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 is 25mm. Correction applied for t > 25mm

Seawater curves use DNV-RP-C203 Table 2-2 parameters

S-N Curve Parameters (DNV-RP-C203):
log(a1) = 12.164, m1 = 3, log(a2) = 15.606, m2 = 5
Calculated Fatigue Life
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cycles
Note: This calculator uses two-slope S-N curves per DNV-RP-C203. The transition from slope m1 to m2 occurs at N = 10^7 cycles.

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 (typically 25mm), a thickness correction is applied to account for increased probability of defects and stress gradient effects:

Scorrected = S x (t/tref)k

Where k = 0.25 for welded joints (per DNV-RP-C203). This effectively 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.

Need Professional Fatigue Analysis?

For complex fatigue assessments involving variable amplitude loading, FEA-based hot-spot stress analysis, or advanced fatigue prediction, we offer professional consulting services.

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