Experimental Investigation And Modeling Of Dynamic Tension Behavior Of Polycarbonate

Polycarbonate is a kind of amorphous polymer. It has been used extensively in automobile and aviation fields due to its excellent transparency and good impact toughness. At present, the researches on the temperature-dependent dynamic properties of polycarbonate mainly focus on its dynamic compression responses. It has been well known that there exists obvious differences between the tension and compression stress-strain responses and the crazing and deformation localization occur during tensile loading for amorphous polymers. Consequently, it is necessary to investigate and characterize the dynamic tension behavior of polycarbonate. However, due to the testing technique difficulties, very little research has been reported on the tension responses of polycarbonate at high strain rates.For low-impendence and low-strength polymeric materials, the specimen geometry used in Hopkinson bar tests is determined and the high signal-to-noise ratio of transmitted pulses is obtained. The dynamic tension experimental technique for polymers is achieved using rotating-disk indirect bar-bar tensile impact apparatus. The quasi-static, moderate strain-rate and dynamic tension tests are carried out to investigate the effects of temperature and strain rate on the tension stress-strain responses of polycarbonate over wide ranges of temperatures (213-393K) and strain rates (0.001-1700s-1). Experimental results indicate that the tension behavior of polycarbonate exhibits complex nonlinear characteristics including viscoelasticity and viscoplasticity such as strain softening and strain hardening. The tensile impact loading-unloading test is conducted to evaluate the elastic behavior of polycarbonate at high strain rates. Experimental results confirm that the stress-strain response prior to the peak stress is elastic and the peak point in the stress-strain curve can be considered as the initial yielding point. It is found that the tension properties of polycarbonate are significantly dependent on the temperature and strain rate. The values of yield strength and yield strain increase with decreasing temperature and increasing strain rate. Furthermore, the yield strength has obvious nonlineat relationship with logarithm strain rate within the rate range investigated in the present paper.Based on the aforementioned experimental work, a phenomenological viscoelastic constitutive model incorporating temperature effect is proposed. The model consists of a nonlinear spring and a Maxwell element with strain-rate dependent relaxation time. The model results agree well with the experimental elastic stress-strain responses, which indicates that the proposed constitutive model can characterize the viscoelastic stress-strain behavior of polycarbonate within a large strain-rate range (from quasi-static and moderate strain-rate loadings to the high strain-rate loading,0.001s-1700s-1). The temperature and strain-rate dependent yield behavior is described using Richeton combination model based on the thermal activation theory. The correlations between the model results and the experimental data are good.According to the viscoplastic stress-strain behavior of polycarbonate, a physically based elastic-viscoplastic constitutive model which accounts for microscopic deformation mechanism of amorphous polymers is established. Its one-dimensional rheological form is a nonlinear Langevin spring parallel connecting with the series of a linear spring and a viscoplastic dashpot. The linear spring describes the intermolecular resistances of polymers corresponding to the elastic responses. The Langevin spring describes the entropy resistances caused by molecular chain rearrangement corresponding to the inelastic responses. The viscoplastic dashpot describes the molecular chain segment sliding corresponding to the yield and post yield strain softening behavior. The comparison between the model results and experimental results indicate that the present constitutive model can accurately describe the temperature and strain-rate sensitivity of the yield stress and capture the typical nonlinear tension behavior characteristics of amorphous polymers such as post-yield strain softening and strain hardening.The numerical simulation on the tensile deformation processes of plate-shaped polycarbonate specimen subjected to the quasi-static and high strain-rate loadings are performed using the finite element method. Especially, the localization phenomenon is simulated by introducing geometric and physical defects in the finite element model. The simulation results show that the specimen geometry defects and material physical nonuniformity can lead to deformation localization. Under quasi-static and dynamic loadings, localized necking is activated from the initiation of shear band, and develops symmetrically along the width direction of the specimen. With increasing the relative displacement of specimen clamping end, the necking gradually extends to both ends of the specimen till the whole specimen testing section is in the necking state. The resulting necking patterns obtained from numerical simulations are close to the test results.