fatigue-welded-joints

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).