Damage accumulation of graded steel

The damage evolution of graded steel is studied. Specimens are taken out of a flange shaft having a graded microstructure resulting from thermo-mechanical forming process. Fatigue experiments are performed for the base material with a ferritic–pearlitic microstructure as well as for the nearly fully martensitic microstructure which is found in the outer area of the flange. The surface damage evolution during fatigue loading is investigated using a long-distance microscope. The base material shows very complex damage accumulation process with a high density of microcracks whereas the martensitic specimens exhibit no pronounced surface damage evolution prior to failure.