Viscoelastic-Plastic Constitutive Model Of Polyamide Under Combined Stress State

Polyamide,a semi-crystalline thermoplastic polymer,has become one of the most important engineering plastics offering the following merits:good damping characteristics,high dissipation performance,easy manufacturing and lightweight.They have wide applications in engineering fields,such as aeronautics and astronautics,mechanical and vehicle engineering.Because of the good mechanical properties and heat resistance,PA66 are widely used as structural components in the field of automotive and electronic appliances,and therefore,the mechanical behavior of PA66 causes the great attention of numerous researchers in the academic as well as engineering circles.In this paper,theoretical analysis and experimental verification are combined to investigate the mechanical behavior of PA66 under various conditions.In addition,a viscoelastic-plastic constitutive model is established,including the strain rate effect.The main contents and conclusions can be stated as follows:(1)The mechanical responses in uniaxial and combined stress state were investigated comprehensively.The results of uniaxial tensile and compressive test indicated that with the increase of strain rate,the yield stress increases,and elastic modulus increases as well.Based on the DIC method,the full-range deformations of tensile test up to failure were collected.The neck formed in the middle of specimen,then to spread toward both ends.The fracture occurred at the root,and the fracture behavior was related to stress triaxiality.Shear stress and normal stress of SCS and modified STS were investigated,which indicated that the stress component changes with the slotting angle,and the yield strength varies with strain rate,showing obvious strain rate sensitivity.Furthermore,the correlation of equivalent stress and equivalent plastic strain can be assessed by experiments and finite element simulations.By comparing with stress-true strain curve in uniaxial compressive experiment,the correlation of equivalent stress and equivalent plastic strain obtained from the SCS can be used to describe the yield behavior of PA66 under complex stress states.(2)A series of dynamic mechanical analysis(DMA)testing at different frequencies over a wide range of temperature were conducted on PA66 specimens.The results show that the storage modulus decreases with the temperature,and the curves shift to higher temperature with increasing frequency.Therefore,we introduced a model of storage modulus which considers the dependence on temperature and dependency.The material parameters of PA66 for the model are determined by the results of DMA testing.Based on a relationship between the DMA and the uniaxial test,a model of elastic modulus was deduced by the expression of storage modulus,and verified by the results of high temperature experiments at different strain rates.Furthermore,based on the modified Maxwell model,a rate-dependent viscoelastic constitutive model of PA66 was obtained.(3)Comparing the existed yield criterions used in present research with experimental yield surface for different stress states,the yield criterion that can describe the asymmetric yield strength of tension and compression and the hydrostatic pressure dependency of PA66 was established.The yield strength for different strain rates were predicted by static and dynamic compressive and tensile yield surface.In addition,the availability was verified by comparing the predicted value with quasi-static SCS and STS test results.Subsequently,quadratic function was introduced to describe the subsequent yield behaviors of PA66 under tension and compression.And then the subsequent yield function can be proposed combined with mixed hardening model.The availability of the subsequent yield function was verified by comparing the experimental flow stress.In addition,based on the Eyring theory,the rate-dependence compressive and tensile flow stresses were obtained,and then the rate-dependence subsequent yield function can be acquired.Finally,on the basis of the classical plastic mechanics,the plastic constitutive relation of the material was derived by the yielding,hardening law and the rate-dependence of the material.Combined with the previous viscoelastic constitutive model,a rate-dependent viscoelastic-plastic constitutive model was finally obtained.The validation of the model was performed by comparison between the experimental and simulation results under uniaxial compressive stress state.