On the structural efficiency of three-dimensional isogrid designs

This investigation explored the structural efficiency of a three-dimensional structural configuration which is based in part on concepts which are similar to those used to derive other efficient structural designs, such as the twodimensional isogrid and three-dimensional space truss. The three-dimensional isogrid truss system described in this paper has multiple symmetries based on repeating units which spiral around the major axis of the system. In this study, a numerical analysis was performed to investigate the structural efficiency of a three-dimensional filament wound, graphite/epoxy isogrid truss system which is subjected to various combinations of tensile, compressive, torsional, and/or bending loads. The results of this analysis clearly indicate the significant potential of three-dimensional advanced composite isogrid structures. A potential weight savings of up to 33 times has been demonstrated for a simple application, while simultaneously achieving increased stiffness. INTRODUCTION The drive to eliminate unnecessary weight in structures transcends from the world of aerospace to the arena of civil infrastructure. The introduction of advanced composite materials coupled with the development of novel structural configurations has enabled much more efficient load transfer in a variety of both simple and complex structures. In particular, composite isogrids developed by government sponsored research yielded highly efficient two-dimensional designs which place the material where it provides the greatest benefit for carrying in-plane loads [1]. In these designs, the skin works in tandem with the integral diagonal stiffeners to carry both the axial and in-plane shear loads in a pseudo-isotropic fashion, with sufficient bending rigidity to prevent local buckling. Unfortunately, although composites can be easily manufactured into virtually any shape or form, the cost of producing high-quality isogrids is still too high to be practical for many two-dimensional applications. This preliminary investigation has explored the structural efficiency of a three-dimensional structural configuration which is based in part on concepts which are similar to those used to derive other efficient structural designs, such as the twodimensional isogrid and space trusses. Like a typical simple truss system, the three-dimensional isogrid design incorporates primarily axial force members which are oriented at angular intervals (such as 30 and 60 degrees) to form stable triangular cells. The three-dimensional isogrid system, however, has multiple symmetries based on repeating units which spiral around the major axis of the system. DESCRIPTION OF SYSTEM The basic three-dimensional isogrid system (see Figure 1) consists of six equally-spaced diagonal members spiraling clockwise about the longitudinal axis, another six equally-spaced diagonal members spiraling counter-clockwise about the longitudinal axis, and another six equally-spaced longitudinal members passing through the intersections of the counter-rotating pseudo-helixes (the basic structure is superimposed over a cylinder Associate Professor, Associate Fellow, AIAA Graduate Student, Student Member AIAA Copyright © 1995 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.