A military application of tungsten carbide cobalt (WC-Co) materials is in armour piercing (AP) small calibre projectiles, where the core material should be very hard and have a high density in order to penetrate the armour effectively. To be able to accurately predict the interaction between projectile and armour, it is important to have reliable criteria for the failure of the WC-Co projectile material. In this report the plastic flow and failure criterias for a cobalt cemented tungsten carbide material are investigated. Emphasis is on the influence of the stress state on the deformation and failure while the effect of strain rate and temperature is not studied in depth. A model for the plastic flow and failure of the tungsten carbide material is developed using a combination of experiments and simulations. The model is tested in simulations of both quasistatic cases and in dynamic loading cases such as ballistic penetration. It is found that the model reasonably well reproduces the deformation and failure of the tungsten carbide material over a wide range of stress states during quasi-static loading. However in combination with the AUTODYN software, the model predicts failure of the material at an earlier stage of penetration than what is found in the ballistic experiments. It is suggested that the strain rate effect on both the strength and the failure model needs to be accounted for and work in this area is on-going. It should be pointed out that different numerical effects may also contribute to the rapid material failure in the simulations and this needs to be further investigated.