In this paper we use Lamb waves to detect and characterize cracks in aluminum plates.
The detection of cracks is performed by a pitch catch method. Although Lamb waves antisymmetric A0 and symmetric S0 modes could be propagated in the plate, only S0 modes are launched by one angle beam probe, from one side of the plate, and detected on the other opposite side by a PZT disk sensor.
The two sensors are moved simultaneously on each side of the plate. The ultrasound beam always points to the PZT sensor. In this way a large portion of the plate is scanned.
If the ultrasonic beam is intersected by a crack, a sudden variation on the PZT signal is observed. In this case part of the original beam is transmitted and another part is reflected. The interaction of the original beam with the defect originates conversion of modes. The analysis of the mode content in the transmitted and in the reflected beam can be used as a tool to infer on the defect characteristics that caused such conversion between propagation modes.
In the experimental tests we used an aluminum plate with a thickness T=2 mm, and a central frequency of the emitted bursts equal to F0=350 kHz.
By using a fft transformation on time and on wavenumber domains it was possible to separate the beams components after interaction with plate cracks, orthogonal to the beam travel direction.
Wavenumber-frequency maps show a show a strong dependence of converted modes with cracks’ depth.