Plate impact and plastic deformation by projectiles

Abstract

The problem of plastic deformation of thin plates struck by blunt projectiles but not perforated is considered. A streak camera and moiré technique were developed to measure the deflection as a function of time for the first few hundred microseconds after impact. The maximum radial strain is about three times the yield strain, the maximum tangential strain is very small. From these and other considerations it was found that there are three main regions of deformation. Since this is a traveling-wave problem, there is a radius beyond which there is no deformation. Inside of this is an annular region traversed by elastic bending waves of the Boussinesq type. Inside of this is an annular region whose dynamic behavior is governed by a membrane stress equal to the tensile yield stress and whose outer boundary travels at a wave velocity governed by the membrane equation. The inner region is a circular region of the diameter of the projectile and traveling with it. The projectile slows and comes to a stop after about 200 or 300 μsec. After this, the material unloads elastically; this elastic vibration behavior was not investigated. Both the time-displacement curve for the projectile and the displacement as a function of radius for specific times can be calculated numerically for dimensionless radius and dimensionless time for one parameter which is governed primarily by the projectile relative mass. The measured and predicted displacements agree within a few percent, except near the boundary between the plastic and the elastic region, for all times up to when elastic unloading commences.