BALLISTIC INVESTIGATION OF STRATEGICALLY TUNED ABSOLUTELY RESILIENT STRUCTURES

This paper investigates the impact properties of Strategically Tuned Absolutely Resilient Structures (STARS) to establish their potential for use in energy transfer and storage applications. Ballistic tests were conducted on six (6), 20.3 cm x 20.3 cm (8 in. x 8 in.) reinforced cementitious composite plates having a constant thickness of 1.27 cm (0.5 in.). Each plate was reinforced with four (4) layers of graphite/Kevlar mesh. The energy absorption and mass reduction were recorded for plates impacted with projectiles traveling with velocities ranging from 9.5 m/s (31 ft/s) to 511 m/s (1678 ft/s). Although the number of samples was limited, a post-impact analysis was conducted to identify the ballistic limit from a limit velocity curve. In cases where the projectile partially penetrated or completely perforated the plate, spall consisted of relatively small particulates. No shear-plugging was observed in any of the tests. PRIOR RELATED RESEARCH Strategically Tuned Absolutely Resilient Structures (STARS) represent a new generation of composites fabricated by placing a low modulus, lightweight matrix over multiple layers of a relatively stiff reinforcement [1]. These remarkable concoctions are designed to morph based on the strength, stiffness, and the position of the component materials in the composite section and their ability to store and release energy depends upon a complex interaction between the shape, modal response, and the forcing function applied to drive the structure [2]. The evolution of STARS [3] began with pioneer research that focused on producing a new generation of thin, lightweight, and structurally efficient panels capable of resisting stresses produced by reverse loading [4]. The study showed that a very efficient composite structure could be fabricated by placing a flexible polymer-enhanced cementitious matrix having a relatively low elastic modulus over two layers of a rigid steel wire mesh having a relatively high elastic modulus. The design relied on the large difference in stiffness between the constituents in the composite section to drive the internal stress from the cementitious matrix to the steel reinforcement and materials were placed symmetrically to form an '”adaptive” section that reacted similarly when bending couples were reversed. Since the compressive strength of the cementitious material was less than its tensile strength [5], it was the modulus of elasticity and tensile strength of the cementitious matrix, as well as the bond strength between the matrix and the reinforcement, that impacted STARS design most. A modified transform section theory was developed to determine the deflections and stresses in such highly compliant cementitious structures [6] and the method was applied to study graphite-reinforced composites. Multilayered composite beams were analyzed by incorporating material properties established from standard tests and finite element modeling was used to verify results. The work fueled another investigation that quantified the dynamic characteristics of laminated plates [7]. In this study, an analytical dynamic finite element model was developed to evaluate the natural frequencies and mode shapes for structures subjected to different boundary conditions. This model was subsequently applied to study the dynamic performance of a larger structure [8]. Numerical results compared favorably with experimental impact hammer test data. As a result, it was concluded that the classical laminated plate theory developed for composite materials could be applied to quantify the dynamic behavior of highly compliant composite structures made from cementitious materials. In short, prior work on STARS included static tests that established material properties and quantified structural behavior, and dynamic analyses that characterized modal parameters. In all of these cases, it was assumed that all constituents remained intact as the service load was applied. However, in some severe test environments, short-term dynamic impact loads can occur, resulting in service overload. In many cases, material deficiencies caused by impact can be overcome by introducing a stiff weave-like reinforcement [9]. Second generation STARS [6-8] fall into this category and this is the first attempt to characterize their impact/ballistic characteristics. TEST SPECIMENS A series of ballistic impact tests were conducted at the University of Alabama in Huntsville Aerophysics Facility, which is located on Redstone Arsenal. This paper focuses on six (6), 20.3 cm x 20.3 cm (8 in. x 8 in.) reinforced cementitious composite plates having a constant thickness of 1.27 cm (0.5 in.). Each plate was reinforced with four (4) layers of a graphite/Kevlar mesh. Figure 1 is a schematic showing the front view of the target plates while Fig. 2 shows how the four (4) layers of mesh were positioned through the thickness. Table 1 describes the mix proportions of the constituents used to create the cementitious matrix. The elastic modulus and Poisson’s ratio of the mix was 793 MPa (115 ksi) and 0.28, respectively. It had a 28-day compressive strength of 4.8 MPa (696 psi) and a 7-day tensile strength of 1.77 MPa (256 psi). As shown in Fig. 3, each layer of reinforcement consisted of a non-impregnated, bi-directional graphite mesh woven together with Kevlar fibers. The graphite mesh had 3,000 fibers per tow, spaced at 3.18 mm (0.125 in.) intervals. Each tow is 0.19 mm (0.0075 in.) thick by 1.07 mm (0.042 in.) wide. The manufacturer reports the elastic modulus and tensile strength of the graphite fiber to be 231 GPa (33.5 Msi) and 3.65 GPa (530 ksi), respectively.