Honeycomb-skin joints

Figure 3.6 shows most of the defects which are likely to occur in an adhesive sheet joint, such as is obtained in a lap or butt joint. (Honeycomb-skin joints are considered separately.)... 

In addition to epoxy-phenolic adhesives three-part epoxy-phenolic-nitrile rubber systems are used in metal-metal edge joints and honeycomb constructions [208], These add toughness not available in most EP systems and improve peel strengths. When used on honeycomb, the NR-P is normally applied to the aluminum skin and the EP to the honeycomb for assembly. Service temperature limitations are those imposed by the NR-P part. 

It is of paramount importance that the adhesive should be capable of forming the so-called fillets during cure. This means that during its cure cycle, the adhesive will melt and flow away from the skin lying at the centre of the honeycomb, first to the cell edges and then down the sides of the cell. This leaves a conical fillet of adhesive on the cell walls and only a very thin coating on the skin. The adhesive between the edge of the cell and the skin should form a butt joint between the two materials. 

The other major form of joint used with structural adhesives is the T joint used in bonding honeycomb to the skins as shown in Fig. 93. The skin and core are bonded by a large number of these lightly-stressed but improbable-looking joints. It is essential that the adhesive forms a generous fillet. Fig. 93(a), and not the apparently more economical but... 

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. 

When bonding honeycomb cores to the skins to make a sandwich construction, it is essential that a generous fillet of adhesive is obtained so that the T joint so formed is properly made (see Adams and Wake"). Even if conditions are met for forming this bond, crushed honeycomb, lack of adhesive or local skin blisters result in a zero bond, providing sites for crack initiation and propagation. 

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