3: Example of Fatigue Failure
Many catastrophic failures have resulted from brittle fracture. However, many have also occurred due to fatigue. While crack growth occurs over time, it is the ultimate failure when a critical crack length is reached that results in catastrophic failure.
An example of fatigue failure was the airline crashes of the De Havilland Comets in 1954. Three of these passenger jets broke up mid-air and crashed within a single year. Sharp corners around the plane’s window openings were found to have acted as stress concentrators which initiated cracks.
Pressurisation of the aircraft during each flight created stress cycles in the fuselage that propagated the cracks over time. At some point a critical crack length was reached and fast fracture of the fuselage shell then occurred.
Stress-corrosion fracture is a third sudden form of failure and occurs from the presence of fine corrosion cracks that penetrate deep into the material.
These cracks are present even though the material, as a whole, may not appear to be corroded.
The cracks act as stress concentrators and failure may occur either through an externally applied stress or through stored residual stresses.