Section Bb

Fig. 8.36. Methods of Reducing Hat Stiffener Peel

Precured Stiffeners

Precured Stiffeners

Fig. 8.37. Principle of Cobonding

• Material Out-time Make Sure It Makes

• Material Out-time Make Sure It Makes

Fig. 8.38. Advantages and Disadvantages of Unitized Cocured Structure certain situations, the amount of tooling required for the curing operation can be reduced.

Cocured unitized structure is a trade-off. As shown in Fig. 8.38, the advantages include fewer parts, less fasteners, and better fit-up at assembly. These advantages must be weighed against the additional tooling costs and span time to produce the tooling, the lay-up costs, and personnel requirements for large complex assemblies, and material out-time issue if the lay-up requires a long time.

Summary

Adhesive bonding is often preferred if thin sections are to be joined where bearing stresses in bolted joints would be unacceptably high, or when the weight penalty for mechanical fasteners is too high. In general, thin structures with stiff and well-defined load paths are good candidates for adhesive bonding, while thicker structures with complex load paths are better candidates for mechanical fastening. It is important to load the adhesive in shear. Tension, cleavage, and peel loading should be avoided to prevent premature failures.

Adhesively bonded structure offers the potential to reduce the number of parts and fasteners required for final assembly, and therefore lowers the cost of structures. The most critical fabrication aspect of bonding is surface preparation. Without proper surface preparation, the long-term durability of the joint will not be realized. Fortunately, the surface preparation for composites is quite a bit simpler than that for aluminum or titanium. When adhesive bonding composite parts, it is important that all cured parts be thoroughly dried prior to bonding. In addition it is important that prefit operations are conducted to make sure that the parts to be bonded will fit together without excessive high or low areas; otherwise, thick glue lines and unbonds may result.

Sandwich construction is very structurally efficient, particularly in stiffness critical applications. Unfortunately, honeycomb assemblies are difficult to build, have frequently experienced in-service durability problems, and can be difficult to repair. Aluminum honeycomb has been prone to severe corrosion problems. If the honeycomb contains liquid water, all honeycomb assemblies can be damaged from repeated freeze-thaw cycles. In addition, if liquid water is present and the assembly is heated above 212° F, there is the danger of steam pressure delamination.

Foam core sandwich structures are somewhat easier to build than honeycomb structures, but the current foams do not have as good mechanical properties as the honeycombs. In addition, foam cores are subject to quite high moisture saturation levels. Both types of core materials can be fabricated into composites structures be either precuring the composite details and then bonding them to the core, or by cocuring the assembly with prepreg plies at one time.

Cocured unitized structure is yet another option for reducing assembly costs. Although it eliminates some of the potential durability costs associated with honeycomb, it is more difficult to design and more costly to tool than honeycomb assemblies. A variation of cocuring, called cobonding, can reduce some of the tooling costs for certain types of assemblies.

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