3D-Analysis of surface damage in metallic materials under fatigue loading

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This interdisciplinary project deals with the in-situ analysis of surface damage accumulation under fatigue loading in metallic materials. Here, the focus of the research is on metrological aspects as well as on materials science with the aim of investigating technical steels subjected to fatigue loading with a high-resolution white-light interferometer. Both, the metrological approaches for the investigation of extensive changes in surface topography and the vibration compensation - developed during the first project phase - should be adapted to a high-resolution Linnik white-light interferometer. Local damage processes in terms of slip bands as well as plastic deformation of grain areas will be analyzed with this interferome-ter set-up directly at the servo hydraulic testing machine. The small grain size of the investigated dual phase steel place high demands on the measurement system. Due to the complexity of measurements and the targeted short interrupts between the load cycles, a fast, highly parallelized, computing time optimized and automated measurement and evaluation process will be aspired. Furthermore, from the metrological perspective the use of white-light interferometers in close-to-machine applications will be inves-tigated. From the gained measurement results, the chronological development of the surface topography will be related to specimen loading. Moreover, parameters that characterize the extent of the damage will be defined. The time development of these damage parameters can be reliably determined using interferometric surface analysis in an in-situ set-up, because there are no disturbances caused by mounting and dismounting of specimens, and because a comparatively large area can be easily analyzed. Consequent-ly, a consistent database will be available from which statistical correlations can be derived. Fatigue damage in a dual phase steel occurs both in terms of isolated slip lines and as plastically deformed grain regions (ranging from about 10% to 100% of the grain area). This damage process has gained little interest in the literature up to now, but seems to be characteristic for the body-centered cubic ferrite phase. It typically plays a role in phases with low carbon content and seems to contribute significantly to crack initiation and hence to the damage accumulation process as a whole. Additional investigations with a high resolution scanning electron microscope will elucidate the relation between change in the surface relief and micro-plastic deformation