Ultrasound-lockin-thermography NDE of composite plates with low power actuators. Experimental investigation of the influence of the Lamb wave frequency

Ultrasonic vibrothermography, also called ultrasound-thermography or thermosonics, is a nondestructive technique that is based on the application of a modulated mechanical stress on the tested structure, while an infrared camera is mapping the surface temperature. Regions of imperfection convert energy to heat through enhanced viscoelastic dissipation, collisions and/or rubbing of internal free surfaces present in delaminations and cracks. Surface defects and internal defects appear hotter when the surface temperature is mapped. In the case of plate-like structures, Lamb waves are interesting as they can propagate over long distances. The objective was to evaluate if ultrasound-thermography could be successfully applied by means of small piezoelectric transducers clamped or stuck to the tested piece. The ultrasound frequency ranged between some tens of kHz to 200 kHz. The amplitude was modulated at very low frequency (some tens of mHz) and lock-in thermography was performed in order to increase the thermal signal to noise ratio. The influence of both frequencies on the thermal contrast induced by impact defects was analyzed. A very high sensitivity to the ultrasound frequency was observed. Ultrasound-thermography can be applied with success with low power actuators provided the ultrasound frequency is properly selected.