Microscopic fracture mechanisms observed on Cu–Sn frangible bullets under quasi-static and dynamic compression

The damage behavior of Cu–Sn frangible bullets was characterized in an effort to aid predictions of impact performance of these projectiles with soft body armor through finite element simulations. Fracture surfaces and failed cross sections were examined via light optical and scanning electron microscopy and related to the composite bullet microstructure. Two types of samples were analyzed: (1) those used in quasi-static and dynamic diametral compression testing to determine the effective properties of the composite material, and (2) bullets discharged into soft body armor. Two primary microscopic fracture mechanisms were cleavage and intergranular fracture of the Cu–Sn intermetallic compounds, ɛ(Cu₃Sn) and η(Cu₆Sn₅), which joined the un-bonded copper particles in the composite microstructure. Microvoid coalescence of copper metal was also observed, though infrequently, in places where the spacing between intermetallic phase clusters on a single copper particle was typically no greater than 30 μm. These modes of failure were similar between the samples used in the mechanical testing methods and the discharged bullets. From these results, it is reasonable to assume that the failure strength data measured via diametral compression testing can be used to predict the onset of bullet failure on impact during finite element simulations.