Detection and characterization of fatigue cracks in butt welds of offshore structures using the eddy current method

A reliable assessment of the actual condition of wind turbines is essential for their continuous operation, or if an extension of their service life is considered. To be able to make valid statements about the condition of the welded structure, an inspection concept for the reliable detection of fatigue cracks and damage on welds of the offshore tower and foundation structures is essential. In the present study, five different eddy current differential sensors were evaluated on cyclically fatigued DY butt welds under tensile load with a focus on the signal-to-noise ratio (SNR). Based on previous evaluations, two sensors were selected for semi-automatic weld testing. It is shown that due to the weld crossing necessary for the detection of fatigue cracks, air coils arranged parallel to the test surface have the highest SNR. This must be contrasted with the potential for crack depth determination. In this context, coils with different arrangement with respect to the test surface were analyzed. It is shown that groove depths can be differentiated based on the imaginary part of the measurement signal for groove depths of up to 8 mm, and actual fatigue cracks in welds with a crack depth of 0.5 mm were detected.