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

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.