Compression Behavior of Fluted-Core Composite Panels

Abstract

In recent years, fiber-reinforced composites have become more accepted for aerospace applications. Specifically, during NASA's recent efforts to develop new launch vehicles, composite materials were considered and baselined for a number of structures. Because of mass and stiffness requirements, sandwich composites are often selected for many 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. Flutedcore, which consists of integral angled web members with structural radius fillers spaced between laminate face sheets, is one such construction alternative and is considered herein. Two different fluted-core designs were considered: a subscale design and a full-scale design sized for a heavy-lift-launch-vehicle interstage. In particular, axial compression of flutedcore composites was evaluated with experiments and finite-element analyses (FEA); axial compression is the primary loading condition in dry launch-vehicle barrel sections. Detailed finite-element models were developed to represent all components of the fluted-core construction, and geometrically nonlinear analyses were conducted to predict both buckling and material failures. Good agreement was obtained between test data and analyses, for both local buckling and ultimate material failure. Though the local buckling events are not catastrophic, the resulting deformations contribute to material failures. Consequently, an important observation is that the material failure loads and modes would not be captured by either linear analyses or nonlinear smeared-shell analyses. Compression-after-impact (CAI) performance of fluted-core composites was also investigated by experimentally testing samples impacted with 6 ft.-lb. impact energies. It was found that such impacts reduced the ultimate load carrying capability by approximately 40% on the subscale test articles and by less than 20% on the full-scale test articles. 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 damagearrest capability of fluted core could provide benefits over traditional sandwich designs in certain weight-critical applications.

Evaluation of Analysis Techniques for Fluted-Core Sandwich Cylinders

“…With the exception of the curvature, these are essentially the same dimensions as the subscale test articles discussed in Ref. 3 and the curved panel of Ref. 4., and the cylinder described herein is representative of the test article shown in Fig.…”

“…Therefore, most structural analysts will be able to use it to analyze fluted-core structure. However, it has been shown that detailed analysis of the fluted-core can require very complex models; 3,4 this suggests finite-element analysis is generally well suited for the final detailed design phase of the DAC, where a final design is being analyzed to ensure it satisfies the requirements. It was demonstrated that relatively simple finite-element models can be used to analyze the fluted-core for basic response, such as global and local buckling, and overall strain requirements.…”

“…by 5.75 in. and smaller), flat test articles, Schultz, et al 3 developed very detailed models to simultaneously capture numerous failure modes, such as local buckling and high stresses, seen with the fluted-core construction. Their model used shell elements to represent the skin and flute portions, and solid elements to represent the noodle region.…”

“…Moreover, the cells in a fluted core can be filled 1 or unfilled. [2][3][4][5] The present authors consider corrugated-core sandwich construction (Fig. 1b) to be a subset of fluted-core sandwich construction, however, in many references the terminology has been used interchangeably.…”

“…Fluted-core sandwich concepts have been explored for structures representative of both aircraft and launchvehicle structures, [1][2][3][4][5] and consist of face sheets and a web core arranged in different ways, as shown in Fig. 1b-f.…”