*Compos.Struct.* **25**, 129-137, 1993

### NUMERICAL INVESTIGATIONS OF FREE EDGE EFFECTS

IN INTEGRALLY STIFFENED LAYERED COMPOSITE PANELS

I.C. Skrna-Jakl and F.G. Rammerstorfer

Institute of Lightweight Structures and Aerospace
Engineering,

TU Wien,
Vienna, Austria

**Abstract** -
A linear finite element analysis is conducted to examine
the free edge stresses and the displacement behaviour 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 behaviour 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° the
deformations of the stiffener section are dominated by bending, whereas for
45°<*Phi*<75° 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.

(hjb,960628)