Considerations about the existence or non-existence of the fatigue limit: implications on practical design

In practical design, the question of existence or non-existence of fatigue limit seems little more than an irrelevant academic discussion. Nevertheless, the answer affects many aspects of the fracture mechanics, such as, the calculation of damage accumulation; recognition of the multiplicity of fatigue mechanisms (and consequently, the multiplicity of S–N fields); interpretation of non-propagating cracks; and repercussion on the Kitagawa-Takahashi diagram. Various prestigious scientists deny the existence of a fatigue limit based on the results of failures in very high cycle fatigue (VHCF) regime, far below the high cycle fatigue (HCF) limit. However, the asymptotic extrapolation of the experimental results beyond the scope of testing with S–N models, cannot confirm or deny this hypothesis if those models do not fulfil indispensable physical and statistical requirements. Some phenomenological models based on solid statistical conditions (stability, limit conditions and compatibility), ensure the necessary existence of an asymptotic fatigue limit for each of the possible failure mechanisms. This may or may not be zero according to the experimental results but only the presence of an asymptotic fatigue limit can avoid the absurdity of fatigue failure under zero load value.