NUMERICAL INVESTIGATIONS OF FREE-EDGE EFFECTS IN INTEGRALLY STIFFENED LAYERED COMPOSITE PANELS

A linear finite element analysis is conducted to examine the free edge stresses and the displacement behavior of an integrally stiffened layered composite panel loaded under uniform inplane tension. Symmetric (+PHI, -PHI, 0, -PHI, +PHI) graphite-epoxy laminates with various fiber orientations in the off-axis plies are considered. The quadratic stress criterion, the Tsai-Wu criterion and the Mises equivalent stresses are used to determine a risk parameter for onset of delamination, first ply failure and matrix cracking in the neat resin. The results of the analysis show that the interlaminar stresses at the +PHI/-PHI and -PHI/0 interfaces increase rapidly in the skin-stringer transition. This behavior is observed at the free edge as well as at some distance from it. The magnitude of the interlaminar stresses in the skin-stringer transition is strongly influenced by the fiber orientations of the off-axis plies. In addition, the overall displacements depend on the magnitude of the off-axis ply angle. It is found that for PHI < 30-degrees the deformations of the stiffener section are dominated by bending, whereas for 45-degrees < PHI < 75-degrees the deformations are dominated by torsion. The failure analysis shows that ply and matrix failure tend to occur prior to delamination for the considered configurations.