@phdthesis{,
    author = {Tiedemann, Dominic},
    title = {Zweidimensionale Ausbreitung kurzer Risse unter Berücksichtigung bruchmechanischer und kontinuumsmechanischer Aspekte : Ein Beitrag zur Optimierung von Lebensdauervorhersagen an gekerbten Strukturen},
    editor = {},
    booktitle = {},
    series = {},
    journal = {},
    address = {},
    publisher = {},
    edition = {},
    year = {2016},
    isbn = {},
    volume = {},
    number =  {},
    pages = {},
    url = {},
    doi = {},
    keywords = {Kurze Risse; Zweidimensionale Rissausbreitung; Kerben; Lebensdauervorhersagen; In-situ-Messung mit der Gleichstrompotentialsonde; viertelelliptische Eckanrisse; Spannungstransformationen auf Basis von Wöhler-Kurven},
    abstract = {In this thesis SEN specimens with three different notch depths of the alloy EN AW 7475‐T761 were fatigued in single stage experiments. This was done in order to determine the influence of notch stresses on the crack propagation, as well as to ascertain the moment of the notch stress relief. Thereby the focus was on the analysis of the two‐dimensional short crack propagation. Due to the high resolution of the DCPD technique used for the in‐situ measurements, it was possible to show that the beginning of continuous crack propagation in the HCF regime is within the first 20 % of the total lifetime. Thus, the so‐called crack initiation lifetime becomes negligible and the
total lifetime is determined by crack propagation only. In the context of the calibration of the DCPD technique it could be illustrated that an absolute value of the crack length depends on the notch geometry and in particular on the geometry of the early flaws. As a variation of lifetime prediction continuum mechanical and fracture mechanical stress transformation were discussed based on SN curves. All variations led to nearly the same prediction quality, so that a known dilemma was emphasized: lifetime predictions for different notch configurations could be obtained using just one reference curve (which was experimentally determined) combined with stress transformation factors, even without any knowledge of the crack propagation itself. The required modelling of the crack propagation was firstly carried out in the direction of the long crack propagation. On the one hand a fracture mechanical approach was used where notch depth is equal to crack length. On the other hand a local approach was followed where the notch stress gradient was taken into account. The fracture mechanical simulations underestimated the experimentally determined lifetimes of specimens with shallow notches. Vice versa, the local approach overestimated the lifetime of specimens with deep notches. Hence, a detailed analysis of the two‐dimensional short crack propagation on the specimen surface was conducted. Therefore, the length of the quarter elliptical corner cracks was determined on the specimen sides and on the notch root surface after defined fatigue intervals. According to the simulation of the two‐dimensional crack propagation it could be shown stepwise, that the propagation of short cracks in both surface direction is interdependent and that the crack prolongation along the notch root is dominant. Finally the stresses responsible for the measured crack propagation were calculated. These local stresses decreased from the maximum level caused by the notch stress concentration to the applied stress level in both direction after only 150 microns. The resulting local stresses were always below the theoretical values of notch stress. In conclusion the relief of the notch stress sets in earlier than expected, in fact this relief already takes place due to the propagation of the short corner cracks.},
    note = {},
    
    school = {Universität der Bundeswehr München},
    
}