Visualization of specular reflections with phase coherence imaging using sub-apertures

In recent years, phased array ultrasonic testing (PAUT) has gradually been substituted by approaches such as the Total Focusing Method (TFM), which use the Full Matrix Capture (FMC) acquisition scheme. Phase Coherence Imaging (PCI) has been proposed to mitigate the problems associated with amplitude-based TFM. PCI also holds high potential for dramatically simpler and far more affordable instruments as it only requires the phase of signals. However, in the context of PCI and in a scenario where the amplitude is not acquired, point-like scatterers would easily be identified due to their diffractive nature, while specular reflectors would not be detectable, as specular information is only captured by a specific group of elements known as sub-apertures (SA). In this paper, specular wave paths are analyzed to predict and choose the suitable emitting and receiving SA that are sensitive to specular reflections. The work demonstrates the effectiveness of the proposed method for revealing specular information by comparing PCI images of notches with the corresponding pairs of side-drilled holes (SDH) at their extremities. Results on two fatigue cracks are also presented as more realistic scenarios, and show that, while PCI highlights diffuse reflections, selecting the right SA on PCI mainly exhibit specular reflections, and reduce the image quality owing to the fact that the number of active elements in the aperture is reduced. When dedicated PCI instruments are present, the proposed method should be included in the inspection workflow to obtain images. PCI may not only be a complementary solution to amplitude imaging, but indeed, a solution in its own right. Finally, as Vector Coherence Factor (VCF) imaging can be achieved using binary acquisitions, this method allows to obtain images very similar to those from TFM should one wish to use simplified instruments.