Honeycomb sandwich

Fig. 7. Corrosion of honeycomb sandwich assembly. Reproduced by permission of the Boeing Company.

Adhesive bonding is an integral part of virtually all composite structure. Early composite matrix resins could in some cases act as an adhesive, such as with self-filleting systems used for honeycomb sandwich fabrication. As composite systems became more optimized for minimum resin content and limited flow, supplementary adhesives became more common. Modern-day composite structure relies on adhesives almost as much as bonded metallic structure. 

Table 1 contains the metal-to-metal engineering property requirements for Boeing Material Specification (BMS) 5-101, a structural film adhesive for metal to metal and honeycomb sandwich use in areas with normal temperature exposure. The requirements are dominated by shear strength tests. Shear strength is the most critical engineering property for structural adhesives, at least for the simplistic joint analysis that is commonly used for metal-to-metal secondary structure on commercial aircraft. Adhesive Joints are purposefully loaded primarily in shear as opposed to tension or peel modes as adhesives are typically stronger in shear than in Mode I (load normal to the plane of the bond) loading. 

Table 2 contains the honeycomb sandwich bonding property requirements for the same adhesive. Note that 0.005" thick adhesive has no sandwich bond requirements because it does not contain enough adhesive to form effective fillets and so is not used for sandwich bonding. 

Applications such as the noise attenuation panels that line engine inlets and exhausts require a special form of high temperature adhesive known as a reticulating adhesive. These bond honeycomb sandwich assemblies have inner facesheets with numerous small holes (Fig. 13). The holes and underlying honeycomb core... 

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. 

Environmental factors such as severe hailstorms and human errors such as impact by aircraft service equipment also cause in-service damage to bonded assemblies. Bonded honeycomb sandwich assemblies are particularly prone to such damage because of their customary use as lightly loaded fairings and flight control surfaces and subsequent thin facesheets and relative fragility. 

The compression strength of the foam and honeycomb sandwich composites can be low. 

Epoxy-phenolic adhesives were developed primarily for bonding metal joints in high-temperature applications. Their first major application was to join major aircraft components. They are also commonly used for bonding glass, ceramics, and phenolic composites. Because of their relatively good flow properties, epoxy phenolics are also used for bonding honeycomb sandwich composites. 

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. 

TSB 124, Bonded Honeycomb Sandwich Construction, Hexcel Corp., Dublin CA. (Also in Advanced Composites March/April 1993.)... 

A common example of a proof test is to apply a cleavage load to a bonded honeycomb sandwich by placing an instrument between the face and core and applying a predetermined force perpendicular to the core. If there is no bond disruption due to this test, it is supposed that the product will meet all its service requirements. Other common proof tests used with sealants are leak-testing with a mobile and easily detected gas such as helium or by the application of hydrostatic pressures. 

Other NDT Methods. Radiography (x-ray) inspection can be used to detect voids or discontinuities in the adhesive bond. This method is more expensive and requires more skilled experience than ultrasonic methods. The adhesive must contain some metal powder or other suitable filler to create enough contrast to make defects visible. This method is applicable to honeycomb sandwich structures as well as metal and nonmetal joints. 

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

One application is honeycomb sandwich construction that consists of thin high strength prepreg skins bonded to a thicker honeycomb, foam, or balsa core. The advantages are very low weight, high stiffness, durable, design freedom, and reduced production costs. 

Corrosion can be brought about as a result of water ingress or the coupling of anodic metals and carbon fiber composites in the absence of a barrier coating [29], If the adhesive layer is improperly coated or sealed, it absorbs moisture and may corrode. The core material in honeycomb sandwich materials is prineipally subject to bondline corrosion [29]. Coupling a cathodic material such as carbon with an anodic metal can cause galv anic corrosion if a protective barrier is not present. 

Shiratori. M., Qiang. Y.. Takahashi. Y.. and Ogasawara, N. (1994). Application of infrared thermography to detection of flaws in honeycomb sandwich constructions. JSME International Journal. Series A, 37, 4.. 396 402. 

Soutis C, Spearing SM. Compressive response of notched woven fabric face-sheet honeycomb sandwich panels. J Polym Rubbers Compos 2002 31(9) 392—7. 

Figure 14.3 Typical repair to honeycomb sandwich panel.

In summary, bonded repairs are the preferred approach for manufacturing repairs to both honeycomb sandwich and monolithic secondary stmcture. However, for the FAA and European Airworthiness and Safety Administration, the main reason for withholding certification of bonded repairs for primary structure is the lack of certainty over bond quality as it is not possible to assess strength and durability of bonded joints without destructive testing. 

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