Implementation of frequency-modulated thermal wave imaging for non-destructive sub-surface defect detection

Abstract

Thermal non-destructive testing (TNDT) is a whole field and non-contact technique for defect detection. The present work describes a variant of TNDT for subsurface defect detection based on frequency modulated thermal wave imaging (FMTWI). Use is made of the frequency dependence of thermal diffusion length, to achieve entire depth scanning of a sample in one run. This novel technique overcomes some of the drawbacks associated with traditional pulse and lock-in thermography. Experimental results are presented in support.

Comparative Data Processing Approaches for Thermal Wave Imaging Techniques for Non-Destructive Testing

“…Due to its mono frequency excitation, the generated thermal wavelength inside the test sample is fixed, leading to a fixed depth resolution. Therefore, in order to get good resolution for various defects located at different depths inside the test specimen, it is necessary to repeat LT with different excitation frequencies [22][23][24][25][26][27][28][29].…”

“…Theoretically, the short duration excitation pulse in PPT does launch a large number of frequency components into the test samples, but the higher order frequency components may not have sufficient energy to cause a thermal wave to propagate deep into the sample. In order to detect deeper subsurface defects in test sample, PT and PPT require high peak power heat sources [25][26][27][28], which may damage the surface of the test sample.…”

“…In light of the above described limitations of the widely used conventional methods, frequency modulated thermal wave imaging methods [22,26,27] have been recently introduced in order to improve depth resolution of detecting defects lying at different depths, in less time compared to LT and with lower peak power heat sources compared to pulsed thermographic methods.…”

“…This helps to inject the desired band of frequencies with significant magnitude into the test sample which improves the test resolution [24,26,28].…”

“…In contrast to FMTWI, modulation of the heat sources is much easier in digitized frequency modulated thermal wave imaging (DFMTWI) [27]. Furthermore, in DFMTWI we can inject more energy into the sample by probing higher frequency harmonics along with the desired band of frequencies which may improve the depth resolution for near surface defects [22][23][24][25][26][27].…”