The results of an experimental study conducted to detect subsurface defects in a thick Gr/PPS composite test sample using a non-contact ultrasonic system are presented. Surface waves are generated by a pulsed laser and detected by a air-coupled capacitance transducer. By controlling the surface wave wavelength through a shadow mask, it is possible to control surface wave penetration depth in the sample. Surface wave peak-to-peak amplitude is related to the near-surface material condition. Results indicate that signal amplitude decreases as the width of the defect increases and an approximately linear relation can be deduced.
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