Short crack and long crack propagation in metals based on damage mechanics concepts

The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditionally tackled through experimental methods such as the S-N curve approach. For more complex variable stress states, such as multiaxial stress histories, the fatigue safety can be analysed by employing a physics-based damage mechanics approach. On the other hand, fatigue failure can be recognized as the result of a stable crack propagation up to a critical condition and, in this context, the availability of suitable laws to properly describe and quantify the crack propagation is a crucial aspect. In the present paper, a fatigue crack propagation law for both short (Low-Cycle-Fatigue) and long crack regime High-Cycle-Fatigue) is discussed based on damage mechanics concepts. Fatigue crack growth law and damage mechanics approach are compared in order to determine both the damage value according to a given fatigue crack growth (FCG) law and the crack length associated to a given mechanical damage of the fatigued material. Such two methods are shown to be different formulations of the same physics-based approach to fatigue phenomena.