Characterization of vertical buried defects using lock-in vibrothermography: I. Direct problem

Abstract

The ability of lock-in vibrothermography to distinguish between different sizes, shapes and locations of vertical kissing defects, such as cracks or corrosion, is analyzed in this work both theoretical and experimentally. We have computed the oscillating, AC, surface temperature of samples containing inner modulated heat sources, representing the defect, and analyzed the sensitivity of this oscillating temperature to the defect geometric parameters. Moreover, we have prepared samples with calibrated vertical defects. Vibrothermographic data are usually recorded under non-steady conditions (i.e. while the surface temperature is still rising). As this transient temperature rise has a non-negligible component at the excitation frequency, the measured AC temperature is strongly distorted. By subtracting the Fourier component of the transient temperature rise from vibrothermographic data, we obtain the pure AC temperature (amplitude and phase) in very good agreement with the theoretical predictions.