Aluminum honeycomb cores

In the late 1960s, Army helicopters in Southeast Asia began to develop severe debonding problems in sandwich panels of titanium and glass-reinforced epoxy composite skins bonded to aluminum honeycomb core. These failures were attributed to the ingress of moisture into the interface. The combined effects of moisture and stress would have, thus. 

The Mosquito was a laminated wood monocoque design that although unusual, was not new. More uniquely, the monocoque shell was fabricated by gluing plywood skins to balsa wood core. This construction foreshadowed the popular honeycomb core/facesheet aluminum and composite designs of later years. Fuselage halves of the Mosquito were formed in closed wooden female tools (tools... 

Honeycomb core. Honeycomb core used for aluminum bonded sandwich structure is exclusively aluminum. The core is fabricated by printing offset stripes of adhesive (the node adhesive) on aluminum foil, stacking a large number of these foils and then curing the adhesive in a heated press. The resulting block is called a hobe. Slices are machined from the edge of the hobe and then expanded to... 

During the temperature ramp period, pressure is applied. How much pressure is applied depends on the adhesive and the type of assembly. Honeycomb assemblies are limited by the compression strength of the honeycomb core, so cure pressure is typically limited to 50 psi for aluminum core of standard density. Metal to metal assemblies can withstand higher pressures and usually have fewer bondline voids when cured at higher pressures. Metal-to-metal assemblies bonded with standard modified epoxies are cured at 90 psi. 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

Different core materials are used. They include foam, honeycomb core (plastic, paper, aluminum, etc.), ribs, balsa wood, filler spacers, corrugated sheet spacers, etc. Materials such as polyurethane foam, cellulosic foams, and polystyrene foams are widely used as core materials. Plastics, such as glass-reinforced polyester, are frequently used as the skins for panels. Different skin materials are used such as metallic skins alone or in conjunction with plastic skins. 

A pressure of 25 psi and temperature of 175°C are required for 1 h to cure the epoxynylon adhesive. Because of their excellent filleting properties and high peel strength, epoxy-nylon adhesives are often used to bond aluminum skins to honeycomb core in aircraft structures. In these applications, climbing drum peel strengths in excess of 150 lb/in have been achieved. 

Adhesives with filleting properties are required for honeycomb cores. A modified phenolic is often used with aluminum honeycomb for high strength, while a neoprene- or nitrile-based organic solvent type of adhesive is often used with impregnated paper honeycomb. Epoxy adhesives are also commonly used in the fabrication of honeycomb sandwich panels. 

E 874 Practice for Adhesive Bonding of Aluminum Facings to Nonmetallic Honeycomb Core for Shelter Panels... 

For honeycomb cores, either water- or alcohol-soluble phenolic resins may be used. Many types of facings may be glued to the honeycomb cores veneers, plywood, hard-boards, asbestos board, aluminum, stainless steel, and paper-plastic laminates. Thin sheet... 

As shown in O Fig. 42.9a, and not the apparently more economical but weaker joint shown in O Fig. 42.9b. Some years ago, the author tried to stiffen an aluminum alloy honeycomb core by filling with a polyurethane foam. This was then scraped down to the level of the honeycomb and the bond made. Unfortunately, the foam prevented the formation of the fillet and the joint failed at a low load. With bonded honeycomb structure, the major defects consist of a lack of attachment between the core and the skin. This may be due to several causes such as locally crushed honeycomb O Fig. 42.10a), skin defects O Fig. 42.10b), or lack of adhesive (O Fig. 42.10c). In themselves, none of these defects may reduce the short-term joint strength. However, as for the lap joint, they may show poor preparation and may provide sites for fatigue crack propagation. 

Door panels have a sandwich structure consisting of aluminum or stainless steel surface plates and aluminum or paper honeycomb core bonded with two-part epoxy adhesives. On inspection lids, switchboard covers, and seat side panels, decorated aluminum sheets with melamine resin are bonded with second generation acrylic adhesives (SGAs) to aluminum frames or stiffeners. O Figure 47.9 shows the cross section of a railcar with adhesively bonded parts (Suzuki 2007). 

If the adhesive used to form the integrally bonded sandwich panel is of such a chemistry that volatile matter is evolved during cure, then the potential pressure built up inside the individual honeycomb cells can be alleviated by using perforated honeycomb core this is particularly relevant to aluminum honeycombs. During processing, prior to applying the node bond adhesive stripes, the aluminum foil is passed over a perforating roll, which punches small holes in the foil. 

---