Examples of failure of aircraft structures caused by metal fatigue include the de Havilland Comet accidents (1950s) and Aloha Airlines Flight 243 (1988). Now that the subject is better understood, rigorous inspection and nondestructive testing procedures are in place.
Composite materials consist of layers of fibers embedded in a resin matrix. In some cases, especially when subjected to cyclic stress, the layers of the material separate from each other (delaminate) and lose strength. As the failure develops inside the material, nothing is shown on the surface; instrument methods (often ultrasound-based) have to be used to detect such a material failure. In the 1940s several Yakovlev Yak-9s experienced delamination of plywood in their construction.