Paper number 448

EFFECT OF RIB GEOMETRY ON THE BUCKLING BEHAVIOR OF COMPOSITE ISOGRID CYLINDERS

Hassan Mahfuz, Mojibur Rahman, Earnest Foster, Uday Vaidya, Anwarul Haque and Shaik Jeelani

Tuskegee University's Center for Advanced Materials
Tuskegee, Alabama 36088

Summary

Isogrid is a structural concept that utilizes the use of repetitive equilateral pattern of stiffening ribs. The fact that the triangular grids behave entirely as an isotropic material gives rise to the name 'isogrid'. The idea of isogrid is not entirely new; it has been previously introduced in metallic structures which resulted in significant improvement in structural integrity. Recently, Air Force Phillips Laboratory, Lockheed Martin and NASA are conducting extensive research in developing composite isogrid structures for future shuttle constructions. Especially, in the construction of intertank and external fuel tanks, composite isogrid structures will play an important role. Isogrid can effectively carry combined loads because of the arrangement of the ribs. They also permit local skin buckling in large lightweight thin structures without triggering instabilities. In this investigation, both static and buckling analysis have been performed for composite isogrid cylinders using finite element methods (FEM). Two steps were adopted for the development of the finite element models. First, a unit cell representative of the cylindrical structure has been modeled. Secondly, several unit cells have been generated in multiples to form into a complete cylinder. Three different isogrid cylinders have been modeled with three different rib geometry; rectangular, triangular and taper. Axial loads were then applied on the cylinder, and parametric studies were conducted with various rib geometry. Buckling analysis has also been performed for the three categories of isogrid cylinders. Critical failure loads, and various modes of buckling were determined for each of the three cylinders. The local skin buckling phenomenon for cylindrical structure has been studied by analyzing the various failure modes. Details of the finite element analysis are presented in this paper.

Keywords

composite structures, blucking, Finite Element Method (FEM).