| As a common engineering material,rubber is an important part of military and industrial development for a country.It has been widely used in various fields such as tires,medical apparatus,aerospace.The broad applications of the rubber are closely related to its excellent vibration damping or vibration isolation performance.The damping performance mainly depends on the viscoelastic properties of the rubber in actual use.The viscoelastic properties of the rubber will exhibit both elastic and viscous effects during the deformation process.The viscous response decides the energy dissipation of the rubber under static or dynamic loadings.However,in the application of rubber damping performance,it is necessary to pay attention to the dynamic viscoelastic heat generation of the rubber material and the thermal-oxidative aging problem during the long-term high-temperature use.The viscoelastic hysteresis of rubber is mainly attributed to the internal friction of the macromolecular chains inside the rubber under dynamic loading.The energy consumption caused by the friction of the molecular chains is finally converted into heat dissipation,which causes the significant temperature rising of the rubber components in turn.And it further affects the physical properties of the material and the performance of the rubber components.Furthermore,since the rubber components are always used in the high-temperature environment,the influence of the thermo-oxidative aging of the rubber material on the performance of the components cannot be ignored.The hysteresis heat generation and thermal-oxidative aging of the rubber mentioned above are typical multi-physics coupling problems.For now.it is difficult to simultaneously consider various factors affecting the performance of rubber components by the single experimental method.And single experimental researches will face the problem of a lons experimental cycle.With the development of computer technology,the experiments combined with multi-scale simulation have become an effective means to quantitatively study the viscoelastic hysteresis temperature rise and thermo-oxidative aging process of the rubber components.It not only provides early guidance for the design of the rubber components but also provides a deeper understanding of the viscoelastic deformation and the microscopic mechanism of the aging process for rubber components.Based on the above research background,the experimental and multi-scale simulation methods are used to quantitatively study the hysteresis heat generation behavior and the thermal aging process of rubber materials in long-term use,which is based on the viscoelastic theory.The main works and conclusions of this paper are as follows:Firstly,the tread rubber samples with different cross-linking densities were used to study the viscoelastic hysteresis loss and temperature rise process under dynamic compression conditions.In the research process,the dynamic mechanical behavior changes of rubber during the loading process were analyzed firstly.And changes of the macroscopic dynamic mechanical properties of rubber with different crosslink density were analyzed from the perspective of compositional differences of the molecular chains.Then,by establishing the rubber viscoelastic constitutive equation suitable for the experimental loading conditions,the dynamic modulus,loss factor and hysteresis energy loss of rubber at different temperatures and loading frequencies were studied.On the basis of quantitative simulation of rubber energy loss,the full coupled Thermo-mechanical finite element program and accurate rubber heat generation rate expression were used to quantitatively calculate and predict the hysteresis heat generation and the temperature rise of rubber sample under dynamic loadings.The results show that different rubber samples have a significant effect on the viscoelastic behavior of rubber due to the internal cross-linking network and the difference in the proportion of different types of molecular chains.The high proportion of dangling chain ends of the rubber will have high hysteresis energy loss under dynamic loadings.At lower ambient temperatures,the loading frequency has a significant effect on the loss factor of the rubber,thus the hysteresis energy loss and the steady-state temperature rise of the rubber sample show significant non-linearity with frequency.As the ambient temperature increases,the steady-state temperature field of the sample gradually decreases with the non-linear change of the loading frequency.It is indicated that the frequency dependence of the energy loss of the rubber material at lower ambient temperatures cannot be ignored in the calculation of the hysteretic heat generation of the rubber components.At the same time,through the quantitative analysis of the thermal parameters of the rubber material,it can be concluded that the change of the thermal conductivity has a great influence on the steady-state temperature field of the rubber components,while the influence of other thermal parameters such as thermal convection coefficient and specific heat capacity on the steady-state temperature field is almost negligible.Secondly,taking styrene-butadiene rubber as the research object,the experiment of thermal oxygen aging of rubber materials and the corresponding research on the construction model were carried out.The microscopic molecular chain structure and macroscopic mechanical properties of rubber with different aging time were quantitatively characterized by various microscopic and macroscopic testing methods.Through analyzing the changes of the polymer network and molecular chain during rubber aging process,a mechanism for changing the molecular network of rubber in the aging process is proposed.In the aging process of styrene-butadiene rubber samples,chain scission and cross-linking reactions occur simultaneously in rubber materials.The broken of the polymer chain will seriously damage the polymer network structure,and more dangling chain ends will be formed inside the material.After a long aging time,a random combination of the free radicals causes additional cross-linking reactions.Therefore,the aged rubber samples have a denser crosslinked network than the unaged styrene-butadiene rubber samples,and the proportion of the dangling chain ends are correspondingly increased during the aging process.Then,based on the experimental phenomena of the polymer network change during rubber aging process and the finite deformation theory of elastomer,the mathematical model which considered the coupling of the hyper-viscoelastic behavior and molecular chains changes of aging rubber was established.According to the difference of the characteristic relaxation process of different types of molecular chains inside the rubber,the evolution of the perfect cross-linking network,physical entanglement chains,and the dangling chain ends during the aging process were considered in the model.The change of viscoelastic behavior caused by the evolution of molecular chain structure in rubber under different aging time was simultaneously studied.The established constitutive model was realized by the finite element program.At the same time,the mechanical experiment of rubber samples under various loading conditions was used to verify the accuracy of the established model.The research and the established model provides some help for quantitatively studying the viscoelastic behavior and the using performance changing of the rubber during the aging process.Then,based on the experimental results of thermo-oxidative aging of styrene-butadiene rubber,the micromolecular simulation of the thermal aging of rubber materials was carried out to further understand the aging mechanism of rubber and the influence of the oxidative aging process on the static and dynamic properties of rubber molecular chains.By establishing a molecular chain model consistent with the styrene-butadiene rubber used in the experiments,the difficulty of the scission of the typical chemical bonds in the rubber molecular chains during aging was analyzed,and based on the position where the rubber molecular chain is most prone to oxidation,the oxidized molecular chain models containing different oxidized functional groups were established through the all-atom molecular dynamics simulation method.The variation of the microscopic and macroscopic properties such as density,free volume.self-diffusion coefficient and glass transition temperature caused by the chain scission and secondary crosslinking of rubber during aging was analyzed.Meanwhile,the changes in oxygen permeation and thermal conductivity of the rubber material in the presence of different oxidized molecular chains were also analyzed.The results show that during the aging of the styrene-butadiene rubber,the difference of the bond dissociation energy for the double bond a-H in styrene,cis-1,4 polybutadiene,trans-1,4 polybutadiene and 1.2-polybutadiene structure is small.But the dissociation energy of the alkoxy radicals of the cis-and trans-1.4 polybutadiene structures is significantly smaller than that of the styrene structure.indicating that the degradation of the aliphatic part of styrene-butadiene rubber during aging is faster than the degradation of the aromatic part.During the aging process,in the presence of the oxidized chains caused by chain scission,the density and mean square displacement of the styrene-butadiene rubber system will increase slightly due to the molecular chain size and the dipole interaction between the chains.The glass transition temperature Tg and the radius of gyration are reduced.Compared to the scission oxidized chain system,the size of the crosslinked oxidized molecular chain is significantly larger,and the cluster of the oxidized chain occurs in the system.Furthermore,compared with the unaged styrene-butadiene rubber system,the thermal conductivity and the solubility of oxygen are gradually reduced in the scission oxidized chain system.Due to the interaction between different molecules and the weak polarity of the chain in the system containing crosslinked oxidized chains,the thermal conductivity and oxygen solubility of the system increase slightly.Finally,according to the non-uniform aging of materials caused by diffusion limited oxidation effect in the actual use of rubber components,a numerical simulation study on oxygen diffusion-aging-viscoelastic coupling of rubber aging process was carried out by the multi-scale simulation method.In the analysis process,a suitable constitutive model of aging-viscoelastic coupling is established firstly and the evolution law of viscoelastic response of rubber materials during aging is revealed.In order to evaluate the non-uniform aging of the rubber product caused by the diffusion-controlled oxidation effect.a molecular simulation method was used to determine the oxygen permeation parameter in the rubber material.In order to evaluate the aging non-uniformity of the rubber product caused by the diffusion-controlled oxidation effect,a molecular simulation method was used to determine the oxygen permeation parameter in the rubber material and used as an input to the macroscopic numerical simulation.Then the finite element method was used to quantitatively analyze the oxygen absorption,diffusion,and chemical reaction processes in the rubber sample.Furthermore,the above process was coupled with the aging-viscoelastic constitutive model to obtain the evolution of the aging degree distribution of the rubber sample along the cross-section and the corresponding dynamic viscoelastic behavior.The results show that in the initial stage of aging,the oxygen concentration distribution is limited to the superficial layer of the sample,and the sufficient oxygen supply on the surface of the sample leads to a balanced aging state in the narrow region of the sample surface.For other areas away from the superficial layer of the sample,the aging of the material is affected by the diffusion of oxygen,which exhibits a decrease of the aging degree along the radial direction.Meanwhile,the diffusion-limited oxidation effect will significantly affect the viscoelastic behavior of different parts inside the sample.In a short aging time,the stress concentration of the rubber sample caused by aging is limited to the superficial layer,and the stress concentration area of the rubber surface gradually expanded with the aging time.After a long aging time,the stress inside the sample will increase significantly and the stress distribution of the entire sample will become more complicated than that of the non-aging.In addition,the relaxation time and energy dissipation of rubber samples will gradually increase with the increase of aging time,which can provide theoretical guidance for understanding the propagation of the surface cracks during rubber aging. |
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