Characteristics And Mechanical Behaviors Of Different Level Microstructures Of Cord-reinforced Rubber Composites

The invention of radial aircraft tires represents the highest state of art in the tire industry.As the main structural material of the radial aircraft tire,the cord-reinforced rubber composite material plays a critical role in the tires' performance.It is also a complex and special composite system,which is characterized with the multi-component,different level microstructures.If the radial aircraft tires work in the extreme working conditions such as the instantaneous impact load and the wide temperature field,the different-level microstructures of the cord-reinforced rubber composites tend to be subject to dramatic changes,wh ich influence the performance and lifetime of the radial aircraft tires.Therefore,improving the higher performance of the radial aircraft tires considerably depends on a better understanding of the different-level microstructures and the mechanical properties of the cord-reinforced rubber composites.To solve the critical problems of the the carbon black reinforcing behavior and the cord-rubber interface performance of the cord-reinforced rubber composites,which have an important effect on the performance of radial aircraft tires,based on the molecular dynamics simulation and the continuous medium mechanics analysis method,the mechanical behavior of the rubber and the effect of the branch-structure carbon black aggregate and its surrounding rubber on the mechanical behaviors of rubber matrix were studied by using the coarse-grained model.The modification mechanism of the surface of aramid fiber was analyzed by the molecular dynamics simulation.And the ways of reducing the concentrated stress in the region of cord-rubber interface were also investigated.The main researches of this dissertation are described as follows:The coarse-grained model derived from the standard iterative Boltzmann inversion(IBI)method can not reproduce the mechanical and thermodynamic properties of poly(cis-1,4-isoprene),mainly because it does not take into consideration the effect of the dissipation.Motivated by method of utilizing the dissipative force in the dissipative particle dynamics,the dissipative particle dynamics algorithm was compared with the molecular dynamics algorithm,and the differences caused by the two algorithms are introduced into the nonbonded potential of the standard IBI method by means of the additional function,and then the the ignored friction effect between the superatoms was introduced into the standard IBI method.The method of determining the parameters of the additional function was given.The introduction of additional functions,w hich has a clear physical meaning and a flexible function form,can ensure that the stress-strain behavior,the glass transition temperature,and the self-diffusion coefficient of the coarse-grained model are consistent with those of the target all atom model.The initial modulus of polyisoprene calculated by the improved coarse grained model were in good agreement with the experimental results.Based on the modified CG force field,the CG models of rubber filled respectively by the branched-structure aggregates and by the the equivalent-sphere aggregates,were built,and the relationship of the aggregate structure respectively with the reinforcement mechanism,the Mullins effect,and the Payne effect was simulated.The simulation results indicate that the initial shear modulus of rubber filled by the branch-structure aggregates is the closest to the value predicted by using the Guth equation in which the effective volume fraction of carbon black included the occlued rubber.Under the different maximum amplitudes of shear strain,the regularity of the stress softening and the trend of the dynamic modulus of the rubber filled by the branched-structure aggregate model were consistent with the experimental results.When the strain amplitude was greater than the threshold value(the value is 0.4 in the dissertation),the superatom number of the occluded rubber decreased,which is an important reason for the stress softening and permanent deformation of the aggregate-filled rubber.Under the same loading conditions,the storage modulus of the aggregate-filled rubber model was higher than that of the equivalent-sphere aggregate-filled rubber,besides the loss factor of the former model is less than that of the latter model.The study on the mechanical behavior of the carbon black and its surrounding rubber provided a reference for regulating the mechanical properties of the filled rubber.Based on the COMPASS force field,the modification mechanism of the surface of aramid fiber was studied by molecular dynamics method.Considering the good adhesion of dopamine and its oligomers,the adhesion mechanism of 21 typical dopamine and its oligomers on the surface of aramid fiber was investigated.The simulation results show that the initial loosely-arranged dopamine molecules actively approached to the surface of the aramid under the rich non-bonded interactions such as hydrogen bonds,?-? stacking and van der Waals forces.After sufficient relaxation,the poly(dopamine)molecules adhered closely to the surface of the aramid,which is consistent with the experimental results from the scanning electron microscopy.For the chain oligomers of 5,6-dihydroxyindole,the degree of polymerization increased from two to eight,resulting in a significant decrease in adhesion work,which is consistent with the experimental results.Additionally,the work of adhesion of 5,6-indolequinone,53.35 m J/m2,is the highest in the initial oxidation stage of poly(dopamine),followed by 49.78 m J/m2,which belongs to the annular eumelanin molecules with annular planar structure.The research results provide a basis for industrial production's obtaining the cheap materials that can replace the expensive dopamine.The representative volume units of cord-reinforced rubber composites were established on the mesoscopic scale,and the effect of the different loading conditions,the cord structure,and the mechanical properties of rubber matrix at the extreme temperatures on the stress of the cord-rubber interface was studied by using the the three-dimensional nonlinear finite element model.The results show that the maximum interfacial shear stress of the single strand of cord-reinforced rubber composite element is the smallest in value among the interfacial stress of single-,double-,and triple-stranded cord reinforced rubbers.The model of the circle cord-rubber interface was created based on the theories of elastic mechanics.By adopting the Kolosov constant,the parameters of the zero-thickness imperfect interface were obtained with consideration of the thermal residual stress and neutral inclusion conditions.The results indicated that,without the thermal residual stress,the neutral inclusion exists under the equal-biaxial tensile or the pure shear,and the neutral inclusion does not exist under the uniaxial tension.Since the thermal residual stress has an effect on the interface parameters,the neutral inclusion probably exists under the pure shear or the uniaxial tension through appropriately adjusting the thermal residual stress.However,the neutral inclusion does not exist under the equal-biaxial tensile,no mater how to adjust the thermal residual stress.Due to the fact that the natural interfacial parameters obtained from the zero-thickness imperfect interfacial model was not characterized with the practical operability,the neutral discrete structure interface model with limited thickness was established.The verification of finite element method shows that the truss structure interface model can effectively reduce the stress concentration caused by external load when the number of bars is enough.