While welds and rivets work well in most applications, there are occasions with vibrations and harsh environments, such as sea water, that make the joints and their structures less reliable. For these applications, joints bonded with modern adhesives can perform better. The big plus for adhesives is that they are made with a continuous bond line that distributes stresses over an entire mating surface rather than localizing them at points as do welds and rivets.
Substituting adhesive bonds isn't difficult, but a little training is in order. In a nutshell, the substitution calls for analyzing joint geometry, estimating stresses, preparing the bond surfaces, and following adhesive-application guidelines. Attending to these details at the beginning of a project can ensure a tough, long-lasting assembly that stands up to challenging operating demands and environments.
Joint design is one of the largest factors in the strength of a structure. Unfortunately, when designing a structure, some designers simply fall back on previous methods adjusted for the new conditions. This ignores an adhesive's inherent properties and may not benefit from techniques, such as a continuous bondline.
In practice, bonded structures must sustain a combination of forces. For maximum strength, loading stresses should be directed along the lines of the adhesive's greatest strength. Cleavage and peel stresses should be avoided. Moreover, the type of substrate, its thickness, and the adhesive must be analyzed for the optimum area of overlap.
Joints can compensate for a variety of mechanical stresses, minimize stress concentrations on the bondline, and optimize the characteristics of the substrates being bonded. Lap joints, a simple layer on layer, are generally strongest. The length of an overlap increases strength but by decreasing amounts. Increasing the width of the overlap produces a proportional increase in strength.
Most data on adhesive lap shear and peel strengths is based on simple lap joints made on aluminum-alloy substrates. But many other joint designs are successful. For example, metals such as mild steel are often bent or folded to form durable joints. Sheets that cannot be bent or folded can be bonded together using profiles (a connection method) designed for the application. For instance, tapering the edges of a stiffener reduces high-stress concentrations caused by abrupt sectional changes.
Large, thin sheets of metal and plastic are reinforced by bonding on stiffeners made of the same material in a similar gauge. For instance, a "top-hat" stiffener (the cross section resembles a top hat) can be tapered at the edge of the sheet to perform like a scarf joint for maximum strength. (A scarf joint is made by tapering substrate surfaces to produce a longer or larger bonding area that adds strength to the assembly.)
The long-term performance of bonded assemblies also depends on the adhesive and substrate, bondline thickness, and service conditions. The most widely used adhesives in bonded structures include:
Epoxies. These form extremely strong, durable bonds on most substrates including metals, rigid plastics, composites, ceramics, and glass. Epoxies are available with different physical characteristics. These low-shrinkage materials stand up to challenging conditions with good solvent and chemical resistance. A few applications include composite sandwich panels used on minesweepers and the assembly of composite-to-titanium frames for racing bicycles.
Polyurethanes produce strong, resilient joints that resist impact and abrasion. They are room-temperature curable and maintain good flexibility even at temperatures to 0°F.
Formulations range in hardness from Shore 10A to 70D. Polyurethane adhesives are well suited for bonding thermoplastics and assembling dissimilar materials. They typically require little or no surface preparation, resist sagging, and apply easily from dual cartridge dispensing systems. Polyurethanes are well suited for projects such as multicomponent ABS housing for a personal vehicle and the assembly of aircraft tray tables.
On the downside, polyurethanes cannot handle temperatures above 250°F. And a few polyurethanes weather poorly. Uncured materials have some toxicity, crystallize when wet, and can be attacked by solvents.
Methacrylate adhesives are fast curing and tough. They have epoxylike strength and produce long-lasting bonds on thermoplastics including ABS, polycarbonate, thermoset composites, as well as metals, ceramics, and glass. And methacrylates can withstand environmental extremes.
After selecting an adhesive and identifying service conditions, there are few decisions to make regarding the bondline and surface prep. The adhesive, how it's applied, and bondline thickness affect bond performance. Making the bondline thicker reduces stress by absorbing more load when the adhesive is more flexible than the substrate. Toughened adhesives like polyurethanes, for example, maintain more strength in thicker bondlines than rigid adhesives such as epoxies. An optimum bondline thickness generally falls between 0.004 and 0.01 in. Between 0.02 and 0.04 in., adhesive strength drops. Shear strength is relatively constant in thicknesses greater than 0.04 in. The tensile tests of simple epoxy-adhesive lap joints show a maximum load of about 1,885 psi.
For thicker bondlines, adding fillers to an adhesive lets it fill gaps better. Wind-turbine blades, for example, require a specially formulated epoxy for high-strength joints on blade shells. Such an adhesive fills 1-in. gaps with 2-in.-diameter beads.
Two-component epoxies can withstand high shear stresses often found in paper-manufacturing equipment and printing machines. In these applications, rollers turn at up to 3,000 rpm for three shifts a day. The paper webs can exert forces over 3,600 lb/ft of roller length. Epoxies produce joints that work at these flexural fatigue conditions at operating temperatures of 176°F.
The durability of a bonded assembly is affected by temperature and the presence of solvents, hydraulic fluids, oils, fuels, and even water. For example, some epoxies are tailored to withstand temperatures to 350°F, while others can be immersed in saltwater, handling chemicals, and other liquids without hurting bond performance. These characteristics allow wide use of epoxy adhesives to bond fiberglass-reinforced plastic pipe for chemical pipelines.
It's no surprise that surface preparation is critical to the bond. The four levels of pretreatment (from least to most effective) include a dry wipe, degreasing, degreasing followed by abrasion (sanding or sand blasting) and removal of loose particles, and finally degreasing with a chemical treatment.
Dry wiping with clean, lint-free cloth removes loose particles and dust. A few polyurethanes and methacrylates have been formulated for this minimal pretreatment on specific substrates.
Degreasing removes all traces of oil and grease. Methods include suspending the surfaces in solvent in a vapor-degreasing unit. Washing and rinsing in separate tanks of the same solvent also works. Technicians can brush or wipe the material with a clean brush or lint-free cloth soaked in a clean commercial degreasing solvent. An additional degreasing process scrubs the surface in a solution of liquid detergent and rinses with clean, hot water, although steam drying is preferable. The best degreasing method uses an alkaline agent following manufacturer's instructions, or with ultrasonic equipment.
Abrading surfaces lightly produces stronger bonds than joining polished surfaces. Sandpaper, a wire brush, or for maximum effectiveness, grit blasting works well. When abrading, the substrate must then be degreased or cleaned using dry and filtered compressed air.
Chemical pretreatment improves adhesion. Specific treatments are selected based on substrates. Metals can be acid etched to remove scale. Typical pretreatments include chromic acid for aluminum, sulfuric acid for stainless steel, and nitric acid for copper. Metals are also cleaned by anodizing or priming.
Chemical pretreatment for thermosetting plastics include using a solvent such as acetone or methyl ethyl ketone. Thermoplastics are the most difficult substrates to join and pretreatment is determined by the grade of plastic and molding process that formed the substrates.
Bonding should proceed as soon as possible after surfaces are cleaned and pretreated. To ensure reliable adhesion, accurately weigh the resin and hardener, mix them for 3 min, and apply in a controlled thickness. Once applied, jigs or other fixtures prevent bonded surfaces from moving during curing. Fixturing requires only light pressure, but it should be applied as evenly as possible over the bonded area. Finally, follow curing temperatures and times recommended by manufacturers.
Huntsman Advanced Materials
5121 San Fernando Rd.
Los Angeles, CA 90039
WHAT TO EXPECT FROM A JOINT LIKE THIS
Adhesives are strongest when exposed to shear, compression, and tension stresses. They perform less effectively under peel and cleavage loading.In shear stress, force is applied parallel to the plane of the substrates and distributed over the bonded area to minimize its affect on joint strength. Compression stress applies force perpendicular to the substrate surfaces and toward the bond line. This type of stress is limited and calls for use of a resilient adhesive. Tension stress applies force perpendicular to the substrate surface and away from the bond line. Peel stress produces a loading at either 90 or 180° to the plane of the bondline, and represents the most severe challenge that can be placed on an adhesive. Cleavage stress is also hard on adhesive joints. Its prying forces are exerted perpendicular and away from the plane of the bond line. Cleavage stress typically is concentrated on one edge.
|Bonded joints can be loaded five basic ways. Adhesives can withstand shear, compression, and tensile stresses more readily than peel and cleavage.||Joints can be designed for strength after analyzing or estimating anticipated stresses.|