Novel Correlation Based Analysis for Sub-Surface Sensing and Imaging Using Active Infrared Thermography

In the domain of thermal non-destructive testing and evaluation (TNDT&E), active thermography has emerged as a preferred technique due to its capability for rapid, large-area, and non-contact inspection without compromising material integrity. Among active methods, pulsed thermography (PT) and lock-in thermography (LT) are widely used, while frequency-modulated thermal wave imaging (FMTWI) offers superior defect detectability and depth resolution through pulse compression. This study presents an experimental evaluation on a hardened steel specimen, commonly used in the shipbuilding industry, containing a flat-bottom hole defect, using PT, LT, and FMTWI. The defect detection potential of FMTWI is further assessed through three post-processing strategies: frequency domain phase (FDP), time domain phase (TDP), and cross-correlation coefficient (CCC). Results demonstrate that the FMTWI, CCC combination yields the most reliable and high-contrast defect visualization, outperforming other approaches in both clarity and depth sensitivity. The study emphasizes the critical role of optimizing excitation-processing pairs to enhance the performance of thermal NDT&E techniques.