Petalling of thin, metallic plates during penetration by cylindro-conical projectiles

Abstract

A theoretical and experimental investigation has been undertaken to study the processes of perforation at normal incidence of thin, soft aluminum plates by hard-steel cylindroconical projectiles of 30° half-cone angle. The analysis is executed by means of an energy balance for a rigid/perfectly-plastic or work-hardening material. For an intact plate, where petalling always occurs, consecutive stages of the process involve initial crack propagation followed by plastic hinge motion out to the position of crack arrest, followed by petal bending due to hinge rotation up to and beyond the point of projectile passage. In the case of central impact on an initial hole in the plate, the first stage constitutes an enlargement of the hole, followed by crack propagation and bending of trapezoidal regions of the plate until the projectile has either perforated or is embedded in the target.Tests conducted on 2024-0 aluminum with a thickness of 3.175 mm indicated a condition so that if the initial hole radius was greater than about 2/3 that of the 12.7 mm diameter hard-steel projectile, cracking did not occur and all the energy was absorbed by hole enlargement. Furthermore, for each initial impact velocity, an optimal hole radius was found to exist where a maximum energy absorption of the plate occurred, greater than for the intact target. This phenomenon was qualitatively substantiated by the theory; discrepancies in magnitudes are attributed to the neglect of certain energies in the analysis, particularly that of dishing. Excellent correlation was found between the theoretical prediction for the terminal velocity of a projectile striking the intact plate and test results when an estimate of the dishing energy for the plate was included.