§1Joining without holes
The bonded joint’s case is everything the mechanical joints could not offer: no holes, whole-face load transfer, a built-in seal, and total indifference to what the two parts are made of.
Count the gifts against this library’s earlier pages. No stress raisers: the fastener pages priced every hole and thread in concentration factors; a bond transfers load across the entire faying surface and drills nothing. Uniform, quiet structure: load spread over area instead of pinched through points means thin skins can be structural — the aircraft and vehicle-body logic. Dissimilar and delicate materials: metal to glass, rubber to steel, composite to anything, with the bond-line even insulating the galvanic couple the corrosion pages warned about. And it seals as it joins — one operation, two of this section’s jobs. The bills are equally structural. Bonds are process-made: strength is manufactured at assembly time by surface prep, mixing and cure, not guaranteed by a grade stamp. They are inspection-blind — a bad bond looks like a good one, which is why critical bonding lives inside controlled procedures and test coupons. They dislike heat and time in ways bolts never do. And they obey the hero’s law absolutely — which §3 turns into geometry.
Contents§2How adhesives hold
A bond is a chain of three links — adhesion to each surface, cohesion through the glue — and the surface links are decided before the adhesive ever arrives, by wetting.
Adhesion is molecular intimacy: the liquid adhesive must wet the surface — spread and make contact at the molecular scale — before any of its bonding mechanisms (secondary forces, micro-mechanical keying into roughness, outright chemical linkage on primed surfaces) can act. Wetting is thermodynamics: liquids spread on surfaces of higher energy than themselves, which sorts the material world at a stroke — metals, glass and most engineering plastics welcome adhesives, while the low-energy polyolefins (polyethylene, polypropylene) and fluoropolymers shed them like rain, bondable only after flame, plasma or primer treatment raises their surface energy. It also writes the gospel of §6: a surface film of oil, release agent or loose oxide is what the adhesive actually wets, and the bond is then to the film. Cohesion is the cured adhesive’s own strength, and it carries the counter-intuitive rule of the bond-line: thin beats thick. A film of one or two tenths of a millimetre is constrained by the stiff adherends and strong; a fat, gap-filling slug is just bulk plastic with bulk-plastic strength and its own shrinkage stresses. Clamp to a controlled thin line — but never so hard the joint is squeezed dry.
Contents§3Designing the joint
Adhesive joint design is one worked number and two geometric morals: area is cheap, overlap length is overrated, and every peel edge must be designed to death.
Bond a simple lap of 25 mm width × 12.5 mm overlap — 312.5 mm² — and take a deliberately modest structural design allowable of 15 MPa (a working figure well inside what good structural adhesives certify on prepared metal): the joint carries F = 4.69 kN, near half a tonne, from a footprint smaller than a postage stamp. The first moral hides in the hero’s stress spikes: shear in a lap is not uniform — the overlap’s ends carry disproportionate load while its middle loafs — so past a modest overlap, added length adds mostly loafing middle. Grow the width, not the overlap, and on serious joints taper or scarf the ends so the adherends’ flexing cannot pry the bond. The second moral is the hero’s lower panel made procedure: identify every edge where service loads, impacts or flexing could lift an adherend, and kill the peel there — stiffen the edge, wrap it, radius it, or drop in the classic rivet-bond: one mechanical fastener at the peel edge whose only job is to convert a peel initiation back into the shear the rest of the bond was designed for.
§4The chemistry shelf
Six families cover the workshop, and each is best remembered by what triggers its cure — because the trigger is also the constraint on how you may use it.
| Family | Cures by | Character & home |
|---|---|---|
| Epoxy (2-part) | mixing resin + hardener | the structural benchmark: strong, stiff, gap-filling; needs mix discipline and cure time |
| Polyurethane | 2-part mix or moisture | strong yet flexible — bonded windscreens, panels; tolerates joint movement |
| Structural acrylic (MMA) | 2-part, surface-tolerant | fast, forgiving of light contamination; production bonding of metals and plastics |
| Cyanoacrylate | trace surface moisture | seconds-fast on close-fitting small parts; brittle, hates gaps, heat and peel |
| Anaerobic | metal contact + no air | lives in threads and fitted joints — §5’s whole subject |
| Silicone (RTV) | atmospheric moisture | the elastic sealer: huge movement and temperature range, modest strength — seals, not structure |
| Read the middle column as a checklist of failure modes: a 2-part product mis-ratioed never cures; a moisture-cure product in a deep, sealed, bone-dry joint never cures; an anaerobic left in the open never cures; a cyanoacrylate across a wide gap cures only at its edges. Every mysterious soft bond in a workshop’s history is one of these sentences, lived. | ||
§5Anaerobics on threads
The anaerobic is this section’s own adhesive: cured by the very conditions inside an assembled metal joint, it fills the clearances the fastener pages spent chapters managing.
Confined between metal surfaces with air excluded — precisely the state inside a made-up thread — the anaerobic polymerises into a tough plastic filling every clearance, and each of its trade names maps to a clearance this library has met. As threadlocker it fills the flank clearance whose working-loose the fastener-security page fought with washers and wire: the fastener is locked and the helical leak path sealed in one liquid, in the familiar strength ladder — low (serviceable with hand tools), medium (the general default), high (studs and permanence, released only with heat softening the polymer). As thread sealant it does the pipe page’s job on taper threads, curing solid where PTFE tape merely packs. And as retaining compound it fills the micro-clearance of slip and light-press fits, bonding bearings, bushes and pulleys to shafts with whole-surface shear — a chemical member of the keys page’s “beyond keys” family, rescuing worn housings and turning a sliding fit into a driving one. The disciplines are §4’s row read carefully: it needs active metal and no air, so passive surfaces (stainless, plated, anodised) take a primer, excess outside the joint stays wet forever, and cure wants its hours before full torque or load.
Contents§6Sealant practice
Sealants succeed on procedure: the surface gospel, a joint that lets the bead work, and enough material planned in the caulking gun before the job starts.
The gospel first, because §2 made it law: clean, abrade, degrease — in that spirit and roughly that order — so the sealant wets metal, not oil film; solvent-wipe with clean cloths toward waste, and on critical work treat prep as a recorded step, since the finished bead hides everything. Joint design next: an elastic sealant survives by stretching, so the bead needs cross-section to stretch with — a joint that works thermally is given width, a sensible depth (backer rod setting it in deep joints), and a low-modulus product from §4’s silicone row; a rigid gasket-maker in a moving joint simply tears on schedule. Then the humble arithmetic of the gun: a Ø6 mm bead laid along 2 m of flange is πr²L = 56.5 mL of product — most of a standard cartridge — and knowing that before starting is the difference between one continuous bead and a cold joint where the second cartridge met the skinned-over first. Finish inside the product’s tooling time, respect the difference between skin-over and full cure before pressurising or immersing, and grant clamped gasket-makers their squeeze-out relief. Sealing, like bonding, is manufacturing — the tube is just where the raw material lives.
Contents§7Quick reference
The working core of the page on one card rack.
The case
no holes · whole-face load
seals as it joins
Holding
wetting decides adhesion
thin bond-line beats thick
Design
312.5 mm² × 15 MPa = 4.69 kN
width, not overlap · kill peel
Cure triggers
mix · moisture · contact-no-air
the trigger is the constraint
Practice
clean–abrade–degrease
plan the bead: 56.5 mL per 2 m