Fatigue is the process by which a structure fails under repeated loading at stress levels well below its static ultimate strength. A wing spar that can support 5,000 N in a single static test may fail at 2,000 N after 100,000 loading cycles — not because the material is weak, but because microscopic cracks nucleate at stress concentrations, grow incrementally with each cycle, and eventually reach a critical length at which the remaining cross-section can no longer carry the load.
Fatigue life is typically characterized by S-N curves (stress vs. number of cycles to failure). Aluminum alloys have no fatigue limit — they will eventually fail at any cyclic stress above zero, given enough cycles. Some steels have a fatigue limit (endurance limit) below which they can theoretically survive infinite cycles.
For reusable UAVs, fatigue is a primary design driver:
- A MALE UAV designed for 20,000 flight hours experiences millions of wing-bending cycles from gusts and maneuvers.
- Propeller hubs experience a bending cycle every revolution — at 5,000 RPM, that is 300,000 cycles per hour.
- Landing gear components experience high-stress cycles on every landing.
For expendable airframes, fatigue is not a design driver. A structure that will experience fewer than 10,000 loading cycles in its entire lifecycle (one flight of a few hours) will not fail from fatigue unless it is already near its static strength limit. This is the key insight that allows expendable drones to use materials and safety factors that would be unacceptable for reusable platforms — 3D-printed thermoplastics, foam-core composites, and hot-glue joints can all survive a single flight even though they would fail within weeks of regular use.
3D-printed parts are particularly susceptible to fatigue because cracks preferentially initiate and propagate along the inter-layer boundaries — the weakest link in FDM construction. For reusable printed UAV components (motor mounts, landing gear), fatigue analysis of the inter-layer bond is essential.
Related terms
- Safety Factor — the design margin that accounts for fatigue uncertainty in reusable structures
- Flutter — a dynamic instability that can cause rapid fatigue failure
- Composite — a material class with complex fatigue behavior (no endurance limit, but high fatigue resistance when properly designed)