Amplitude-modulated lock-in vibrothermography for NDE of polymers and composites

We present a nondestructive testing method based on lock-in thermography with mechanical heat excitation. Stresses are generated in the sample by vibrating it with a mechanical shaker. The mechanical energy is converted to thermal energy due to the acoustical damping. The defected regions have a stronger damping and also a stress concentration next to them, both of which result in a higher temperature generation. Because of the changes of the thermal properties, the defects also affect the heat conduction. These phenomena result in thermal anomalies due to the defects. The high-frequency vibration used for excitation is amplitude-modulated with a low frequency. The magnitude and phase of the sample temperature with respect to the modulation are measured with an infrared camera and a software lock-in technique. The use of phase information increases the reliability of the defect detection, and the application of high vibration frequencies results in a good thermal signal even at low stress levels, which helps to keep the test truly nondestructive. The suitability of the method was proved with samples of CFRP and aramid composites, and different polymers. The measurements included detection of impact damages, inclusions, voids, and cracks, and the evaluation of stress level distributions, paint thicknesses, and quality of bondings.