TY - JOUR

T1 - Assessment of corrosion fatigue in welded joints using 3D surface scans, digital image correlation, hardness measurements, and residual stress analysis

AU - Shojai, Sulaiman

AU - Brömer, Tim

AU - Ghafoori, Elyas

AU - Woitzik, Christian

AU - Braun, Moritz

AU - Köhler, Markus

AU - Schaumann, Peter

N1 - Funding Information: The research project “Influence of corrosive media on the fatigue strength of offshore wind turbines (CorroFAT)”, grant number 37 LN/1, of the Research Association for Steel Applications (FOSTA) e. V. was funded by the German Federal Ministry of Economics and Climate Action (BMWK) via the German Federation of Industrial Research Associations “Otto von Guericke” (AiF) e. V. as part of the program “Leittechnologien für die Energiewende” and as part of the joint project “Offshore Wind Energy Systems for Hydrogen Supply” to promote joint industrial research (IGF) on the basis of a resolution of the German Bundestag. The authors would like to express their sincere gratitude for the experienced financial support. In addition, special recognition is given to the project partners involved in the production of the specimens: Salzgitter AG, Muehlhan AG, Fraunhofer IGP.

PY - 2023/11

Y1 - 2023/11

N2 - Corrosion can significantly reduce fatigue resistance of steel constructions including offshore support structures. This can be attributed to either localized stress concentration caused by pitting corrosion or to embrittlement of the material during the corrosion process. In addition to the stress concentrations that can arise from pitting corrosion, offshore steel structures are characterized by a significant number of notches at weld seams, which may also cause stress concentrations. Therefore, it is of great importance to study the interaction between the pre-existing notches from welds and the notches from corrosion. Thus, reference material specimens as well as butt- and fillet-welded specimens from a mild steel S355 were investigated in this study. Before being stored in a salt spray chamber, the specimens were clean blasted as usually carried out for offshore support structures. After one month of exposure, the specimens were tested against fatigue and monitored by digital image correlation (DIC). The specimens were scanned with high-resolution 3D-scanners before and after corrosion exposure. In addition, material hardness and residual stresses were investigated to quantify the influence of corrosion on the material side and the influence from the welding process. It is shown that corrosion strongly influences the weld geometry. Both, individual pits and uniform corrosion are observed at the weld toe, which is relevant for fatigue. It was also shown that the fatigue strength of welded specimens depends not only on the geometry and its degradation by corrosion, but to a greater extent on the residual stresses present after corrosion. The residual compressive stresses applied by clean blasting were partly relieved by corrosion. The fatigue tests have shown increased fatigue strength after clean blasting and subsequent reduction due to corrosion. The fatigue strength of fillet welded specimen, for example, were increased from 74 N/mm2 in its as-welded condition to 158 N/mm2 through clean blasting. However, due to corrosion, fatigue strength decreases to 98 N/mm2.

AB - Corrosion can significantly reduce fatigue resistance of steel constructions including offshore support structures. This can be attributed to either localized stress concentration caused by pitting corrosion or to embrittlement of the material during the corrosion process. In addition to the stress concentrations that can arise from pitting corrosion, offshore steel structures are characterized by a significant number of notches at weld seams, which may also cause stress concentrations. Therefore, it is of great importance to study the interaction between the pre-existing notches from welds and the notches from corrosion. Thus, reference material specimens as well as butt- and fillet-welded specimens from a mild steel S355 were investigated in this study. Before being stored in a salt spray chamber, the specimens were clean blasted as usually carried out for offshore support structures. After one month of exposure, the specimens were tested against fatigue and monitored by digital image correlation (DIC). The specimens were scanned with high-resolution 3D-scanners before and after corrosion exposure. In addition, material hardness and residual stresses were investigated to quantify the influence of corrosion on the material side and the influence from the welding process. It is shown that corrosion strongly influences the weld geometry. Both, individual pits and uniform corrosion are observed at the weld toe, which is relevant for fatigue. It was also shown that the fatigue strength of welded specimens depends not only on the geometry and its degradation by corrosion, but to a greater extent on the residual stresses present after corrosion. The residual compressive stresses applied by clean blasting were partly relieved by corrosion. The fatigue tests have shown increased fatigue strength after clean blasting and subsequent reduction due to corrosion. The fatigue strength of fillet welded specimen, for example, were increased from 74 N/mm2 in its as-welded condition to 158 N/mm2 through clean blasting. However, due to corrosion, fatigue strength decreases to 98 N/mm2.

KW - Corrosion fatigue

KW - Digital image correlation

KW - Digital scans

KW - Offshore-wind

KW - Residual stress, hardness

KW - Stress concentrations

KW - Welds

UR - http://www.scopus.com/inward/record.url?scp=85169902763&partnerID=8YFLogxK

U2 - 10.1016/j.ijfatigue.2023.107866

DO - 10.1016/j.ijfatigue.2023.107866

M3 - Article

AN - SCOPUS:85169902763

VL - 176

JO - International journal of fatigue

JF - International journal of fatigue

SN - 0142-1123

M1 - 107866

ER -