Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of fatigue limit (endurance limit). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Some steels have a genuine fatigue limit — below about 40% of tensile strength they survive indefinitely. Aluminium and most non-ferrous metals do not, and their fatigue strength must be quoted at a specified number of cycles, usually 10⁷ or 5×10⁸.
The practical asymmetry matters: a steel part can be designed for infinite life, and an aluminium part cannot. It is a large part of why aircraft structures are life-limited rather than infinite-life.
Common misuse
Assuming aluminium has an endurance limit like steel. It does not — specify a fatigue strength at a cycle count instead.