• Vancoillie, Robin
• Hectors, Kris
• Saeed, Hasan
• Mehri Sofiani, Farid
• Waele, Wim De

Abstract

Offshore wind turbine support structures are exposed to maritime conditions, which can lead to corrosion fatigue. This work is part of the FATCOR project funded by the Belgian Energy Transition Fund, aiming to develop a qualitative and quantitative understanding of the mechanisms of corrosion fatigue in seawater. Localized corrosion generates a geometrical defect, raising the local stresses and reducing the fatigue life. The transition from pit growth to short fatigue crack propagation occurs at a critical pit size, which depends upon the microstructure, the applied stress level and the geometry of the pit. In linear elastic fracture mechanics, the stress intensity factor is used to describe the magnitude of the stress singularity near a crack tip caused by remote stresses and is useful for establishing a failure criterion. Literature lacks stress intensity factor solutions for cracks emanating from a three-dimensional semi-ellipsoidal pit. Fig. 1 (a) shows a schematic representation of a plate subjected to axial tensile stress with a semi-ellipsoidal pit at the center of the top surface. Two cracks in the shape of a circular arc are introduced at the pit mouth perpendicular to the loading direction (see Fig. 1 (b)). Finite element analysis is used to calculate the stress intensity factor (K₁) at the crack tip (see Fig. 2). The displacement extrapolation method is used to quantify the effect of different pit configurations and crack lengths on K₁. This method determines K₁ from the displacement field near the crack tip. A parametric study is performed on a range of relative geometrical parameter values (a/2c, b/c) and crack lengths (r/a). It is observed that changes in the pit geometry can drastically affect the stress gradient in the vicinity of the pit, which directly influences the magnitude of K₁. For example, (a/2c) equal to 1, 0.5 and 0.25, resulting in K₁ values of 74.4, 71.1 and 56.6 MPa√mm respectively, for a remote stress of 100 MPa. In future work, regression analysis will be performed to develop an equation to calculate the K₁ for a wide range of pit configurations and crack lengths.

Topics

• impedance spectroscopy
• microstructure
• surface
• corrosion
• crack
• fatigue
• finite element analysis