A terminal ballistics application of transmission electron microscopy: The anatomy of a bullet hole

Abstract

A novel technique was devised for the selected area observation of residual microstructures in the deformation zones and the detached cap sections of 0.22 calibre bullet holes in thin sheets and laminate thicknesses ranging from 0.03 to 0.54 mm of 304 stainless steel and Inconel 600 by bright- and dark-field transmission electron microscopy. Microhardness measurements over the detached cap and petals, extending radially into the undeformed material, indicated the zone of action to be confined to a region bounded by the deviation of the impact zone from the plane of the plate. This feature was confirmed by electron microscope observations which indicated no defects beyond the bounds of the zone of action so defined. Residual microhardness was observed to increase from the radial bound of the zone of action toward the axis of projectile motion, with maximum hardness values roughly twice the undeformed sheet hardness observed in the petal edges and the detached caps. Dislocation densities were observed to increase correspondingly over this range (∼ 10⁶ to 10¹¹ cm⁻²), with deformation twins occurring in the petal edges and the detached caps of both materials. An analytical treatment of the specific ballistic perforation examined in this investigation based on the plastic stretching of the target zone of action, fracture and detachment of a cap, and the subsequent petalling of the crater as perforation commences, indicated an absence of high pressure shock effects.