Aircraft material failures are one of the leading causes of aircraft engine failure. One of the most prevalent failures involves aircraft metal fatigue. The term refers to the exposure of metal aircraft components to continuous load stress, which leads to the accumulation of microcracks, and thus the weakening of said components. Determining whether aircraft metal fatigue will occur is not a matter of if, but when. Let’s examine how metal fatigue is caused over the life cycle of an aircraft, and how manufacturers determine fatigue limits and inspection.
During each flight cycle, an aircraft is subject to intense pressurization cycles and varying temperature fluctuations. The entire airframe will expand, contract, and shrink during the take-off and landing phase, increasing the stress load on metal components. Structures on the aircraft that experience higher levels of corrosion and pressure are increasingly vulnerable to metal fatigue. Additionally, aircraft that are used for shorter, more frequent flights, are more likely to experience metal fatigue. Regardless, due to the various stressors, an aircraft is exposed to, all aircraft are susceptible to metal fatigue at some point.
The initial development of micro-cracks is typically not visible to the human eye. Micro-cracks begin to develop on a molecular level, are difficult to see as they expand, and usually only reveal themselves when a part breaks completely. As a result, structural engineers and parts manufacturers have developed parameters in which to test for metal fatigue and the potential for failure.
Stress tests, or load tests, are conducted to determine how many stress cycles, and what applied a load, will cause metal fatigue or failure of a particular component. The data collected from these tests can be used to calculate the longevity and load limit of a part. Tests of this nature can also predict which parts will start to fail first, and when they will need to be inspected.
Another fascinating approach to detect fatigue is the monitoring of photoelasticity. A thin coat of the material in question can be applied on top of the component. Any change in optical properties can identify areas of deformation and stress throughout the component’s life cycle. This process can help maintenance professionals predict what the condition of the underlying component might be at any given time.
Even with numerous testing procedures and specified load limits, it is difficult to predict metal fatigue. Incidents of detrimental metal fatigue and failure still occur and account for around 20% of aircraft losses. The 1989 British Midland crash was caused by a fatigued fan blade. Similarly, Southwest Airlines is haunted by their 2018 metal fatigue incident, where it is suspected a stressed fan blade separated from the turbine and caused the left engine of the aircraft to fail.
With all of the above in consideration, it is extremely important that stringent inspection parameters and replacement recommendations set forth by the manufacturer are adhered to over the life cycle of an aircraft.
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