Fatigue Strength of Welded Ultra High Strength Steels Improved by High Frequency Hammer Peening

Existing design recommendations for the influence of high frequency hammer peening (HFHP) on the fatigue strength are limited to maximum steel strengths of S960 and plate thicknesses of 5 mm and higher. The influence of HFHP on the fatigue strength of welded ultra high strength steels with yield strengths of 960 N/mm2 and higher - loaded in low cycle fatigue (LCF) respectively upper finite fatigue life region - has not been investigated sufficiently so far. For this reason, the Institute for Metal and Lightweight Structures of the University of Duisburg-Essen has performed fatigue tests on four different welded details a) butt weld, b) transversal stiffener, c) longitudinal stiffener and d) cover plates made of ultra high strength steels S960, S1100 and S1300 to determine the influence of HFHP on the fatigue strength. The fatigue strength of HFHP treated specimens was at least twice the fatigue strength of the as welded toe condition. In comparison with existing investigations for steel strengths S960 and lower, the results of the fatigue tests at HFHP-treated specimens showed the same trend: the slope of the S-N-curve increases to approximately m = 5. Furthermore, after HFHP treatment in some cases the location of crack initiation changes from the weld toe to the weld root or to notches in the base material.