Preliminary Nonlinear Structural Analysis of Advanced Composite Tow-Steered Shells with Large Geometric Imperfections

The structural performance of two advanced composite tow-steered shells with and without tow overlaps, and with large geometric imperfections, are predicted using linear and geometrically nonlinear finite element analyses. These shells, 35 in. long and approximately 16.3 in. diameter, are fabricated using automated fiber placement from IM7/8552 graphite/epoxy prepreg. The 8-ply,[±45/±Θ]s. shell layup incorporates a steered fiber angle Θ that varies from 10 deg. to 45 deg. periodically over the shell circumference. Shell analysis models are evaluated using geometric imperfections normalized to ±1 shell wall thickness (±0.040 in.), which are then superposed and rotated incrementally around the shell longitudinal axis. Using these nominal imperfections, the shell prebuckling axial stiffnesses and buckling loads predicted with linear and nonlinear analyses are close to reference values from linear analyses with no imperfections. The linear and nonlinear analyses are then repeated for scaled imperfections that are larger by up to a factor of 10. For these larger imperfections, linear analyses predict reductions in axial stiffnesses and buckling loads of up to 5 and 30 percent, respectively, from reference values. The nonlinear analyses predict even larger reductions in axial stiffnesses and buckling loads of up to 10 and 55 percent, respectively.