Determining the stress-strain behaviour at large strains from high strain rate tensile and shear experiments

To characterise the high strain rate mechanical behaviour of metals, split Hopkinson bar experiments are frequently used. These experiments basically yield the force and elongation history of the specimen, reflecting not only the specimen material behaviour but also the specimen structural behaviour. Calculation of the real material behaviour from this global response is not straightforward, certainly for materials such as Ti6Al4V where due to low strain hardening, the specimen deformation is very inhomogeneous. However, for fundamental material research and constitutive material modelling, knowledge of the true effective stress versus plastic strain, strain rate and temperature is essential.In this contribution, a combined experimental-numerical approach for extraction of the strain rate and temperature dependent mechanical behaviour from high strain rate experiments is presented. The method involves the identification of the material model parameters used for the finite element simulations. The technique is applied to determine the stress-strain behaviour of Ti6Al4V using both high strain rate in-plane shear and tensile test results. For the tensile tests, even stress-strain data beyond diffuse necking are retrieved. A comparison is made between the material behaviour extracted from the tensile and the shear experiments. The material behaviour is modelled with the Johnson-Cook constitutive relation. It is found that the simultaneous use of tensile and shear tests to identify the model parameters gives a more generally applicable model. Validation of the material model and the finite element simulations is done by local strain measurements in the shear and tensile test by means of digital image correlation.