An example NDT apprenticeship project – remote phased array ultrasonic inspection of thin-walled pipes

D B O’Hare 

As part of the new NDT apprenticeship trailblazer schemes, apprentices must complete a project that demonstrates many of the skills, knowledge and behaviours that are expected. The project requires the apprentice to carry out research, determine inspection methodology and limitations and report the findings in both a written paper and a presentation. This is presented during the end-point assessment (viva), determining whether the apprenticeship has been successfully completed.

This paper discusses one such project, where phased array ultrasonics were used to remotely inspect the condition of thin-walled pipes for wall thickness loss. The pipes in question had a diameter of approximately 50 mm and a wall thickness of 1 mm. Damage to the pipe was found to be caused by fretting under a clip that was costly to remove. Hence an on-wing inspection capable of detecting damage beneath the clip without the need for removal was required. The topology of the defect in question was determined, suitable test-pieces designed, data collected and reported. Different NDT methods were considered, with multi-skip phased array ultrasound being determined the most sensible method to use.

The investigation determined that 50° (optimised angle) shear waves in multi-skip mode provided the best performance to remotely detect a representative area of wall thickness loss (pipe wear) on a test-piece, with a linear relationship between the signal amplitude and the depth of wall thickness loss. It was also found that the pipe contents at the time of inspection caused a 6 dB drop in signal when testing with shear waves, but had little to no impact on testing with longitudinal waves. Wall thickness loss as small as 50 µm was detected using this method.