Numerical studies of ejecta characteristics behind thin plates

Abstract

A study of hypervelocity impact of spheres upon thin targets is described. The study was based upon numerical solutions utilizing the STEEP code. (STEEP is a two-dimensional Eulerian technique, based upon a hydrodynamic-e lastic-plastic model.) The cases considered are Al, Cu, and Cd spheres impacting at 0.75 and 1.5 cm/jusec (24,600 and 49,200 fps) upon like-material target plates, with the plate thickness being J of the projectile diameter. Results can be scaled to any impact case having the same ratio. From the numerical solutions, spatial plots showing the time-resolved response of the projectile and target are shown in the form of velocity, and principal stress fields. The primary objective of the solutions was to determine the characteristics of the debris clouds behind the targets. Curves are presented showing debris mass, axial momentum, and kinetic energy per unit solid angle intercepted on witness plates behind the targets. Information regarding the residual states of the debris materials is also given. YPERVELOCITY impacts produce extreme pressures in the projectile and target materials. When the target is thin relative to the projectile dimensions, these propagate through to the rear surface, thereby accelerating the target material. Subsequent pressure rarefactions shatter the projectile and target materials, and a debris cloud is ejected at high velocity behind the target. The effectiveness of thin metal plates in breaking up and dispersing compact hypervelocity projectiles into a cloud of tiny debris particles is the basis of bumper concepts for protecting structures against such impacts. With a properly selected bumper, the cumulative effect of the impacts of the debris particles on the protected structure will be less damaging than the effect of the original proj ectile. However, the debris itself will cause some damage to the protected structure, and means for absorbing the debris energy and for disbursing its momentum are sought. For the design of such absorbers, knowledge regarding the debris characteristics is needed. Experimental observations can be used to assess some debris characteristics, but such measurements are often difficult, and do not yield as complete a description as may be desired. Numerical techniques offer an alternative source of data, and can further be used to identify important mechanisms which operate in the formation and dispersion of the debris. In addition, numerical techniques can be applied to obtain data in experimentally inaccessible regions, e.g., extremely high impact velocities. Two-dimensional numerical techniques have been developed and applied to a range of hypervelocity impacts into semiinfinite and relatively thick targets,13 as well as into targets where the thickness is just sufficient to defeat the projectile.4 Using the STEEP code, detailed solutions have now been obtained to study the debris characteristics in the seven thin plate impact cases in Table 1. The hydrodynamic-elasticplastic model used in this code is described in Read.5 The numerical solutions of the seven cases treated permit a limited analysis of material property effects, impact velocity effects, and elastic-plastic strength effects. For these solu