Load Spectrum Counting

Fatigue Load Spectrum Counting (structures/fatigue/load-spectrum-counting)

Use when the task is building or reducing a fatigue load spectrum: rainflow cycle counting, exceedance and level-crossing spectra, mission spectrum assembly, truncation, and spectrum damage.

Domain quick reference

• Rainflow counting (ASTM E1049-85 section 5.4.4 practice) extracts cycles from the turning points of a load history: an excursion is a full cycle when the following excursion in the opposite direction is at least as large; the residual stream forms half cycles.
• The exceedance spectrum counts, per load level, how many peaks reach or pass the level; the level-crossing spectrum counts crossings of a level in the history.
• A mission spectrum aggregates per-phase (level, cycles) blocks and sums repeated levels across phases.
• Spectrum truncation removes cycles below a cutoff level; they contribute little damage and are dropped before damage summation.
• Miner's rule sums n/N over the spectrum blocks. This leaf derives N from a Basquin S-N curve N = C * S_a**(-b); the miner-damage leaf sums (n, N) blocks directly.
• Fatigue load spectra sit in the FAR-25.571 damage tolerance context for transport aeroplanes.

Workflow

1. Reduce the mission history to cycles with rainflow_cycles.
2. Merge the counts with rainflow_spectrum.
3. Count level activity with exceedance_counts and upcrossing_count.
4. Aggregate mission phases with mission_spectrum.
5. Drop small cycles with truncate_spectrum.
6. Sum damage with spectrum_damage (Basquin lives, Miner fractions).
7. Feed the damage fraction into the miner-damage leaf's life check.

Pitfalls

• Counting cycles by naive peak counting instead of rainflow.
• Summing exceedance over raw samples instead of turning points.
• Truncating away the cutoff level itself (keep levels >= cutoff).
• Mixing alternating stress (half the range) with the full load range in the Basquin curve.
• Double counting half cycles as full cycles in the spectrum.

Behavior contract (gate 3)

The counting, spectrum, truncation, and damage logic is exercised by the gate 3 contract test: scripts/test_load_spectrum.py against scripts/load_spectrum_logic.py (stdlib unittest, offline). Run: python3 scripts/test_load_spectrum.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download. The rainflow and Miner rules are common fatigue methodology, summary-only per standards-map.yaml. ASTM E1049 names the counting practice; the method text here is summary, not standard text.
• compliance: STANDARDS-REF, gated: false.