Early fatique damage from the NDE point of view

It is obvious - components under fatique load fail cracking. Crack initiation is followed by crack propagation up to the break. It is also in no controversial discussion that - depending on the material and the load regime - surface breaking of nearsurface subsurface microcracks often occur very early, after the second or third load cycle, especially for low cycle fatigue. Removing the surface discontinuities after a certain number of load cycles generally leads to a life time extension of the component. Therefore to find an answer to the above mentioned questing seems to be very erasy: Early detection and characterisation of fatigue damage will be achieved by the detection of microcrack initiation - surface breaking and/or subsurface - and the characterisation of their propagation as a function of the load cydle number of usage factor; a lifetime prediction follows the general rules of fracture mechanics, i.e. a Paris law, taking into account an accumulated crack length. But the answe r is not as easy: Microscopic investigation by using through transmission SEM, show eventful changes in the microstructure, in the dislocation density and cell structure. However, physical measurement techniques exist to be sensititve for the detection of cracks as well as for the detection of changes of the properties of the materials in the bulk volume on a microscopic scale in order to deal as a ND precursor to identify and indicate damage and to follow the damage accumulation. The contribution on one hand discusses the potential of these techniques and reports on the other hand to the difficulties of NDT experts to organise the interdisciplinary research. Special emphasis is on the discussion of a neuw - mesomechanical- approach