The company that provided the epoxy blamed in Boston’s fatal Big Dig tunnel collapse was indicted yesterday in the death of a motorist crushed by falling ceiling panels. Testing fasteners can help avoid injury, litigation, a bad reputation and myriad other headaches. Here are some things to consider.

Testing adhesives correctly requires special equipment. Actually, “testing adhesives” can be a misnomer. What you really want to test is the bond, which involves the adhesive and the substrates being joined.

Bond strength varies greatly under different conditions. For example, the substrate surface matters greatly. The ability of the adhesive to adhere depends on the surface roughness or smoothness, the presence of any contaminants such as oils and temperature. In some cases, it’s even necessary to use a scanning electron microscope to see the surface and learn which material will provide the best bond.

Above or below certain temperature ranges adhesives become unreliable for bonding. Thus, temperature must be taken into account. Some adhesive designers use ovens to test adhesives under various temperature regimes. “Thermoset adhesives, such as epoxies and anaerobics, can be formulated to retain much of their strength over a broad temperature range, up to 300°F. Selecting a curing temperature close to the operating temperature can reduce the effect of thermal-expansion differences between materials,” Machine Design explains.

Glues are designed for a specific humidity range, as well. Not only does humidity affect bond formation, but it can affect the substrate, too. Controlled humidity chambers enable some glue makers to determine the humidity limits of an adhesive in its ability to maintain a bond.

Just as some materials are better performers under impact and vibration, so too are adhesives. Only testing can help an adhesive buyer predict bond integrity. Why does this matter? None of us wants to see a recreational vehicle fall apart in front of us on a busy highway. And residents of mobile homes don’t want their abodes coming apart, either. When designed, manufactured and applied correctly, adhesives can hold unusual substrates together. For example, holding jet engines to a race car is no small feat for an adhesive.

The application of adhesives is important to achieving a long-lasting and reliable bond. The ideal surface preparation, temperature, humidity and volume per unit area all play a role. Thus, if the people who applied the epoxy in Boston’s Big Dig didn’t apply it correctly, they’re just as liable as the manufacturer.

Reputable adhesive manufacturers will demonstrate how to apply adhesives and perhaps pass along some test results. You may want to replicate these tests, as the manufacturer may have tested under a different condition that a specific application calls for.

Like adhesives, mechanical fasteners can be reliable or unreliable depending on their design, installation and material performance characteristics. Metal fatigue can occur with age, so it is crucial to know what type of service life can be expected. This was learned the hard way when Aloha Airlines’ Flight 243 Boeing 737 lost its skin in April 1988. The disaster caused almost all major United States air carriers to retire their oldest airplane models, as Wikipedia notes.

For mission-critical applications, from my experience at an adhesives firm for six years, use of an X-ray machine to check for imperfections in metal is essential. Just as a bridge engineer would specify a special type of steel for a girder, discussion of the formula used for a metal (or polymer) fastener helps prevent product failure. (By formula, I mean the percentages of the various elements in the metal.)

Whether the fastener is an adhesive, mechanical metal or polymer, the key to product success is knowing the material and the manufacturer’s reputation for quality. Matching the fastener specifications to the application calls for critical thinking and careful calculations.

For a variety of joining methods, check out Machine Design.

Resources

Basics of Design Engineering — Fastening & Joinging
Machine Design

Epoxy Company in Big Dig Tunnel Indicted for Manslaughter
by Steve LeBlanc
The Ledger, Aug. 8, 2007