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In the present paper, some interesting questions concerning 3D fatiguecrack propagation are discussed. The main focus is on experimental investigations of3D fatigue crack propagation under Mode I loading conditions. Transparent specimensof PMMA are used, in order to be able to observe and to document accurate sequencesof the real 3D crack front evolution profiles via in situ photographic measurement.Such well documented experimentally observed sequences of real crack front profilesare necessary to identify an appropriate crack propagation criterion via comparing theexperimental observations with corresponding numerical simulations. The aim of thispaper is to present a new 3D crack propagation criterion, based on simple 2D fatiguetests on CT specimens, which allows to predict the correct distribution of the incre-mental crack extension also for complicated loading conditions under which only partsof the crack front propagate. Another focus of the experimental investigations concernsthe influence of the additional corner singularities in the case of surface breakingcracks. Results of both, numerical simulations of 3D fatigue crack propagation an cor-responding experimental investigations, are presented.