Crack detection with wireless inductively-coupled transducers

In earlier work we developed inductive coupling for surface-mounted Lamb wave transducers operating at relatively low frequencies, such as 300 kHz. We now report on similar inductively-coupled transducers at higher (multiple MHz) frequencies. Our investigation was motivated by a particular application, to examine a box girder top flange, using transducers mounted along the flange edge. We employ a pair of transducers in pitch-catch mode, offset to create a diagonal path, and show that a shadow is detected when the path is intercepted by a through-thickness crack. We compare results obtained using conventionally wired transducers and using inductively-coupled transducers, showing that effective performance can be achieved with wireless (inductively-coupled) operation. Superior performance is obtained if plexiglas wedges are used to direct the beam along the diagonal path. Reflection from the crack is evident, as is the shadow effect along the direct diagonal path.