The American Society for Testing and Materials defines "fatigue life", "Nf", as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. For some materials, notably steel and titanium, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit or endurance limit. However, in practice, several bodies of work done at greater numbers of cycles suggest that fatigue limits do not exist for any metals.