Quantification and fusion of thermography and ultrasonic non-destructive testing results for composites with low-velocity impact damage

This study presents a comparative evaluation of four infrared thermography (IRT) techniques, namely step-heating, C-CheckIR, flash, and lock-in thermography, as well as ultrasonic C-scan for the detection and quantification of low-velocity impact damage (LVID) in thin laminated composites. The assessment relied on the quantitative hit/miss metric analysis, along with the dependency of thermal response on impact energy and impactor tip geometry under 20 different scenarios. The qualitative and quantitative analyses highlighted C-CheckIR and lock-in thermography were effective partially but illustrated inconsistent performance across most LVID scenarios. Flash thermography, in contrast, was identified as the most reliable thermography technique, offering consistent detection capability across a wide range of impact scenarios. Its superior performance is attributed to higher sensitivity to energy level and impactor tip size, as well as consistently lower error magnitudes in the hit/miss quantities, including relative area error, detected and missed area fractions. Furthermore, a dual-tree complex wavelet transform (DTCWT)-based algorithm was developed to fuse flash thermography and ultrasonic C-scan results, enabling complementary visualization of surface cracks and subsurface delamination. The fusion framework improved the interpretability of damage patterns, reduced false interpretations, and provided more comprehensive assessment of impact damage compared to individual non-destructive testing techniques.