Tip splitting and ring formation in impact of hollow Al-6061 cylinders on Al-6061 plates

Abstract

Numerous ductile failure models have been proposed in literature and it is useful to compare them for test problems. We compare three simple failure models on projectile impact problem in which hollow cylindrical Al-6061 projectiles, strike 5 mm thick Al-6061 plates at 403 m/s and undergo fracture in the projectile tip with high speed ejection of a ring-shaped fragment. High resolution simulations employing an axisymmetric formulation of smoothed particle hydrodynamics (SPH) are carried out using three different failure models: (1) a standard Johnson–Cook fit of Leuser (JC); (2) a novel Johnson–Cook fit of Ghahremaninezhad and Ravi-Chandar (GR) based upon grain-level plastic strain measurements; and (3) the Bao-Wierzbicki (BW) failure model. Numerically, the projectile tip splits and a ring is ejected when the SPH interparticle spacing 62 μm or lower, with JC or BW failure. However, simulations using JC model show further splitting of projectile tip contrary to the experiment. Grain-level GR fit produces failure of projectile tip with SPH interparticle spacing of 24 μm. This length is roughly the grain size in Al-6061. Thus, it confirms the idea of Ghahremaninezhad and Ravi-Chandar that high resolution simulations require caliberation of the failure model at the same scale. The ring dimensions obtained through simulations have a discrepancy of about 20 percent with experimental values. Hence, simple BW and GR ductile failure models reproduce experimental results in a relatively complex projectile impact. However, residual discrepancies show need for further investigation of axisymmetric SPH and failure models.