DIRECT OBSERVATION OF FATIGUE CRACK TIP DEFORMATION IN THE MIDSECTION OF A SPECIMEN

摘要

A simple direct observation of the crack tip deformation in the midsection of a specimen is not possible and the surface observation is not representative for the processes along the crack front. This lack of information is the main reason for the discrepancies about the crack propagation mechanisms under cyclic loading. We applied a stereophotogrammetric technique to measure the real shape of the crack tip in the midsection of the specimens at different loads of a fatigue cycle. An overview of our observations of the crack tip deformation during constant and after single overloads is presented. It is shown that in both cases the crack tip deformation can be described by a blunting and re-sharpening of the crack tip which leads to a cycle per cycle growth similar to the models proposed by Pelloux or by Neumann.