Understanding Air Safety in the Jet Age/Metal Fatigue
Without exception modern airliners rely almost exclusively on metal for their structure. All metals suffer from fatigue to some degree. Fatigue occurs when repeated loading leads to progressive structural damage and the growth of cracks. Once a fatigue crack has started, it will grow slightly with each loading cycle. The crack will continue to grow until it reaches a critical size at which point it will grow rapidly and lead to the complete fracture of the structure. Because of the dangers of fatigue, the concept of a failsafe was introduced. A failsafe is a secondary structure that will carry the load if the primary mechanism fails. Unfortunately it is usually weaker than the primary structure and provides only a short window for the failure to be found if disaster is to be avoided.
Accidents involving metal fatigue have been happening since the very first jet airliner took to the sky. They all have at least one of the following characteristics: poor design, flawed maintenance or inadequate repairs. Unfortunately the industry as a whole doesn't seem to have learnt and fatigue induced accidents continue with frightening regularity. Therefore vigilant maintenance is the only solution for an aircraft with a metal structure.