Laser vibrometry of air-coupled Lamb waves

A new non-contact methodology combining air-coupled ultrasonic excitation and laser detection of the Lamb waves is developed and applied for advanced material characterization. The flexural modes are generated in a specimen at normal and slanted incidence of a focused air-coupled beam. A laser scanning vibrometer is used for dynamic visualization of the Lamb wave field. As the local flexuaral wave velocity depends on the thickness of a specimen, the methodology enables remote imaging and profiling of thickness at high accuracy. Direct imaging of the flexural wave-defect interaction is capable of visualizing the damaged areas in fibre-reinforced composites. The shape of the detected flexural waveforms is shown to reflect the in-plane material anisotropy. The experiments proved a high sensitivity of the technique in remote imaging and measuring of local fibre orientation in composite materials.