JOURNAL OF MATERIALS SCIENCE 37 (2 0 0 2 ) 5197 – 5207 Experimental observations and computer simulations of spherical aluminum-alloy projectiles impacting plane limestone targets F. M. RANDRIANARIVONY, S. LAIR, S. A. QUINONES, L. E. MURR Department of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79968, USA E-mail: fekberg@utep.edu The stress distribution within spherical aluminum-alloy (2024) projectiles impacting plane limestone (calcite) targets was observed by converting residual microhardness maps obtained for cross-sections of recovered projectiles impacting from 0.8 to 1.3 km/s. A maximum residual yield stress zone was observed to migrate toward the rear of the impacting projectiles with increasing impact velocity. The maximum occurred at a normalized depth z ′ /a m ∼ = 0.5 (where a m is the contact radius); consistent with the theoretical result for elastic impacts. Computer simulations showed good agreement with experiment, and demonstrated that elastic assumptions were valid well into the plastic deformation regime. C  2002 Kluwer Academic Publishers 1. Introduction Impacts, including multiple impacts involving very small particles (0.02 to 0.2 mm) have a wide range of interests which encompass ballistic and hypervelocity impacts (with projectile diameters ranging from 0.3 to 30 cm). These can include military targets and a variety of structural systems in space, shot peening to improve the mechanical behavior of surfaces of structural parts, and a variety of cavitation-erosion damage to surfaces. This range of impact produces a wide variety of surface and subsurface damage effects which, on a larger scale, include cratering and deep penetration of structural tar- gets and armor. Particular interest in many related impact studies centers on the issues of subsurface deformation and residual stresses in the target materials, including the evolution or progression of microstructure [1–6]. In many instances, the surface or near surface region in the impacted, plane target exhibits some softening as a consequence of dynamic recrystallization followed by a hard zone which gradually declines toward the original target hardness [3, 5, 6]. These residual hardness zones increase in size and shift slightly into the target from the impact surface with increasing penetrator velocity, penetrator size, angle of impact, etc. In many studies, contact is characterized simply as the impact of rigid, elastic spheres producing a correspondingly, idealized target geometry, but in actual crater or penetration development, the actual crater geometries may become considerably exag- gerated, especially where the projectiles have very large densities (>7 g/cm 3 ) [7], and severe plastic deformation of both the projectile and target occur. In many impact systems which are treated as ide- ally elastic regimes, a maximum residual stress occurs below the impact (or target) surface and moves away from the surface and deeper into the target material with increasing impact velocity [2–5]. The practical impli- cations of elastic regimes include impacts on ceramic or other brittle surfaces which respond elastically up to fracture [8, 9]. However, at sufficiently high veloc- ities for finite target thicknesses, shock-induced spal- lation and other mitigating issues can intervene, and of course systems which can respond plastically do so. In this context, the impacting particles or projectiles are always of finite dimensions and for spherical pro- jectiles the geometry is particularly unique since from the instant of impact, the associated shock wave (or corresponding peak pressure) propagating through the projectile successively reflects from its surface. In ad- dition, the impacting projectile can undergo plastic de- formation which, at sufficiently high impact velocities can contribute to projectile flow and fragmentation. At hypervelocities, some impacting projectiles are often considered to melt or vaporize during crater formation. In retrospect, very little is known about the behav- ior of impacting projectiles, and in some cases this may be even more important than the target behav- ior. The issues of interest involve the development and movement of residual stresses in the impacting projec- tile with increasing impact velocity and the role these residual stresses may play in the actual deformation and fate of the projectile. Since the projectile has a fi- nite volume and idealized spherical shape at the outset, it can serve macroscopically and microscopically as both a quantitative and qualitative measure of elastic or elastic-plastic behavior. It is the intent of this study to examine idealized, spherical projectiles which have impacted relatively brittle/ceramic limestone targets over a range of impact 0022–2461 C  2002 Kluwer Academic Publishers 5197