Epoxy Adhesives Hold Their Own

When you look out your airplane window in a few years, you may not see the familiar rows of rivets. Believe it or not, adhesive bonding has advanced to the point where it can compete with high-performance traditional fastening technologies. Adhesives can lighten structures and extend service life in aerospace, marine, electronic, and automotive applications.

Advanced adhesive-bonding technologies are increasingly pulling their weight in high-performance structural assemblies. Besides replacing traditional mechanical fasteners like rivets, nuts and bolts, and screws, they are competing with other bonding techniques like brazing and welding.

Adhesives can improve product aesthetics by eliminating attachments, protrusions, punctures, and scruffs. Without having to allow for mechanical fasteners, product designers may have the option of having panels painted or finished before they are joined.

Adhesive joints don’t have the stress concentrations and heat-affected zones that welding or drilled holes cause. Loads spread over a wide area, improving reliability. Designers may even be able to scale back requirements for attachment strength because tough adhesives resist vibration, mechanical shock, and fatigue better than mechanical fasteners.

Adhesives chemically separate dissimilar materials so they can be bonded without the threat of galvanic corrosion or chemical incompatibility. The resins can also seal the joint against strong acids and bases, fuels and lubricants, and moisture.

This sealing affect can also protect the joint from prolonged exposure to high vacuums or pressures, thermal cycling, and extreme temperatures. Designers can now take advantage of formulations that can withstand sustained structural service at –150 to 600°F.

Factory bonding
Adhesive bonding can also eliminate manufacturing steps and cut assembly costs. Workers need less training and skill to assemble a properly designed joint than to drill, countersink, and pound rivets or to create a clean weld line.

Of course, that doesn’t mean you’ll get the best performance if you just slap together an adhesive joint. You need to know the best way to store, dispense, apply, and cure the adhesive as well as properly prepare the substrates.

Designers can opt for one or twocomponent adhesive systems. Singlecomponent systems have heat-activated curing agents and needn’t be mixed. The resin and curing agent of two-component systems must be mechanically mixed before cure. These systems can be stored more than a year at room temperature, while one-component systems should be used within three to six months, preferably after refrigerated storage.

Both the one and the two-component systems need a heat cure for a structural and chemically resistant bond. Cures vary from resin to resin but take between 1 and 3 hr and temperatures between 200 and 400°F. Postcure heating can further extend the mechanical properties for some systems.

Many adhesive users fail to optimize their bonded assembly’s performance because they don’t prepare the surface properly. Pretreatment methods generally consist of chemical etching or physical abrasion with sandpaper or grit-blast media. It’s also important to ensure that prepared surfaces are free of oils, greases, lubricants, waxes, and other likely contaminants. The accompanying table lists recommended pretreatments for various substrates.

Designs that bond
Designers must consider the joint itself before getting out the sandpaper and applying glue. Adhesives joints are not geometrically limited like mechanical fasteners, so designers are free to focus on the mechanical and chemical stresses the joint will face.

Tensile stresses pull the two substrates directly away from each other, perpendicular to their surfaces, while compressive stresses act in the opposite direction. Structural adhesives have tensile strengths in the 8,000 to 11,000-psi range and compressive strengths between 20,000 and 40,000 psi.

Shear stresses act in plane with the two substrates to move them in opposing directions. Most structural adhesives can take 2,500 to 3,500 psi of shear stress at room temperature. This property is the one most commonly affected by water or chemical attack, so screening tests often take place both with and without water or fluid immersion.

Designers should pay more attention to cleavage and peel stresses in adhesive bonds than would be the case in mechanically fastened systems. Cleavage arises when tensile force is unevenly applied to one edge of the joint, forcing it open. In peel failure, one of the substrates itself deforms and is pulled away from the bond line. Both failures cut down on the effective surface area of the adhesive bond and lead to unzipping of the bond.

Designers must weigh the stresses the joint will see against assembly considerations to choose the joint best suited to a particular application. The butt joint is the simplest, with two parts bonded end to end. Scarf joints are similar, but have the ends of the joining parts beveled at matching angles for more surface area and shear resistance.

Lap joints allow even more bonding area, but result in offset surfaces that can be susceptible to peel. This can be resolved with an offset or joggle lap where one part is formed to compensate for the bond line thickness. Machined laps and double laps are other options, but they entail additional manufacturing steps. A strap joint combines the end-to-end butt joint with the greater surface area of a double lap joint.