Mechanical Characteristics And Constitutive Relation Of Strain Rate And Temperature Dependence Of Typical Viscoelastic Materials

With the rapid development of material science,the researchers found that the deformation process of some materials under external force is a time-dependent process,which shows the mechanical characteristics of solid materials and also shows some properties of viscous fluids,such as polymer materials,engineering geological materials,biological materials and so on.These materials have been widely used in national defense,aerospace and civil engineering due to their unique viscoelastic mechanical properties,such as military helmet,shock absorption for aircraft,ship anti-collision and explosion-proof wall etc..Since these viscoelastic materials are mainly used in extreme environments such as high pressure and high temperature,it is found that these materials bring specific functions to the structural design,but also cause some thorny problems.For example,the viscoelastic material is very sensitive to the strain rate and temperature,and its mechanical behavior has the hysteresis and timeliness,causing the failure of structural parts often occurs at a moment after loading,which is unpredictability and suddenness.Therefore,the studies on the strain rate and temperature dependent mechanical behavior of these viscoelastic materials and their constitutive relations can provide a reliable experimental and theoretical basis for the application of materials in extreme environments.Limited to specific research fields,this thesis mainly studies the the strain rate and temperature dependent mechanical behavior and their constitutive relations of two kinds of solid viscoelastic materials.In order to ensure the novelty of the research,and taking into account the diversity of the materials and the universality of the models,a new type of polyurea elastomer is selected as the test object for the polymer viscoelastic materials,while a homogeneous granite is selected as the test object for the rock viscoelastic materials.The strain rate and temperature dependent mechanical behavior of these materials under uniaxial loading and confined pressure is tested based on the electronic universal testing machine,split Hopkinson pressure bar,active and passive confining devices as well as high and low temperature systems.The changing laws and failure modes of compressive mechanical behavior of the materials under different strain rates,different temperatures and different stress states are analyzed.The three-dimensional visco-hyperelastic constitutive model of polymer viscoelastic materials and the three-dimensional viscoelastic-plastic damage constitutive model of rock viscoelastic materials are established based on the experimental data of polyurea elastomer and granite.The specific research contents of this thesis are as follows:?1?The strain rate and temperature dependent mechanical properties of two kinds of polyurea elastomer under non-confined and confined pressure are tested by DNS-100 electronic universal testing machine and split Hopkinson pressure bar.The uniaxial and multi-axial compressive stress-strain curves of the materials are obtained over the strain rate range of10^-310⁴ s^-1 and the temperature range of-40°C²0°C.The results show that the uniaxial compressive stress-strain curves of polyurea elastomer are significantly dependent on the strain rate and temperature.For the same deformation,the flow stress increased with the increase of strain rate,and decreases with the increase of temperature.The uniaxial compressive stress-strain curves of polyurea elastomer exhibit the transition from rubbery behavior at low strain rates or high temperatures to glassy behavior at high strain rates or low temperatures.Compared with that of the material under non-confined pressure,the strain rate and temperature dependence effect of the multi-axial compressive stress-strain curves of polyurea elastomer under confined pressure are weakened.Under non-confined and confined pressure conditions,the damage morphology of polyurea samples also has some differences.In addition,the compressive strength of PU605?Versalink P-650 reacted with 105%molar equivalent Isonate143L?is slightly higher than that of PU105?Versalink P-1000 reacted with 105%molar equivalent Isonate 143L?under the same loading condition.?2?Under confined pressure,the polyurea samples can be considered as a quasi-one dimensional strain state due to the confinement of the confining tube device.Based on the linear elastic theory and the boundary conditions under confined pressure,an approximate calculation method for the bulk modulus of polyurea elastomer is proposed.The results show that the bulk modulus of polyurea elastomer increases with increasing strain rate,and decreases with increasing temperature.Under the same loading conditions,the bulk modulus of PU605 and PU105 has little difference.?3?The energy dissipation rate of the materials is derived on the basis of the law of conservation of thermodynamics.Then,a three-dimensional visco-hyperelastic constitutive model suitable for describing the strain rate and temperature dependent mechanical behavior of viscoelastic polymer materials is proposed based on the theory of viscoelasticity and energy dissipation rate.The three-dimensional constitutive model is simplified by the boundary conditions under different loading conditions.The comparison with the test results of polyurea elastomer and rubber materials shows that the three-dimensional viscous-hyperelastic constitutive model has good prediction ability.?4?The large-diameter split Hopkinson pressure bar and active confining device are used to test the strain rate and temperature dependent mechanical properties of granite with different heat treatment temperature under non-confined and confined pressure.The uniaxial and multi-axial compressive stress-strain curves of granite at different strain rates and heat treatment temperature are obtained.The results show that the compressive mechanical properties of granite under different conditions are closely related to the heat treatment temperature,confined pressure and strain rate.Its peak stress and elastic modulus decrease with the increase of heat treatment temperature,and the variation law can be expressed as the functional form of temperature;while its peak strain increases with the increase of heat treatment temperature,and the variation law under uniaxial loading can be expressed as a function of temperature shift factor.In addition,the elastic modulus of granite increases with the increase of confined pressure,but is less affected by the strain rate.The microscopic analysis shows that the main failure mode of granite under uniaxial impact loading is longitudinal splitting along the thickness direction of the specimen.?5?A three-dimensional viscoelastic-plastic damage constitutive model suitable for describing the the strain rate and temperature dependent mechanical behavior of rock-like viscoelastic materials has been evolved based on the theory of viscoelastic-plastic mechanics.The applicability of the three-dimensional constitutive model is verified based on the test data and loading boundary conditions of granite under different conditions.