The Optimization For Tensile Split Hopkinson Bar Tests And The Study On Constitutive Models And Dynamic Fracture Of Oxygen-free High Conductivity Copper

The optimization of tension Hopkinson bar (TSHB) tests has been studied. The structure and loading conditions of TSHB have evident influence on test results including the gauge length and geometry of specimen, interconnecting linkage, contact boundary, stress waves and so on. In order to determine the mechanical parameters in specimen using the strains obtained from incident bar and transmitted bar, the numerical simulation of TSHB test processes should be performed to result in uniaxial stress and uniform stress and strain in the specimen. The numerical results shown that the length to diameter ratio of tensile specimen should be 1 ~ in specimen with specifical transition radius at ends.Using optimized TSHB tests, the effects of heat-treatment process and tension-compression asymmetry on constitutive relations for Oxygen-free high conductivity copper (OFHC) have been studied. The micro structure and the initial yield stresses of OFHC in annealed and as-received states have been analyzed. In compared with as-received specimen, the toughness of annealed specimens increase remarkably and the stress-strain curve of it has much distinction. The differences between dynamic tension and compression constitutive relations of OFHC are more evident with increasing of strain and strain rate. It is indicated that the determined Z-A type constitutive model of OFHC is more suitable than J-C type model for representing the strains on incident and transmitted bars in numerical simulations of TSHB tests.The dynamic necking and fracture of OFHC specimen in TSHB tests are also investigated. The computed deformation and non-localized necking of OFHC specimen using the determined constitutive model are consistent with that recorded by high-speed photography. However, the criterion for localized necking and failure of bars based on the computed variation rate of the cross section of the specimen could not match experimental results. It seems that the criteria for localized necking and failure should be based on dynamic constitutive relations with involving the damage evolution. A dynamically computational void cell model is presented to investigate void growth and coalescence process in uniaxial impact tension. It seems that the criterion of void coalescence based on void shape evolution is coincident with experimental results.