All fatigue is dynamically induced. That is, there must be some level of dynamic loading in order for fatigue damage to occur. It is probably a true statement to say that nothing in real life is actually static, or not moving at all. Even slight changes in temperature will cause stress fluctuations in an otherwise apparently static structure. Some dynamic loading is hardly detectable, changes very slowly, and is quite repeatable while other types are quite noticeable and very random in nature such as engine noise from an automobile.

The pseudo-static approach for calculating a stress time response, where unit stresses are associated with load time histories, is valid if the frequency of the input loading is below the lowest natural frequency of the structure. However, for cases where the dynamic response of the structure comes into play, the usage of transient response or random response is appropriate to compute fatigue life.