Study On The Mechanical Behavior And Constitutive Model Of Uncured Rubber Under Different Deformation Modes

In the molding process of various rubber products,the material has not been vulcanized,so it usually exhibits the mechanical properties at a high-elastic state.In another word,the un-vulcanized rubber will produce both the elastic deformation caused by the stretching of polymer chains and the viscous flow due to its alignment of molecular chains.This nonlinear characteristic of mechanics between ideal elasticity and viscosity leads to great difficulty in forming control of rubber products.In order to improve the design precision and quality of the finished rubber components,we wish to fully understand the deformation response and mechanism of uncured rubbers,so as to achieve effective adjustment or control in their manufacturing process.From the view of this point,our paper mainly focuses on the visco-and hyper-elastic behavior of such rubbery elastomer under different deformation modes,by employing the method in combination with experimental investigation and theoretical analysis.By using the non-contact Automated Grid Method,the loading-unloading cyclic tests of uncured rubber under quasi-static uniaxial and planar tensile deformations were carried out respectively.Test results demonstrate the obvious elastic-viscoplastic and rate-dependent effect on the large strain tensile behavior of uncured rubber.In these two deformation modes,the material stiffness decreases rapidly after the initial loading stage,showing a yield-like feature which is more evident with the increase of strain rate.The stress softening and hysteresis loop can be easily observed during the cyclic loading-unloading process,and they generally increase with applied strain rate.After several cycles,the material responses gradually stabilize.Besides,the internal damage of uncured rubber is hardly to recover because of its strong viscosity,where residual strain accumulates step by step with preceding strain level.Based on the obtained accurate experimental data,firstly,the linear viscoelastic generalized Maxwell model and nonlinear viscoelastic Bergstrom-Boyce model were used to describe the mechanical response of uncured rubber and its rate dependence.It is found that the linear viscoelastic theory is only suitable for a single strain rate case,while Bergstrom-Boyce model is not adequate to accurately capture the viscoplastic flow of material.Consequently,according to the evaluation on above two classical models,a parallel network framework composed of a rate-independent hyperelastic element and two rate-dependent viscoelastic-plastic elements is presented,which takes into account the plastic flow criterion.By comparing the test and characterization results of both uniaxial tension and planar tension at different strain rates,it concludes that this new model can capture the overall large deformation behavior of uncured rubber well.At meantime,it is capable to predict the monotonic tensile and multi-step relaxation responses of the material.By considering the difference between tension and compression mechanism of uncured rubber,when the model is adopted to separately characterize the cyclic compression deformations,it also gives a good effectiveness.These factors verified the characterization ability and validity of our three-elements parallel constitutive model.Lastly,the quasi-static and dynamic compression tests of uncured rubber were carried out by using fatigue machine.The obtained results show that the quasi-static compressive behavior of the material is rate-dependent as well.However,the loading stress-strain relations are close to linear elasticity without yielding phenomenon,which is distinct with the tension property.The raise trends on the static stiffness,hysteresis loss and residual strain are all with the increase of strain rate.On the other hand,the dynamic compression experiments illustrate that the storage modulus and energy loss of uncured rubber are obviously affected by the pre-strain amount.But with the increase of frequency,just the storage modulus shows a growth,where the energy loss does not change significantly.