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Cycles to Failure Fig. 7.2. Fatigue Properties of Aerospace Materials1

and delaminations or ply separations can occur; and composites are more difficult to repair than metallic structure.

The major cost driver in fabrication for a conventional hand layed-up composite part is the cost of laying-up, or collating, the plies. This cost (Fig. 7.3)

Tool Prep

(12%) Collate Plies

Tool Prep

(12%) Collate Plies

Fig. 7.3. Cost Drivers for Composite Fabrication2

Fig. 7.3. Cost Drivers for Composite Fabrication2

generally consists of 40-60% of the fabrication cost depending on part complexity. Assembly cost is another major cost driver, accounting for about 50% of the total part cost. As previously stated, one of the potential advantages of composites is to cure or bond a number of detail parts together to reduce assembly costs and the number of required fasteners.

7.1 Materials

Continuous fiber composites are laminated materials (Fig. 7.4), in which the individual layers, plies, or lamina are oriented in directions that will enhance the strength in the primary load direction. Unidirectional (0°) laminates are extremely strong and stiff in the 0° direction but are also very weak in the 90° direction because the load must be carried by the much weaker polymeric

Fig. 7.4. Laminate Construction1

matrix. While a high strength fiber can have a tensile strength of 500 ksi or more, a typical polymeric matrix normally has a tensile strength of only 5-10 ksi. The longitudinal tension and compression loads are carried by the fibers, while the matrix distributes the loads between the fibers in tension, and stabilizes and prevents the fibers from buckling in compression. The matrix is also the primary load carrier for interlaminar shear (i.e., shear between the layers) and transverse (90°) tension. Since the fiber orientation directly impacts the mechanical properties, it would seem logical to orient as many of the layers as possible in the main load carrying direction. While this approach may work for some structures, it is almost always necessary to balance the load-carrying capability in a number of different directions, such as the 0°, +45°, -45°, and 90° directions.

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