Hollow cylindrical Al-6061 T6 projectiles, driven in a coilgun system, suffer radial pinching and are deformed into pentagonal cross-sections. In some cases, the projectiles develop longitudinal cracks in the pinched region. This is studied as an instance of plastic buckling under radial compression. Dominant buckling modes are identified from the final projectile configurations obtained through modelling and compared with an analytic estimate. The dominant mode from simulations are κ = 3 and 4 while the analysis, assuming buckling of a ring, gives κ = 7 where κ is the wavenumber. A magnetic Rayleigh-Taylor (MRT) analysis is carried out in conjuction with magnetohydrodynamics simulations of the coil gun projectile system. An initial perturbation in projectile thickness is unstable provided the amplitude of the perturbation is more than 20 μm. The MRT-unstable projectiles show evidence of longitudinal cracks.

Keywords: Magnetic Rayleigh-Taylor instability, Buckling failure, Coil gun.