Fatigue cracking is the formation and gradual growth of cracks in a material caused by repeated cycles of stress. Unlike a single overload, fatigue damage builds invisibly over thousands of cycles until a crack appears and grows.
How It Works#
Every time an aircraft is pressurized and depressurized, its fuselage expands and contracts slightly. This cycle applies stress to the metal skin, frames, and fastener holes. Over thousands of flights, that repeated stress weakens the material at its most vulnerable points.
Cracks almost always start at a stress concentrator: a fastener hole, a notch, a scratch, or a structural joint. At these points, stress is locally higher than in the surrounding material. A tiny crack forms and then grows a small amount with each new stress cycle.
This growth follows a predictable pattern. For most of its life, a fatigue crack grows very slowly. As it gets longer, the stress at its tip increases, and growth accelerates. If left unchecked, the crack eventually reaches a critical length and the structure fails suddenly.
Aluminum alloys, which make up most of a transport aircraft's primary structure, are especially susceptible to fatigue. Engineers account for this by setting fatigue life limits, expressed in flight cycles or flight hours, for individual components and the airframe as a whole.
Example in Aviation#
The Aloha Airlines Flight 243 accident in 1988 is the most cited real-world example of fatigue cracking in aviation. The Boeing 737 had accumulated nearly 90,000 pressurization cycles, far above its design threshold. Widespread fatigue cracking at multiple fastener holes in the fuselage skin joined together, a condition called multiple-site damage (MSD). An 18-foot section of the upper fuselage tore away in flight.
That accident directly drove new inspection requirements and accelerated research into aging aircraft structures worldwide.
Why It Matters#
Fatigue cracking is invisible to the naked eye until a crack is already significant. Pilots and maintenance personnel cannot rely on visual checks alone. Scheduled inspections using techniques such as eddy current testing and ultrasonic scanning are required to catch cracks before they reach a dangerous size.
Understanding fatigue helps pilots appreciate why airframe hours and cycles are tracked so carefully. An aircraft that looks fine externally may be approaching a structural life limit that demands mandatory component replacement or retirement.
Key Takeaways#
- Fatigue cracks grow under repeated stress cycles, not from a single overload.
- Cracks initiate at stress concentrators such as fastener holes and notches.
- Growth is slow at first, then accelerates as the crack lengthens.
- Multiple-site damage occurs when many small cracks link together across a structure.
- Airframes carry mandatory life limits in cycles and hours to control fatigue risk.