Study On Viscoplasticity Constitutive And Damage Models Of Tungsten Heavy Alloy

Tungsten heavy alloys are widely used in various industries due to a useful combination of properties such as high strength,high densities,high ductility,good corrosion resistance,high fracture toughness as well as high melting point.Since tungsten heavy alloys are often used in the process of high temperatures and high pressures,the mechanical properties of tungsten heavy alloys at different temperatures and strain rates have always been the focus of researchers.In this thesis,an electronic universal testing machine and a split Hopkinson pressure bar apparatus are used to study the mechanical properties of 95W-3.5Ni-1.5Fe（95W）at various temperatures,strain rates,and number of impacts.Further,viscoplastic constitutive models,ductile damage models,and a multiple impacts based constitutive model are studied to describe the mechanical properties of 95W under different loading conditions.The main research content of this thesis consists of the following parts:（1）Based on the quasi-static compression(0.001～0.1 s^-1,298 K)and dynamic impact(2500～11000 s^-1,298～773 K)experimental data of 95W cylindrical specimens（Φ4 mm×3mm）,the plastic behaviors of 95W under different loading conditions were analyzed.Five viscoplastic constitutive models,Johnson-Cook（JC）,Zerilli-Armstrong（ZA）,Mechanical Threshold Stress（MTS）,Modified ZA（M-ZA）and Modified MTS（M-MTS）,were fitted using the experimental data combined with Grey Wolf Optimizer,respectively.The description performance of these five constitutive models was compared,and it was found that M-MTS model had the best description performance.（2）The damage behaviors of 95W under quasi-static and dynamic loading conditions were studied based on experiments.The relations between 95W failure strain and stress triaxiality were obtained from the quasi-static tensile experimental data of smooth and notched round bars.Further,dynamic fracture experiments were conducted to obtain the relations between failure strain and Lode angle parameter,the relations between failure strain and strain rates,and the relations between failure strain and temperatures.Then JC damage parameters were obtained and a ductile damage model considering strain rates,temperatures,stress triaxiality,and Lode angle parameter was proposed.The proposed damage model was implemented by user-defined material subroutine（VUMAT）in ABAQUS.Eventually,the dynamic fracture experiments combined with the finite element simulations were employed to validate JC damage model and the proposed damage model.（3）The split Hopkinson pressure bar apparatus was employed to conduct the dynamic multiple impacts of 95W under the impact pressures of 0.2 MPa,0.3 MPa,0.4 MPa,and 0.5MPa.The impact numbers were 8 times for 0.2 MPa,7 times for 0.3 MPa,7 times for 0.4 MPa,and 6 times for 0.5 MPa.Then the stress-strain curves of multiple impacts under the four impact pressures were obtained.The mechanical properties of 95W under different impact numbers（the same impact pressure）and the mechanical properties of 95W under different impact pressures（the same impact number）were studied.Moreover,a constitutive model considering viscoplastic theory and damage behaviors was proposed to describe the mechanical behaviors of 95W under dynamic multiple impacts.The model description results were in good agreement with the experimental data.