the development of active mechanical metamaterials with reconfigurable and adaptive properties.4D printing was a combination of 3D printing and stimulus-responsive materials.The printed objects can adaptively change the configuration/properties under external stimuli(thermal,magnetic,electrical,etc.).Shape memory polymer(SMP)was a type of stimulus-responsive material that had the ability to retain a temporary shape and recover to its original shape.The mechanical metamaterials fabricated by SMP exhibited the characteristics of reconfigurable configuration,adjustable mechanical properties and adaptive,which further improved the design freedom of mechanical metamaterials.The ligament configuration of the existing ligament type chiral mechanical metamaterials was linear,which led to the single geometric configuration and mechanical properties of this type of metamaterials.In addition,the linear ligament configuration also limited its deformation ability,and the lack of reconfigurability of metamaterials restricted its design freedom.In this background,this work proposes the design,fabrication and mechanical properties of 4d printing mechanical metamaterials,aiming to address the limitations of the weak configuration/mechanical reconfigurability properties of mechanical metamaterials and improve their design freedom.The specific contents were as follows:(1)To address the above limitations of chiral metamaterials,wave ligaments were introduced into the hexa-and tetra-ligament chiral structures inspired by the conformations and deformation mode of collagen fibers,and combined with 4D printing technology to develop tetra-and hexa-ligament chiral metamaterials(twodimensional structures)with arc-shaped and crescent-shaped ligaments.Based on the energy method,the effective mechanical prediction models for hexa-and tetraligament chiral structures with arbitrary ligament configurations were developed,and the effects of geometric parameters(e.g.,ligament circular angle,circular diameter,etc.)on the effective Poisson’s ratio of the metamaterials were investigated.The large deformation behavior of the designed mechanical metamaterial was characterized by the combination of experimental and finite element methods,and the effects of geometric parameters and strain on the nonlinear stress-strain relationship and Poisson’s ratio-strain relationship of the metamaterial were analyzed.The shape memory performance,reconfigurable properties of geometric configuration and mechanical properties of the metamaterial were characterized.During the stretching process,the deformation mode of the designed two-dimensional auxetic chiral metamaterial transformed from bending-dominated to stretch-dominated,thus showing "J"-shaped stress-strain curves similar to that of biological tissues,demonstrating its promising application in biomaterials,flexible electronics.(2)The configuration of the tetra-chiral mechanical metamaterial with arcshaped and crescent-shaped ligaments(two-dimensional)from Content 1 was extended to study its mechanical properties under compressive loading.The TPE/PLA-SMP composite chiral metamaterials were prepared by combining a soft material(thermoplastic elastomer,TPE)with a hard material(shape memory polylactic acid,PLA-SMP)based on the multi-material 3D printing technology.The temperature-dependent properties of TPE,PLA-SMP and interfacial bond strength were experimentally characterized.The effects of structural geometric parameters,soft material distribution direction,and soft material content on their nonlinear mechanical properties were investigated by the combination of finite element method and experiment.Chiral metamaterials with gradient distribution of ligament configuration were designed,and the effects of ligament arc angle,soft material distribution direction/content and loading direction on the mechanical properties of the metamaterials were investigated.(3)Three-dimensional chiral structures have tension-torsion coupling deformation properties,while they usually consist of periodically interconnected linear ligaments and circular nodes.This design mode limited the tensile and tensiontorsion coupled deformation ability of metamaterials.it also reduced the maintainability of the metamaterials.To address this limitation,this work introduced wavy ligaments inspired by collagen fibers into three-dimensional chiral structures to design tensile-torsional coupled mechanics metamaterials(three-dimensional structures).Inspired by the array mode of hedgehog spines,the internal redundant constraints in the tension-torsion coupled mechanical metamaterials were removed,and pixel mechanical metamaterials with uncoupled constraints between array individuals(i.e.,mechanical pixels)were developed.Similar to screens that adjusted images by changing the color of pixels,pixel mechanics metamaterials adjusted their mechanical properties by changing the geometric configuration of the mechanics pixels.The model predicting the tensile-torsional effect of the mechanical pixel was developed,and the effect of geometric parameters on its tensile-torsional effect was investigated.The large deformation behavior of the mechanical pixel was characterized,and the effects of geometric parameters and strain on its nonlinear stress-strain and torsional angle-strain were investigated.The shape memory properties,reconfigurable of conformation and mechanical properties of mechanical pixels were characterized,and the mechanical pixels were integrated into pixel mechanics metamaterials.In addition,the prospects of the designed pixel mechanics metamaterials in biomaterials,information encryptions,kinematics controllers,and protection devices were demonstrated.