2. The ply angles
As can be seen from the figure, the ply angle plays an important role in determining the strength of the hat panel. Small angles are stronger for axial loads, but angles around 45 degrees improve shear strength.
Design and Manufacturing of a Composite, Hat-Stiffened Panel
Design Specifications:
The hat panel must be designed to be as light and as strong as posssible.
The design variables are:
| OPTIMUM NUMBER OF PLIES SHOULD BE CHOSEN. |
1. Number of plies The number of plies has to be determined. Keep in mind that increasing the number of plies gives more strength, but it also increases the cost and weight of the hat panel. |
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2. The ply angles As can be seen from the figure, the ply angle plays an important role in determining the strength of the hat panel. Small angles are stronger for axial loads, but angles around 45 degrees improve shear strength. |
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3. Internal ply configuration. The internal ply configuration, or the "lay-up" of the hat panel is an important design consideration. The way each ply is oriented with the global X direction affects the strength of the hat panel in any particular direction. The figure shows a layup of 0/90/90/0. The circles represent the fibers. Thus, you can see that when the fiber direction is along the X-direction, it is a 0 degree ply, and if it is perpendicular to the X-direction it is a 90 degree ply. |
The design specifications are:
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1. The dimensions of the hat panel. |
The first part of the experiment requires each group of students to submit a design proposal. The proposal is reviewed by the instructor and then he gives the go ahead for the manufacture of the hat panel.
Manufacturing
There are three main steps involved in the manufacturing process:
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1. Lay-up |
Lay-up
The initial raw composite material consists of fiber and matrix. The matrix is like a glue holding the fibers together. In order that the matrix remains stable ( i.e. does not cure - the term is defined below ) the composite material is stored in a freezer.
The main steps involved in the lay-up are:
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1. Cutting the ply angle according to the designed specifications. |
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2. Laying up the cut plies on the caul plate to form the desired shape of the hat panel. |
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3. Inserting the entire lay-up in a vacuum bag |
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4. Sealing the vacuum bag completely. |
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5. Placing the entire setup in the autoclave |
Curing
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Curing is a process in which the natural chemical reaction between the fibers and the matrix is accelerated with the help of a catalyst. Usually the catalyst is heat and/or pressure. In our lab, the hat panel is cured by an& autoclave. Think of the autoclave as a big pressure cooker. It provides high temperature and pressure so that the resin flows between the fibers and then hardens. But, unlike a pressure cooker, the autoclave is completely computer controlled! |
Machining
Final machining may consist of grinding or, more commonly, simply trimming the panel (using a diamond tipped saw blade) to obtain the desired dimensions.
