Mechanical And Shape Memory Behaviors Of Viscoelastic Snapping Structures

Shape memory polymer(SMP)as a kind of smart material,has the capability to respond to the environment actuation,and recover to its original shape.Recently,it is possible to fabricate complex structures based on SMP,such as spherical shell,triangular and hexagonal honeycomb,thanks to the generation of 4D printing method,which further broaden the application field of SMP.It is well recognized that spherical and honeycomb are typical structures with negative stiffness and snap-through instability,allowing large deformation between different configurations.So novel behaviors and phenomena in mechanical responses and functional application brought by the combination of nonlinear material and nonlinear structures are worth investigating.In the nonlinear structures based on SMP,the inherent viscoelasticity of SMP could contribute to time-dependent or rate-dependent performance,which allows the system to response to the external stimulation and be programmable besides the original bearing capability and mechanical behaviors.The material nonlinearity has profound influence on structural behaviors,such as the structural stability.In view of the background,the purpose of this paper is to combine SMP with some nonlinear structures that have negative stiffness or snap-through instability.The pure viscoelasticity of SMP under constant temperature is studied,and the temperature-dependent constitutive relations are derived.Together with the nonlinear strain-displacement relation and the equilibrium equations of the system,the mechanical responses of complex structures based on SMP are predicted including the bistability and snap-through instability.First,this work analytically,numerically and experimentally investigates the effects of material viscoelasticity on snap-through instability and behaviors in different stability regimes.A discrete model of viscoelastic dome is developed with standard linear solid(SLS)springs,which establishes analytical understanding of the role viscoelasticity played in pseudo-bistability and provide quantitative prediction of different stability configurations.Sensitivity analysis is carried out based on the discrete model to quantify the relative importance of input variables including geometry,relative stiffness and relaxation time in determining the output value of recovery time via Sobol’s variance-based method.We perform finite element analysis using Generalized Maxwell-Wiechert model fitted with relaxation test data to characterize the force and energy responses of dome snap-through instability.Finally,experimental validation is conducted on silicone rubber with different viscoelasticity and compared with finite element analysis,showing good agreement with each other.Then,we examine the spherical viscoelastic domes with predesigned geometric imperfection that can tune the structural stability and control snap time without external triggers.Finite element model is established using generalized MaxwellWiechert material model fitted with relaxation test data to characterize the effect of imperfection on force responses and recovery performance of spherical dome.Experimental validation is conducted on silicone rubber dome with the geometry and magnitude of the defect precisely designed and fabricated through 3D printed mold,which is compared with finite element method(FEM),showing good agreement with each other.The results show that even small magnitude of imperfection could play a significant role in pseudo-bistability,and positive and negative imperfections affect the snap time towards to different directions.A discrete model of viscoelastic dome is developed with standard linear solid(SLS)springs to explore analytical understanding of the geometric role of imperfection in both instantaneous and dynamic behaviors and provide explanations for FEM and experiments.Further,this work investigates the anisotropic characteristics of honeycombs caused by different infill strategies via both experimental and theoretical methods,which are known to have negative stiffness and typical snap-through instability.Uniaxial tensile tests and compressive tests are performed to study the effect of infill patterns on mechanical properties.Classical laminate theory and honeycomb equivalent modulus theory are modified to predict the parameters of straight-line and polygonal patterns,respectively.Generalized Maxwell-Wiechert model is applied to describe the viscoelasticity of each printing pattern and Prony Series are fitted through relaxation tests to provide references to implement in Abaqus.For triangular and hexagonal honeycombs based on SMP and typical viscoelastic material,general discrete model is developed to predict the deformation and recovery processes.The discrete model separates the bending effect and stretching effect,which could describe the mechanical behaviors of both triangular and hexagonal elements.Results shows that the bearing capability decreases due to the viscoelasticity,and the structure shifts to monostable direction with the increase of relaxation coefficients,altering the structural stability.The phase diagram includes bistability,monostability,S shape,and pure monotonous type.SMP behaves differently when exposed to different temperature conditions under pinned boundary condition,achieving the control of recovery step by temperature.Finally,based on the arch shape that has the snap-through instability,the details of an integrative hinge fabricated by carbon fiber reinforced shape memory epoxy composites in the sequence of material selection,structure design and manufacture,material and structure experiments,and application are presented.