EXPERIMENTAL AND NUMERICAL INVESTIGATION INTO THE EFFECTS OF PANEL CURVATURE ON THE HIGH VELOCITY BALLISTIC IMPACT RESPONSE OF ALUMINUM AND COMPOSITE PANELS

Abstract

Determining how a material responds to a ballistic impact is important for designing improved penetration-resistant structures. Historically, the majority of research into the effects of ballistic impact has been for flat geometries. However, in aerospace applications, the surfaces that would most likely be subjected to high velocity impacts, the fuselage and the wing sections, are not flat. A need therefore exists to systematically examine and understand the effect, if any, of panel curvature on the ballistic response of both aluminum and composite panels. For this dissertation, a hybrid combination of experimental testing and numerical modeling which was employed to examine the effects of panel curvature on the ballistic limit, the dynamic panel response, and the impact-induced damage in the target material is discussed. Panels of varying curvature were impacted by ½-inch diameter steel spheres for a range of impact velocities that bracketed the experimentally-determined ballistic limit. AS4-3501-6 graphite-epoxy composite panels with two varying curvatures, a 4.4-inch radius of curvature and a 12-inch radius of curvature, and 2024-T3 aluminum panels with four varying curvatures, a 4.4-inch radius of curvature, an 8-inch radius of curvature, a 12-inch radius of curvature, and an infinite radius of curvature (flat plate), were tested. Non-linear finite element models consistently and reliably modeled the ballistic impact event, for both the flat and the curved panels, when the specified elastic modulus correctly captured the characteristics of the wave propagation behavior for the panel material being modeled. For the composite panels, dynamic deformation measurements and strain-gage-instrumented impact tests indicated that an effective elastic modulus on the order of the tensile modulus of the matrix material was more appropriate than a "rule-of-mixtures" effective modulus. The combined experimental-numerical results also identified a parabolic relationship between the panel curvature and the ballistic limit. More importantly, an optimal panel curvature with respect to maximizing the ballistic limit was shown for both the aluminum and the composite panels. Preliminary results from non-destructive and destructive post-impact evaluations suggest that the severity of impact damage may also vary with panel curvature.