Designing adhesive bonded joints: adhesive types, joint geometry, surface preparation and process control

Adhesives hold together more of the modern world than most people realise. Vehicle bodies, aircraft panels, wind turbine blades, electronics, appliances, signage, furniture and countless industrial products rely on bonded joints. Adhesives join dissimilar materials, spread load over a whole area instead of concentrating it at bolts or welds, seal as they join, avoid heat distortion and holes, and can produce clean, smooth exteriors. For many products, bonding is the only practical way to join thin, lightweight or mixed-material parts.

Bonded joints also have a reputation for unreliability, and that reputation is usually earned by poor design and process control rather than by the adhesives themselves. Joints loaded in peel instead of shear, surfaces contaminated by oils or release agents, adhesives chosen without regard to the service environment, mix ratios and cure conditions left to chance and no evidence of long-term durability are the common causes of failure. Unlike a weld or a bolt, a bonded joint cannot easily be inspected after it is made, so quality must be built in through design and process.

This article explains, in general terms, the main families of structural and engineering adhesives, how to design joint geometry, why surface preparation matters so much, how to control the bonding process, how to prove durability and how to read failures. It is general information. Adhesive manufacturers’ technical data and application engineers should guide specific selections, and safety data sheets must be followed.

The main adhesive families

No adhesive is best for everything. Selection depends on the materials being joined, loads, temperature range, chemical exposure, gap, production rate, cure method and health and safety considerations.

Designing the joint

Load joints in shear, avoid peel and cleavage

Adhesives are strongest when the load is spread over the whole bonded area, which happens in shear (sliding the surfaces relative to each other) and compression. They are weakest in peel and cleavage, where the load concentrates at the edge of the bond and the joint opens like a zip. A joint that is strong in shear can fail at a fraction of that load in peel.

Good joint designs therefore:

• use overlapping lap joints, scarf joints or strap joints that load the adhesive in shear;
• avoid geometries where loads lever the joint open;
• add features such as flanges, tabs or a mechanical fastener at the end of the joint to resist peel where it cannot be avoided; and
• stiffen thin adherends near the joint, because flexible parts bend and create peel stresses at the ends of the overlap.

Overlap length has diminishing returns

In a lap joint, stress is not uniform along the overlap. It concentrates at the ends, while the middle carries less. Doubling the overlap length does not double the strength. Beyond a certain length, adding overlap adds little. Wider joints, tapered adherend ends and fillets of adhesive at the joint edges reduce the end stress concentrations more effectively.

Control the bondline thickness

Each adhesive performs best within a range of bondline thickness. Too thin, and the joint may be starved of adhesive; too thick, and strength and stiffness may fall. Methods to control thickness include glass beads or wire spacers in the adhesive, fixtures with stops and features moulded or machined into the parts.

Allow for differential movement

Different materials expand by different amounts with temperature. Bonding a metal to a plastic or composite over a large area can create high stresses as temperatures change. Flexible adhesives, such as polyurethanes and silicones, or thicker bondlines, can accommodate movement.

Surface preparation: the most important step

Most bonded joint failures that are not design-related trace back to surfaces. Adhesion depends on intimate contact between the adhesive and a clean, receptive surface. Contamination from oils, cutting fluids, fingerprints, release agents, silicones or loose oxides prevents that contact.

Typical preparation steps include:

1. Cleaning and degreasing with suitable solvents or cleaners.
2. Mechanical abrasion to remove weak surface layers and increase surface area, followed by removal of dust.
3. Chemical treatments, primers or conversion coatings for some metals, especially for durable aluminium bonds.
4. Surface activation for some plastics, using methods such as plasma, corona or flame treatment.
5. Bonding within a defined time after preparation, before the surface becomes contaminated or oxidised again.

Low-surface-energy plastics, such as polyethylene and polypropylene, are notoriously difficult to bond and usually require surface treatment or adhesives specifically formulated for them. Silicone contamination, from sprays, release agents or some gloves, is particularly damaging and hard to remove.

Controlling the process

Because bonded joints are difficult to inspect afterwards, consistent quality depends on controlling the process. Key variables include:

• Mix ratio and mixing for two-part adhesives, ideally using static mixing nozzles or metered dispensing equipment rather than hand mixing.
• Pot life and open time, the time available to apply and assemble before the adhesive starts to cure.
• Quantity and placement of adhesive, so the joint is fully covered without excessive squeeze-out.
• Clamping or fixturing to hold parts in position and control bondline thickness during cure.
• Cure time and temperature, distinguishing between fixture time, when parts can be handled, and full cure, when the joint reaches its design strength. Cold workshops can slow curing significantly.
• Humidity, which affects some adhesive types.
• Material shelf life and storage, including temperature limits.

Process records, such as batch numbers, mix checks and cure conditions, provide traceability if problems arise. Witness samples, small test joints made alongside production parts and tested destructively, give evidence that the process produced sound bonds. The proving a process is ready for production article explains how to demonstrate process capability before relying on it.

Proving durability

Datasheet strengths are measured on standard test specimens, usually at room temperature, shortly after cure. Service conditions are rarely so kind. Bonded joints can lose strength over time with heat, moisture, chemicals, ultraviolet light, sustained load (creep) and cyclic load (fatigue). Proving durability means testing joints that represent the real design, materials and surface preparation under conditions that represent service, including ageing where appropriate.

For critical joints, design values should be based on such testing, with appropriate safety factors, rather than on headline datasheet figures.

Reading bond failures

When a bonded joint fails, the fracture surface is informative:

Cohesive or substrate failure generally indicates a well-made bond; adhesive failure usually points to surfaces or process.

Anaerobic adhesives in machinery

Anaerobic adhesives deserve particular mention because they are used in almost every workshop, often without much thought. They cure when confined between close-fitting metal surfaces, away from air, and are used to:

• lock threads, preventing fasteners loosening under vibration;
• retain bearings, bushes, gears and pulleys on shafts and in housings, supplementing or replacing interference fits;
• seal threaded pipe connections; and
• form gaskets between rigid machined flanges.

Products are graded by strength. Low-strength threadlockers allow disassembly with hand tools; high-strength grades may need heat to remove. Choosing a grade stronger than needed makes maintenance difficult, while a grade too weak may not hold.

Anaerobics have limits. They fill only small gaps, so loose fits may not cure fully. Cure is slower on some metals, such as stainless steel, zinc-plated and anodised surfaces, and in cold conditions; activators or primers recommended by the manufacturer can help. Surfaces should be clean and free of oil. They are not suitable for most plastics. Used correctly, they are inexpensive and highly effective, and specifying the grade on drawings and assembly instructions avoids ad hoc choices on the shop floor.

Hybrid joints

Combining adhesives with mechanical fasteners, such as rivet-bonding or screw-bonding, can provide immediate handling strength, resist peel at joint ends and provide a fail-safe load path. Weld-bonding is used in some sheet metal structures. The choosing a joining method article compares bonding with welding, bolting, riveting and brazing.

Health and safety

Many adhesives contain chemicals that can irritate skin and eyes, cause sensitisation or release vapours. Epoxies and some acrylics are common sensitisers, and some polyurethanes contain isocyanates that require particular care. Safety data sheets, appropriate ventilation, gloves and eye protection and training are essential, and these sit within an employer’s work health and safety duties.

A worked example

This is an illustrative example. The products and events are hypothetical.

A manufacturer of composite equipment enclosures bonds aluminium mounting brackets to moulded glass-fibre panels using a two-part epoxy. After several months in the field, some brackets detach from the panels. The failed joints show clean separation of the adhesive from the composite surface: an adhesive failure.

The investigation finds two causes. The panels are moulded with a release agent that leaves a residue on the surface, and the bonding area is wiped but not abraded. Also, in winter, the workshop temperature falls overnight, and brackets are handled and packed before the epoxy reaches full cure.

The business changes its process. Bond areas on the panels are abraded and solvent-wiped immediately before bonding, and a simple water-break test checks for contamination. Two-part adhesive is dispensed through static mixers. Bonded assemblies cure in a heated area to a defined schedule before handling. Witness lap-shear coupons are made with each batch and tested weekly. The joint geometry is also revised to add a small flange that reduces peel at the bracket edge. Field failures stop, and the witness tests show consistent cohesive failure, indicating sound bonds.

How this applies to a small Australian business

• Design for shear, and add features to resist peel.
• Choose adhesives for the materials and environment, with supplier support.
• Treat surface preparation as a controlled process, not a wipe.
• Control mixing, quantity, fixturing and cure, especially in variable workshop temperatures.
• Make witness samples and keep process records.
• Test durability for critical joints under service-like conditions.
• Follow safety data sheets and manage chemical risks.

Common mistakes

• Joints loaded in peel or cleavage.
• Inadequate surface preparation, especially with release agents or oils present.
• Hand mixing two-part adhesives without ratio control.
• Handling parts before full cure.
• Relying on datasheet strengths without testing the real joint.
• Ignoring temperature, moisture and creep in service.
• Using general-purpose adhesives on low-surface-energy plastics.

Frequently asked questions

Can we inspect a bonded joint without destroying it? Some methods can detect voids or disbonds, but they are limited. Most assurance comes from controlling the process and testing witness samples.

How long can mixed adhesive be used? Only within its stated pot life or open time, which shortens at higher temperatures and with larger mixed quantities.

Are adhesives strong enough for structural use? Many structural adhesives are, when the joint is designed for shear, surfaces are properly prepared and durability has been proven for the service environment.

Questions to ask

• How are our bonded joints loaded, and could any of them peel?
• Was the adhesive chosen for the materials, temperatures and chemicals involved?
• How are surfaces prepared, and how do we know they are clean?
• How are mixing, cure time and temperature controlled and recorded?
• What evidence do we have that our joints will last in service?
• When a joint fails, do we examine the failure mode?

Bringing it together

Adhesive bonding joins dissimilar and thin materials, spreads load and seals, but it rewards discipline. Choose the adhesive family for the materials and environment, design joints to load the adhesive in shear and resist peel, control bondline thickness and allow for movement. Above all, prepare surfaces consistently and control mixing, application and cure, because bonded joints cannot easily be inspected afterwards. Witness samples, durability testing and careful reading of failures turn bonding from a source of field failures into a dependable joining method.

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Source: KEVOS editorial notes. Examples in this article are illustrations. Always follow adhesive manufacturers’ technical data and safety data sheets.