Constitutive Model,Failure Mechanism And Fatigue Behavior Of High Density Polyethylene Under Cyclic Loading

It is significant and challenging to investigate constitutive behavior,explore failure mechanism as well as predict failure mode,and evaluate fatigue lifetime of high density polyethylene(HDPE)under cyclic loading since its applications are widely utilized in industries due to the excellent shape flexibility,light weight,cost effectiveness,corrosion resistance,electric resistance,and recyclability.For instance,the plastic fuel tanks made up of HDPE become more and more popular in automobile industry.Technically,in order to reduce cost and improve robustness of related designed products,it is important to study long-term durability behavior of HDPE.Theoretically,for the purpose of enriching the comprehension of traditional mechanics and understanding the mechanical properties of HDPE,it is essential to study the deformation mechanism of HDPE since this material always performs complicated coupled elastic-viscoplastic mechanical behavior when it is applied forces.In general,the mechanical properties of HDPE are strongly dependent on strain rate and temperature,which introduces difficulties into investigating long-term durability behavior,particularly under cyclic loading.Although there are plenty of investigations concentrating on the long-term mechanical behavior including creep under constant stress and stress relaxation under constant strain,the researches on mechanical behavior of HDPE under cyclic loading are preliminary and limited.Constitutive model is the fundamental of failure mode prediction and fatigue life evaluation,and meanwhile,the failure mode prediction and fatigue life evaluation are the application of constitutive model.The main body of the dissertation consists of the following works and contributions.1)Tests under quasi-static and static loadingMechanical behavior of HDPE under quasi-static and static loading are examined by performing uniaxial tensile tests at different strain rates and temperatures,creep tests and stress relaxation tests.It is confirmed that both elastic modulus and yield stress increase with the growth of strain rate or decrease of ambient temperature on the basis of tensile experimental data.There is no initial linear portion in the tensile stress-strain curve.According to creep test data,the primary creep lasts longer when the applied stress is higher,while the steady-state creep where the creep strain rate is close to zero can be observed at the very lower applied stress.The stress always becomes stable after an initial dramatic decay in the stress relaxation test.The stress-strain data at the steady state of a relaxation test is in accordance with the static tensile test data.The stress-strain data at the steady-creep state of a creep test under the very lower stress is also consistent with the static tensile test data.2)Elastic-viscoplastic constitutive modelThe parallel rheological frame work can account for the consistency among the stress-strain data of steady state of relaxation,steady-creep state and static tensile curve.Therefore,a nonlinear elastic-viscoplastic constitutive model is developed based on the parallel rheological framework to predict the stress-stain response of HDPE under different loading conditions.The constitutive model consists of an elastic-plastic network in parallel with three viscoelastic networks to simulate the nonlinear coupled elastic-viscoplastic deformation behavior.The elastic-viscoplastic network directly determined by the relaxation-unloading tests is governed by nonlinear elasticity and kinematic hardening plasticity with two back stresses.The power-law viscosity is developed to ba applicable for simulating cyclic-loading behavior of HDPE,which is fulfilled by coding UCREEPNETWORK subroutine in Abaqus.The relaxation-unloading test is specially designed for HDPE to distinguish the time-independent elastic-plastic deformation from the overall time-dependent elastic-viscoplastic deformation.The viscoelastic networks ruled by nonlinear elasticity and power-law(Norton-Bailey law)viscosity are calibrated based on uniaxial tension,creep,stress relaxation and cyclic-loading tests,which is accomplished by parameter optimization and constitutive behavior simulation thanks to embedding the Abaqus FEM model into the ModeFrontier main program.As a validation,the prediction is well matched with the test data for HDPE under various loading conditions,especially cyclic loading.3)Failure of HDPE under cyclic loadingThe failure modes of HDPE under cyclic loading is predicted successfully based on the developed constitutive model.There can be three failure modes,i.e.necking,buckling and fatigue cracking,when HDPE specimen is applied cyclic loading.It is proved that the failure mode for a cyclic-loading test is strongly dependent on the specimen temperature increase as a result of the competition between heat accumulated by inelastic deformation energy and heat transferred with the surroundings.The specimen temperature increase with test cycle results in the gradual reduction of yield stress and elastic modulus.If the yield stress falls below the applied peak stress,the specimen will fail by necking.And if the buckle stress which is proportional to elastic modulus declines below the absolute of applied valley stress,the specimen will be buckling.Otherwise,the specimen can fracture by fatigue cracking or run out cycles.In order to predict the failure mode for a cyclic-loading test of HDPE,the specimen temperature increase is calculated first by heat transfer analysis in which the heat source is input as the area of hysteresis ellipse computed by the developed constitutive model,then the numerical relationship based on extended Eyring's theory for mechanical properties dependent on temperature are acquired by experiments,and finally,the yield stress and buckle stress considering temperature effect are calculated and compared with the applied peak stress and the absolute of the applied valley stress,respectively.4)Numerical model for fatigue life predictionA numerical fatigue model based on the developed constitutive model,Walker method and damage evolution as well as accumulation is proposed to predict fatigue life of HDPE under complicated loading conditions.It is verified by the experimental results that the stress response dominates the fatigue life of HDPE for both stress-controlled test and strain-controlled test.Hence,it is concluded that it is the stress state characterized by stress amplitude and stress ratio determine the fatigue life of HDPE under complicated loading conditions.In the numerical methodology,the stress history with cycle is known for a stress-controlled test or calculated by constitutive model for a test under complicated loading conditions,then the equivalent stress amplitude for Walker method is computed as a function of stress amplitude and stress ratio,and finally,the fatigue life can be predicted as the corresponding cycle number if the damage accumulation is equal to 1 by integrating damage evolution.The damage evolution law which relates the fatigue damage to the equivalent stress amplitude can be defined by the test data of the fully reversed stress-controlled fatigue.The Walker method is effective to evaluate the stress ratio influence on the fatigue life of HDPE,which is validated by stress-controlled fatigue data.