This study investigates the fatigue performance of HFMI-treated welded joints in offshore wind turbine monopile foundations. Fatigue tests were carried out on longitudinal butt welds (LB) and T-butt welds (TB) made from S355J2 + N, S355ML, and S500ML in both as-welded (AW) and HFMI-treated conditions. In addition, single-sided transverse stiffeners (TS) were tested after blast cleaning to simulate conventional surface preparation prior to coating. The results show that HFMI significantly improves the fatigue strength of TB welds, shifting the failure location away from the critical weld toe when all weld toes, including intermediate layers, are treated. In contrast, LB welds exhibited a high fatigue resistance already in AW condition, indicating limited benefit of HFMI treatment under axial loading. Blast cleaning, whether applied before or after HFMI, was found to be compatible with organic coatings and thermal sprayed zinc and did not reduce fatigue performance; a slight positive effect was even observed. These findings support the selective use of HFMI treatment for critical details in offshore monopiles and confirm its compatibility with standard corrosion protection systems.

This paper analyses the applicability of a modified strain approach to predict the fatigue life of HFMI-treated transverse stiffeners under variable amplitude loading (VAL) with random load sequences of a p(1/3) and linear shaped spectrum. Local stresses are determined using linear-elastic finite element analyses. The measured weld geometry and component imperfections are considered. From the hardness of the HFMI-treated zone and the base material, the elastic–plastic material behaviour and Coffin-Manson parameters to describe the damage parameter Woehler curve are estimated. Based on a hysteresis counting method (HCM), the damage for each closed hysteresis is calculated. The applied notch strain approach includes the impact of residual stresses and the influence of surface roughness. Thus far, the application of similar approaches has only been validated for welded components with comparatively low residual stresses and HFMI-treated welds subjected to constant amplitude loading. To validate the accuracy of the approach for HFMI-treated welds under variable amplitude loading, the approximated fatigue life is compared to the number of cycles derived from experimental investigations. In this study, it is shown in conjunction with experimental results that it is essential to consider the strength of the base material near the weld when assessing the service life. This area can be more critical than the HFMI-treated weld toe.

Welded joints show large variation of the weld toe geometry along the weld seam, which is one important reason for the comparably large scatter in fatigue life. Therefore, it is crucial to take the local geometry at the weld toe into account, to reduce the conservatism in fatigue assessment of welded joints. This study is based on the IBESS procedure for the calculation of the fatigue strength, whereby the evaluation of local geometrical parameters is carried out by means of 3D surface scans. The approach is validated against 26 fatigue test series. The fatigue life is in general overpredicted, whereas good agreement is achieved for high stress ratio ( R=0.5 R=0.5 ). A sensitivity analysis conducted with IBESS shows that weld toe radii rho < 2 mm and flank angle alpha < 30(degrees) have a significant influence on the calculated fatigue strength. In contrast to this, no strong correlation between rho and the fatigue strength was determined experimentally in this study.

Due to notches, welds are most critical regarding fatigue failure within cyclic loaded constructions. Therefore, various post-weld-treatment techniques like post-weld treatment by high-frequency mechanical impact (HFMI) treatment have been invented to improve the fatigue strength of welded details. The benefit, resulting from HFMI treatment, has already been proven by numerous studies. Since a manual HFMI treatment must be performed by a skilled and trained person to ensure an acceptable treatment quality, an automated application of HFMI treatment is supposed to result in a more reliable and consistent treatment result, which does not depend on the operator. Furthermore, a robotic application of HFMI treatment enables an economic implementation of HFMI treatment of automated welded constructions like offshore wind energy converters and various mechanical components, as these parts do not have to be taken out of the production chain to manually perform HFMI treatment. This paper focuses on the experimental investigation of the fatigue behaviour of automated HFMI-treated welds, using a developed robotic application of the HiFIT device (specific HFMI tool).

Stub column compression test of a cold-formed C-shaped steel-section used for the validation of a new design approach which combines the AISI

Die Wirksamkeit der Schweißnahtnachbehandlung durch höherfrequente Hämmerverfahren (HFH) zur Steigerung der Ermüdungsfestigkeit wurde bereits durch eine Vielzahl von Studien belegt. Durch die robotergesteuerte Anwendung der HFH‐Verfahren soll die Schweißnahtnachbehandlung in den Herstellprozess automatisiert geschweißter Bauteile integriert werden können und ein wirtschaftlicher Einsatz der HFH‐Behandlung innerhalb der Serienfertigung ermöglicht werden. Zudem kann eine automatisierte Qualitätskontrolle in den Nachbehandlungsprozess eingebunden werden, die eine genauere Vermessung der Nahtgeometrie als konventionelle Messmethoden ermöglicht. Dieser Beitrag befasst sich mit der gerätespezifischen Entwicklung einer robotergesteuerten Anwendung der HFH‐Nachbehandlung mit dem HiFIT‐Gerät sowie einer automatisierten Qualitätskontrolle. Anschließend wird die Verifizierung der Wirksamkeit der automatisierten HFH‐Nachbehandlung anhand von Ermüdungsversuchen und der Vermessung der Eindruckgeometrie dargestellt.

The potential of HFMI treatment to increase fatigue life under service loading remains in debate. However, some recent studies show that even under variable amplitude loading (VAL), fatigue strength is increased compared to untreated welds. Discussions generally focus on the stability of initial compressive residual stresses, which may be reduced during VAL due to high peak stresses. In this context, the potential of HFMI treatment is often only attributed to residual stress stability. This study presents further results on the effect of VAL with a P(1/3) and a linear load spectrum on the residual stress stability of HFMI-treated transverse stiffeners (TS) made of mild steel (S355) and high-strength steel (S700M). The impact of random, High-Low and Low-High loading sequences on the fatigue strength as well as on the residual stress behaviour of HFMI-treated joints will be discussed.

The effectiveness of post weld treatment by High Frequency Mechanical Impact Treatment (HFMI) to increase the fatigue strength has already been proven by numerous studies. An automated application of HFMI-treatment can be integrated in the manufacturing process of automated welded constructions and enables an economical use of HFMI-treatment within series production. Furthermore, an automated quality control can be embedded in a robot-controlled HFMI-process, which enables a more precise measurement of the weld geometry than conventional measuring methods. This article covers the device-specific development of a robot-controlled application of HFMI-treatment by the HiFIT-tool as well as an automated quality control. Following, the verification of the effectiveness of an automated HFMI-treatment by fatigue tests and the measurement of the geometry of the HFMI groove is presented.

It has been shown in several studies that methods to improve the fatigue strength of welded structures, such as high-frequency impact treatment (HFMI), can increase the fatigue life of welded joints [1–6]. The results of these investigations led to current guidelines and recommendations for the fatigue assessment of HFMI-treated welded joints. Nevertheless, in practice, there are reservations regarding the efficiency of HFMI-treated welded steel joints under variable amplitude loading. Recent results [7] from studies on transverse attachments of the material S355 and S700 under variable amplitude loading show that the fatigue strength increasing effect of the HFMI-treatment is maintained compared to the as-welded state. The aim of this study is to analyse the sequence effect on the fatigue strength of HFMI-treated transverse attachments and to validate the applicability of linear damage accumulation hypotheses for the design of as-welded and HFMI-treated welded details. In this paper, fatigue test results with random variable amplitude loading (VAL) and high-low VAL and low–high VAL with linear spectrum for the two states as-welded (AW) and HFMI-treated joints will be presented.

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.