Study On Mechanical Behavior And Constitutive Model Of Semi-crystalline Polymer:polypropylene

The research on the mechanical properties of polymer materials involves intersection and penetration of chemistry,physics,mechanics and materials science.Polymer molecular is very weight,and molecular chain is very long.Thus,intermolecular interactions affect the polymer aggregation state,stack and modulus,viscosity and strength and mechanical properties,mainly reflected on the temperature and time dependence of the mechanical properties of polymer materials,with obvious viscoelastic behavior.A typical semi crystalline polymer polypropylene(PP)is selected as the research object in this study.In order to analyze the strain rate sensitivity and temperature sensitivity of polypropylene in uniaxial compression,the quasi-static and dynamic mechanical response of polypropylene under uniaxial compression at at different strain rates and temperatures were experimentally studied.The quasi-static experimental results show that under uniaxial compression,the mechanical properties of polypropylene materials can be divided into a viscoelastic zone and a plastic flow zone.The viscoelastic response characteristics of Standard Linear Solid Model(SLSM)is used to fit the experimental results,and it is found that in case of large deformation,PP mechanical response has deviation of the SLSM results,indicating that there is some unrecoverable plastic deformation of polypropylene.In this case,polypropylene should be regarded as a visco-elastic visco-plastic material rather than a simple viscoelastic material.Experiments of different strain rates and different temperatures show that PP has obvious strain rate sensitivity and temperature sensitivity.The shock mechanical response of polypropylene is similar to that of the quasistatic state,and the strain rate effect and temperature effect are obvious.However,the viscoelastic stage under impact is almost linear,which shows that the viscous effect is not obvious and the material response is close to that of the elastic plastic response.In addition,since the shock process is adiabatic process,in the case of large deformation,the material will appear adiabatic softening phenomenon.The time temperature stress equivalence principle(TTSSP)of polymer materials during deformation is further deduced and discussed.According to the characteristics of the microstructure of semi crystalline polymer and the mechanical properties of polypropylene reflected by the results of one-dimensional compression experiments,the visco-elastic visco-plastic rheological model of semi crystalline polymer with strain rate and temperature effects was established by improving Holmes’ viscoelastic viscoplastic rheological model.The model parameters are determined by the results of quasistatic compression experiment and dynamic experiment,and the physical meaning of the model is discussed.The quasi-static mechanical response and dynamic mechanical response of polypropylene were predicted by the model,and the predicted results were compared with the experimental results.The results show that the proposed viscoelastic viscoplastic rheological model of semi crystalline polymer with temperature and strain rate effects can better describe the mechanical response of polypropylene in a large strain rate and temperature range.In order to study the yield behavior of polypropylene under combined compression and shear loading,the shear-compression specimens with different tilt angles were designed and processed.Experimental study on quasi-static compressive loading of specimens was carried out.The results show that the response of PP under compressive and shear loading can be divided into two parts:viscoelastic response and plastic flow,but with the increase of the deformation of the specimen,the specimen will be local failure.The split Hopkinson pressure bar(SHPB)device was used to conduct the compression shear composite dynamic impact loading experiment.The experimental results show that polypropylene still behaves as a viscoelastic plastic material under impact,but with the increase of impact speed,local damage and failure are prone to occur,resulting in a drop in the load displacement curve.Under the combined compression and shear loading,polypropylene shows a three-stage transformation of "no yield→yield→failure".The yield rule of polypropylene shear-compression specimens was summarized as:it is found that the commonly used von Mises yield criterion to describe the polypropylene has deviation with experimental results when the shear force is larger;it is more reasonable to use the Hu-Pae yield criterion to describe polypropylene yield criterion,indicating that the yield properties and hydrostatic pressure are related and it is helpful to study the effect of hydrostatic pressure on the yield law of polymers.The model parameters of Hu-Pae yield criterion and the strain rate correlation of parameters are obtained by using quasi-static and dynamic experimental data.Finally,the tensor form of the thermodynamic constitutive model of semi crystalline polymers is derived.The 1D rheological model is extended to a 3D tensor form for describing the mechanical properties of semi solid crystalline polymer under complex stress conditions.According to the need of finite element model simulation,the recurrence relation of stress and strain in tensor form model is given through 3D rheological model.Based on the free energy function,the irreversible thermodynamic theory of the viscoelastic plastic constitutive model describing the mechanical properties of solid semi crystalline polymers is deduced.On the basis of the 3D rheological model of polymer solids,considering the flow characteristics of the melt forming polymer melt at high temperature,a 3D solid-melt constitutive model was developed to describe the mechanical behavior of semi crystalline polymers before and after the solid-liquid transition process.The thermodynamic properties of polypropylene in the vicinity of melting point were determined by Differential Scanning Calorimetry(DSC)tests.The description that polypropylene in near the melting point of thermodynamic function change,using mathematical form near the melting point of the thermodynamic transition function is described.