What Is Metal Fatigue in Aircraft?
As aircraft are subjected to a wide-range of environmental extremes and unprecedented stressors, they may experience fatigue or structural failure over time. To be considered airworthy, aircraft undergo rigorous testing before being deemed safe and certified for flight. According to Boeing, equipment and structural failure accounts for about 20% of aircraft accidents, while mechanical failure is attributed for nearly 80%.
With safety at stake, metal fatigue and failure should be taken into consideration with every maintenance inspection. Metal fatigue is a common occurrence among all metal airframes. As a result of continuous flight cycles and frequent use, the metal elements in aircraft eventually weaken and necessitate attention and repair. Weakness usually manifests itself in microscopic cracks that become visible and larger with time.
An aircraft starts to age after its first flight, and the effects of corrosion and fatigue appear almost immediately. Aging becomes a serious issue when the aircraft can no longer be repaired or sustain flight. The signs of fatigue become more apparent in aging aircraft as it is continually exposed to atmospheric pressure. After a certain number of flight cycles, aircraft should be retired to prevent catastrophic failure. Accumulated fatigue damage is a reality for all metal airframes as they are constantly exposed to varying atmospheric pressure, G-loads, turbulence, and other conditions.
Similar to any metal vehicles, excessive use and wear weaken aircraft over long periods. Pressure is the leading cause of weakened metal components. Outside exposure, turning, accelerating, decelerating, and other maneuvers can also deteriorate the load bearing capacity of the wings and other surfaces. Furthermore, the regular pressurization and depressurization of flying at high altitudes causes the metal skin to expand and contract. As metal is bent from its original shape and re-bent back, it becomes weaker.
Fatigue cracks originate in three different areas. Internally, fatigue is found in load-bearing structural elements; while, externally, it may also be found in load-bearing aircraft skins. Lastly, it may also manifest itself around the edges of fastener holes, such as those for rivets, screws, bolts, or other areas of concentrated stress. Not only are these areas of high pressure more prone to cracking, but they are often deemed the original sites of failure.
Large aircraft components moving at high speeds and experiencing excessive vibration, like engines, are also prone to cracking. Other systems and aircraft components that endure flight stress and fatigue include engine components, fan blades, wing attachments, brakes, and landing gears. Recently, aerospace engineers have devised ways in which to prevent aircraft fatigue. For instance, laser peening, which is a surface engineering process that provides protection of compressive residual stresses on metal surfaces, is garnering popularity.
Laser peening offers technicians an opportunity to perform maintenance in hard-to-reach areas and hazardous environments. More than that, aerospace OEMs are taking advantage of laser peening to extend the fatigue life of the rotors in jet engines, bulkheads, wing spars, landing gears, wing lufs, arresting hooks, and more. It also reduces operational downtimes, which saves airlines money over time.
Another key way of preventing fatigue from accelerating is through the implementation of non-destructive testing (NDT) to find points of fatigue. NDT involves the non-invasive inspection of metal elements for damage. Some of these techniques include ultrasound, X-ray scattering, X-ray absorption, electrical eddy currents, magnetic particles, liquid penetrants, and optics. Beyond such methods, battling fatigue can be accomplished by keeping an aircraft’s surface polished. Polished surfaces can protect flight control surfaces and other areas from fatigue as well.
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