Study On Constitutive Model Of Shape Memory Polymers And Applicatinos In Analyses Of Intelligent Structures

Shape memory polymer(SMP)is a new class of polymer material with shape memory effect that can change and fix its shape.When subjected to a specific stimulus,it can recover its original shape reversibly.SMP has the characteristics of large deformation,high recovery rate,easy to shape and to adjust the response temperature and a variety of incentives(heat,humidity,light,electricity,magnetic)and other characteristics,thus becoming one of the most popular functional materials.SMP has broad application prospects in various areas such as complex sensing devices,biomedicine,aerospace and other smart devices.The constitutive theory is a mathematical model reflecting the mechanical properties of materials.It plays an important role in the evolution of memory behavior and performance characterization of SMP.This thesis mainly focuses on constitutive theory,finite element method and smart structure(space expansion shell,intelligent reversible diaphragm and SMP vascular stent)of SMP.The detailed contents are as follows.(1)A three-dimensional thermodynamic viscoelastic constitutive model of SMP was established and implemented by finite element method.The total strain of the one-dimensional constitutive model of SMP was decomposed into viscoelastic strain,reversible strain and thermal strain,and the equation expressions of each strain were given.Subsequently,the one-dimensional constitutive equation was extended to three-dimensional form and finite element subroutine(UMAT)that can be called by ABAQUS was encoded.The finite element subroutine was used to simulate the tensile test of SMP.The numerical results are in good agreement with the experimental data.The correctness of the constitutive equations and the finite element subroutine were verified.(2)Based on the finite element subroutine,the folding deformation and shape memory effect of spatially deployable SMP shell were studied,and the SMP shell was optimized.The forward/backward folding and auto-expanding process of the SMP shell structure were simulated.The influence of structural parameters and temperature on the folding behavior was analyzed.The forward and reverse folding processes of metal/SMP composite shell were investigated.Based on full factorial design method,an optimized model of SMP shell was established with the maximum expansion drive performance as the objective function,and structural parameters as variables of design under the instability bending moment constraints.The optimization model was solved to obtain the SMP shell structure with the best driving performance.(3)An intelligent reversible SMP diaphragm structure was studied.Based on the characteristics of SMP large deformation and shape memory,an intelligent reversible diaphragm was designed.The finite element simulations of the overturning process and recovery effect of the SMP diaphragm were performed.The pressure and strain energy during the overturning deformation process were obtained.The influence of geometric parameters and temperature on the overturning behavior was analyzed.Based on the response surface method,an optimized model was established with minimum overturning pressure as target function and thickness,height,radius and temperature as variables of design under the critical pressure constraints.These studies provide theoretical guidance for the design and application of new SMP diaphragm.(4)A three-dimensional thermodynamic constitutive equation of SMP and a finite element subroutine for finite deformation of SMP were developed.Based on the previous constitutive model,the volume fraction calculation formula was improved,so that the conversion process of glass-rubber phase can be described more rationally.By using obtained three-dimensional finite deformation constitutive equations,a UMAT subroutine was developed and the deformation and shape memory behavior of SMP during stretching and bending were simulated.The validity of the constitutive theory and the finite element program was verified.The constitutive model requires less material parameters and the finite element program is suitable for engineering applications.(5)The finite deformation of SMP mesh and helical vessel stents were modeled.The contraction and self-expansion process of these vascular stents were simulated,and the process of mesh stents expand blood vessels was discussed.The present constitutive theory and finite element subroutine for the finite deformation can effectively describe the shape memory behavior of SMP structures.