Adhesive Bonding
Adhesive bonding joins parts with a cured polymer layer that spreads load over the whole bond area instead of concentrating it at fastener points.
- Part
- Joining
- Revised
- 2026-08-11
At a glance
- Family
- Mechanical
- Typical tolerances
- Bond line 0.005–0.010 in (0.13–0.25 mm) held with glass beads or shims; assembly position set by the bond fixture, commonly ±0.010 in (±0.25 mm)
- Typical volumes
- 1 to 1,000,000+
- Lead time
- Seconds to fixture with cyanoacrylate; 4–24 h to handling strength and about 7 days to full cure at room temperature; 30–60 min at 250–350 °F (120–175 °C) for heat-cured structural epoxy
- Materials
- Metal, Plastic, Wood, Glass, Ceramic, Composite
What it is
Adhesive bonding joins parts with a cured polymer layer that carries load across the entire overlap instead of concentrating it at fastener holes. That distribution is the reason to use it: a bonded lap joint has no drilled holes, no stress risers, no local bearing stress, and a fatigue life that a riveted joint in the same panels cannot match.
It also joins what nothing else will — metal to plastic, composite to metal, glass to steel — and the bond line electrically isolates dissimilar metals that would otherwise form a galvanic couple.
Structural epoxies reach 2,500–5,000 psi (17–35 MPa) in lap shear on properly prepared metal, toughened grades higher; structural acrylics 2,500–4,000 psi (17–28 MPa); polyurethanes 1,000–2,500 psi (7–17 MPa) with far more elongation. The catch is that adhesive joints are weak in peel and cleavage, and almost impossible to inspect non-destructively — which makes surface preparation and process control the real engineering work.
How it works
- Surface preparation. This determines whether the joint works. Degrease with solvent, abrade — 80–180 grit paper or grit blast — to break the weak oxide layer and increase surface area, then degrease again to remove the debris and bond promptly. Aerospace aluminum bonding goes further, to phosphoric acid anodizing or a sol-gel treatment, because a mechanically abraded aluminum oxide layer rehydrates and the bond degrades over years.
- Surface energy check. The adhesive must wet the substrate, which requires the substrate's surface energy to exceed the adhesive's surface tension. Low-surface-energy plastics — PP, PE, PTFE, POM, silicone — sit below about 36 dyn/cm and will not bond without flame, corona or plasma treatment, or a dedicated primer.
- Apply and set the bond line. Dispense a controlled bead or film. Bond line thickness is a design variable, not an accident: 0.005–0.010 in (0.13–0.25 mm) is the structural sweet spot, held with glass beads mixed into the adhesive, wire spacers or molded standoffs.
- Close and fixture. Parts are clamped or fixtured so the bond line is uniform and the joint does not move while the adhesive gels.
- Cure. Two-part epoxies reach handling strength in 4–24 h at room temperature and full properties in about 7 days; heat curing at 250–350 °F (120–175 °C) for 30–60 min gives higher glass transition temperature and strength. As a rule of thumb, every 10 °C increase roughly halves cure time. Cyanoacrylates fixture in seconds; anaerobics cure only when confined between metal surfaces with air excluded.
Adhesive families at a glance
Epoxy for the highest structural strength and temperature resistance. Structural acrylic where the surfaces cannot be perfectly cleaned — it tolerates light oil far better than epoxy. Polyurethane where the joint must flex or absorb differential expansion. Silicone for sealing and gasketing from −65 to 400 °F (−55 to 205 °C) rather than for strength. Cyanoacrylate for fast fixturing of small parts, with poor peel strength and a temperature ceiling near 180 °F (82 °C). Anaerobic threadlockers and retaining compounds for fastener locking and cylindrical press-fit augmentation.
Design guidelines
Load the joint in shear, never in peel
Adhesives are strong in shear and compression and weak in peel and cleavage — often by an order of magnitude. Convert every peel-loaded joint into a shear-loaded one: use a lap, double lap, scarf or joggle rather than a butt or a right-angle T. Where peel cannot be designed out, add a mechanical arrest — a rivet, a screw or a formed lip at the end of the overlap — to stop a peel crack from propagating.
Set the overlap and the bond line
Start at an overlap of 4–8x the thinner adherend's thickness. Beyond about 20x thickness, added overlap buys very little, because load transfers at the two ends of the joint and the middle carries almost nothing. Hold a bond line of 0.005–0.010 in (0.13–0.25 mm) with beads or shims: starved joints are weak, and thick bond lines lose strength and cure unevenly. Add a spew fillet at the overlap ends — a small radius of squeezed-out adhesive measurably reduces the peel stress peak there.
Design for differential expansion
Bonding steel (CTE roughly 6.5 µin/in/°F, 11.7 µm/m/°C) to aluminum (13, 23) or to CFRP (near 1, 2) means the joint sees strain every thermal cycle. A more flexible adhesive and a thicker bond line both absorb it; a rigid epoxy in a thin bond line between mismatched materials will eventually fail at the interface. Coefficients of expansion and moduli for the substrates are on the material properties chart.
Substrate preparation and compatibility
| Substrate | Prep | Notes |
|---|---|---|
| Steel, stainless | Degrease, abrade 80–120 grit, degrease | Bond promptly — flash rust starts within minutes |
| Aluminum (structural / long life) | Phosphoric acid anodize or sol-gel | Abrasion alone gives a bond that degrades with humidity over years |
| PP, PE, POM, PTFE, silicone | Flame, corona or plasma, or a primer | Surface energy below ~36 dyn/cm — untreated joints fail at the interface |
| ABS, PC, PVC, acrylic | Solvent wipe, light abrade | Bond well; check adhesive compatibility for stress cracking |
| CFRP, GFRP | Peel ply then abrade, or grit blast | Never bond over release agent |
| Glass, ceramic | Clean, silane primer | Silane coupling agents dramatically improve durability |
| Galvanized, painted, plated surfaces | Bond to the coating, not the metal | The joint is only as strong as the coating's adhesion |
Abrasion targets a specific roughness rather than a polish — see the surface finish chart for the Ra values that correspond to common grit grades.
Anaerobics on threaded fasteners
Threadlockers cure in the confined, airless gap between engaged threads. During assembly they act as a lubricant, so torque to the lubricated values rather than the dry ones — see the bolt torque chart. Retaining compounds do the same job on cylindrical press fits, and can let a looser, cheaper machined fit carry the same torque.
Inspection
This is adhesive bonding's genuine weakness. Ultrasonic through-transmission and thermography reliably find voids, porosity and disbonds, but neither detects a kissing bond — surfaces in intimate contact with no chemical adhesion, usually caused by contamination or bad surface prep. Because the worst defect is invisible, quality is controlled by process rather than inspection: documented surface preparation, adhesive lot traceability, controlled time between prep and bonding, and witness coupons bonded alongside every batch and tested destructively.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Bond line thickness | 0.005–0.010 in (0.13–0.25 mm) | Held with beads or shims | Starved joints are weak, thick ones cure unevenly |
| Overlap length | 4–8x thinner adherend thickness | Gains plateau past ~20x | Load transfers at the overlap ends |
| Loading mode | Shear or compression | Never peel or cleavage | Peel strength is an order of magnitude lower |
| Spew fillet at overlap ends | Small radius, retained | Do not wipe off | Reduces the peel stress peak |
| Time from surface prep to bonding | Minutes to a few hours | Per adhesive datasheet | Prepared surfaces re-oxidize and re-contaminate |
| Low-surface-energy plastics | Flame, corona or plasma treat | Untreated will not bond | Substrate energy must exceed adhesive surface tension |
| Quality method | Witness coupons per batch | NDT alone is insufficient | Kissing bonds are not detectable |
Cost drivers
Adhesive bonding has almost no tooling cost and a significant process cost. There is no die, no fastener tooling, no weld fixture beyond a simple bond jig — but there is surface preparation labor, cure time, and floor space consumed by parts waiting to cure.
Cure time is usually the real constraint. A room-temperature epoxy that needs 4–24 h to reach handling strength forces work-in-progress inventory and fixtures that are tied up for the duration; a heat cure buys speed at the cost of oven time and energy. That trade — fixture count against cure schedule — is where bonded assembly economics are won or lost.
Surface preparation is the other cost, and it is the one most often cut, always with the same result. Abrasion and solvent wipe are labor; anodizing or plasma treatment is a process step with its own equipment. Neither is optional if the joint is structural.
- Match cure chemistry to takt time. A faster adhesive often costs less overall than a cheaper one that ties up ten fixtures.
- Design self-fixturing joints. A lip, tongue or molded standoff that holds alignment and bond line during cure deletes a fixture entirely.
- Use beads in the adhesive rather than shims or spacer parts to set the bond line.
- Combine bonding with a few mechanical fasteners at the overlap ends — this arrests peel and provides fixturing during cure at the same time.
- Bond witness coupons with every batch. Destructive testing on coupons is far cheaper than the field failures that unverified surface preparation produces.
Questions
5 questionsHow strong is a structural adhesive bond?
Two-part structural epoxy typically reaches 2,500–5,000 psi (17–35 MPa) in lap shear on properly prepared metal, with toughened grades higher; structural acrylics 2,500–4,000 psi (17–28 MPa); polyurethanes 1,000–2,500 psi (7–17 MPa) with much greater elongation. Those are shear numbers — peel and cleavage strength is roughly an order of magnitude lower and must be designed out.
What bond line thickness should I design for?
0.005–0.010 in (0.13–0.25 mm) for structural adhesives. Thinner than that and the joint is starved, with the adherends effectively in contact and the adhesive unable to distribute load; much thicker and strength falls, cure becomes uneven and shrinkage stress rises. Control it with glass beads mixed into the adhesive, wire spacers or molded standoffs rather than by clamp pressure.
Why won't adhesive stick to polypropylene?
Polypropylene, polyethylene, PTFE, acetal and silicone have surface energies below about 36 dyn/cm — lower than the surface tension of most adhesives — so the adhesive beads up instead of wetting. Flame, corona or atmospheric plasma treatment raises the surface energy, and dedicated polyolefin primers do the same chemically. Untreated joints fail cleanly at the interface.
Can bonded joints be inspected non-destructively?
Only partly. Ultrasonic through-transmission and thermography find voids, porosity and disbonds reliably, but neither detects a kissing bond — surfaces in intimate contact with no actual adhesion, which is what contamination or bad surface prep produces. Because the most dangerous defect is invisible, quality control rests on documented surface preparation and destructively tested witness coupons bonded with each batch.
Do threadlockers change bolt torque values?
Yes. An anaerobic threadlocker wets the threads during assembly and acts as a lubricant, lowering the friction that torque has to overcome, so torquing to dry values will overload the fastener. Use the lubricated column on the bolt torque chart. The adhesive only develops its locking function after it cures in the airless gap between the engaged threads.