Bonding Procedures1

Some general guidelines for adhesive bonding are summarized in Table 8.2. The basic steps in the adhesive bonding process are:

• collection of all the parts in the bonded assembly, which are then stored as a kit;

• verification of the fit to bondline tolerances;

• cleaning the parts to promote good adhesion;

• application of the adhesive;

• mating of the parts and adhesive to form the assembly;

• application of force concurrent with application of heat to the adhesive to promote cure, if required; and

• inspection of the bonded assembly.

8.8.1 Prekitting of Adherends

Many adhesives have a limited working life at room temperature, and adherends, especially metals, can become contaminated by exposure to the environment.

Table 8.2 General Considerations for Adhesive Bonding1

• When received, the adhesive should be tested for compliance with the material specification. This may include both physical and chemical tests.

• The adhesive should be stored at the recommended temperature.

• Cold adhesive should always be warmed to room temperature in a sealed container.

• Liquid mixes should be degassed, if possible, to remove entrained air.

• Adhesives which evolve volatiles during cure should be avoided.

• The humidity in the lay-up area should be below 40% relative humidity for most formulations. Lay-up room humidity can be absorbed by the adhesive and is released later during heat cure as steam, yielding porous bondlines and possibly interfering with the cure chemistry.

• Surface preparation is absolutely critical and should be conducted carefully.

• The recommended pressure and the proper alignment fixtures should be used. The bonding pressure should be great enough to ensure that the adherends are in intimate contact with each other during cure.

• The use of a vacuum as the method of applying pressure should be avoided whenever possible, since an active vacuum on the adhesive during cure can lead to porosity or voids in the cured bondline.

• Heat curing systems are almost always preferred, because they yield bonds that have a better combination of strength and resistance to heat and humidity.

• When curing for a second time, such as during repairs, the temperature should be at least 50° F below the earlier cure temperature. If this is not possible, then a proper and accurate bond form must be used to maintain all parts in proper alignment and under pressure during the second cure cycle.

• Traveler coupons should always be made for testing. These are test coupons that duplicate the adherends to be bonded in material and joint design. The coupon surfaces are prepared by the same method and at the same time as the basic bond. Coupons are also bonded together at the same time with the same adhesive lot used in the basic joint and subjected to the same curing process simultaneously with the basic bond. Ideally, traveler coupons are cut from the basic part, on which extensions have been provided.

• The exposed edges of the bond joint should be protected with an appropriate sealer, such as an elastomeric sealant or paint.

Thus, it is normal practice to kit the adherends so that application of the adhesive and buildup of the bonded assemblies can proceed without interruption. The kitting sequence is determined by the product and production rate. Prefitting of the details is also useful in determining locations of potential mismatch such as high and low spots. A prefit check fixture is often used for complex assemblies containing multiple parts. This fixture simulates the bond by locating the various parts in the their exact relationship to one another, as they will appear in the actual bonded assembly. Prefitting is usually conducted prior to cleaning so that the details can be reworked if necessary.

8.8.2 Prefit Evaluation

For complex assemblies, a prefit evaluation ("verifilm") is frequently conducted, as depicted in Fig. 8.11. The bondline thickness is simulated by placing a vinyl plastic film, or the actual adhesive encased in plastic film, in the bond lines. The assembly is then subjected to the heat and pressure normally used for curing. The

Skin

Skin

Fig. 8.11. Prefit of Details Using Verifilm3

Skin

Fig. 8.11. Prefit of Details Using Verifilm3

parts are disassembled, and the vinyl film or cured adhesive is then visually or dimensionally evaluated to see what corrections are required. These corrections can include sanding the parts to provide more clearance, reforming metal parts to close the gaps, or applying additional adhesive (within permissible limits) to particular locations in the bondline. Verification of bondline thickness may not be required for all applications. However, the technique can be used to validate the fit of the mating parts prior to the start of production, or to determine why large voids are produced in repetitive parts. Once the fit of mating parts has been evaluated, any necessary corrections can be made. For cases in which the component parts can be dimensionally corrected, it is much more efficient to make the correction than risk having to scrap the bonded assembly, or, worse yet, having it fail in service.

8.8.3 Adhesive Application

The most commonly used adhesives are supplied as liquids, pastes, or prefabricated films. The liquid and paste systems may be supplied as one-part or two-part systems. The two-part systems must be mixed before use and thus require scales and a mixer. The amount of material to be mixed should be limited to the amount needed to accomplish the task. The larger the mass, the shorter the pot or working life of the mixed adhesive. To prevent potential exotherm conditions, excess mixed material should be removed from the container and spread out in a thin film. This will prevent the risk of mass-related heat build-up and the possibility of a fire or the release of toxic fumes.

Skin

One factor that must be considered in adhesive application is the time interval between adhesive preparation and final assembly of the adherend. This factor, which is referred to as pot, open, out-time, or working life, must be matched to the production rate. Obviously, materials that are ready to bond quickly are needed for high rate applications, such as those found in the automotive and appliance industries. It should be noted that many two-part systems that cure by chemical reaction often have a limited working life before they become too viscous to apply. Application of liquid adhesives can be accomplished using brushes, rollers, manual sprays, or robotically controlled sprays. Application of paste adhesives can be accomplished by brush, by spreading with a grooved tool, or by extrusion from cartridges or sealed containers using compressed air.

Film adhesives are high quality but costly and thus are used mainly in aircraft applications. They consist of an epoxy, a bismaleimide or polyimide resin film, and a fabric carrier. The fabric guarantees a minimum bondline thickness, because it prevents adherends from contacting each other directly. These adhe-sives are manually cut to size, usually with knives, and placed in the bondlines. When applying film adhesives, it is important to prevent or eliminate entrapped air pockets between the adherend and adhesive film by pricking bubbles or "porcupine" rolling over the adhesive prior to application.

8.8.4 Bond Line Thickness Control

Controlling the thickness of the adhesive bondline is a critical factor in bond strength. This control can be obtained by matching the quantity of available adhesive to the size of the gap between the mating surfaces under actual bonding conditions (heat and pressure). For liquid and paste adhesives, it is a common practice to embed nylon or polyester fibers in the adhesive to prevent adhesive starved bondlines. Applied loads during bonding tend to reduce bondline thickness. A slight overfill is usually desired to insure that the gap is totally filled. Conversely, if all of the adhesive is squeezed out of a local area due to a high spot in one of the adherends, a disbond can result.

For highly loaded bondlines and large structures, film adhesives are used that contain a calendared film with a thin fabric layer. The fabric maintains the bondline thickness by preventing contact between the adherends. In addition, the carrier acts as a corrosion barrier between carbon skins and aluminum honeycomb core. In the most common case, the bondline thickness can vary from 0.002 to 0.010 in. Extra adhesive can be used to handle up to 0.020 in. gaps. Larger gaps must be accommodated by reworking the parts, or by producing hard shims to bring the parts within tolerance.

8.8.5 Bonding

Theoretically, only contact pressure is required so that the adhesive will flow and wet the surface during cure. In reality, somewhat higher pressures are usually required to (1) squeeze out excess adhesive to provide the desired bondline thickness, and/or (2) provide sufficient force to insure all of the interfaces obtain intimate contact during cure.

The position of the adherends must be maintained during cure. Slippage of one of the adherends before the adhesive gels will result in the need for costly reworking, or the entire assembly might be scrapped. When a paste or liquid adhesive is used, it is usually helpful to have a load applied to the joint to deform the adhesive to fill the bondline. C-clamps, spring loaded clamps, shot bags, and jack screws are frequently used for simple configurations. But, if elevated temperature curing is required, some care is required that these pressure devices do not become heat sinks.

Liquid and paste adhesives that are cured at room temperature will normally develop enough strength after 24 h. so that the pressure can be removed. For these adhesives that require moderate cure temperatures (e.g., 180° F), heat lamps or ovens are frequently used. When using heat lamps, some degree of caution is necessary to insure that the part does not get locally overheated. If the contour is complex, it may be necessary to bag the part and employ the isostatic pressure of an autoclave. Instead of using the positive pressure of a vented bag in an autoclave, a vacuum bag (<15psia) in an oven is quite commonly used. The disadvantage of this process is that the vacuum tends to cause many adhesives to release volatiles and form porous and weak bondlines.10

When elevated temperature (e.g., 250-350° F) curing film adhesives are used, autoclave pressures of 15-50 psi are normally used to force the adherends together. The majority of these adhesive systems cure in 1-2 h at elevated temperature. Autoclave bonded parts are made on bond tools very similar to the ones used for cure tooling. The bagging procedures for autoclave bonding are also very similar to those used for composite curing except that bleeder is not required since we are not trying to remove any excess resin during cure. Both straight heat-up and ramped (intermediate hold) cure cycles are used. A typical autoclave cure cycle for a 350° F curing epoxy film adhesive would be:

• Pull a 20-29 in. of Hg vacuum on the assembly and check for leaks. If the assembly contains honeycomb core, do not pull more than 8-10 in. of Hg vacuum.

• Apply autoclave pressure, usually in the range of 15-50 psi. Vent the bag to atmosphere when the pressure reaches 15 psi.

• Heat to 350° F at a rate of 1-5° F/min (Option: an intermediate hold at 240° F for 30 min is sometimes used to allow the liquid resin to thoroughly wet the adherend surfaces).10

• Cure at 350 ± 10° F for 1-2 h under 15-50 psi.

• Cool to 150° F before releasing autoclave pressure.

During cure, the adhesive flows and forms a fillet at the edge of the bond. It is important not to remove this fillet during clean-up after bonding. Testing has shown that the presence of the fillet significantly improves the joint strength.11

8.9 Sandwich Structures

Because it is an extremely lightweight structural approach that exhibits high stiffness and strength-to-weight ratios, sandwich construction is used extensively in both aerospace and commercial industries. The basic concept of a sandwich panel12 is that the facesheets carry the bending loads (tension and compression), while the core carries the shear loads, much like the I-beam comparison shown in Fig. 8.12. As shown in Fig. 8.13, sandwich construction, especially honeycomb core construction, is extremely structurally efficient,12 particularly in stiffness critical applications. Doubling the thickness increases the stiffness over 7X with only a 3% weight gain, while quadrupling thickness increases stiffness over 37X with only a 6% weight gain. Little wonder that structural designers like to use sandwich construction whenever possible. Sandwich panels are typically used for their structural, electrical, insulation, and/or energy absorption characteristics.

Facesheet materials that are normally used include aluminum, glass, carbon, or aramid. Typical sandwich structure has relatively thin facesheets (0.010-0.125 in.) with core densities in the range of 1-30 pcf (pounds per cubic foot). Core materials include metallic and non-metallic honeycomb core, balsa wood, open and closed cell foams, and syntactics. A cost versus performance comparison13 is given in Fig. 8.14. Note that, in general, the honeycomb cores are more expensive than the foam cores but offer superior performance. This

Facesheets Carry Tension and Compression Loads

Core Carries Shear Loads

Web Carries Shear Loads

Flanges Carry Tension and Compression Loads

Web Carries Shear Loads

Sandwich Panel Fig. 8.12. Why Sandwich Structures Are So Efficient12

-Beam

Fig. 8.13. Efficiency of Sandwich Structure12

Relative Performance

PS - Polystyrene PP - Polypropylene

PU - Polyurethane PMI - Polymethacrylimide

Fig. 8.14. Cost vs. Performance for Core Materials13

explains why many commercial applications use foam cores, while aerospace applications use the higher performance but more expensive honeycombs. It should also be noted that the foam materials are normally much easier to work with than the honeycombs. A relative strength and stiffness comparison of different core materials is given in Fig. 8.15.

Aluminum: 3003/ACG, 5052, 5056 Nomex: HRH-10, HRH-78 Fiberglass: HRP

Fig. 8.15. Strength and Stiffness of Various Core Materials12

Aluminum: 3003/ACG, 5052, 5056 Nomex: HRH-10, HRH-78 Fiberglass: HRP

Fig. 8.15. Strength and Stiffness of Various Core Materials12

Foam core sandwich assemblies can be bonded together with supported film adhesives, but the more common case is either to use liquid/paste adhesives or to do wet lay-up of the skin plies directly on the foam surface. More recently, foam cores with dry composite skins are impregnated and bonded with liquid molding techniques, such as RTM or low pressure VARTM. Supported film adhesives are normally used to bond composite structural honeycomb assemblies.

8.9.1 Honeycomb Core

The details of a typical honeycomb core panel are shown in Fig. 8.16. Typical facesheets include aluminum, glass, aramid, and carbon. Structural film adhesives are normally used to bond the facesheets to the core. It is important that the adhesive provide a good fillet at the core-to-skin interface. Typical honeycomb core terminology is given in Fig. 8.17. The honeycomb itself can be manufactured from aluminum, glass fabric, aramid paper, aramid fabric, or carbon fabric. Honeycomb manufactured for use with organic matrix composites is bonded together with an adhesive, called the node bond adhesive. The "L" direction is the core ribbon direction and is stronger than the width (node

Facesheet

Face Bond Adhesive

Honeycomb Core

Node Fillet Core Foil

Bond Adhesive

Fig. 8.16. Honeycomb Panel Construction1

Face Bond Adhesive

Honeycomb Core

Facesheet

Node Fillet Core Foil

Bond Adhesive

Fig. 8.16. Honeycomb Panel Construction1

Ribbon or "L" Dimension

"W" Dimension

Ribbon or "L" Dimension

"W" Dimension

Thickness "t"

Node Bond

Fig. 8.17. Honeycomb Core Terminology12

Cell Size

Thickness "t"

Node Bond

Fig. 8.17. Honeycomb Core Terminology12

Cell Size bond) or "W" direction. The thickness is denoted by "t" and the cell size is the dimension across the cell as shown in the figure.

Although there are a variety of cell configurations available,14 the three most prevalent (Fig. 8.18) are hexagonal, flexible-core, and overexpanded core. Hexagonal core is by far the most commonly used core configuration. It is available in aluminum and all non-metallic materials. Hexagonal core is structurally very efficient, and can even be made stronger by adding longitudinal reinforcement (reinforced hexagonal) in the "L" direction along the nodes in the ribbon direction. The main disadvantage of the hexagonal configuration is limited forma-bility; aluminum hexagonal core is typically rolled formed to shape, while non-metallic hexagonal core must be heated formed. Flexible-core was developed to provide much better formability. This configuration provides for exceptional formability on compound contours without cell wall buckling. It can be formed around tight radii in both the "L" and the "W" directions. Another configuration with improved formability is overexpanded core. This configuration

Hexagonal Flexible-Core

Fig. 8.18. Types of Honeycomb Core Cell Configurations12

Overexpanded

Hexagonal Flexible-Core

Fig. 8.18. Types of Honeycomb Core Cell Configurations12

Overexpanded is hexagonal core that has been overexpanded in the "W" direction, providing a rectangular configuration that facilitates forming in the "L" direction. The "W" direction is about twice the "L" direction. This configuration, as compared to regular hexagonal core, increases the "W" shear properties but decreases the "L" shear properties. An excellent source of more detailed information on honeycomb core is available.15

Honeycomb core is normally made by either the expansion or the corrugation process shown in Fig. 8.19. The expansion process is the one that is the most prevalent for lower density (<10pcf) honeycomb core used for bonded assemblies. The foil is cleaned, corrosion protected if it is aluminum, printed with layers of adhesive, cut to length, stacked, and then placed in a press under heat

Sheet Cleaned, Corrosion Protected, and Printed with

Sheet Cleaned, Corrosion Protected, and Printed with

Sheet Cleaned, Corrosion Protected, and Corrugated with Rolls

Sheet Cleaned, Corrosion Protected, and Corrugated with Rolls

Corrugation Method

Fig. 8.19. Fabrication Methods for Honeycomb Core14

Corrugation Method

Fig. 8.19. Fabrication Methods for Honeycomb Core14

and pressure to cure the node bond adhesive. After curing, the block, or HOBE (honeycomb before expansion), is sliced to the correct thickness and expanded by clamping and then pulling on the edges. Expanded aluminum honeycomb retains its shape at this point due to yielding of the aluminum foil during the expansion process. Non-metallic cores, such as glass or aramid, must be held in the expanded position and dipped in a liquid epoxy, polyester, phenolic, or polyimide resin, which then must be cured before the expansion force can be released. Several dip and cure sequences can be required to produce the desired density.16 Since phenolics and polyimides are high temperature condensation curing resins, it is important that they are thoroughly cured to drive off all volatiles. If the volatiles are not totally removed during core manufacturing, they can evolve during sandwich curing, creating enough pressure to potentially split the node bonds. Therefore, after the initial cure, it is common practice to post-cure the phenolic or polyimide core at higher temperatures to insure that the reactions are complete. Corrugation is a more expensive process reserved for materials that cannot be made by expansion, or for higher density cores such as >10pcf. For example, high temperature metallic core (e.g., titanium) is made by corrugation and then welded together at the nodes to make the completed core sections.

The comparative properties of some of the commercial honeycomb cores are given in Table 8.3. Aluminum honeycomb has the best combination of strength and stiffness. The higher performance aerospace grades are 5052-H39 and 5056-H39, while the commercial grade is 3003 aluminum. Cell sizes range from 1/16 to 3/8 in. but 1/8 and 3/16 in. are the ones most frequently used for aerospace applications. Glass fabric honeycomb can be made from either a normal bi-directional glass cloth or a bias weave (±45°) cloth. It is usually impregnated with phenolic resin, but for high temperature applications a poly-imide resin is used. The advantage of the bias weave is that it enhances the shear modulus and improves the damage tolerance of the core. There are currently three types of aramid core. The original Nomex core is made by impregnating aramid paper with either a phenolic or a polyimide resin. However, an issue with Nomex is that the resin cannot fully impregnate the paper. Therefore, Dupont developed Korex paper that is thinner and more easily saturated leading to better impregnation, resulting in a core material that has improved mechanical properties and less moisture absorption.17 Kevlar honeycomb is made by impregnating Kevlar 49 fabric. Finally, bias weave carbon fabric core is a high performance but expensive material that was developed for special applications requiring high specific stiffness and thermal stability when bonded with carbon reinforced facesheets.

The good news about honeycomb core is that it does offer superior performance compared to other sandwich cores. A comparison of strength and stiffness for several core types was previously shown in Fig. 8.15. Note that aluminum core has the best combination of strength and stiffness, followed by the non-metallic honeycombs, and then polyvinyl chloride (PVC) foam. The

Table 8.3 Characteristics of Typical Honeycomb Core Materials18

Name and Type of Core

Strength/Stiffness

Maximum

Typical Product

Density

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