The Thermomechanical Tests And Phase Transition Constitution Model Of Epoxy Shape Memory Polymer

Shape Memory Polymers(SMPs),a new type of smart materials,have become the focus of attention of researchers in recent years because of their various driving modes,strong deformation ability,low density,high recovery and easy processing.At the same time,shape memory polymers have very strong development prospects in many fields(such as aerospace)for their unique shape memory characteristics.Therefore,the research on shape memory polymer materials will greatly promote the development of intelligent structures in various fields.Based on the application prospects and economic benefits of this kind of materials,the mechanical behaviors and constitution model of shape memory polymers represented by epoxy were studied systematically in this thesis.The details are as follows:Firstly,the shape memory properties of epoxy-based shape memory polymers were tested systematically.(1)The shape memory recovery of SMP under tensile,bending and torsion conditions was tested.The shape memory fixation rate and recovery rate of SMP under different deformation conditions were measured;(2)In this thesis,the free and constrained recovery processes of different pre-strain under tensile deformation are further developed.The variation of the recovery strain with temperature and the recovery stress with temperature during constrained recovery are obtained;(3)In order to study the effect of stress on the critical temperature of shape memory polymer,three groups of temperature-increasing loading tests under constant load were carried out,which provided the experimental basis for the constitutive model of material.Secondly,the research about the thermodynamic properties of epoxy based shape memory polymers has been carried out.(1)The phase transition temperature of the epoxy-based shape memory polymer was measured by dynamic thermomechanical analyzer,and relationship of the tangent value,the storage modulus,and loss modulus of the material varied with temperature was measured too.(2)The strain rate tests,creep tests and relaxation tests of epoxy-based shape memory polymer at different temperatures were carried out by using electronic universal testing machine.The thermodynamic properties of epoxy-based shape memory polymer at different temperatures were studied.Then,based on the new phase transition model proposed by predecessors,the shape memory properties of shape memory polymers during thermodynamic cycling are described in depth,and the variation of volume fraction of glass and rubbery phases with temperature is explained accurately.On the basis of the new phase transition model,a constitutive model based on the classical viscoelastic theory and free energy theory is proposed.The model can not only describe the volume fraction of glass and rubbery phases during phase transition,but also characterize the viscoelastic and shape memory properties of shape memory polymers.In the new phase transition model,the shape memory polymer material includes glass phase mechanical strain,rubbery phase mechanical strain,storage strain and thermal expansion strain.Among the model,the variation of storage strain with temperature in the stage of shape fixing and temperature recovery is the key points discussed in this thesis.Based on the viscoelastic theory and the rubbery free energy theory,the stress-strain relationship of materials in different stages of typical thermodynamic cycling process is given.Finally,based on the new phase transition constitutive theory,the thermal cycling shape memory process and the strain response of SMP under constant load are simulated and analyzed.The rationality and accuracy of the new phase transition model are verified by comparison with the experimental results.