Phase inversion for accurate extraction of the harmonic thermal response in active infrared thermographic NDT

Infrared thermography is an efficient non-destructive testing technique in which heat waves are actively stimulated in the test piece by an external device, and an infrared camera measures the evolution of the surface temperature for revealing hidden defects as localized anomalies. The excitation amplitude is often sinusoidally modulated for a few cycles to obtain an improved detectability and to tune the detection depth. However, the commonly used excitation sources are heating-only (i.e. mono-polar) and thus constantly heat up the sample during the excitation, which leads to an undesired ascending trend in the measured thermal signal. This paper experimentally demonstrates the concept of phase inversion in infrared thermography as a physics-based technique for filtering this ascending trend (i.e. DC component) and extracting the desired harmonic (i.e. AC) component of thermal signal. The technique is validated for optical infrared thermography and low-power vibrothermography of a carbon fiber reinforced polymer aerospace component with impact damages