TFM+ for improved defect detection in heterogeneous media


The Total Focussing Method (TFM) is the ‘gold-standard’ phased array imaging algorithm within the ultrasonic non-destructive testing community. However, the algorithm (in its most basic form) assumes that the waves propagate along straight lines at a constant wave speed. Consequently, when the medium under inspection exhibits a heterogeneous and locally anisotropic microstructure, the resulting defect reconstructions are often misplaced and poorly characterised. These errors can be attributed to the scattering and refraction of the input waveforms by the underlying material microstructure, causing disparity between the collected data and the calculations made within the TFM framework. In this work, we present a novel and efficient model for wave propagation in polycrystalline materials (which can be easily generalised for application to other complex media) and demonstrate how this can be incorporated within the TFM framework (we refer to this new algorithm as the TFM+). In cases where the underlying material structure is known a priori, our algorithm accounts for the refraction caused by the spatial variation in material properties throughout the medium. Our initial results show a 4-fold improvement in defect location for the case where a 3mm side drilled hole is embedded within an austenitic steel weld.