| name | adhesive-joints |
| description | Adhesive joint design — lap joint stress (Volkersen, Goland-Reissner), peel stress, failure modes, surface preparation, structural adhesives, design guidelines. |
| metadata | {"priority":7,"promptSignals":{"phrases":["adhesive joint","adhesive bonding","bonded joint","lap joint stress","peel stress","structural adhesive"],"minScore":3}} |
Adhesive Joint Design — Complete Skill
Joint Types
- Single-lap: simple; peel and bending stresses at overlap ends
- Double-lap: balanced; reduces peel; still has shear non-uniformity
- Scarf joint: low peel; best stress uniformity; requires precision machining
- Butt joint: poor shear/peel; avoid for structural loads
- Stepped-lap: most efficient; used in composite repair (F/A-18, B-2)
Volkersen Shear Stress Distribution (Elastic)
For single-lap joint, adhesive shear stress:
τ(x) = T/(2bt) × ω × cosh(ωx/2) / sinh(ωL/2) + T/(2bt) × [non-uniform terms]
Simplified (uniform substrates, symmetric):
τ_max/τ_avg = ω L/2 × coth(ω L/2)
where ω = √(2G_a / (t_a × E_s × t_s))
G_a = adhesive shear modulus; t_a = adhesive thickness; E_s t_s = substrate stiffness
Adhesive shear stress concentration: highest at ends of overlap
Goland-Reissner (Includes Bending)
Single-lap joint with eccentric load path → bending moment at overlap ends
Peel stress σ_y (tension or compression) superimposed on shear
Peak occurs at free ends of overlap
Bending moment factor: k = [1 + 2√2 tanh(...)]/... (see Goland-Reissner 1944)
Peak peel stress often governs failure — adhesives weak in tension
Design Rules for Lap Joints
- Keep peel stress low: extend overlap length; use tapered (spew) fillet at ends
- Overlap length: L ≥ 25–30 mm minimum; typically L/t_s ≥ 50 for structural
- Increasing overlap beyond effective length: no strength gain (peak shear unchanged)
- Adhesive thickness: t_a = 0.1–0.3 mm optimal; thin = brittle, thick = low strength
- Scarf angle: 5°–10° for structural — distributes shear+peel uniformly
- Avoid tension-butt loading — use fillets, doublers, or step joints instead
Failure Modes
| Mode | Cause | Prevention |
|---|
| Cohesive failure | Adhesive bulk yields/fractures | Use tougher adhesive |
| Adhesive failure | Bond line separates from substrate | Better surface prep |
| Substrate failure | Adherend tears | Increase substrate thickness |
| Peel failure | End stress concentration | Taper, longer overlap |
| Environmental degradation | Moisture uptake | Sealant, compatible primer |
Structural Adhesive Types
| Type | Shear Strength | T-peel | Temperature | Notes |
|---|
| Epoxy (2-part) | 15–40 MPa | Low-Med | –55 to 120°C | Most common structural |
| Epoxy-film (structural) | 30–50 MPa | High | –55 to 150°C | Aerospace (Cytec, Hexcel) |
| Acrylic (structural) | 15–30 MPa | High | –55 to 120°C | Fast cure; works on oily surfaces |
| Polyurethane | 5–15 MPa | Very high | –50 to 90°C | Flexible; crash-resistant |
| Cyanoacrylate | 10–25 MPa | Very low | –50 to 80°C | Brittle; small area only |
| Silicone | 0.5–5 MPa | Moderate | –65 to 250°C | Sealant; low structural |
Surface Preparation (Critical)
Adhesive failure most common cause: inadequate surface prep
Steel: blast to Sa 2.5 (SIS 05 59 00), apply primer within 4 hours
Aluminum: etching (FPL or P2 etch), phosphoric anodize, or grit blast + silane
CFRP: light abrade + solvent wipe; avoid deep scratching of fibers
Ti: CAA (chromic acid anodize) or PASA-JELL + primer
Contact angle: ≤ 10° water contact = excellent; ≥ 30° = poor prep
Environmental Effects
Moisture reduces strength 20–50% for epoxy adhesives (long-term immersion)
Design knock-down factor: use 50% of dry strength for wet/hot environment
Primer selection critical for hot-wet durability (BR-127, BR-6747-1 aerospace)
Fracture Mechanics Approach
Mode I fracture toughness G_IC of bond line governs peel failure
Typical G_IC: brittle epoxy 0.3–0.8 kJ/m²; toughened epoxy 1–3 kJ/m²; structural film 2–4 kJ/m²
DCB (double cantilever beam) test: G_IC = 12F²a²/(E b² h³)
Output
Provide: joint type, overlap length L [mm], τ_max [MPa] vs. shear strength, peel stress σ_peel [MPa] vs. allowable, adhesive selection, surface preparation specification, environmental knock-down applied, failure mode predicted.
