The buckling and postbuckling behavior of laminated composite structures are affected by novel factors, and new failure modes appear that are not experienced in structures made of conventional materials. Elastic couplings are introduced into the mechanical behavior when unbalanced and more pronounced unsymmetrical laminates are employed into the design. These couplings can be exploited to achieve critical performances advantages; however their presence degrades the load-carrying capacity and stiffness like in the case of a drop-off in a laminate construction [1,2] and when local delaminations exists. Delaminations may also stem from impact damage and poor fabrication, or result from significant interlaminar stresses developed in highly stressed and deformed elements and is the case of the present study. In particular the delaminations develop in the web of stiffened structures designed to operate in the postbuckling region to enhance their structural efficiency, thus avoiding complete exploitation of the postbuckling strength capacity of these structures. High interlaminar stresses leading to delaminations are also experienced along free (exposed fibers) unstiffened/unreinforced edges of laminates, as well as discontinuities (cut-outs).

Another degrading factor is stringer/skin separation and is associated with buckling and postbuckling of stiffened composite structures. Microbuckling phenomena [3,4], which reduce the compressive strength of laminated composite structures and lead to premature failure, may also significantly decrease the buckling load of laminated composite structures as well as affect their postbuckling characteristics. The microbuckling phenomena may stem from presence of manufacturing imperfections in the form of voids, poor interlaminar bonding and misaligned fibers, or from inherent characteristics of the fiber-reinforced composites that influence their comprehensive strength capability: fiber volume fraction, fiber array, specimen geometry, fiber diameter, fiber and matrix properties and fiber end fixity within the laminated material.

In the present study, the alleviation of stress concentrations in a stiffened composite torsion box is analyzed using a methodological approach. The subject of buckling of laminated composite plates and shells can be found in the excellent review of Leissa [5].

To enable the investigation of buckling and postbuckling behavior of laminated composite curved stringer stiffened panels under torsion moments, and their experimental testing, two curved panels were assembled together using two straight aluminum plates to form a torsion box . In the experiments, the collapse of the torsion box could not be reached due to local failure of the panel skin at its eight corners at a relatively low moment. This was predicted by the analysis and experienced in the tests. The failure mode is a shear type one, due to high stress concentration at the corners of the structures induced by the warping of the torsion box. To solve the problem, and to alleviate the stresses to allow the collapse of the torsion box without local failures, a methodological investigation was initiated. The investigation was accompanied by a FE analysis, using the MSC NASTRAN code [6].

It was decided to perform the reduction of the stress concentration in sequential steps, each step accompanied by a FE analysis. The proposed methodological approach yielded the expected results without altering the postbuckling behavior of the torsion box. The above cited procedure was successfully applied on a subsequent torsion box, BOX 3, yielding the collapse of the box under torsion, without any local failures at the corners of the box.

1

J.M. Curry, "Effect of ply drop-offs on the strength of graphite-epoxy laminates", M.Sc. thesis , VPI and state university, May 1986.

2

M.T. DiNardo, and Lagace, P.A., "Buckling and postbuckling of laminated composite plates with ply drop-offs", AIAA J., 27(10), Oct. 1989, 1392-1398. doi:10.2514/3.10276

3

J.R. Lager, and June, R.R., "Compressive strength of Boron-epoxy composites", J. of Composite Materials, 3, Jan. 1969, 48-56. doi:10.1177/002199836900300104

4

L.B. Greszczuk, "Microbuckling failure of circular fiber-reinforced composites", AIAA J., 13(10), Oct. 1975, 1311-1318. doi:10.2514/3.60544

5

A.W. Leissa, "Buckling of laminated composite plates ands shells panels", Report AFWAL-TR-85-3069, AF Wright Aeronautical Laboratories, June 1985.

6

The MacNeal-Schwendler Corporation. MSC/PATRAN MSC/NASTRAN, Preference Guide, MacNeal-Schwendler Corporation.Electronic book, 2000.

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