Prebuckling and postbuckling behavior of stiffened composite panels with axial-shear stiffness coupling

Abstract

To advance structural tailoring methods in composite structures, an experimental and numerical investigation of the prebuckling and postbuckling responses of flat rectangular graphite-epoxy composite panels with a centrally located I-shaped stiffener subjected to a uniform end shortening is presented. Axial-shear stiffness coupling is introduced by rotating the stiffener and/or prescribing skin laminates with membrane and bending stiffness coupling. A panel’s axial-shear coupling response is defined as the ratio of the panel’s shear load to its compression load when a simple end shortening is applied.

Experimental results are reported for five panels. The baseline test panel has an unrotated stiffener and a [±45/∓45/0₃/90]_s skin laminate. Two panels have either the stiffener or the entire skin laminate rotated 20°, and the remaining two panels have both the stiffener and the skin laminate rotated by 20°, either in the same direction, or in opposite directions. Extensive experimental data are obtained electronically during quasi-static tests. Finite element models are defined which accurately represent the conditions in the experiment, and geometrically nonlinear analyses are conducted. Measured and predicted responses are compared to verify the numerical models. The panels’ stiffness, buckling parameters, load vs. end shortening relations, out-of-plane deformations, and axial-shear coupling responses are reported. The finite element analyses, based on two-dimensional plate elements, are utilized to address failure due to skin-stiffener separation by estimating the skin-stiffener attachment forces and moments at failure.

The results of a parametric study which isolates the mechanisms which contribute to axial-shear stiffness coupling are reported. It is found that rotating the stiffener or introducing skin anisotropy typically reduces the axial stiffness and buckling loads. The axial shear coupling response due to rotating the stiffener is constant in prebuckling and increases after skin buckling, and the magnitude of the response can be adjusted by varying the stiffener rotation and rigidity. Skin membrane stiffness coupling creates axial-shear coupling responses that are constant in prebuckling and decrease in magnitude after skin buckling. Skin bending stiffness coupling creates axial-shear coupling responses that are zero in prebuckling and increase in magnitude after skin buckling. Examples are presented which demonstrate how different mechanisms can be tailored independently and then superimposed to effectively tailor a stiffened panel’s axial-shear coupling response in the pre buckling and postbuckling load ranges.