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Expensive high performance closed cell foam that can be thermoformed. High temperature grades (WF) can be autoclaved at 350° F/100psi. Used for secondarily bonded or cocured high performance aerospace structures.

widely used structural foams are summarized in Table 8.4. It is important to thoroughly understand the chemical, physical, and mechanical properties of any foam considered for a structural application, particularly with respect to solvent and moisture resistance, and long-term durability. Depending on their chemistry, foam core materials can be used in the temperature range 150-400° F.

Polystyrene cores are lightweight, of low cost, and easy to sand but are rarely used in structural applications due to their low mechanical properties. They cannot be used with polyester resins, because the styrene in the resin will dissolve the core; therefore, epoxies are normally employed.

Polyurethane foams are available as either thermoplastics or thermosets, with varying degrees of closed cells. There are polyurethane foams that are available as finished blocks, and formulations that can be mixed and foamed in place. Polyurethane foams exhibit only moderate mechanical properties and the resin-to-core interface bond tends to deteriorate with age, which can lead to skin delaminations. Polyurethane foams can be readily cut and machined to contours but hot wires should be avoided for cutting since harmful fumes can be released.

Polyvinyl chloride (PVC) foams are one of the most widely used core materials for sandwich structures. PVC foam can be either uncrosslinked (thermoplastic) or crosslinked (thermoset). The uncrosslinked versions are tougher, more damage resistant, and are easier to thermoform, while the crosslinked materials have higher mechanical properties, are more resistant to solvents, and have better temperature resistance. However, the crosslinked foams are more brittle and more difficult to thermoform than the uncrosslinked materials. Because they are not highly crosslinked like the normal thermoset adhesives and matrix systems, they can be thermoformed to contours. The crosslinked systems can be toughened with plasticizers, in which some of the mechanical properties of the normal crosslinked systems are traded for some of the toughness of the uncrosslinked materials. PVC foams are often given a heat stabilization treatment to improve their dimensional stability and reduce the amount of off-gassing during elevated temperature cures. Styrene acrylonitrile (SAN) foams are also available that have mechanical properties similar to crosslinked PVC but have the toughness and elongation of the uncrosslinked PVCs. Patterns of grooves can be scribed in the surfaces of foams to act as infusion aids for resin transfer molding processing.

Polymethylmethacrylimides (PMIs) are lightly crosslinked close-cell foams that have excellent mechanical properties and good solvent and heat resistance. They can be thermoformed to contours and are capable of withstanding autoclave curing with prepregs. These foams are expensive and are usually reserved for high performance aerospace applications.

Due to the inherently lower mechanical properties of foam compared to honeycomb core, manufacturer's are looking at methods of reinforcing foam cores to improve the mechanical properties. Two of these methods include stitching through both the skins and foam core,29 and the insertion of high strength pultruded pins (carbon, glass, or ceramic) into the foam core to form a truss configuration.30

8.9.5 Syntactic Core

Syntactic core consists of a matrix (e.g., epoxy) that is filled with hollow spheres (e.g., glass or ceramic microballoons), as shown in Fig. 8.25. Syntactics can be supplied as pastes for filling honeycomb core or as B-staged formable sheets for core applications. Syntactic cores are generally of much higher density than honeycomb, with densities in the range of 30-80 pcf. The higher the percentage of the microballoon filler, the lighter but weaker the core becomes. Syntactic core sandwiches are used primarily for thin secondary composite structures where it would be impractical or too costly to machine honeycomb to thin gages. When cured against precured composite details, syntactics do not require an adhesive. However, if the syntactic core is already cured and requires adhesive bonding, it should be scuff sanded and then cured with a layer of adhesive.

Glass microballoons are the most prevalent filler used in syntactic core, ranging in diameter from 1 to 350 ^m, but typically in the range of 50-100 ^m.

Glass microballoons have specific gravities 18 times lower than fillers like CaCO3; however, there have been issues in the past with moisture absorption into the glass microballoons. Ceramic microballoons have properties similar to glass but better elevated temperature properties, while polymeric microballoons (e.g., phenolic) are lower density than either glass or ceramic but have lower mechanical properties. The properties of the microballoons can be improved by increasing the wall thickness at the expense of higher densities. In most commercial applications, the microballoons have a size distribution to improve the packing density. Packing densities as high as 60-80% have been achieved.

8.9.6 Inspection

Adhesively bonded joints and assemblies are normally non-destructively inspected after all bonding operations are completed. Radiographic and ultrasonic inspection methods are typically used to look for defects in both the bondlines and the honeycomb core portions of the assemblies. In addition, it is quite common practice to leak check honeycomb bonded assemblies by immersing the assembly for a short time in a tank of hot water (e.g., 180° F). The hot water heats the residual air inside the honeycomb core and any leaks can be detected by air bubbles escaping from the assembly.

8.10 Integrally Cocured Structure

Integrally cocured or unitized structure is another manufacturing approach that can greatly reduce the part count and final assembly costs for composite structures. The process flow for an integral cocured control surface31 is shown in Fig. 8.26. In this particular structure, the spars are cocured to the lower skin. The upper skin is cured at the same time as the spars are cocured to the lower but is separated from the lower skin and spar assembly by a layer of release film. This is necessary to allow removal of the upper skin for mechanical installation of the ribs and center control box components. After substructure installation, the upper skin is attached to the spar caps with mechanical fasteners.

The plies for the spars are collated and then hot pressure debulked on their individual tooling details. Hot pressure debulking is required to remove excess bulk from the plies so that all of the tooling details will fit together. While the spars are being prepared, both the upper and the lower skins are collated and hot

Fig. 8.26. Process Flow for Integrally Cocured Control Surface31

pressure debulked on separate plastic lay-up mandrels (PLMs). The lower skin is placed on the tool first, followed by the spar details and the tooling filler blocks that go in the bays between the spars. As previously mentioned, the upper skin is separated from the lower skin and spar assembly by a layer of release material. The completed assembly is bagged, leak checked, and then autoclave cured. Several of the key lay-up and tool assembly sequences are shown in Fig. 8.27. Note that since this type of structure does not contain honeycomb core, drain holes can be drilled in strategic locations to allow any water to drain out of the assembly while in-service.

Pressure is provided by both the autoclave and the expansion of the aluminum substructure blocks, as shown in Fig. 8.28. The autoclave applies pressure to the skins and spar caps, while the expansion of the aluminum substructure blocks applies pressure to the spar webs. If required, the expansion of the aluminum substructure blocks can be supplemented by the presence of silicone rubber intensifiers. The unit after cure and removal from the tool is shown in Fig. 8.29.

Composite Skin Lay-up

Center Box Tooling on Skin

Composite Skin Lay-up

Center Box Tooling on Skin

Locating Spar on Skin

Fig. 8.27. Key Process Steps for Integrally Cocured Control Surface Source: The Boeing Company

Locating Spar on Skin

Spars and Filler Blocks Located on Skin

Fig. 8.27. Key Process Steps for Integrally Cocured Control Surface Source: The Boeing Company

Pinned Ramp Block

Autoclave Pressure I

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