Application of Golay-based total focusing method using a high-frequency, lead-free, flexible ultrasonic array for inspection of thick non-planar industrial components

The compromise between axial resolution and penetration depth in ultrasound imaging poses a challenge for high-frequency ultrasonic arrays, limiting their ability to effectively inspect thick components in industrial applications. In this work, a commercial 20 MHz, 64 element, 1 mm pitch lead-free flexible linear array was characterised in terms of its performance. The array was subsequently evaluated using Golay-coded excitation techniques to enhance the signal-to-noise ratio (SNR) and operability on non-planar thick components. The SNR improvement verification results were acquired with the array deployed on a 100 mm thick flat aluminium test specimen. As expected, an increase in SNR was observed as the Golay code length increased. The imaging strategy employed a combination of Full Matrix Capture (FMC) and Total Focusing Method (TFM) to assess the performance variations between the conventional pulse excitation and Golay-coded excitation. The Golay-based TFM demonstrated superior performance compared to the conventional pulse-based TFM, with an SNR improvement of 4.95 dB when using the full array aperture to inspect the non-planar steel S355 specimen. A sub-aperture selection approach, based on the effect of the array element beam spread, offered additional SNR improvement of up to 8.2 dB. Greater imaging penetration depth was achieved, with an increase of > 40 % compared to conventional pulse-based TFM. Thus, for inspection of thick non-planar industrial components using a lead-free high-frequency array, Golay-coded excitation schemes show excellent potential to enhance SNR, penetration depth and imaging quality.