Honeycomb core sandwich structure

Figure 7.46 Honeycomb core sandwich structure (courtesy of Plastics FALLO)...

Sandwich structure n. A term often employed for a laminate comprising at least three layers for example, a cellular-plastic or honeycomb core sandwiched between two layers of glass-reinforced laminate. 

A 179 C cure, flame retarded epoxy designed for use in structural aircraft laminates and honeycomb core sandwich panels. Although formulated for autoclave processing, components can be press moulded and vacuum bag cured. 

A 121°C cure, self-extinguishing, self-adhesive, solvent resistant, epoxy resin with a good dry service temperature, suitable for autoclave, press or vacuum-bag moulding. Co-curable with 7714A, which provides considerable design flexibility with respect to the use of hybrid lay-ups. Designed for use in structural laminates and honeycomb core sandwich panels for aircraft. 

A honeycomb structure consists of honeycomb sandwich panels attached either to a frame or in some instances to each other. The honeycomb panels consist of a honeycomb core sandwiched between metallic or composite face sheets. This arrangement permits use of stabilized, lightweight panels to carry most of the structural loads. Secondary supports would mount to inserts bonded within the honeycomb structure in various locations and orientations. Fittings or load spreaders are frequently required to transfer loads into and out of the honeycomb structure. 

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... 

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. 

Repair. Repairs for damaged bonded structure can be either mechanical or adhesively bonded. Mechanical repairs are metallic doublers on one or both sides of a damaged component, held on by fasteners. The fasteners transfer the load through the doubler around the damaged site and restore part functionality. Although common for metal-to-metal bonded structure, mechanical repair of sandwich structure is rare because of the risk of further delamination. Unless the doubler and fasteners are perfectly sealed, water can travel into the honeycomb core eventually causing freeze-thaw damage and delamination. 

In addition to developing solid RP structures, work has been conducted on sandwich structures such as filament-wound plastic skins with low-density foamed core or a plastic honeycomb core to develop more efficient strength-to-weight structures. Sandwich structures using a syntactic core have been successfully tested so that failures occurred at prescribed high-hydrostatic pressures of 28 MPa (4,000 psi). 

E 900 Specification for Core Splice Adhesive for Honeycomb Sandwich Structural Panels... 

A typical layered structure consists of two thin, glass-fibre-reinforced polymer skins bonded to a thick, lightweight honeycomb core (Fig. 4.5a). Such sandwich panels are used in railway carriages and aircraft there are similar structures inside many skins. Other examples are less obvious the space between the outer container and the toughened polystyrene liner of a refrigerator is filled with rigid polyurethane foam. 

Also available are whisker reinforcements with exceptional high performances (Chapter 2). Also used are non-fibrous materials, such as steel wire (Table 1.6), and surface-treated mineral fillers that include mica platelets, talc, fibrous and finely divided minerals, glass flakes, and hollow and/or solid glass micro spheres. Lightweight expanded materials, such as sheets of reinforced foam or honeycomb, are used as cores in sandwich structures (Chapter 7). 

Producing airplanes at lower costs is another aspect of advanced RP structural applications. In many cases, the carbon and aramid fiber RP components compare fevorably with the cost of conventional component structures, in spite of the rather high material costs. An important aspect here is the possible simplification of the design. For example, the complicated leg fairing of the Airbus was replaced by a simple all-RP sandwich (honeycomb core) panel reinforced by two RP beams. Besides a weight savings of about 30%, the production hours were reduced by 27%. 

Innovations range from individual parts to the complete plane. There is the armored flight deck door to help secure flight decks. The one-inch thick door includes an RP sandwich structure using a phenolic honeycomb core between phenolic-glass fiber laminated facing sheets. 

X-radiography is used to detect problems in the honeycomb cores of bonded sandwich structures. Thermography can locate flaws in continuous... 

Honeycomb core—A sheet material, which may be metal, foamed into cells (usually hexagonal) and used for sandwich construction in structural assemblies, especially in aircraft construction. 

The major drawback, as far as their use in structural adhesives is concerned, is that they are difficult to use in bonded sandwich structures. Perforated honeycomb core has to be used because of the volatiles produced on cure and they do not possess good filleting behaviour. In the latter case, a core primer has to be used. The other problem is the general need to use a solvent route when formulating and/or applying these adhesives. 

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