Lockin Thermography with Optical or Ultrasound Excitation

Abstract

Thermography is a well established non-destructive testing (NDT) technique providing images of temperature distributions. If the temperature field on a sample surface is modulated by periodical injection of heat either from outside or inside the sample, the time dependence of the temperature field provides information on thermal features hidden underneath the surface. Using the Fourier-transform, this information is finally compressed into a phase and an amplitude image. Phase images are more robust than amplitude images, because surface features and reflections are effectively suppressed. With excitation by ultrasound, the heating mechanism is local conversion of elastic energy into heat which occurs preferably due to local friction losses caused e.g. by the relative motion of boundaries in a crack. As intact material and boundaries are suppressed in such an image, it displays defects selectively. The techniques and their applications are illustrated by examples that were obtained on various industry-relevant components.