Aerospace Adhesives.

Adhesives have existed for thousands of years but only in limited form prior to the 1900s. Most were formulated from organic materials such as animal or vegetable byproducts or inorganic minerals. These primitive adhesives included natural gums and waxes such as rubber and beeswax, which were selected for their good moisture-resistance. Traditional woodworking adhesives were made by boiling animal bones to produce granules or flakes that were dissolved in water prior to use. These had limited resistance to moisture and temperature, in contrast to modern woodworking adhesives made from synthetically produced polymers. Soluble sodium silicate is an inorganic adhesive used for bonding porous materials while providing high temperature and moisture-resistance properties. In general, these primitive adhesives produce bonds with limited strength and variable performance.
The development of modern adhesives has paralleled the plastics industry because of their common science, polymer chemistry. As plastics began to be produced in varied forms in the early 1900s, so did adhesives. Initially, adhesives were
an adhesive bond involving two elements, the adhesive and related adherents. Additional elements may be required to improve the bond, including primers or activators to assist the initial bonding process and accelerators (applied to the exposed adhesive) to help complete the cure.

Aerospace fasteners have undesirable limitations. Screws and rivets create unwanted stress concentrations which can result in premature failure due to overload or fatigue. Welding and brazing offer a similar solution but their application process is significantly different. Welding involves a great amount of heat which can result in material property changes and distortion. Brazing, although it requires less heat, produces a bond of lesser strength.

Aerospace Adhesives offer an attractive alternative to other fastening methods for several reasons, including even distribution of applied loads, shock and vibration isolation, sealing, weight reduction and cost savings. Additionally, adhesives provide the ability to bond dissimilar materials without significant preparation or alteration and, in the case of metallics, bonding without fear of galvanic corrosion. Fastening materials without alterations to the substrate can also be a strong argument for their use. Often, a combination of these provides the motivation behind the selection of an adhesive over other fastening methods.
The disadvantages of adhesive bonding must not be overlooked. If the substrate is not properly prepared or if an adhesive is not properly applied to a substrate that is correctly prepared, no amount of adhesive or cure time can salvage the bond. Unlike a threaded fastener which can be retorqued if
improperly assembled, or if it becomes loose, adhesives provide only one chance. The consequences can be costly if done incorrectly and this fact alone is often the only reason to discourage their selection.

Are adhesives load bearing???

When compared to other mechanical fastening methods, the load carrying ability of adhesives are typically less for the same working area. This limitation can often be surmounted by proper joint design and adhesive selection. It is also acceptable to create a hybrid joint which includes both a threaded fastener and an adhesive. This approach can eliminate the concerns associated with the sole use of an adhesive by offering a mechanical backup, while also providing a clamping method that is necessary during joining. Adhesives cannot be disassembled for the purpose of inspection or if an error was made.

Advantages of adhesives

Provides more uniform distribution of stress and larger stress-bearing area than conventional mechanical fasteners
Joins thick or thin materials of any shape
Joins any combination of similar or dissimilar materials
Minimizes or prevents electrochemical corrosion between dissimilar metals
Resists fatigue and cyclic loads
Provides smooth contours
Seals joint, insulates (heat and electricity), and damps vibration Frequently faster and less expensive than conventional fastening
Heat required to set adhesive is usually too low to affect strength of metal parts
Post assembly cleanup of parts is not difficult

Disadvantages of adhesives

Requires careful surface preparation of adherends
Relatively long times are sometimes required for setting adhesive Limitation on upper service temperature is usually 175°C (350°F), but materials are available for limited use to 370°C (700°F)
Heat and pressure may be required for assembly Jigs and fixtures may be required for assembly
Rigid process control is usually necessary.