Structures primarily fail because of fatigue damage.
Typical sources of fatigue are restrained thermal expansion, repeatedly applied outside forces, and internal vibratory sources. Failure analysis uses stress analysis and strain gage testing to reveal the stress state in the region of failure initiation. Coupled with the material's properties, it can be determined if the failure was initiated by low cycle fatigue, high cycle fatigue, or if the crack grew to a critical size and catastrophically failed. As applied to rotating and reciprocation equipment, failure analysis can be used to identify contributory factors of product non-performance, such premature component breakage or reduced bearing life. With the incorporation of strain gage testing, failure analysis identifies the source of fatigue damage that, for example, caused a pressure vessel to crack or fan blades to break.