Appendix 4F Review Of Adhesives, Fasteners, And Fitting
There exist a number of alternatives to welding, brazing, and soldering which might be employed in space industry for joining metals and especially nonmetals. The most important of these are adhesives, metals fasteners, interlace fasteners for stitching or stapling, shrink fitting, and press fitting. Each has been considered for space applications in terms of manufacturing processes, required materials, possible compatible substitute techniques, and the degree of automation attainable. In addition, the unique impact of zero-g (more properly, "free-fall"), hard vacuum, and intense radiation is considered for each joining process examined.
4F.1 Glues and Other Nonmetallic Bonding Agents
Adhesives are used to fasten two surfaces together, usually producing a smooth bond. This joining technique involves glues, epoxies, or various plastic agents that bond by evaporation of a solvent or by curing a bonding agent with heat, pressure, or time. Historically, glues have produced relatively weak bonds. However, the recent use of plastic-based agents such as the new "super-glues" that self-cure with heat has allowed adhesion with a strength approaching that of the bonded materials themselves. As a result, gluing has replaced other joining methods in many applications - especially where the bond is not exposed to prolonged heat or weathering.
A large fraction of modern glues are carbon-based petrochemical derivatives. These can be used to bond almost any combination of surfaces, either by direct contact or by fastening both surfaces to a third as with adhesive tapes. Glues can serve as bonding agents in strong structural materials - one of the earliest, and still common, such use is the fabrication of plywood (a wood composite). Other related composites include fiberglass and various fiber-epoxies such as boron-epoxy and carbon-epoxy. Many of these materials make superior stress-bearing components.
Composite structures often are far less massive than comparable metal components and may be used in structural locations. Some of the early plans for beam construction on Shuttle flights call for carbon-epoxy materials. Composites may be the major use of glue/epoxy adhesives in space. For macroscopic bonding, alternatives such as welding, stapling, bradding, stitching, and other fasteners can replace adhesives if necessary. But although composites in theory can be replaced by metal parts it is far more likely that in space metal parts will give way to composites.
The space application of adhesives includes the following considerations:
- Zero-g - Although some adhesives must bond and cure under pressure, variations on clamping could compensate for the lack of gravity. Application of adhesives also should not demand gravity feed, although squirting and injection techniques have been perfected.
- Vacuum - Many resins and glues used on Earth are fairly volatile and deteriorate under vacuum. But some plastics, once cured, no longer are volatile and may continue to be used in vacuo. Silicate-based waxes and bonding epoxies employed in composites are just two examples of currently available vacuum-compatible adhesives.
- Radiation - Most hydrocarbon-based plastics weaken under the influence of infrared and higher-frequency electromagnetic radiation. These would not be suitable for exposed space use without shielding. More research is needed to develop radiation-resistant adhesives and bonding agents.
The application of glues to complex shapes already is automated in many industries, particularly fabric applications. Composite mixing and curing is now done by machines with a high level of reliability. Further automation of these processes should present no unusual difficulties.
4F.2 Metal Fasteners
Metal fasteners are of two kinds - those producing a permanent bond and those requiring either a releasable or a sliding bond. Screws, nuts and bolts, rivets, brads. retaining rings and clamps are examples from the first category. These are used for permanent fastening where stress loads preclude gluing but do not require welding or where the possibility exists of undoing the bond for some future purpose such as repair. Nonpermanent fasteners include quick-release couplers and clamps intended for removal at a specified time, and pins which allow relative movement of fastened parts. Pins are used where conditions of movement
