EARLY FATIGUE DAMAGE FROM THE NDE POINT OF VIEW

The motivation for an early fatigue damage characterization by using NDE is easy to understand. Technical components are designed observing in most of the cases conservative design rules for a certain lifetime under fatigue loads. However, during service a mechanical and/or thermal overloading can occur, unexpected at the design phase, and the component locally sometimes is damaged. During further operation the component does not follow the predicted fatigue behaviour, lifetime is shortened and risk for early failure has to be taken into account. Therefore, as a part of the in service inspection, it is relevant to detect locations and regions of early damage and to quantify the state of damage is the NDE challenge, i.e. to predict the actual lifetime. Repairing the component or by adjustment of the loads the component is made to follow a safe behaviour again up to the designed end of life. It is obvious- components under fatigue load fail by cracking. Crack initiation is followed by crack propagation up to the break. It goes without saying that- depending on the material and the load regime- surface breaking or nearsurface subsurface microcracks often occur very early, after the second or third load cycle, especially in th