Date of Graduation

2003

Document Type

Thesis

Degree Type

MS

Committee Chair

Samir N. Shoukry

Committee Chair

Samir N. Shoukry

Abstract

A novel closed-form analytical model is described and applied to predict the stiffness and stress distributions around the circumference of cylindrical composite ducts with upright bolted flanges. The primary capability provided by the model is the interactive evaluation of alternative combinations between various geometric, processing, material, lay-up and loading characteristics in the design of connection flanges between cylindrical or conical ducts. Each ply in such a laminated flange is laid-up of circular segments, or "gores", which are stamped out of a prepreg unidirectional tape, or woven fabric, through a "cookie-cutter" type operation. The gores are then butted in the circumferential direction, constrained by tooling in the radial direction, and the "through-thickness" plies of the laminate are laid-up in an interleaved sequence, where they cover each other in every transition region from one gore to another to complete the circular contour of each ply. The individual plies are stacked over each other in an interleaved sequence, so that every transition region from one gore to another is fully covered by the continuous gores of the adjacent plies.

The duct and the flange are made as an integral unit as a whole i.e., the gores are bent to form the flange from the duct. When two gores meets there will be discontinuities in the fiber orientation, which leads to variation in the stiffness and stress acting on the flanged duct. These discontinuities have been considered in this study. The effects of holes on the stiffness and stress variations are also studied. These flanges are used in connecting two ducts that carry high-pressurized fuels to the aircraft engines. High-pressurized fuels induce high stress on the bolted flanges, which tend to separate the flanges. So there should be a design to specify the maximum stress induced and the material that can withstand these high stresses. The size of such gores, their interleaving pattern through the thickness of the laminate, as well as the ply stacking sequence within each gore play an important role in the resulting stiffness and stress distributions. This study is concentrated in finding out the best possible design which is cost effective, easy to manufacture and which can withstand the stress due to the flow of high-pressurized fuels. The margins of safety, the maximum stress induced in the flange for various loading scenarios are also found out.

An example of a closed-form parametric analysis of the stiffness and stress characteristics predicted by the model is presented in this study for a carbon T-300/Bismaleimide material system, for various sizes of the gore pieces and various interleaving patterns between adjacent plies. The results indicate that the size of the gore pieces can be increased in order to reduce manufacturing costs without degrading important structural characteristics, if the lay-up architecture is properly tailored to specific loading scenarios. The non-linear trends and interactions of the numerical results highlight the importance of performing similar parametric studies during the preliminary design of such flanges, in order to enhance the tradeoffs between their structural performance and manufacturing cost.

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