Glue/Adhesive Bonded Joints

As was previously discussed (in the section on Composite and Non-Metallic Materials), these are permanent joints in which an adhesive is used to join two or more materials together. The materials can be any of the large variety of fabrics found in the aerospace industry (metal, paper, plastic, rubber or wood).

Some advantages of using adhesives to make joints are that the materials being joined may or may not be similar and the joints can be made proof against the leakage of gases and liquids.

Adhesives are normally good electrical insulators, which can greatly reduce dissimilar corrosion on metal joints and are not normally affected by temperature changes.

Joining with adhesives not only saves the weight (and costs) associated with threaded fasteners (and rivets), but also eliminates the need to make holes in the structure for those fasteners, which avoids the possibility of potential stress raisers.

The absence of fasteners in an aircraft’s skin results in a smoother airflow around the aircraft and thus contributes to its aerodynamic efficiency.

Adhesive bonded joints also provide greater stiffening to the structure, compared to that achieved with mechanical fastenings.

However, there are some disadvantages in that the surfaces of the items to be stuck together (the adherends) must be free from grease, oil or dust, and the type of adhesive must be suitable for the conditions or environment in which it is intended to be placed.

Fumes from adhesives can be narcotic, toxic and extremely flammable, so that great care must be taken when applying adhesives. This entails working in well-ventilated conditions, wearing the appropriate personal protective equipment and observing the relevant safety precautions to prevent the outbreak of fire.

Locking by Adhesives

Applying Shellac, Araldite etc. to DTD 900 specification may be used to lock many small components, particularly those in instruments, valves, switches etc. Adhesive is applied to the outside of the nut face and the protruding screw thread or to the component and screw head, after tightening and prevents movement between relevant parts.

It is good practice when using Araldite, to mix a separate sample under similar conditions, to check that it hardens within the specified time period.

Loctite

Loctite is the trade name for a liquid sealant, used to lock metal threads. It is an approved, proprietary material, which hardens in the screw threads after assembly. Loctite is supplied in various grades to give a predetermined locking strength in a variety of applications from stud locking to retaining bearing housings.

When using Loctite, it is advisable to degrease the parts to achieve maximum strength. If the threads are not degreased about 15% of the locking strength is normally lost. Loctite should only be used when specified by the approved drawings or instructions and applied in accordance with the manufacturer’s directions.

Synthetic Resin Adhesives

Synthetic resin adhesives are used extensively for joints in wooden structures to avoid the localised stresses and strains, which may be set up, following the use of mechanical methods of attachment.

Synthetic resin adhesives that are used for gluing aircraft structural assemblies must comply with the requirement prescribed in an acceptable specification.

Synthetic resin adhesives usually consist of two separate parts, namely the resin and the hardener. The resin develops its adhesive properties only as a result of a chemical reaction between it and the hardener.

Testing of Adhesive Joining Techniques

Frequent tests would be made to ensure that joining techniques are satisfactory. Whenever possible, tests should be done using off-cuts of actual components from each batch. Where off-cuts are not available, tests should be done on representative test pieces.