S-N Curve Fatigue Life Calculator

Calculate fatigue life for welded steel joints using industry-standard S-N curves

DNV-RP-C203 API RP 2A BS 7608

Input Parameters

Typical fillet welded joints for offshore structures

Hot-spot or nominal stress range depending on assessment method

Reference thickness tref is 25 mm (32 mm for the T curve). Correction applied for t > tref with the class exponent k

Curves per DNV-RP-C203 (2011) Table 2-1 (air), Table 2-2 (cathodic protection) and Table 2-3 (free corrosion)

S-N Curve Parameters (DNV-RP-C203):
log(a1) = 12.164, m1 = 3, log(a2) = 15.606, m2 = 5
Calculated Fatigue Life
--
cycles
Note: In air and in seawater with cathodic protection this calculator uses the two-slope S-N curves of DNV-RP-C203: slope m1 changes to m2 = 5 at N = 10^7 cycles in air and N = 10^6 cycles with cathodic protection. Free-corrosion curves are single slope, m = 3. Below 10^5 cycles, air and cathodic-protection lives are capped at the B1 curve of the same environment.

About S-N Curve Fatigue Analysis

S-N curves (also called Wohler curves) describe the relationship between applied stress range and the number of cycles to failure for a given structural detail. The fatigue life N at a constant amplitude stress range S is calculated using:

N = a / Sm
or in logarithmic form:
log(N) = log(a) - m x log(S)

Where:

  • N = Number of cycles to failure
  • S = Stress range (MPa)
  • a = S-N curve intercept (material/detail dependent)
  • m = S-N curve slope (typically 3 for high-cycle fatigue)

S-N Curve Classifications

The S-N curve classification (B1 through W3, plus T for tubulars) represents the fatigue strength of different weld geometries and structural details. Key classifications include:

  • B1-B2: Base material and ground butt welds (highest fatigue strength)
  • C-C2: Butt welds with different finishing quality
  • D: Most common for fillet welded offshore structures
  • E-F3: Attachments and details with geometric stress risers
  • G: Partial penetration welds
  • W1-W3: Weld root details (most critical)
  • T: Tubular joint hot-spot stress method

Thickness Correction

For plates thicker than the reference thickness tref, a thickness correction is applied to account for increased probability of defects and stress gradient effects:

Scorrected = S x (t/tref)k

The correction applies only for t > tref. Per DNV-RP-C203 (2011) section 2.4.3, tref = 25 mm for welded connections other than tubular joints and 32 mm for tubular joints (T curve). The exponent k depends on the curve class (Tables 2-1 to 2-3): k = 0 for B1 and B2 (no correction), 0.15 for C, C1 and C2, 0.20 for D and E, and 0.25 for F to W3. For the T curve, k = 0.25 for SCF ≤ 10 and 0.30 for SCF > 10; this calculator has no SCF input and assumes SCF ≤ 10 (k = 0.25). The correction reduces the allowable fatigue life for thicker sections.

Environment Effects

Seawater environment significantly affects fatigue performance:

  • In Air: Baseline S-N curve applies
  • Seawater with CP: Approximately 2-3x reduction in fatigue life
  • Free Corrosion: Approximately 3-4x reduction (avoid in design)

Limitations

This calculator provides preliminary estimates based on constant amplitude loading. For actual design:

  • Variable amplitude loading requires rainflow counting and damage accumulation (Miner's rule)
  • Stress concentration factors may need to be applied for hot-spot stress method
  • Mean stress effects may be significant for high R-ratios
  • Final design should be verified by qualified engineers
Disclaimer: This calculator is for educational purposes only. All final engineering calculations should be performed by qualified engineers and verified against the applicable design codes.

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