Performance Limits for Stiffness-Critical Graphitic Foam Structures. Part II: Comparisons of Foams,Foam-Core and Honeycomb-Core Sandwiches in Bending/Shear

Abstract

Part I of this study [1] described the concept of graphitic structural foams and indicated their substantial potential for saving mass in stiffness-critical structures. Upper bounds, considered achievable, on ligament moduli were assumed in a well-known, empirically-based relation [2] modeling the foam stiffness versus porosity. The resulting hypothetical graphitic foams were compared to commercially available, high-modulus solids and foams for simple structural applications, and were found to offer the potential for dramatic mass reductions. The present paper compares the potential stiffness-critical performances of neat graphitic foams in plate cylindrical bending to those of honeycombcore sandwich materials, as well as evaluating the foams as potential sandwich core materials. These evaluations require estimates of the foam shear modulus and shear deformation, which were not included in Part I, in order to model the foam deflection to the order standard for sandwich modeling. Three separate foam constitutive models ([2], [3] and elements of [2]-[4]) are considered in the plate bending comparisons, based on upper bound ligament stiffnesses in analogy to [1]; all three assumptions produce similar results, especially for foam porosities exceeding 90 percent. It is found that the influence of shear deformation [5] grows dramatically at porosities exceeding 97 percent, where higher-order theories should be appealed to for accuracy. As the construction of a sandwich of a given stiffness is not unique for given facing and core materials, only mass-optimum sandwiches were compared; each sandwich incorporated the same graphite-epoxy material for face sheets. The neat graphitic foams and foam core sandwiches were found to be competitive on mass and thickness bases with the honeycomb-core sandwich materials for porosities exceeding roughly 90 percent. The graphitic foams and foam-core sandwiches offer the potential for increased in-plane stiffnesses together with the bending stiffnesses associated with honeycomb sandwiches. Finally, the graphitic foams and foam-core sandwiches are briefly compared to the solid materials considered in Part I, including now the effects of shear deformation, and the foams are found to offer mass advantages similar to those predicted in Part I. In summary, Parts I and II indicate that graphitic foams offer exciting potential as additional materials for the design engineer's menu, and may be especially advantageous for stiffness-critical applications involving complex loadings/geometries where minimum mass drives the design.