Evolution of Strategically Tuned Absolutely Resilient Structures (STARS)

This paper describes how Strategically Tuned Absolutely Resilient Structures (STARS) are being designed for energy transfer and morphing applications based on the strength, stiffness, and the position of the component materials in the composite section. The discussion ranges from on-line health monitoring and smart diagnostics/prognostics strategies developed to improve the readiness and performance of STARS to integrated sensors and structural information systems designed to keep the stress in the reinforcement below yield and prevent cracks from propagating through the surrounding matrix. INTRODUCTION 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]. Pioneer research involving STARS focused on producing a new generation of thin, lightweight, and structurally efficient panels capable of resisting stresses produced by reverse loading [3]. 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. A modified transform section theory was developed to determine the deflections and stresses in such highly compliant cementitious structures [4] 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 verity results. The work fueled another investigation that quantified the dynamic characteristics of laminated plates [5]. 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 [6]. 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. COMPOSITE DESIGN AND STRESS TRANSFER STAR structures are fabricated from composite materials with constituents that are purposefully stressed and deformed to the largest extent possible so that the maximum amount of strain energy can be stored and released. Their design relies on large differences in stiffness between the constituents in the composite section to drive the internal stress from a flexible matrix to relatively stiff reinforcement. In most cases, reinforcements are strategically placed, symmetrically through the thickness, to form an “adaptive" section that reacts similarly when mechanical loadings are reversed. This stress transfer can be visualized using the method of photoelasticity by observing the isochromatic fringe patterns shown in Figure 1 [3].