Design And Properties Of Negative Stiffness Mechanical Metamaterials

As an important branch of metamaterials,negative stiffness metamaterials show great application potential in many fields,such as energy absorption,vibration control,deployable structures and actuators owing to their unique mechanical properties.However,many problems have also been exposed during the development of negative stiffness metamaterials,such as weak mechanical properties,apparent direction dependence(most of them exhibit negative stiffness behavior only under uniaxial tension and compression),and single structural form(mostly plane structure),etc.To address above problems,this study conducted the following research from the aspects of structural design,mechanism innovation and optimization methods:First,we investigated the mechanical properties of the beam structures with different configurations and the influence of the variable cross-section design using the parameter scanning analysis method.We draw the geometric parameter boundary of positive stiffness,negative stiffness,and bi-stable regions for these beam structures,and compare their mechanical properties as well as energy trapping capacity.Research results indicate that the curved beam and the tilted beam respectively show better performance in different parameter regions,and are both more suitable than the arched beam in constructing the metamaterials.The variable cross-section design can be applied to tune the basic mechanical properties and shift the boundary for various mechanical behaviors.Secondly,considering that the above beam type negative stiffness mechanical metamaterials can only achieve negative stiffness behavior in the uniaxial loading direction,a shear induced multi-stable mechanical metamaterial based on multi-magnet system is proposed inspired by the interaction between magnets.The multi-stable mechanism and energy trapping principle of the multi-magnet system are demonstrated by theoretical method,and the influence of the configuration of the multi-magnet system on the mechanical properties of the metamaterial is systematically studied.The configuration of multi-magnet system with the best energy absorption property under end shear loading is determined.In order to overcome the direction dependence of negative stiffness metamaterials(i.e.,negative stiffness behavior can only be achieved in a single loading direction or in a single loading mode),combined with the truncated conical shell elements,a novel tridirectional negative stiffness mechanical metamaterial is proposed,and a semi-empirical and semi-theoretical model for predicting metamaterial’s local and global response is established.Based on the above results,the feasibility of all-directional negative stiffness metamaterials(with compression and shear induced negative stiffness behavior)is discussed theoretically,and a design concept of all-directional negative stiffness mechanical metamaterial based on negative rational stiffness element is presented.Research results demonstrated the assumption of three-dimensional negative stiffness metamaterials.It is found that the all-directional negative stiffness mechanical metamaterials can exists under ideal conditions,and the presented mechanical metamaterial based on negative rational element can infinitely approach all-directional negative stiffness property.Finally,to deal with some special working conditions,a novel negative stiffness cylindrical shell structure,different from the above-mentioned plane negative stiffness structure,is proposed.The static and dynamic mechanical properties of the proposed structure are studied through theory,experiment and simulation.The impact response characteristics of the novel structure are described.Based on the above results,a universal performance optimization scheme of negative stiffness mechanical metamaterial is proposed,and the optimization scheme is proved in detail.The results show that the negative stiffness cylindrical shell structure has excellent cushioning performance and the introduction of filler can effectively improve the performance of negative stiffness mechanical metamaterials.