The limit velocity and limit displacement of nanotwin-strengthened metals under ballistic impact

Abstract

A new category of structural metals with high strength and good ductility is coarse-grained metals strengthened by nanotwinned (NT) regions. This unique combination makes them particularly suitable for applications in bulletproof targets. The extent of this improvement, however, depends on the volume fraction of the NT regions and multiple other microstructural features. Here, a numerical model based on the strain gradient plasticity and the Johnson–Cook failure criterion is developed to study the effects of these attributes. The ballistic performance is quantified by two indices: the limit velocity that measures its ability to resist failure and the limit displacement that measures its ability to resist deformation. The results obtained indicate that, in general, a relatively small twin spacing and a moderate volume fraction of NT regions can achieve both excellent limit velocity and limit displacement. Moreover, array-arranged NT regions are more effective than staggered NT regions in enhancing the limit velocity, but the influence of the array group tends to depend more on the volume fraction of NT regions than the latter one. Mechanism analyses based on the three stages of the impact process and two categories of low-speed impact are performed. The simulated results could provide significant insights into the NT structures for a superior class of nanotwin-strengthened metals for ballistic protection.