Ultrasonic detection of spark eroded notches in steel plates

A long-standing challenge in integrity management of gas pipelines is stress corrosion cracking (SCC). Such cracks are difficult to detect, calling for sensitive detection methods with good coverage. Recent advances in gas-coupled broad band ultrasound provide a potential method that is based on guided plate waves. The objective of this work is to describe and experimentally evaluate this method based on Halfwave's ART Scan® tool. A water-immersed 12 mm thick steel plate with machined crack-like, spark eroded notches (SEN) of 2 and 6 mm depth, and 10 and 20 mm lengths, were acoustically investigated with broadband (0.3–1.3 MHz) transducers in a pitch-catch setup. Received signals were acquired at 2D grid locations, and pointwise processing parameters were extracted. Summed signal power and a spectral parameter demonstrated the detection of the SENs. The received energy in a selected frequency band was 2.5 and 5.5 dB lower for the 2 and 6 mm deep SENs, respectively, compared to that of an undamaged part of the plate. Using a tomographic image inversion method, the received signals were combined and converted into images. Results of the 6 mm deep SEN showed good agreement with the actual size and location of the SEN. In conclusion, the potential to detect SCC by using an ART Scan® based set-up, combined with pointwise and image formation processing, was successfully demonstrated on test plates with machined crack-like defects.