Fractographic evaluation of a failed component’s fracture surface is an important step in establishing if fatigue was a mode of fracture.  However, the three steps of a fatigue fracture are initiation of a crack, growth of the crack, and then uncontrolled growth which leads to the final fracture. The initiation mechanism, therefore, is very important in determining the cause of a fatigue fracture. Although fatigue can be the cause of an "initiated crack", the fracture "mode" can be fatigue, but the cause of the fracture was a pre-existing crack that was formed by another mechanism. 
Often the validity of a failure theory can be tested using fracture mechanics with the guidance of, for example, API-579 - Fitness for Service.