Modified differential absolute contrast using thermal quadrupoles for the nondestructive testing of finite thickness specimens by infrared thermography

Infrared thermography is a nondestructive evaluation technique in which the specimen surface is thermally stimulated to produce a temperature difference between "sound" (free of defects) areas and eventual defective regions. It is well known that the thermographic methods based on the thermal contrast are strongly affected by non-uniform heating at the surface. Hence, thermal contrast-based results considerably depend on the chosen reference point. The differential absolute contrast (DAC) method was developed to eliminate the need of determining a reference point by defining the thermal contrast with respect to an "ideal" sound area. The DAC technique is based on the ID solution of the Fourier diffusion equation for homogeneous and semi-infinite materials stimulated with a Dirac heat pulse. Although very useful at early times, this assumption considerably decreases DAC accuracy when the heat front approaches the sample rear face. We propose a modified DAC version by explicitly introducing the sample thickness using the thermal quadrupoles theory. We demonstrate that taking into account the sample thickness, the DAC validity range considerably extends for long times after excitation while preserving its performance for short times.