Numerical simulation of ballistic performance of bimodal nanostructured metals

Nanostructured (NS) metals with bimodal grain size distribution, composed of coarse grained (CG) and nanograined (NG)/ultrafine-grained (UFG) regions, not only have high strength and good ductility, but they can also provide excellent ballistic performance. In this work we use numerical simulations to study the influence of microstructural attributes such as the distribution and shape of the CG inclusions on the ballistic performance of bimodal NS Cu. Our formulation is based on the mechanism-based strain gradient plasticity theory and the Johnson-Cook failure model. We also consider the effects of constitutive and failure parameters of the NG phase as well as boundary constraint of the specimen. Our results on the effect of inclusion attributes indicate that microstructures can significantly affect the velocity history of the bullet as well as limit velocity and maximum displacement of the specimen. The analysis also suggests that, to improve the ballistic performance of the bimodal NS metal, the CG inclusions need to have regular distribution and, under the condition of same distribution, they also need to have a longer projection perpendicular to the direction of impact. The simulations show that when the abrasion effect is not considered, the ballistic performance of the microstructures depend heavily on their ductility rather than their strength. The results on the boundary constraint show that when this constraint is released, the ballistic performance is reduced tremendously. It is believed that these results could provide useful insights into the development of advanced NS materials for ballistic protection.