Analysis Of The Pull-in Behavior Of Viscoelastic Microbeam And Microplate With Small Scale Effect

Microbeam or microplate is one of the core components of micro-electro-mechanical system(MEMS).In general,Geometry size of MEMS devices in micron or sub-micron,in metal materials,composite materials,polymer materials and silicon materials of small scale experiments have confirmed that: in the micron and submicron scale,micro component mechanical performance has obvious size effect,and the size effect phenomena is the classical continuum mechanics can't explain.Therefore,it is very important to develop and perfect the theory and model that can explain and describe the phenomenon of size effect of microstructure.Many scholars have studied the mechanical behavior of microstructure,and the microstructure of elastic materials has been fully studied,mainly from experimental methods,numerical simulation,micro-scale theory and other methods to study the mechanical properties of microstructure.Moreover,many research results show that the material parameters of some materials used in fabricating microstructures show strongly time-dependent viscoelasticity.Meanwhile,some MEMS devices such as sensors work in a complex coupling field environment including such as flow field and electric field.In this thesis,the viscoelastic continuum theory,modified couple stress theory,Euller-Bernoulli beam theory and Kirchhoff plate theory are used to investigate the governing equation of electrically actuated viscoelastic microbeams/microplates with considering the squeeze film damping.The specific contents of this thesis are shown as follows:First,the effect of the flow field on the microbeam structure in the fluid-solid coupling model is simplified as the pressure film damping force.Based on the theory of viscoelasticity,Euller-Bernoulli beam theory and the modified couple stress theory,the dynamic governing equation of the microbeam with the film damping was derived.The Galerkin method and Runge-Kutta method were used to solve the equation numerically.The phenomenon of static pull-in under direct current(DC)voltage was analyzed and the influence of damping force on the pull-in time was discussed.The fringing field effect has significant influence on the pull-in time and pull-in voltage of the microbeam structure.The assumption of small deformation will cause the stiffness of the microbeam structure to be smaller than the actual stiffness,which will lead to the decrease of the pull-in voltage and the pull-in time.Influence of damage effect on absorption time and voltage of structure has been investigated.Based on viscoelasticity theory,Kirchhoff plate theory and modified couple stress theory,the equation governing the dynamics of viscoelastic microplates were derived.The Galerkin method together with the Runge-Kutta method was employed to solve the equation numerically.The pull-in behaviors of viscoelastic microplates under the action of DC voltage were analyzed.The effects of aspect ratio and the material length scale parameter on the pull-in time and the pull-in voltage were discussed.The influence of small deformation hypothesis on structural stiffness is studied.The influence of fringing field effect on the structure of microplates is discussed and it is found that the fringing field effect has less influence on the structure of microplates than the structure of microbeams.The influence of damage factors on the pull-in characteristics of structures was analyzed.