  Calibrating and verifying crystal plasticity material models is a significant
challenge, particularly for materials with a number of potential slip and twin
systems. Here we use digital image correlation on coarse-grained
$\alpha$-uranium during tensile testing in conjunction with crystal plasticity
finite element simulations. This approach allows us to determine the critical
resolved shear stress, and hardening rate of the different slip and twin
systems. The constitutive model is based on dislocation densities as state
variables and the simulated geometry is constructed from electron backscatter
diffraction images that provide shape, size and orientation of the grains,
allowing a direct comparison between virtual and real experiments. An
optimisation algorithm is used to find the model parameters that reproduce the
evolution of the average strain in each grain as the load is increased. A
tensile bar, containing four grains aligned with the load direction, is used to
calibrate the model with eight unknown parameters. The approach is then
independently validated by simulating the strain distribution in a second
tensile bar. Different mechanisms for the hardening of the twin systems are
evaluated. The latent hardening of the most active twin system turns out to be
determined by coplanar twins and slip. The hardening rate of the most active
slip system is lower than in fine-grained $\alpha$-uranium. The method
developed in the present research can be applied to identify the critical
resolved shear stress and hardening parameters of other coarse-grained
materials.
