• Leitner, Martin
• Eichlseder, Wilfried
• Stoschka, Michael

Abstract

It is well known that the fatigue strength of welded structures is in general independent from the material strength. In case of high-strength steels, however, a significant improvement in the fatigue behaviour can be realised through post-treatment processes. This paper deals with the effect of high-frequency mechanical impact (HFMI) on the fatigue behaviour of a range of steels starting from mild construction steel (S355) to ultra high-strength steel (S960). The experiments involve fatigue tests at a stress ratio of R = 0.1 on butt welds, T-joints, and longitudinal attachments on 5 mm, thin-walled specimens. The fatigue assessment was performed in accordance to the nominal and the notch stress approach taking the HFMI condition into account. Finally, a novel method is outlined to evaluate the notch stress fatigue behaviour of HFMI-treated joints made of high-strength steel. Applicability of this new HFMI notch stress approach is shown through fatigue assessment of about 330 HFMI post-treated specimens taken from both literature and own test results. Further work focuses on the expansion of the introduced HFMI notch stress model for load spectra influence covering overloads and multiaxial fatigue.<br/><br/>Access provided by Technische Universität Graz Universitätsbibliothek<br/>1 Introduction<br/><br/>In the finite lifetime area, the fatigue behaviour of welded high-strength steel joints is mostly beneficial due to the high yield strength of the base material. In regard to the high-cycle fatigue zone, the notch topography, microstructure in the heat-affected zone, and the residual stress state have a significant influence on the fatigue lifetime. According to the International Institute of Welding (IIW) recommendation [1], the fatigue life is independent of the yield strength of welded steel components. However, additional high-frequency mechanical impact (HFMI) treatment offers the possibility to increase the fatigue life, especially for high-strength steels. To assess the local fatigue strength of welded and HFMI post-treated high-strength steel joints experimentally, fatigue tests using three different thin-walled (t eff = 5 mm) joints are investigated. Figure 1 illustrates the investigated specimen types, ranging from butt joint, root surface in grinded condition, over T-joint to longitudinal attachment.<br/>Fig. 1<br/>figure 1<br/><br/>Investigated joint types<br/>Full size image<br/><br/>Two low-alloyed high-strength steels S690 and S960, and for comparative purposes a common construction steel S355, with a sheet thickness of 5 mm are used as different base materials. All plates were sand blasted before welding. For the as-welded condition, the structural detail-dependent nominal stress range is defined in [1] as FAT, characteristic fatigue strength at two million cycles considering a probability of survival of 97.7 %. Figure 2 depicts the nominal stress FAT classes of the three probed joints.<br/>Fig. 2<br/>figure 2<br/><br/>Recommended nominal stress FAT values for the investigated steel joints [1]<br/>Full size image<br/><br/>In the nominal stress concept, the fatigue strength enhancement for higher strength steel welds (f y  &gt; 355 MPa) improved by hammer or needle peening is expressed by a bonus factor of 1.5 [1]. This factor is applied to the recommended stress range. An overview of the existing post-weld treatment methods, their application, and the proposed benefit in fatigue is given in [2]. Recent research results [3–5] observed that the fatigue strength of improved HFMI-treated welds increases with material yield strength. In [3], an extensive study including 228 experimental test data points showed that the fatigue benefit can be expressed by an increase of 12.5 % for every 200 MPa increase in the material strength, choosing f y,0 = 355 MPa as base material strength reference.<br/>2 Objectives<br/><br/>In this contribution, an evaluation of the fatigue behaviour of HFMI-treated joints using the nominal and the notch stress concept for the investigated joints is presented. The goal is to assess methodically the benefit of the HFMI treatment for thin-walled high-strength steels. The work packages undertaken in this study are:<br/><br/>Experimental fatigue tests to ascertain the influence of the base material yield strength and the stress concentration factor at the weld toe on the fatigue behaviour of as welded (without post-treatment) and HMFI-treated steel joints.<br/><br/>Application of an existing procedure [3] to consider HFMI in the nominal stress concept and verification of the own experimental data compared to the published results.<br/><br/>Proposal of a novel S/N model to consider the HFMI treatment by the notch stress approach. Validation of this HFMI-enhanced notch stress approach by extensive experimental data.<br/><br/>3 Experimental work<br/><br/>Preliminary investigations [6–8] concerning the fatigue testing of welded joints, showed that a ratio 10:1 betwee...

Topics

• impedance spectroscopy
• microstructure
• surface
• experiment
• strength
• fatigue
• yield strength
• fatigue testing
• structural steel