Widespread Fatigue Damage

Widespread Fatigue Damage (structures/damage-tolerance/widespread-fatigue-damage)

Use when the task is screening transport airplane structure for widespread fatigue damage (WFD) per FAR 25.571(b)/(c): classify multiple site damage (MSD) versus multiple element damage (MED), run the WFD susceptibility screening over cracked fastener-hole sites, and decide whether a supplemental inspection applies to fatigue critical baseline structure.

Domain quick reference

• WFD (FAR 25.571(b)/(c)) is the simultaneous cracking of multiple sites in a structure element such that the structure can no longer sustain the required loads between normal inspections. Transport airplane evaluations must show that WFD will not occur before the design service goal is reached.
• MSD: multiple site damage. Small fatigue cracks at multiple adjacent fastener holes within one element (for example a lap splice row). Each crack may be short on its own, but the population can link up.
• MED: multiple element damage. Cracks in adjacent load-path elements (for example adjacent stringers or stiffeners) that together reduce the redundancy of the structure.
• Fatigue critical baseline structure: the structural elements whose failure from fatigue would directly reduce the residual strength of the airplane below the required level. This baseline is the scope for WFD screening.
• WFD susceptibility screening: compare the detected crack lengths at the fastener-hole sites against a screening threshold, count how many sites exceed it, and judge whether the population is WFD susceptible. A single worst-site crack is not the screening question; the site population is.
• Supplemental inspections: for designs that cannot show WFD resistance up front, a supplemental inspection program (for example the SID/SLWFD style program) is applied to fatigue critical baseline structure so cracking is caught before it becomes widespread.
• Certification date thresholds: the WFD rule applies to transport airplanes by certification date, and fatigue critical baseline structure for earlier designs is handled through supplemental inspections rather than a full up-front WFD demonstration.

Workflow

1. Identify the fatigue critical baseline structure in scope per FAR 25.571(b)/(c): the elements whose fatigue failure would cut the residual strength below the required level.
2. Collect the detected crack lengths at the adjacent fastener-hole sites of each element, plus the count of cracked load-path elements.
3. Classify the damage: MSD (cracks at adjacent fastener holes of one element), MED (cracks in adjacent load-path elements), both, or neither. Use scripts/wfd_logic.py classify_damage() with the cracked-site and cracked-element counts.
4. Run the MSD susceptibility screening with screen_msd(): count the sites whose crack length exceeds the screening threshold; the verdict is "susceptible" when at least two sites exceed it.
5. Apply the certification date threshold: decide whether the design must show WFD resistance up front or whether fatigue critical baseline structure is covered by a supplemental inspection program.
6. Set the supplemental inspection required flag with supplemental_inspection_required(): required for fatigue critical baseline structure when the screen is susceptible or WFD resistance has not been shown.
7. Report the classification, the site-count verdict, and the supplemental inspection flag together (wfd_screen_report()) and file the result in the damage tolerance assessment.

Pitfalls

• Screening only the largest site crack. WFD is a population question: multiple adjacent sites matter, not the single worst site.
• Confusing MSD with MED. MSD is multiple cracks in adjacent fastener holes of one element; MED is cracks in adjacent load-path elements. Count sites for MSD, elements for MED.
• Treating the screening threshold as an exact boundary. A site at exactly the threshold is not counted; the verdict flips on sites strictly above it and on the two-site minimum.
• Leaving fatigue critical baseline structure out of the screening scope, which hides the elements the WFD rule targets.
• Ignoring the certification date threshold. New designs must show WFD resistance; older designs rely on supplemental inspections, and the flag logic changes accordingly.
• Reporting a supplemental inspection flag without checking the fatigue critical baseline flag first: non-baseline structure never triggers the flag.

Behavior contract (gate 3)

The MSD/MED classification, susceptibility screening, and supplemental inspection flag logic is exercised by the gate 3 contract test: scripts/test_wfd.py against scripts/wfd_logic.py (stdlib unittest, offline). Run:

python3 scripts/test_wfd.py

Compliance

• FAR 25 (14 CFR Part 25) is public-domain US regulation; CS-25 is an EASA free-download publication. Both are referenced by id only (reference-only: true per standards-map.yaml); no verbatim text is reproduced, paraphrase only.
• compliance: STANDARDS-REF, gated: false.