fatigue-analysis

What it does

Provides comprehensive fatigue analysis capabilities for offshore structures, including mooring lines, risers, and structural components, using S-N curves (DNV and general), rainflow counting, Miner's rule, and design-factor-based life assessments.

How it works

• Uses S-N curve fundamentals to map stress ranges to cycles to failure, including a bilinear approach with a and m parameters for different curve segments.
• Retrieves DNV S-N curve parameters via a function that selects a class (e.g., B1, C, D, E, F, G, W1-W3) and applies thickness corrections, returning a1, m1, a2, m2, and thickness.
• Calculates cycles-to-failure (N) from a1/(Δσ)^m1, switching to the second segment if N1 > 1e7 using a2/(Δσ)^m2.
• Performs Rainflow counting on a stress time series to extract stress ranges and cycle counts, producing a histogram of ranges (bin centers and counts).
• Applies Palmgren-Miner damage accumulation: total_damage = Σ(n_i / N_i), where N_i comes from the SN curve, then applies a design factor to get damage_with_design_factor and fatigue_life estimates.
• Includes a narrow-band spectral fatigue method using a stress spectrum to compute damage via zero-crossing frequency and Rayleigh assumptions, yielding damage, damage_with_design_factor, and fatigue_life.
• Provides a mooring chain fatigue workflow that combines tension time series, chain properties, rainflow, SN curves for chain class F3, and Miner’s rule with a design factor to produce fatigue damage and life metrics.
• Offers a complete example pipeline that reads tension data, runs rainflow counting, computes chain fatigue with DNV curves, and outputs results.

When to use it

• Mooring line fatigue analysis
• Riser fatigue analysis (flexible and rigid)
• Structural fatigue of hull joints and connections
• S-N curve analysis for various materials and weld types
• Rainflow counting of stress/time series to identify cycle counts
• Miner’s rule for cumulative fatigue damage and life estimation
• Fatigue design verification with design factors and life predictions

What it can touch

• Tools: claude-code
• Functions and code blocks named and described exactly as in the skill's content, including:
  • get_dnv_sn_curve(curve_class: str, thickness: float = 25) -> dict
  • calculate_cycles_to_failure(stress_range: float, sn_curve: dict) -> float
  • rainflow_counting(time_series: np.ndarray, bin_width: float = None) -> tuple[np.ndarray, np.ndarray]
  • calculate_fatigue_damage_miners_rule(stress_ranges: np.ndarray, cycle_counts: np.ndarray, sn_curve: dict, design_factor: float = 10.0) -> dict
  • spectral_fatigue_narrow_band(spectrum: np.ndarray, frequencies: np.ndarray, sn_curve: dict, duration: float, design_factor: float = 10.0) -> dict
  • mooring_chain_fatigue_analysis(tension_time_series: np.ndarray, chain_diameter: float, chain_grade: str = 'R4', design_life_years: float = 25, time_step: float = 0.1) -> dict
  • complete_fatigue_assessment(tension_file: str, output_dir: str = 'reports/fatigue') -> dict

Caveats

• License: MIT
• Declared design-factor values (e.g., 10.0) and specific DNV curve mappings are explicit in the models; actual applicability depends on project compliance.
• Some code relies on external libraries (numpy, scipy) and assumes inputs in specific units (e.g., MPa for stress, kN for tension) as described in the examples.
• The examples show simplified conversions (e.g., stress from tension using an area) and may require adaptation for real-world materials and geometries.