Study On The Constitutive Models Of Shape Memory Polymers And Dynamic Behaviors Of A Shape Memory Polymer Membrane

Shape memory polymers(SMPs)are a kind of new smart material that can change from temporary shape to permanent shape(original state)under appropriate stimulation(such as heat,light,electricity,magnetism,etc.).At present,thermal stimulation is the most common stimulation of shape memory polymer and most of the light and magnetic stimulation are finally converted into heat to stimulate shape recovery.Thus,this paper mainly studies the shape memory behaviors(SMBs)of thermally induced SMPs.As a new type of smart material with great application prospect,its deformation mechanism and application design are under-researched.Therefore,the deformation theory and application design of thermally induced SMPs are studied in this paper.For theoretic research,the constitutive models for different types of SMPs were established by using viscoelastic theory and phase transition theory.We then implemented the proposed models by user defined subroutine UMAT into ABAQUS software to study the general deformation behavior of SMPs.We further propose another two more reasonable constitutive models by combining the viscoelastic modeling with the phase change modeling.For application research,we study the dynamic behavior of SMP membrane to solve the dynamic problems that may be encountered in the application of space deployable structure.Firstly,a three-element model with simple form is proposed and implemented into UMAT subroutine to investigate the thermo-mechanical small strains behavior of polyurethane.To validate the model,simulated and predicted results are compared with Tobushi’s experimental results and good agreement can be observed.Secondly,we establish a new phase-evolution-based thermomechanical constitutive model for amorphous SMPs by considering the materials as a mixture of the rubbery phase and glassy phase.The shape memory effect(SME)is captured under the assumption that the rubbery phase can transform into the glassy phase and that part of the strain will be frozen during the glass transition.To make the model be more feasible,furthermore,we improve the model by introducing a time factor and considering the influence of frozen strain release rate.To validate the robustness and applicability of the proposed new model,we reproduce the shape memory behaviors(SMBs)of two different materials under different constraints and conditions.The results show a remarkable consistency between the new model simulation and experimental data.Furthermore,we have developed the UMAT subroutine to study the SMBs of approximately pure elastic SMPs.Thirdly,based on the formation and disappearance of a reversible phase,we propose a novel phase-transition-based viscoelastic model including the time factor for SMPs,which has a clearer physical significance.To describe the phase transition phenomenon of SMPs,our new model defines different constitutive structures for above and below transformation temperature separately.As the proposed viscoelastic model is based on phase transition,it can not only be used for different types of SMP materials,but also can be used to treat large strain problems.Forthly,to establish a more reasonable constitutive model,we propose a new model of SMPs combining phase transition and viscoelasticity.The SMP materials are treated as a mixture of rubbery phase and glassy phase.Thus,it can physically relate the SME to glass transition.As the proposed model is also based on viscoelastic theory,it can successfully characterize the viscoelastic properties and predict the rate-dependent shape memory response of SMPs.By ignoring the viscosity of the material,the proposed model could be degenerated to a purely elastic phase transition model.We further investigate the influence of strain rate and heating rate on SMBs.Analysis of the results show that the proposed model possesses an evident advantage in studying the rate-dependent response of SMPs.Especially in the cooling process of shape memory cycle,the constitutive model proposed in this work can fully embody all four effect factors on stress evolution.Lastly,based on a thermomech anical constitutive model and the Euler–Lagrange equation,we establish a dynamic model to study the dynamic behaviors of an SMP membrane in isothermal and variable temperature processes.We also investigate the influence of dynamics on SMBs of SMPs.These results and discussions may provide guidance for exploring the vibration and dynamic performances of SMPs aerospace deploy structures.