Modeling And Simulation Of The Large Deformation And Fracture Behavior Of Polymers

Polymers have been widely used in automobile,home appliances,national defence and aerospace fields,due to their high specific strength,good processability,low price,etc.Polymers often experience large deformations,even fractures under impact conditions,such as dropping and crash,and the strain rate ranges from low to high strain rate.Nowadays,there are still problems as follows in the existing researches related to the large deformations and fracture behavior of polymers:(1)the arrangement and fracture of molecular main chains and the disentanglement and deformation of molecular branched chains are different under low,moderate and high rate loading,leading to complex large deformations and deformation mechanism of polymers,and it is difficult to find such a model which can accurately describe their large deformations and deformation mechanism;(2)because the impact process is highly nonlinear and the direction of crack propagation can not be predicted in advance,fracture mode during impact process is not simple as mode ?,?,? or mixed mode during quasi static process,and it is difficult to accurately predict the fracture behavior;(3)the deformation behavior of polymers depends on the strain rate,temperature and molding process,while the interaction between processing simulation and mechanical behavior simulation has not been built,leading to the fact that the influence of molding process on the mechanical behavior can not be considered.On account of the problems above,the paper is devoted to the following researches:In this paper,polycarbonate(PC)and its blends PC/ABS were studied.Uniaxial compression tests under low,moderate and high strain rate loading were performed,their mechanical behavior was observed to be strongly dependent on the strain rate and temperature,and the initial nonlinear elastic deformation and strain hardening deformation under high rate loading were more significant than that under low and moderate rate loading.Uniaxial tensile tests and falling weight impact tests were performed,to analyze their large deformation and fracture behavior.Dynamic mechanical analysis(DMA)tests were conducted to discuss their rate-dependent glass and secondary transitions.In conjunction with the DMA results,their deformation mechanism at low and moderate strain rates can be explained with the glass transition regime,and that at high strain rates can be explained with the glass and secondary transitions regime.To describe the complex large deformations and deformation mechanism of polymers,the existing M-B physical model was modified by allowing for the evolution of the internal shear strength related to the secondary transition under high rate loading;meanwhile,based on the existing M-B,G'Sell-Jonas and DSGZ constitutive models,a new phenomenological constitutive model was proposed,which considers the effect of the glass and secondary transitions on the large deformation.Stress-strain updating algorithms in constitutive models have been built,and predicted results of the original M-B model,the modified M-B model,the DSGZ model and the new phenomenological model have been compared.Compared with the original M-B model,the modified M-B model is verified to more accurately explain the deformation mechanism of PC/ABS and predict its large deformation.Among the four constitutive models,the new phenomenological model is the most accurate to explain its deformation mechanism and predict its large deformation.Based on the results above,the new phenomenological model was popularized.In comparison with experimental results,the new phenomenological model can accurately expound the deformation mechanism of PC and PC/ABS under low,moderate and high strain rate loading,accurately predict their large deformations over a wide range of strain rates during uniaxial compression and cyclic loading,and accurately predict the large deformation of the PC plane at impact energy of 100 J during falling weigh impact.Using the fracture criterion of the maximum equivalent strain,the new model can explain the necking initiation and propagation of dumbbell-shape PC specimen,and simulate its large deformation and fracture behavior.Combining the two fracture criterion of the maximum equivalent strain and the maximum chain stretch,the new phenomenological model can expound the crack initiation and crack extension of PC plane at impact energy of 120 J during falling weight impact loading,and accurately predict its button type fracture.To characterize the influence of thermal history during injection molding on the large deformation of polymers,a thermal history(T-H)constitutive model was proposed based on the new phenomenological model.PC tensile specimens processed at different molding temperatures were annealed at various temperatures and time,to obtain the influence of mold temperature and annealing process on its yield stress.T-H was obtained by processing simulation,and the yield stress of PC during molding process could be accurately simulated;the T-H constitutive model was implemented in ABAQUS,and the large deformation of PC under different T-H conditions could be accurately simulated,to accomplish the combined simulation of injection molding and mechanical behavior.Hence,the large deformation behavior and deformation mechanism of polymers over a wide range of strain rates were expounded,a new phenomenological constitutive model was proposed,and it has been compared with the original M-B model,the modified M-B model and the DSGZ model.The new phenomenological model was verified that it can accurately predict the large deformation and fracture behavior of polymers over a wide range of strain rates,and accurately expound their deformation mechanism.Moreover,the T-H model based on the new phenomenological model can accurately predict the influence of thermal history during molding process on the large deformation.