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bad news about honeycomb core is that it is expensive and difficult to fabricate complex assemblies, and the in-service experience, particular with aluminum honeycomb, has not always been good. It can also be very difficult to make major repairs to honeycomb assemblies.

Aluminum honeycomb assemblies have experienced serious in-service durability problems, the most severe being moisture migrating into the assemblies and causing corrosion of the aluminum core cells. Honeycomb suppliers have responded by producing corrosion inhibiting coatings that have improved durability. The newest corrosion protection system, called PAA core, is shown in Fig. 8.20. The core foil is first cleaned and then phosphoric acid anodized. It is then coated with a corrosion inhibiting primer before printing with node bond adhesive. PAA core has demonstrated an approximate three-fold (3X) increase in corrosion protection when compared to typical (non-PAA) corrosion resistant aluminum honeycomb. However, even the most rigorous corrosion protection methods will not stop core corrosion but only delay its onset.

If liquid water is present in the honeycomb cells, freeze-thaw cycles encountered during a typical aircraft flight can cause node bond failures.19 At high

altitudes, the standing water in the core freezes, expands, and stresses the cell walls. After landing, the water thaws and the cell walls relax. After a number of these freeze-thaw cycles, the node bonds fail and the damage propagates. It should be noted that this freeze-thaw cyclic damage is not confined to aluminum honeycomb but can occur in the non-metallic cores. In addition, water in the honeycomb can also cause disbonds and delaminate the facesheets, particularly if the temperatures exceed the boiling point of water (212° F), as can happen during operation or repair.

Liquid water normally enters the core through exposed edges, such as panel edges, closeouts, door and window sills, attachment fittings, or almost any location that the skin and core bond terminates. The majority of the damage is often found at the edges of panels.20 Adhesive bond degradation will lower the skin-to-core bond strength, the fillet bond strength, and the node bond strength. Node bond degradation can reduce the core shear strength so that the assembly fails prematurely by core failure. In addition, water will enter the assembly through any puncture in the facesheets. Since some honeycomb assemblies contain extremely thin skins, water has been known to pass through the skins and then condense on the cell walls. Interconnected microcracks in thin skin honeycomb panels can also allow water ingression.21 Although absorbed moisture affects the properties of any composite assembly, it is the presence of liquid water in the cells that does the majority of the damage. Many field reports blame water ingression on "poor" sealing techniques. While there is a great deal of truth to the statement that good sealing practices are important, it is the author's opinion that it is just a matter of time before water will find its way into the core of most honeycomb designs and initiates the damage process.

8.9.2 Honeycomb Processing

Honeycomb processing before adhesive bonding includes: perimeter trimming, mechanical or heat forming, core splicing, core potting, contouring, and cleaning.

Trimming. The four primary tools used to cut honeycomb to dimensions are serrated knife, razor blade knife, band saw, and a die. The serrated and razor edge knives and die cutter are used on light density cores, white heavy density cores and complex shaped cores are usually cut with a band saw.

Forming. Metallic hexagonal honeycomb can be roll or brake formed into curved parts. The brake forming method will crush the cell walls and densify the inner radius. Overexpanded honeycomb can be formed to a cylindrical shape on assembly. Flexible core usually can be shaped to compound curvatures on assembly. Non-metallic honeycomb can be heat formed to obtain curved parts. Usually the core is placed in an oven at high temperature for a short period of time (e.g., 550° F for 1-2 min). The heat softens the resin and allows the cell walls to deform more easily. Upon removal from the oven, the core is quickly placed on a shaped tool and held in place until it cools.

Splicing. When large pieces of core are required, or when strength requirements dictate different densities, smaller pieces or different densities of core can be spliced together to form the finished part. This is usually accomplished with a foaming adhesive, as shown in Fig. 8.21. Core splice adhesives normally contain blowing agents that produce gases (e.g., nitrogen) during heat-up to provide the expansion necessary to fill the gaps between the core sections. Foams are one-part epoxy pastes that expand during heating. They are used for core joining, insert potting, and edge filling applications. Foaming film adhesives are thick unsupported films (0.04-0.06 in.) that expand 1.5-3 times their original thickness when cured. Although some of these products can damage the core by overexpanding if too much material is used in the joint, most just expand to fill the gap and then stop when they meet sufficient resistance. A normal practice is to allow up to three layers of foaming adhesive to fill gaps between core sections. Larger gaps call for rework or replacement of the core sections. It is also important to not process some foaming adhesives under a vacuum, or excessive frothing of the foam bondline may occur.

Potting. Potting compounds are frequently required for fitting attachments where fasteners must be put through the honeycomb assembly. As shown in Fig. 8.22, the cells are potted with a high viscosity paste that is cured either during core splicing operations or during final bonding. These compounds usually contain fillers, such as milled glass or aramid fibers, silica, or glass or phenolic

■ Locations of Core Splice Bonds

Fig. 8.21. Complex Structure with Different Core Densities Source: The Boeing Company

■ Locations of Core Splice Bonds

Fig. 8.21. Complex Structure with Different Core Densities Source: The Boeing Company

Potting Compound

Potting Compound

Fig. 8.22. Core Potting in Honeycomb Core3

microballoons. They can be formulated to cure at room temperature, 250, or 350° F depending upon the intended use temperature for the structure.

Machining. In most applications, honeycomb must have its thickness machined to some contour. This is normally accomplished using valve stem type cutters on expanded core. Occasionally, before expansion, the solid honeycomb block is machined using milling cutters. Typical machines used for contour machining (carving) are gantry, apex, 3-D tracer, or NC five-axis. With five-axis NC machining, the cutting head is controlled by computer programs, and almost any surface that can be described by x-, y-, and z-coordinates can be produced. These machines can carve honeycomb at speeds of up to 3000in./min with extreme accuracy. A standard contour tolerance of an NC machine is ±0.005 in. Many core suppliers will supply core machined to contour ready for final bonding.

Cleaning and Drying. It is preferable to keep honeycomb core clean during all manufacturing operations prior to adhesive bonding; however, aluminum honeycomb core can be cleaned effectively by solvent vapor degreasing. Some manufacturers require vapor or aqueous degreasing of all aluminum core prior to bonding; however, most part manufacturers accept "Form B" core from the honeycomb suppliers, and bond without further cleaning. Non-metallic core, such as Nomex or Korex (aramid), fiberglass, and graphite core, readily absorbs moisture from the atmosphere. Similar to composite skins, non-metallic core sections should be thoroughly dried prior to adhesive bonding. A further complication is that since the cell walls are relatively thin and contain a lot of surface area, they can reabsorb moisture rather rapidly after drying, and therefore should be bonded into assemblies as soon as possible after drying.

Honeycomb Bonding. Honeycomb bonding procedures are similar to regular adhesive bonding with a few special considerations. Unlike many composite assemblies, honeycomb assemblies require special closeouts, several of which are shown in Fig. 8.23. During bonding, these require filler blocks in cavities and ramp areas to prevent edge crushing during the cure cycle. Again, closeouts are areas for potential water ingression so special care is required during both the design and the manufacturing process.

Pressure selection is an important consideration during honeycomb bonding. The pressure should be high enough to push the parts together, but not be so high that there is danger of crushing or condensing the core. The allowable pressure depends on both the core density and the part geometry. Common bonding pressures can range anywhere from 15 to 50 psi for honeycomb assemblies. As previously discussed for adhesive bonding, the positive pressure of an autoclave, with a vented bag, gives superior quality to that of a bond produced in an oven under vacuum bag pressure. The amount of pressure, as well as the adhesive selected, are important in forming fillets at the core-to-skin bondlines. The degree of filleting to a large extent determines the strength of the assembly.

Outward Facing Channel

• Inexpensive

• Skin-to-Channel Bondline Weak

Z-Closure

• Inexpensive

• Requires Machined Step in Core (Potential for Mismatch)

• One Strong Skin-to-Closure Bondline (Bottom of Sketch)

Inward Facing Channel

• Strong Skin-to-Channel Bondlines

• Difficult to Stuff Core and Difficult to Inspect

Integral Closure

• Kick Loads on Bondlines

• Core Machining Difficult

• Often Used on Cocured Structure

Integral Closure

• Inexpensive

• Core Subject to Crushing on Cure

• Good Moisture Sealing

• Often Used on Cocured Structure

Fig. 8.23. Examples of Honeycomb Structure Close-outs3

Pressures that are applied on the sides of the core can easily condense the cells. Since honeycomb is stronger in the longitudinal ("L") direction than the width ("W") direction, the core is more prone to crushing in the "W" direction. Even when the initial vacuum is pulled, vacuum pressure alone has been known to cause core migration and cell crushing. Some manufacturers limit the vacuum level to 8-10 in. of Hg to help in preventing differential pressures within the cells. Autoclave processing of honeycomb assemblies is more sensitive to bag leaks than regular adhesive bonding. If pressure enters the bag through a leak, it can literally blow the honeycomb apart due to the large differential pressure.

Honeycomb assemblies can also be made by cocuring the composite plies onto the core. In this process, the composite skin plies are consolidated and cured at the same time they are bonded to the core. Although a film adhesive is normally used at the skin-to-core interface, self-adhesive prepreg systems are available that do not require a film adhesive. To prevent core crushing and migration, this process is normally conducted at approximately 40-50 psi, as opposed to the normal 100 psi used for regular laminate processing. This can produce skins that are somewhat more porous than those processed under higher pressures, but the biggest drawback is the pillowing or dimpling that occurs in the skins (Fig. 8.24), due to the skin being only supported at the cell walls. Although the schematic is somewhat exaggerated in the amount of pillowing

Fig. 8.24. Pillowing Effect in Composite Cocured Honeycomb Panels3

usually experienced, pillowing does create a serious knockdown in mechanical properties, as much as 30% in some cases.22 The amount of pillowing can be reduced by using a smaller cell size (e.g., 1/8 vs. 3/16in.).

Although core migration and crushing can be a problem when bonding pre-cured composite skins, it is an even bigger problem with cocured skins. A considerable amount of work has been done to solve this problem.23-25 Potential solutions include: (1) reducing the ramp angle (20° or less is recommended); (2) increasing the core density; (3) using grip, or hold down, strips to restrain the plies; (4) potting the cells in the ramp area to increase the core rigidity; (5) encapsulating the core with a layer of adhesive prior to cocuring; (6) bonding fiberglass plies into the center of the core (septums) to increase core rigidity; (7) adjusting the temperature and pressure during heat-up; and (8) even using "high friction" prepregs2627that minimize ply movement during curing.

8.9.3 Balsa Wood

Balsa wood is one of the oldest forms of core materials and was used in early aircraft but now primarily in the boat industry. The core is manufactured by first cutting sections transverse to the grain direction, which are then cut into rectangles, and adhesively bonded together. This results in the grains running perpendicular to the facesheets and is known as end-grained balsa. Typical cell sizes are 0.002 in. in diameter with densities in the range of 6-19 pcf.

End-grain balsa has good mechanical properties, is fairly inexpensive, and is easy to machine and bond to facesheets. The main disadvantages include severe moisture sensitivity, lack of formability, and variable mechanical properties since the block is made by bonding together smaller sections of variable density. Since balsa can absorb large amounts of resin during cure, it is a common practice to seal the surface prior to the lay-up operations.

8.9.4 Foam Cores

A third type of core material frequently used in adhesively bonded structure is foam core. While the properties of foam cores are not as good as honeycomb core, they are used extensively in commercial applications such as boat building and light aircraft construction. The term "polymer foam" or "cellular polymer" refers to a class of materials that are two-phase gas-solid systems in which the polymer is continuous and the gaseous cells are dispersed through the solid. These polymeric foams can be produced by several methods including extrusion, compression molding, injection molding, reaction injection molding, and other solid-state methods.28 Foam cores are made by using a blowing or foaming agent that expands during manufacture to give a porous, cellular structure. The cells may be open and interconnected or closed and discrete. Usually, the higher the density, the greater the percentage of closed cells. Almost all foams used for structural applications are classified as closed cell, meaning almost all of their cells are discrete. Open cell foams, while good for sound absorption, are weaker than the higher density closed cell foams and also absorb more water, although water absorption in both open and closed cell foams can be problematic. Both uncrosslinked thermoplastic and crosslinked thermoset polymers can be foamed with the thermoplastic foams exhibiting better formability, and the thermoset foams better mechanical properties and higher temperature resistance. Almost any polymer can be made into a foam material by adding an appropriate blowing or foaming agent.

The blowing agents used to manufacture foams are usually classified as either physical or chemical blowing agents. Physical blowing agents are usually gases, mixed into the resin, that expand as the temperature is increased, while chemical blowing agents are often powders that decompose on heating to give off gases, usually nitrogen or carbon dioxide. Although there are foams that can be purchased as two-part liquids that expand after mixing for foam-in-place applications, the majority of structural foams are purchased as pre-expanded blocks that can be bonded together to form larger sections. Sections may be bonded together using either paste or adhesive films. Sections can also be heat formed to contour using procedures similar to those for non-metallic honeycomb core. Although the uncrosslinked thermoplastic foams are easier to thermoform, many of the thermoset foams are only lightly crosslinked and exhibit some formability. Core densities normally range from about 2 to 40 pcf. The most

Table 8.4 Characteristics of Some Foam Sandwich Materials3

Name and Type of Core

Density (pcf)

Maximum Temperature (° F)

Characteristics

Polystyrene (Styrofoam)

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