When metal parts fail after being subjected to cyclic stresses over a relatively long period of
time, the likely cause is “fatigue”.
Fatigue failure is progressive… small flaws get incrementally larger over time until they become
large enough to cause the component under stress to fail. Early in the progression, the slowly
growing flaw can be very hard to detect.
When a fracture is examined under a microscope, the fatigue portion of the fracture has a
relatively smooth looking surface and a characteristic pattern from the point of origin to the final
fracture.
Applications where stresses are cyclic by nature make fatigue more likely. Parts are designed to
use materials that resist the stress cycles predicted for the lifetime of the component.
Periodic inspections are also used to further assure that fatigue cracks are not present or are
still small enough as not to be hazardous.