Plane Wave Imaging Techniques for Immersion Testing of Components with Non-Planar Surfaces

Plane Wave Imaging (PWI) is an ultrasonic array imaging technique used in non-destructive testing, that has been shown to yield high resolution with few transmissions. Only a few published examples are available of PWI of components with non-planar surfaces in immersion. In these cases, inspections were performed by adapting the transmission delays in order to produce a plane wave inside the component. This adaptation requires prior knowledge of the component geometry and position relative to the array. The current paper proposes a new implementation, termed PWI Adapted in Post-Processing (PWAPP), which has no such requirement. In PWAPP the array emits a plane wave as in conventional PWI. The captured data is input into two post-processing stages. The first reconstructs the surface of the component, the latter images inside of it by adapting the delays to the distortion of the plane waves upon refraction at the reconstructed surface. Simulation and experimental data are produced from an immersed sample with a concave front surface and artificial defects. These are processed with conventional and surface corrected PWI. Both algorithms involving surface adaptation produced nearly equivalent results from the simulated data, and both outperform the non-adapted one. Experimentally, all defects are imaged with Signal-to-Noise Ratio (SNR) of at least 31.8 and 33.5 dB for respectively PWAPP and PWI adapted in transmission, but only 20.5 dB for conventional PWI. In the cases considered, reducing the number of transmissions below the number of array elements shows PWAPP maintains its high SNR performance down to number of firings equivalent to a quarter of the array elements. Finally, experimental data from a more complex surface specimen is processed with PWAPP resulting in detection of all scatterers and producing SNR comparable to that of the Total Focusing Method.