Ermüdungsverhalten HFH-nachbehandelter Kerbdetails des Mobilkranbaus

Fatigue behaviour of HFH-treated notch details of mobile crane structures.

In mobile crane structures, ultra high strength fine grained structural steels (UHS) with yield strengths up to 1100 N/mm² and 1300 N/mm² respectively are used. The fatigue life of welded notch details which can be classified into the upper finite fatigue life region is critical. One possibility to improve the fatigue behaviour is the application of post weld treatment methods like high frequency hammer peening (HFH). Existing design recommendations for the consideration of HFH are limited to maximum steel grades of S960 and plate thicknesses of 5 mm and higher. The influence of HFH-treatment on the fatigue behaviour of welded ultra high strength steels with yield strengths of 960 N/mm² and higher - loaded in the upper finite and low cycle fatigue (LCF) life region respectively - has not been investigated sufficiently so far. For this reason, fatigue tests have been performed at four typical welded notch details of mobile crane structures made of S960, S1100 and S1300 to determine the influence of HFH on the fatigue strength. The results of the HFH-treated specimens showed a significant improvement of the fatigue strength in comparison with the as welded toe condition. Furthermore, a fatigue life improvement due to HFH treatment can be observed at loading cycles of 10000 and higher. In accordance with existing investigations, the slope of the S-N-line increases to approximately m ~ 5 due to HFH treatment if the fatigue cracks start from the treated weld toes. The classification of the test results for the HFH-treated toe condition shows, that FAT classes of existing design proposals are conservative.