Australian Fatigue Design — AS 4100 Clause 11 Guide
Quick access: [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
Reference for fatigue design of steel structures per AS 4100:2020 Clause 11. Covers detail categories, S-N curves, stress range calculation, Palmgren-Miner cumulative damage rule, and fracture-critical applications.
Detail Categories — AS 4100 Table 11.5.4
| Category | Description | Constant Amplitude Limit (MPa) |
|---|---|---|
| 10 | As-welded rolled section, butt weld trimmed flush | 52 |
| 20 | Butt weld in plate (full pen, ground) | 40 |
| 30 | Butt weld in plate (as-welded) | 33 |
| 36 | Longitudinal fillet weld end | 27 |
| 45 | Transverse fillet weld (load-carrying) | 22 |
| 50 | Cruciform joint, full-pen weld | 18 |
| 56 | Cruciform joint, partial pen weld | 15 |
| 63 | Attached plate by fillet welds | 12 |
| 71 | Welded cover plate end | 10 |
| 80 | Welded shear stud | 7 |
Higher category number = better fatigue resistance (lower stress concentration).
Fatigue Strength — S-N Curves
Constant amplitude fatigue limit (CAFL): Δσc at 2×10⁶ cycles
Fatigue life: N = (Δσc / Δσ)³ × 2×10⁶ (for Δσ > CAFL)
Infinite life: No fatigue check required if Δσ ≤ CAFL
| Category | Δσc at 2×10⁶ | ΔσCAFL (5×10⁶) | Δσ (1×10⁷) |
|---|---|---|---|
| 10 | 125 MPa | 52 MPa | 40 MPa |
| 20 | 92 MPa | 40 MPa | 30 MPa |
| 36 | 71 MPa | 27 MPa | 20 MPa |
| 50 | 56 MPa | 18 MPa | 14 MPa |
Cumulative Damage — Palmgren-Miner Rule
D = Σ(ni / Nfi) ≤ 1.0
Where:
- ni = number of cycles at stress range Δσi
- Nfi = number of cycles to failure at Δσi from S-N curve
Example: Two stress ranges: Δσ1=50 MPa (2,000 cycles, Nf1 for cat 36), Δσ2=30 MPa (10,000 cycles)
- D = 2000/Nf1 + 10000/Nf2
- If D < 1.0 → OK for design life
Worked Example
Problem: Crane runway girder with Category 36 detail. Stress range Δσ=60 MPa, 500 cycles/day, 250 days/year. Design life 30 years.
Solution:
- Total cycles: N = 500 × 250 × 30 = 3.75×10⁶ cycles
- For Cat 36: Δσc=71 MPa at 2×10⁶
- Fatigue life: Nf = (71/60)³ × 2×10⁶ = 1.65 × 2×10⁶ = 3.30×10⁶ cycles
- Cumulative damage: D = 3.75/3.30 = 1.14 > 1.0 FAIL
- Solution: Use Category 20 detail (better detail). Nf = (92/60)³ × 2×10⁶ = 7.2×10⁶ cycles → D = 3.75/7.2 = 0.52 < 1.0 OK
Design Resources
- [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
FAQ
When is fatigue design required per AS 4100? Clause 11.1 requires fatigue design when the number of stress cycles exceeds 20,000 and the stress range is significant. Crane girders and bridges are typical.
What detail category is best for fatigue? Category 10 (trimmed flush butt weld) has the highest fatigue strength. Category 80-90 (attached elements) have the lowest. Specify detailed design to minimize stress concentrations.
How is cumulative damage assessed? Using Palmgren-Miner rule: D = Σ(ni/Nfi) ≤ 1.0. Each stress range level contributes proportionally to the total damage.
Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.