Although the stresses and strains may be well below the static resistance level of the metal, the damage is accumulating cycle by cycle and after a certain number of load fluctuations a failure will occur. For structures in service, the damage accumulation may take several years to reach a critical level with resulting failure. The fatigue process is usually divided into three phases:
– phase 1: crack initiation; – phase 2: crack growth;
– phase 3: final fracture.