Thermographic nondestructive evaluation: Data inversion procedures

In Part I of this paper, a one-dimensional procedure was described for nondestructive thermographic evaluation. In Part II, a two-dimensional approach is presented to retrieve the subsurface defect depth, lateral size, and thermal resistance from the time-variable surface temperature distribution as recorded by a thermographic camera. The algorithm performance is tested on experimental data obtained under transient lateral-scanning surface heating conditions on a variety of artificial defects including bottom-open cavities of different geometries as well as Teflon™ implants in CFRP plates. In general, the defect depth and lateral size are inferred with satisfactory accuracy. The thermal resistance inferred through our inversion routine in the artificial delamination case was always higher than the incremental resistance due to the implants, suggesting the presence of resistive interfaces between the inclusions and the host material. Assuming a constant relative contrast error, the precision on the defect thermal resistance decreases as the defect resistance and depth/lateral size ratio increase.