Searching for the first sign : crack initiation under fatigue loading in seawater-corroded structural S355 steel

Corrosion caused by direct contact with seawater is the most important problem for technical systems and components made of steel in the maritime sector (quote). The loss of material associated with general corrosion leads to a weakening due to the associated reduction in cross-section. Besides, stress concentrations will occur at localized areas of corrosion called corrosion pits. With repeated loading, such pit may act as a nucleation site for fatigue cracks, which ultimately lead to the degradation of the fatigue strength of the material (quote). Due to the random nature of the spatial occurrence of such localized corrosion, the prediction of service life under fatigue loading is made even more difficult. Research shows that a significant portion of fatigue life is spent in the transition of a corrosion pit to a short fatigue crack and in the growth of such short cracks [1]. As a part of the MAXWind project, numerical models are developed that incorporate the evolution of pit morphology due to corrosion [2], associated geometric stress concentration [3], and pit to crack transition [4]. Experiments that represent actual corrosion pits are thus required to verify and validate such models and to determine the model parameters. As part of the project, structural steel coupons (used for manufacturing wind turbine monopile structures) were directly exposed to seawater in an offshore facility for 22 months. Subsequently, dog-bone shaped specimens were extracted from coupons from different corroded contact areas (underwater, water level, splash zone) for fatigue strength evaluation. This article describes selected results of fatigue tests (purely tensile at different load levels. The sample surfaces were monitored with an IR camera during the entire fatigue test. The aim of this observation was to record the spatial temperature distribution which can be correlated to the stress distribution on the sample surface. The occurrence of cracks is characterized by the appearance of closely neighboring regions with reduced (at the crack flanks) and increased (at the crack tip) internal stress [5]. The resulting temperature differences are very small but can be quantified using the lock-in technique (i.e. TSA - thermal stress analysis). In this study, different thermal signatures are considered, which can be used as an indicator for the first occurrence of macroscopic cracks.