P 938 - Influence of high-frequency hammering on the fatigue strength of ultra-high-strength steels of strength classes S960, S1100 and S1300 using the example of welded notch details

For this reason, the Wöhler lines of untreated (m ~ 3) and HFH-treated (m ~ 5) notch details theoretically intersect in the upper fatigue strength range. Existing design proposals for recording HFH only take into account steel grades up to a maximum of S960 and sheet thicknesses greater than or equal to 5 mm. The influence of HFH on the fatigue strength in the upper fatigue or short-term strength range for steel grades S960 and higher is not adequately covered. To determine the influence of higher frequency hammering processes on the fatigue strength of welded ultra-high-strength steels, fatigue tests were carried out on four typical crane notch details made of the ultra-high-strength fine-grained structural steels S960, S1100 and S1300. Compared to the untreated seam condition, at least a doubling of the fatigue strength and increases in service life from a number of load cycles of around 10,000 were observed as a result of HFH treatment. In the short-term strength range with numbers of cycles at break of the test specimens left in the seam condition between 10,000 and 40,000, depending on the notch detail and yield strength, a two to tenfold increase in service life is achieved as a result of HFH treatment. As a result of the HFH treatment, some of the crack initiation locations shift from the seam transition notches to the seam root area or to existing notches in the base material. The evaluation of the results for the notch details longitudinal stiffness and butt joint reveal an influence of the material yield strength on the effectiveness of the HFH treatment. Different sheet thicknesses and seam geometries influence the comparability of the results when analysing the results using the nominal stress concept. The classification of the test results for the HFH-treated seam condition in existing design approaches for the consideration of HFH shows that the notch classes of all design approaches are conservative compared to the existing results. For the notch details of the lamella and the butt joint with plate thickness jump, a further increase in the notch class as a result of HFH treatment appears possible. The fatigue behaviour of HFH-treated, ultra-high-strength fine-grained structural steels S960 and higher, taking into account preloads, variable load collectives and increased mean stresses, has not yet been adequately covered and requires further investigation. The research project P 938 of FOSTA - Forschungsvereinigung Stahlanwendung e. V., Düsseldorf, was carried out at the Institute for Metal and Lightweight Construction at the University of Duisburg-Essen with technical support and financial assistance from FOSTA.