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' Silicone Rubber

/ Composite Skin ^ j-Composite Substructure

Silicone Rubber (as required)

Fig. 8.28. Pressure Application for Unitized Cocured Structure

Silicone Rubber (as required)

Steel Bond Tool

Fig. 8.28. Pressure Application for Unitized Cocured Structure

As shown in Fig. 8.30, the upper skin is mechanically fastened to the spar caps after the substructure is completed.

The advantages of this type of structure are obvious: fewer detail parts, fewer fasteners, and fewer problems with part fit-up on final assembly. The main disadvantages are the cost and accuracy of the tooling required, and the complexity of the lay-up that requires a highly skilled workforce. To help control tool accuracy, the substructure tooling is usually machined as a single block (Fig. 8.31) and then sectioned into the individual tooling details.

One potential problem with this type of structure is spring-in. As shown in Fig. 8.32, both the spar cap and the web will spring-in during cool down after cure. The web spring-in can largely be eliminated by placing a couple of plies on the backside to support the web. However, the spar cap cannot be compensated by increasing the angle on the tooling block, because it would make an indentation in the concurrently cured upper skin. It is therefore necessary to shim this joint during final assembly.

Another type of structure that is frequently cocured is skins with cocured hats,32 such as the one shown in lay-up in Fig. 8.33. Although matched die tooling can be used to make this type of structure, the more common practice is to use localized tooling only at the hat stiffener locations. A typical bagging arrangement, shown in Fig. 8.34, contains an elastomeric mandrel to support the hat during cure; an elastomeric pressure intensifier to insure that the radii obtain sufficient pressure; and some plastic shims to minimize mark-off on the skin from the pressure intensifier. In this figure, the mandrel contains a hole in the center to equalize the pressure. However, some mandrels are solid elastomer or frequently elastomer reinforced with carbon or glass cloth. If the stiffeners require exact location, it may be necessary to use cavity tooling similar to that shown in Fig. 8.35.

Matching Upper Skin

Fig. 8.29. Cocured Unitized Control Surface Source: The Boeing Company

Matching Upper Skin

Fig. 8.29. Cocured Unitized Control Surface Source: The Boeing Company

One of the key design areas for cocured stiffeners are the terminations. Since the bond holding the stiffener to the skin is essentially a resin or an adhesive bond, any peel loads induced at the stiffener ends could cause the bondline to "unzip" and fail. The most prevalent method for preventing this is by installing mechanical fasteners near the stiffener terminations, as shown in the hat design of Fig. 8.36. Note that the hat is also thicker at the ends and scarfed to further help reduce the tendency for bondline peeling. Other methods include stitching and pinning in the transverse (Z) direction. However, stitching of pregreg lay-ups is expensive and can damage the fibers. Z-pining is a technology in which small

Composite Skin

Mechanical Fasteners

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