Fatigue behaviour of high frequency hammer peened ultra high strength steels

• 119 fatigue tests on S960, S1100 and S1300 with as welded and HFHP condition.
• The negative inverse slope of the S–N-line increases to m ∼ 5 due to HFHP treatment.
• An improvement due to HFHP can be observed at load cycles of 10,000 and higher.
• Increase of FAT classes of design proposals is possible if HFHP is applied at UHSS.
• By HFHP treatment, other untreated notch details can be relevant for design.

Existing design recommendations for the consideration of high frequency hammer peening (HFHP) are limited to steel grades of S960 and plate thicknesses of 5 mm and higher. The influence of HFHP treatment on the fatigue behaviour of welded ultra high strength steels with yield strengths of 960 MPa and higher – loaded in the upper finite and low cycle fatigue life region – has not been investigated sufficiently so far. For this reason, fatigue tests have been performed on four typical welded notch details of mobile crane structures made of 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. A fatigue life improvement due to HFHP treatment can be observed at load cycles of 10,000 and higher. In accordance with existing investigations, the slope of the S–N-line increases to approximately m ∼ 5 due to HFHP treatment if the fatigue cracks start from the treated weld toes. The classification of the test results for the HFHP treated toe condition shows, that fatigue classes (FAT) of existing design proposals are conservative. Further improvements of the proposed FAT classes are possible which shows the potential use of UHSS with steel grades higher than S960 in combination with HFHP treatment.