ความล้าโลหะ (Metal Fatigue) (Metal Fatigue)

What Is Metal Fatigue?

Metal fatigue is the phenomenon by which a metallic structure develops microscopic cracks under repeated loading and unloading cycles. Over time these cracks grow until the remaining cross-section can no longer support the applied load, resulting in sudden fracture. Critically, fatigue failure can occur at stress levels far below the material's published tensile strength, making it a hidden and dangerous failure mode. Aviation airworthiness regulations are heavily shaped by lessons learned from fatigue-related disasters.

How It Works

Fatigue damage accumulates at stress concentration points—holes, notches, surface scratches, or poorly formed rivets. Each pressurization cycle of a fuselage expands and then relaxes the skin; each landing and takeoff adds bending load cycles to the wing root and wingbox. The fatigue life of a component is expressed in:

• Cycles to failure (S-N curve): A plot of stress amplitude versus number of cycles before crack initiation.
• Damage tolerance: Modern design requires that a detectable crack must grow slowly enough to be found at a scheduled inspection before reaching critical size.
• Safe-life vs. fail-safe: Safe-life components are retired after a fixed number of cycles; fail-safe structures are designed so one element can crack without causing total failure.

Applications in Aviation

The de Havilland Comet disasters of 1954 were the defining fatigue events in commercial aviation. Square window corners created stress concentrations in the pressurized pressurized fuselage that caused catastrophic in-flight breakup. Subsequent investigation by the Royal Aircraft Establishment established the science of fatigue as a core airworthiness discipline. Today, Boeing and Airbus conduct full-scale fatigue tests—cycling airframes through tens of thousands of simulated flights—before certifying any new type. The Aloha Airlines Boeing 737 incident (1988), where a fuselage section peeled away due to widespread fatigue in lap-joint rivets, reinforced mandatory aging-aircraft inspection programs.

Future Developments

Structural health monitoring (SHM) systems using embedded piezoelectric sensors and fiber-optic strain gauges are being developed to detect crack growth in real time, enabling condition-based maintenance rather than fixed inspection intervals. The growing use of composite materials reduces fatigue concerns for primary structure, since carbon fiber does not exhibit the same crack-propagation behavior as aluminum—though delamination and matrix cracking present different inspection challenges.