Performance Limits for Stiffness-Critical Graphitic Foam Structures. Part I: Comparisons with High-Modulus Foams, Refractory Alloys and Graphite-Epoxy Composites

Abstract

Graphitic foams offer exciting potential for mass savings in structures. The present investigation seeks to identify the potential mass savings and corresponding dimensional tradeoffs for application of such materials to simple, stiffness-critical structural elements, in support of an in-house materials development program. A hypothetical family of graphitic foams having ligament stiffness properties identical to those of P120 graphite fiber is considered; the stiffness properties of the hypothetical foams are viewed as realistic upper bounds for the performance of this class of materials. A representative refractory alloy, AS4/3501-6 unidirectional and [0/90], composites, and commercially available SiC and amorphous carbon foams are compared to the graphitic foams for application to plates in bending and buckling, beams in bending and buckling, and tension bars, in analogy to Gordon [1] and Ashby [2,3]. The mass and thickness ratios of the foams to the comparison materials for the chosen applications depend primarily on the Young's modulus of the foam, which is modeled according to a well-established relation due to Gibson and Ashby [4]. The hypothetical foams are found to offer stiffness properties leading to structural masses ranging from competitive to dramatically improved versus the described comparison materials. Increased structural dimensions are often associated with the savings in mass. Strength properties have not been considered in the present modeling, as processing issues still need to be resolved and the resulting defect distributions are unknown; foams of substantial promise are, however, being fabricated in small quantities.