Research On Constitutive Models Of Thermally Activated Amorphous Shape Memory Polymers And Their Composites

As a promising class of smart materials,shape memory polymers(SMPs)and their composites(SMPCs)have drawn considerable attention in recent years for their broad applications in many fields including morphing aircraft,self-deployable structures in spacecraft and intelligent medical devices.Therefore,it is critical to develop the constitutive models that can be used to describe and predict their shape memory effect(SME).The intended functions of the SMPs and SMPCs can be realized by the SME which can be triggered by a stimulus such as temperature,light,solution,electric field or magnetic field.The thermally activated amorphous SMPs and SMPCs have great potential in many aspects for their relative simple trigger mechanism.Currently,modeling efforts generally have lagged those for the development of new polymer systems and devices.In fact,the precise constitutive model may have a significant effect on comprehending and controlling of the SME of SMPs and SMPCs.The main purpose of this study is to develop new constitutive models for these materials by using the phase transition approach and the viscoelastic approach.The main contents are as follows:1.A phenomenological constitutive model for SMPUs based on the phase transition approach is proposed.SMPUs are a subclass of SMPs that can recover a finite pre-deformed shape in response to thermal stimuli,due to the combined action of hard and soft segments in the molecule.Affine network models are used to describe the mechanical behaviors of the soft segment phases.Further,a new four-element recovery model is constructed for the hard segment phase in consideration of the structure change during the recovery process.In addition,as SMPUs are temperature-sensitive materials,the influence of temperature on the parameter viscosity is investigated.2.A thermoviscoelastic-themoviscoplastic finite deformation constitutive model incorporating structural and stress relaxation is developed for thermally activated amorphous SMPs.In this study,the nonlinear Adam-Gibbs model is used to describe the structural relaxation.Further,a modified Argon scalar equation is proposed for the viscous flow as the temperature crosses the glass transition temperature(Tg).In addition,Arruda-Boyce eight-chain model is used here to capture the hyperelastic behavior of the SMPs up to large stretches.To further feature the macroscopic post-yield strain softening behavior,the phenomenological evolution rule is also implemented.3.A thermoviscoelastic modeling approach is developed to predict the recovery behaviors of the thermally activated amorphous SMPs based on the generalized finite deformation viscoelasticity theory.In this paper,a suitable series of moduli and relaxation times of the generalized Maxwell model is estimated from the stress relaxation master curve by using the nonlinear regression(NLREG)method.The hyperelastic response of the materials is well modeled with a hyperelastic model in Ogden form.For simplicity,only the Williams-Landel-Ferry(WLF)equation is used to describe the horizontal shift factor obtained by using the time-temperature superposition principle.Moreover,the possibility of developing a temperature-responsive stent using the material in this study is investigated in the thermomechanical aspect and the SME of a cantilever beam made of this material is studied.4.A multi-branch thermoviscoelastic-themoviscoplastic finite deformation constitutive model incorporated with structural and stress relaxation is developed for a thermally activated SMP based syntactic foam.In this paper,the total mechanical deformation of the foam is divided into the components of the SMP and the elastic glass microballoons by using the mixture rule.The nonlinear Adam-Gibbs model is used to describe the structural relaxation of the SMP as the temperature crosses the Tg.Further,a multi-branch model combined with the modified Eying model of viscous flow is used to capture the multitude of relaxation processes of the SMP.The deformation of the glass microballoons could be split into elastic and inelastic components.In addition,the phenomenological evolution rule is implemented in order to further characterize the macroscopic post-yield strain softening behaviors of the syntactic foam.Moreover,a parametric study is conducted to examine the predictability of the model and to provide guidance for reasonable design of the syntactic foam.5.A unified constitutive model is developed for both the SMPs and the SMPCs by using the multiple relaxation viscoelastic modeling approach in the Abaqus from the macroscopic aspect.A novel method for the determination of the relaxation Young moduli and relaxation times of each branch is proposed,in which the parameters can be determined by fitting the dynamic mechanical analysis(DMA)experiment with a single frequency.Obviously,the method can raise efficiency in the work of parameters determination.For temperature above the reference temperature,the horizontal shift factor can be calculated by the WLF equation,and for temperature below the reference temperature,the horizontal shift factor can be calculated by the Arrhenius equation.The hyperelastic response of the materials is well modeled with a hyperelastic model in Arruda-Boyce form which is also used as the reference elastic model in Abaqus.