Abstract
In the meantime, it’s well known that post-weld fatigue strength improvement techniques for welded structures like high-frequency mechanical impact (HFMI) treatment increase the fatigue live of welded joints. Although the current design recommendations for HFMI-treated welded joints give first design proposals for the HFMI-treated welds, in practice the application of HFMI treatment and the associated increase in fatigue resistance are still being discussed. There are, for example, reservations regarding the efficiency of HFMI-treated welded joints under variable amplitude loading (VAL). This paper analyses first results for the sequence effect of VAL of a p (1/3) spectrum on the service fatigue strength of HFMI-treated transverse stiffeners (TS) of mild steel (S355). Fatigue test results with random and high-low loading for the two states as-welded (AW) and HFMI-treated joints will be presented. The modified linear damage accumulation and the failure locations will be discussed. The experimental results show a clear change in the slope of the S-N curve from the as-welded (AW) state to the HFMI state and additionally in the HFMI state from constant amplitude loading (CAL) to variable amplitude loading (VAL). It was particularly noticeable in the experimental results of all tested HFMI series that the specimens failed exclusively in the base material 2–4mm before the HFMI-treated welds. The presented results of the investigations show that with application of the nominal stress concept, no sequence effect was recognizable.
Highlights
For the fatigue strength of welded structural steel constructions, the geometrical conditions and local boundary layer conditions at the weld seams are decisive
This paper presents results of the research project “HFHBetriebsfestigkeit” [21] concerning the sequence effect of variable amplitude loading (VAL) of a p (1/3) spectrum on the service fatigue strength of high-frequency mechanical impact (HFMI)-treated transverse stiffeners of mild steel (S355)
The stress range ratio transverse stiffeners (TS), (99.92%/0.08%)=1:1.264 is estimated from the statistically evaluated lower prediction bound with 95% of confidence
Summary
For the fatigue strength of welded structural steel constructions, the geometrical conditions and local boundary layer conditions at the weld seams are decisive. In order to present a safe, economical and resource-saving design proposal for steel structures, a reliable and realistic estimation of the influence of operating service loads as well as of the influences of notch sharpness and the condition of the surface layer after the mechanical treatment on the fatigue behaviour of welded constructions is necessary. The efficiency of HFMI treatment under real operating conditions is currently questioned This reservation is based on the assumption that the residual compressive stresses generated during operation can be reduced and lose their initial effect, i.e. in [16]. It has not yet been clarified how the effect of such postweld treatment can be taken into account in a reliable service fatigue strength analysis. No studies are available focussing on sequence effects for single load spectra
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