Input Parameters

S-N Curve Classification

Typical fillet welded joints for offshore structures

Stress Range (MPa)

Hot-spot or nominal stress range depending on assessment method

Plate Thickness (mm)

Reference thickness is 25mm. Correction applied for t > 25mm

Environment

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

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 / S^m
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:

S_corrected = S x (t/t_ref)^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.

Thickness Correction Factor	1.000
Environment Curve	In-air curve
Corrected Stress Range	-- MPa
S-N Curve Regime	--
log(N)	--

S-N Curve Fatigue Life Calculator