Termination of the ability to perform its required function by a device, component, circuit, or any part or subsystem. The failure mechanism is the physical, chemical, metallurgical, or other process resulting in a failure. The predicted or observed result of a failure mechanism on a particular item in relation to the operating condition at the time of failure is the failure mode. The predicted or observed result on the function of the item or of related items is the failure effect.

Failure is classified according to cause, suddenness, and degree. There are various causes of failure:

• • misuse failure is attributable to the application of stresses beyond the stated capabilities of the item;

• • inherent weakness failure is attributable to a weakness inherent in the item itself when subjected to stresses within its stated capabilities;

• • primary failure is not caused either directly or indirectly by the failure of another item whereas secondary failure is;

• • wear-out failure results from deterioration processes or mechanical wear, the probability of occurrence increasing with time.

Failure can be either sudden or gradual, i.e. unanticipated by prior examination or anticipated. It can also be partial, complete, or intermittent. A failure that is both sudden and complete is termed catastrophic failure; one that is both gradual and partial is called degradation failure. See also failure rate; mean time between failures; mean time to failure.