Structural nanocomposites for aerospace applications
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Introduction Carbon-fiber-reinforced polymer (CFRP) composites are the state-of-the-art material systems for lightweight aerospace structures. Because of their mass advantage over metallic material alternatives, their use in commercial aircraft has increased steadily over the past 50 years, as shown in Figure 1.1–3 The increasing weight fraction of CFRPs in commercial aircraft is consistent with the S-curve pattern associated with technology maturation, where the use of technologies reaches a plateau following a rapid rise in adoption.4 Just as the development of carbon fibers was spurred by the need for strong, lightweight materials in aerospace applications,5 projections of carbon nanotubes (CNTs) being a game-changing structural material were based on their superior mechanical properties enabling the next generation of lightweight, efficient aerospace vehicles.6 The promise of CNTs being a revolutionary material was triggered by Iijima’s seminal paper,7 which captured the imagination of the research community. In the United States, research was fueled by focused funding infused by the National Nanotechnology Initiative.8 Today, a cursory search for publications associated with CNTs results in over 100,000 publications, attesting to the scale of investment in CNT-related research. The attention garnered by these reports kindled a surge in expectations that this unique material would have an imminent impact. The interest in CNTs follows the generic technology
life-cycle phases depicted by Gartner, Inc. as the Gartner Hype Cycle.4 In Figure 2, a publications plot is overlaid on the Gartner Hype Cycle, annotated with some key CNT events relevant to the advancement of CNTs toward structural applications. With what appears to be the beginnings of the “trough of disillusionment” on the Gartner Hype Cycle being coincident with the greater commercial availability of bulk formats of CNTs, the time is right for a perspective on the realistic impact CNTs can have. The purpose of this article is to examine how close CNTs have come to fulfilling expectations for lightweight aerospace structures in the 24 years since Iijima’s report7 stimulated intense interest in this material and to propose areas of study to bridge knowledge gaps that can realize the potential for this material to be part of the lightweight structures technology suite. This review is limited to CNT composites and hybrid composites of CNT and CFRPs having engineering polymeric matrixes such as epoxies, bismaleimides, and polyimides.
Carbon nanotubes CNTs are high-aspect-ratio cylinders of graphene sheets.9 Figure 3 shows that, at the nanoscale, the specific strength of CNTs is 10 times greater than that of Hexcel’s polyacrylonitrile(PAN-) based IM7 carbon fiber, and their specific modulus is five-fold higher. At the time this assessment was conducted,6 the promise of structural CNTs was based on theoretical calculations for single-walled nanotubes. Experimental validation of
Emilie J. Siochi, Advanced Materials and Processing Branch, NASA Langley Research Center,
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