Lock-in thermography with mechanical loss angle heating at ultrasonic frequencies

In this paper the mechanical loss angle heating as an internal thermal wave source is described. Acoustic energy at high frequency (ultrasound) can be coupled into the sample. The high hysteresis loss in the defect region results in increased heating of the defect. When the intensity of the ultrasound is modulated at a low frequency the defect itself emits a thermal wave at this low frequency. The range of this thermal wave is large enough for detection at the surface with lock-in thermography tuned to the intensity modulation. This technique combining ultrasonic waves and lock-in thermography provides a fast imaging of imaginary part of Young's modulus in materials. Results obtained with this selective heating on various real components are presented.