Mechanical Behaviors And Damage Evolution Of Tungsten Alloys At High Shear Strain Rates

Tungsten heavy alloys are widely used in civil and military fields due to their high density,high strength.The experiments found that ultrasonic elliptical vibration cutting can obtain tungsten alloy parts with higher surface quality than common cutting.However,the mechanisms of the high shear strain rates induced by ultrasonic excitation on the mechanical behavior of tungsten alloys are still unclear,which limit the full use of tungsten resources in our country.Therefore,in order to explore the mechanical responses and failure mechanisms of tungsten alloys at high shear strain rates,a combination of experimental observation and finite element analysis were used in this thesis.The hat-shaped specimens were selected for impact experiments of tungsten alloys under dynamic loads.Then,based on finite element method,impact experiments were simulated for specimens with different loads and sizes,the adiabatic shear phenomena and damage evolution were studied.The main contents include the following three aspects.1.Based on a universal testing machine（UTM）and a split Hopkinson pressure bar（SHPB）apparatus,the mechanical tests of tungsten alloys were carried out in ranges of shear strain rates0.001～22700 s^-1 and temperatures 25～600℃.The experimental results show that the tungsten alloys have strong sensitivity to shear strain rates and temperatures.When the shear strain rates are less than 20000 s^-1,the strain rates have obvious hardening effects on the flow stress.As the temperatures increas,the flow stress decreases and the initial shear failure strains tend to increase.Dynamic failure energy can be used as the basis for judging whether adiabatic shear failure occurs in tungsten alloys.It can be considered as a constant at different strain rates,but it increases with increasing temperatures.In addition,the microstructures of the tungsten alloys after the experiments were observed by scanning electron microscope.The microstructure observations show that with the increase of strain rates,the main failure modes of tungsten alloys change from intergranular fracture between tungsten particles and ductile avulsion of the matrix to transgranular cleavages of tungsten particles.The width of the adiabatic shear band is negatively correlated with the strain rates.while the effects of temperatures are opposite to strain rates.As the temperature increases,the tungsten alloys changes from brittle fracture to ductile fracture,and the width of the adiabatic shear band is positively correlated with temperatures.2.Based on the empirical model Johnson-Cook（JC）constitutive and damage model,the impact simulation models of tungsten alloys hat-shaped specimens with different loads and sizes were established,the adiabatic shearing phenomenon of tungsten alloys and the relationships between specimen sizes and ideal experimental data and stress state were studied.The results show that the shear deformations of hat-shaped specimen are non-uniform localized process of strains and temperatures,and the shear strain rate and temperature affect the failure mechanism of tungsten alloys by affecting the local temperature rise.The force equilibrium condition and shear strain rate gradient of tungsten alloys during the test can’t be optimized for the change of shear zone size simultaneously.With increasing width,height and fillet radius of the shear zone,the force equilibrium condition generally decreases,and the shear strain rate gradient generally decreases with the increase of the width,but increases with height or radius.Whether or not the corner radius is 0 mutates the above rules.The results of variance analysis show that the width has the greatest influence on the stress state,and the established artificial neural network modeling can accurately describe the mathematical relationship between the width,height,fillet radius and shear stress fraction,stress triaxiality and lode angle.3.In order to analyze the damage evolution of tungsten alloys based on actual physical significance,the numerical implementation of shear-modified GTN meso-damage model describing material damage by porosity coefficient is reviewed.Based on the elastic prediction-plastic correction stress backoff algorithm,the VUMAT subroutine is programmed.After compared the unit test results of VUMAT and the ABAQUS’s built-in GTN model under uniaxial tension,uniaxial compression and simple shear loads,the accuracy of the VUMAT is verified,the applications of JC constitutive model and shear-modified GTN damage model is realized.Based on the experimental data of simple shear hat-shaped specimens,the damage parameters of the tungsten alloys under high shear strain rates are identified by the trial and error method.The evolution of the porosity during the failure process of the tungsten alloys and the effects of the strain rates on the porosity were analyzed based on physics by means of simulations.