Constitutive Model And Space Application Verification Of Epoxy-based Shape Memory Polymer Composite

Shape memory polymers(SMPs)and their composites(SMPCs)are new types of smart materials that can be actively deformed by external stimuli.They have characteristics of shape memory,active and controllable large deformation,variable stiffness,etc.They can be designed and manufactured as components that integrate driving and bearing capacities.Such components possess simple and reliable structures,and have great prospects in space deployable structure,expecting to partially replace complex mechanical structures.In this dissertation,a series of studies has been conducted on the mechanical behaviors of epoxy-based SMP and its fiber-reinforced composite,including the characterization of the thermo-mechanical properties of materials,establishments of the constitutive model of SMP and the elastic constants prediction model of fiber-reinforced SMPC,and development of a sunlight-stimulated SMPC substrate.First of all,since the shape memory behavior characterization of existing thermosetting SMPs and their composites are mostly in small deformation and have not been characterized by the constrained stress recovery performance.Therefore,three types of shape memory behavior characterization experiments are performed on epoxy-based SMP with different pre-extensions and unidirectional carbon fiber-reinforced epoxy-based SMPC with different fiber fractions,including free recovery,constrained deformation recovery and constrained stress recovery,to obtain temperature-stress-strain data.During the free recovery process,the recovery ratio-temperature curve is in a hyperbolic tangent form;during the constrained deformation recovery process,the maximum recovery stress is proportional to the amount of pre-deformation and fiber fraction;during the constrained stress recovery process,the maximum recovery ratio has a linear relationship with the value of external force/maximum recovery force.In addition,the glass transition temperature,storage modulus and thermal expansion coefficient of the epoxy-based SMP are obtained by using the dynamic mechanical analyzer.The flexural modulus and strength of the unidirectional carbon fiber-reinforced epoxy-based SMPC at different temperatures are characterized by the three-point bending experiment,indicating that the modulus and strength are temperature-dependent.Besides,the cyclic loading and unloading experiment at 393 K of the unidirectional carbon fiber-reinforced epoxy-based SMPC is carried out,resulting in the loss factor and stiffness loss decrease rapidly in the first three cycles.The above thermo-mechanical characterizations provide necessary material parameters for the subsequent theoretical modeling.Secondly,the existing SMP constitutive models only focus on the free recovery and constrained deformation recovery,but the material is subjected to external loads in practical applications,and there is currently no research on constrained stress recovery.This dissertation proposes a constitutive model of SMP based on phase transition and viscoelasticity theory.The SMP is composed of the rubbery phase and the glassy phase,and the volume fraction of the glassy phase can be described by an empirical equation based on the normal distribution function.A kinematic framework,integrating thermal deformation gradient and mechanical deformation gradient,is constructed.And the evolution equations of deformation gradients under cooling and different heating recovery processes(free recovery,constrained deformation recovery,and constrained stress recovery)are derived.The Zener three-element model is used to establish the viscoelastic constitutive equation of the rubbery phase,the Hooke's law is used to establish the general elastic constitutive equation of the glassy phase.Numerical fitting of the uniaxial tensile experiment confirms the correctness of the constitutive equation for each phase.Furthermore,a one-dimensional simplification of the constitutive model is carried out and compared with the experimental results of shape memory behavior characterization of the epoxy-based SMP to verify the correctness of the constitutive model.This model theoretically explains the thermo-mechanical behavior of SMP during the characterization of different shape memory behaviors.Next,the condition of the spacecraft moving in and out of the earth's shadow causes the periodical temperature change of the space structure components.Besides,the SMP is temperature-sensitive.Therefore,it is necessary to study the temperature dependence of elastic constants of SMPC.A prediction model for elastic constants of the fiber-reinforced SMPC is developed,in which the phase transition between the rubbery phase and the glassy phase with temperature causes the temperature dependence of the elastic constants.Two glassy phase volume fraction equations are established,one based on the Eyring equation,the other based on the normal distribution function.Analytical expressions of the longitudinal modulusE₁₁,transverse modulusE₂₂,axial shear modulusG₁₂,axial Poisson's ration₁₁,and transverse shear modulusG₂₃ are derived by using the modified rule of mixture and glassy phase volume fraction function.Then,the changing regularities of elastic constants over temperature and fiber fraction are obtained.ModuliE₁₁,E₂₂,G₁₂andG₂₃ decrease with the increase of the temperature,and increase with the increase of fiber fraction at a certain temperature;while then₁₂ increases with the rising temperature,and at a certain temperature,the higher the matrix fraction,the larger then₁₂.An inverse identification by comparing the theoretical and experimental modal analysis of carbon fabric-reinforced SMPC laminate at different temperatures is conducted to verify the rationality of the prediction model.This model gives elastic constants of SMPCs with various fiber fractions at different temperatures only by the matrix and fiber parameters.Finally,a sunlight-stimulated substrate based on carbon fiber-reinforced epoxy-based SMPC(named Mission SMS-I)is designed,manufactured,conducted ground and geostationary orbital qualification.The substrate can be deployed from the“?”packaged configuration to the“-”deployed configuration once the temperature reaches or exceeds the material's glass transition temperature.The substrate integrates the conventional substrate,support structure,and deployment function together.It can be deployed by sunlight without satellite energy supply.The bending and mechanical properties of the carbon fiber-reinforced epoxy-based SMPC are tested.The material used for the substrate is determined to be carbon fiber reinforced epoxy-based SMPC with the fiber fraction of 43 vol.%,and the bending radius is 10mm.The design and manufacturing process of the Mission SMS-I is briefly introduced.Sine vibration,random vibration,shock experiment,vacuum thermal cycling experiment and ground-based deployment are performed to confirm that the structure can withstand the mechanical conditions during launching and the thermal conditions at space,and can be deployed by light.Finally,the orbital verification of the SMPC substrate has been completed.It has been successfully deployed by sunlight and has a good long-term anti-irradiation capability.The result preliminarily proves the space applicability of SMPC.