Scales of metal fatigue cracking

A new paradigm is proposed for considering metal fatigue cracking based on the principles of synergetics and physical mesomechanics. Fatigue cracking is described as a three-stage process. Metal evolution is studied with stress growth from the micro- (ultrahigh cycle fatigue) to meso- (high cycle fatigue) and then macroscale (low cycle fatigue). The notion of two effective stress concentration factors on the metal surface and in its bulk is introduced; their variation pattern with stress growth is discussed. In the general case, the propagation of through-the-thickness cracks is shown to also occur in three stages—on the micro- (shear), meso- (rotation with the formation of triangular fatigue striations) and macroscale (rotation plus shear which lead to the formation of fatigue striations of complex shape), consecutively.