Abstract: Shear failure is an important mode of dynamic failure of structural members under impulsive loading.In this paper the non-dimensional criterion of the shear failure of a free-free beam subjected to impact by a projectile at the mid-span is presented.It is illustrated that whether a shear failure occurs at the interface, where the initial velocity has discontinuity, depends on the initial kinetic energy of the projectile, the mass ratio and the non-dimensional thickness of the beam.