Mode conversion of the fundamental shear horizontal wave at a defect

It is essential to detect defects in plates, and ultrasonic guided waves offer the potential for detection of defects ranging from porosity in welds, through to larger corrosion patches. Shear-horizontal (SH) waves generated by electromagnetic acoustic transducers (EMATs) have previously been shown to interact with both types of defect, with the reflection properties dependent on the defect geometry and the wavelength and wavemode chosen. This paper investigates the behaviour of the mode converted reflections that occur when an incident SH0 wave interacts with a defect in a plate. The properties of the reflected mode converted fundamental symmetric Lamb wave, S0, are shown to depend on the defect width, length and depth, which determine the angle at which a peak reflection occurs. This paper aims to develop fundamental understanding of the behaviour, showing that the defect width reflection behaviour follows a modified single-slit-diffraction behaviour, while the defect length dependence is due to interference between waves reflected from the front and back faces of the defect. Finite element simulations are primarily used, and verified experimentally. The S0 reflection is small, but results could be combined with analysis of SH0 reflections, enabling a higher probability of detection for small (mm-sized) defects.