Testing and Analysis of Advanced Composite Tow-Steered Shells

The structural performance of two advanced composite tow-steered shells, manufactured using a fiber placement system, is assessed using both experimental and analytical methods. The fiber orientation angles vary continuously around the shell circumference from 10 degrees on the shell crown and keel, to 45 degrees on the shell sides. The two shells differ in that one shell has the full 24-tow course applied during each pass of the fiber placement system, while the second shell uses the fiber placement system s tow drop/add capability to achieve a more uniform shell wall thickness. The shells are tested in axial compression, and estimates of their prebuckling axial stiffnesses and bifurcation buckling loads are generated using linear finite element analyses.

Cutouts, scaled to represent commercial aircraft passenger and cargo doors, are then machined into one side of each shell. The prebuckling axial stiffnesses and bifurcation buckling loads of the shells with cutouts are then computed using linear finite element analyses. When retested, large deflections were observed around the cutouts, but the shells carried an average of over 90 percent of the axial stiffness, and 85 percent of the buckling loads, of the shells without cutouts. These relatively small reductions in performance demonstrate the potential for using tow steering to mitigate the adverse effects of typical design features on the overall structural performance.

Previous studies have typically shown poor correlation between experimental buckling loads and supporting linear bifurcation buckling analyses. The good correlation noted for these tow-steered shells may result from their circumferential axial stiffness variation, which may reduce sensitivity to geometric imperfections. A numerical investigation was performed using measured geometric imperfections from both shells. Finite element models of both shells were analyzed first without, and then, with the measured imperfections, superposed in different orientations around the shell longitudinal axis. Small variations in both the axial prebuckling stiffness and global buckling load of the shells were noted for the range of orientations studied.