For steel products which are subject to moving loads, such as automobile springs, crankshafts, and power takeoff shafts, it is important to know the endurance, or fatigue limit.
Specimens are prepared for testing and are subjected to bending as they revolve. In every revolution of the test machine the stress is reversed twice-under load.
With the initial specimen, stress is applied which is far beyond the breaking point. Testing is repeated with different specimens and the applied stress is gradually reduced until a specimen will undergo ten million reversals without breaking. This stress is referred to as the fatigue limit.
The Fatigue Limit is the maximum stress that a material can endure for an infinite number of cycles without breaking. It is also referred to as the Endurance Limit.
Ten million cycles is the engineering community’s testing approximation for infinite.
Image source: Steelways chart dated 1955 on steel testing from my personal archive.