Compression Response of Fluted-Core Composite Panels

In recent years, fiber-reinforced composites have become more accepted for aerospace applications. For example, during NASA's recent efforts to develop new launch vehicles, composite materials were considered and baselined for a number of structures, including dry barrel sections, which are primarily loaded in longitudinal compression. Because of mass and stiffness requirements, sandwich composites are often selected for these applications. However, there are a number of manufacturing and in-service concerns associated with traditional honeycomb-core sandwich composites that in certain instances may be alleviated through the use of other core materials or construction methods. A fluted core, which consists of integral angled web members with structural radius fillers spaced between laminate facesheets, is one such construction alternative. In this paper, two different fluted-core composite designs were considered: a subscale design and a full-scale design sized for a heavy-lift-launch-vehicle interstage. In particular, the longitudinal compression behavior of fluted-core composites was evaluated with experiments and finite-element analyses. Detailed branched-shell finite-element models were developed, and geometrically nonlinear analyses were conducted to predict both buckling and material failures. Good agreement was obtained between test data and analysis predictions for both failure types. Though the local buckling events are not catastrophic, the resulting deformations contribute to material failures. Consequently, neither the local buckling behavior nor the material failure loads and modes can be predicted by either linear analyses or nonlinear smeared-shell analyses. Compression-after-impact performance of fluted-core composites was also investigated experimentally. Nondestructive inspection of the damage zones indicated that the detectable damage was limited to no more than one flute on either side of any given impact. More study is needed, but this may indicate that an inherent damage-arrest capability of fluted core could provide benefits over traditional sandwich designs in certain weight-critical applications.