| name | fatigue-welded-joints |
| description | Fatigue of welded joints — IIW recommendations, FAT classes, hot spot stress method, effective notch stress, structural stress (Battelle), S-N curves for welds, weld detail selection. |
| metadata | {"priority":7,"promptSignals":{"phrases":["fatigue welded","weld fatigue","FAT class","hot spot stress","IIW","structural stress","effective notch","weld detail","fatigue life weld"],"minScore":3}} |
Fatigue of Welded Joints — Complete Skill
Why Welds Are Fatigue-Critical
Welded joints have:
- Stress concentrations at weld toe and root (Kt = 2-6+)
- Tensile residual stresses equal to yield strength (from solidification)
- Geometric imperfections (undercut, misalignment, porosity)
- Microstructural changes in HAZ
Key consequence: R ratio (mean stress effect) largely irrelevant — residual stresses dominate.
Most weld S-N data assume R = 0 (pulsating tension) or high R = +0.5.
Walker/Goodman corrections not applied to weld S-N curves.
IIW Recommendations (IIW-2259-15)
FAT Classes (Structural Steel)
FAT class = allowable stress range Δσ at 2×10⁶ cycles (50% survival, reference slope m=3)
Higher FAT = better fatigue resistance
Selected FAT classes:
FAT 160: base material (smooth surface, no defects)
FAT 125: transverse butt weld (flush ground, full penetration)
FAT 112: transverse butt weld (as-welded, full penetration, inspected)
FAT 100: transverse butt weld (partial penetration or fillet)
FAT 90: load-carrying fillet weld in shear
FAT 80: T-joint fillet weld, toe of weld on flange
FAT 71: non-load-carrying attachment fillet weld
FAT 63: gusset plate, lap joint, cover plate
FAT 36: weld root of fillet weld (no root penetration, throat)
Shear stress range: FAT class/√3 (von Mises conversion)
S-N Curve Equation
log N = log C - m × log Δσ
m = slope (m=3 for m≤5×10⁶; m=5 above knee at 10⁷ cycles; m→∞ below FAT/2.4 threshold)
Design equation:
Δσ_d ≤ FAT / (γ_Mf × √(n_i/2×10⁶)) [fatigue resistance]
Or: fatigue damage D = Σ(n_i/N_i) ≤ 1.0 (Palmgren-Miner rule)
Fatigue Enhancement Treatments
Toe grinding: FAT × 1.3 (removes stress concentration at toe)
TIG/plasma dressing: FAT × 1.3 (re-melts toe, smoother geometry)
UIT/UIP (Ultrasonic Impact Treatment): FAT × 1.5-2 (introduces compressive residual stress)
Hammer peening: FAT × 1.3 (compressive residual stress)
HFMI (High Frequency Mechanical Impact): up to FAT × 2 for higher yield steels
Stress Assessment Methods
Nominal Stress Method (Simplest)
Use calculated nominal stress in the section: σ_nom = F/A or M/Z
Apply appropriate FAT class for the detail as-drawn
FAT class accounts for local stress concentration implicitly
Apply when: well-defined section; detail matches tabulated descriptions exactly
Hot Spot Stress (Structural Stress at Weld Toe)
Extrapolate stress to weld toe from measurement points away from singularity
Linear surface extrapolation:
Plate: read σ at 0.4t and 1.0t from weld toe, extrapolate linearly
Fine: read at 0.4t, 0.9t, 1.4t (3-point for curved distributions)
σ_hs = 1.67σ(0.4t) - 0.67σ(1.0t) [linear, 2-point]
FAT classes for hot spot stress:
Type A (plate surface, stress extrapolated to toe): FAT 100
Type B (plate edge, stress extrapolated to toe): FAT 100
IIW: FAT 90-100 for hot spot method (removes scatter of detail-specific FAT)
Effective Notch Stress (ENS)
Fictitious rounding ρ_f = 1 mm at weld toe and root (for t ≥ 5 mm)
FEA with 1mm radius at notch → compute peak σ_ENS
Apply FAT 225 (steel) for all weld geometries — single S-N curve!
Requirements: fine FEA mesh (element ≤ 0.25 mm at notch), linear elastic
Battelle (Structural Stress) Method
Used in ASME Div 2 Annex 5.5.5 and API 579
σ_s = σ_membrane + σ_bending (derived from nodal forces, mesh-insensitive)
Equivalent structural stress: ΔS_s = Δσ_s / (t^(2-m)/2m × I(r))
Master S-N curve: single curve for all weld geometries (eliminates FAT class selection)
m = 3.6 (steel), I(r) = loading mode correction factor
Variable Amplitude Loading
Palmgren-Miner Rule
D = Σ(n_i / N_i) ≤ 1.0 (failure when D = 1)
N_i from S-N curve at stress range Δσ_i
Conservative: D_allowable = 0.5 (offshore) to 1.0 (general)
Cycle Counting
Rainflow counting (ASTM E1049): closed hysteresis loops → cycle pairs
Output: stress range histogram [Δσ_i, n_i]
Effective Stress Range
Equivalent damage stress range: Δσ_eq = (Σ n_i Δσ_i³ / Σ n_i)^(1/3) [for m=3]
Fatigue Life Estimate
N = C / Δσ^m
C from FAT class: at N=2×10⁶, Δσ=FAT → C = FAT^m × 2×10⁶
Weld Root Fatigue
Root failure often hidden (no surface crack to detect).
Throat stress: Δτ_⊥ = F / (a × L) (a = throat, L = weld length)
IIW: FAT 36 for weld root (transverse load on full-penetration fillet)
Prevention: increase throat, use full penetration, examine root by UT/TOFD
Root vs. toe competition: typically root governs for thin throat welds;
toe governs when throat ≥ 70% of plate thickness
Multiaxial Fatigue in Welds
Normal + shear combined: Δσ ≥ FAT_σ and Δτ ≥ FAT_τ/√3
IIW interaction formula:
(Δσ / (f_σ × FAT_σ))^3 + (Δτ / (f_τ × FAT_τ))^5 ≤ 1.0
f_σ, f_τ = partial safety factors (typically 1.0 for mean line)
Exponents 3 and 5: m=3 for normal, m=5 for shear (IIW convention)
In-phase vs. out-of-phase:
In-phase: use equivalent stress from interaction formula
Out-of-phase: more conservative; use critical plane methods or IIW with amplitudes
Fatigue Assessment Standards
| Standard | Domain | Method |
|---|
| IIW-2259-15 | General welded structures | Nominal / Hot spot / ENS |
| EN 1993-1-9 (Eurocode 3) | Steel structures | Nominal stress, detail categories |
| AWS D1.1 | Structural steel, USA | Stress category (A-F) |
| BS 7608 | UK welded joints | S-N classes A-W |
| ASME Div 2 (App 5.5.5) | Pressure vessels | Battelle structural stress |
| DNV-RP-C203 | Offshore structures | Hot spot / nominal |
| FKM (Germany) | Machine components | Local stress, Kt correction |
Output
Provide: FAT class for detail [from table], stress range Δσ [MPa], fatigue life N [cycles], damage D (Miner's rule), required throat thickness, treatment upgrade recommendation, R-ratio note (whether correction applies).
