Defect Characterization from Lock-in Vibrothermography Data

Abstract

A method to characterize vertical cracks which combines lock-in vibrothermography experiments and a stabilized inversion algorithm is presented. The capability of the method to retrieve the shape and location of square cracks buried at increasing depths is analyzed by inverting synthetic data with added white noise. Moreover, the ability of the method to resolve two square buried cracks as a function of depth is studied. A relationship between the depth of the cracks and the minimum distance between them in order to obtain separate reconstructions is found. The predictions of the inversions with synthetic data have been checked by inverting experimental data obtained from steel samples containing calibrated cracks. In order to prevent the sample from being damaged and to reach the steady state faster, low ultrasound excitation powers are used. The signal-to-noise ratio corresponding to the resulting weak signal is improved by processing a large number of infrared images using the lock-in method. The experimental results confirm that the method is able to reconstruct square heat sources whose depth is twice their lateral dimension, and that the resolution condition holds.

Bayesian Inference for 3D Volumetric Heat Sources Reconstruction from Surfacic IR Imaging

“…Then, Figure 4 enables the obtained probability to be determined for three different signal-to-noise ratios: SNR = 10 (a), SNR = 50 (b), and SNR = 100 (c). One can note that the method is not so sensitive to the noise compared to literature methods [8,12,15]: for SNR = 50 and 100, the probability at the source position is superior to 0.95, while for SNR = 10, the probability is 0.30, which can be questionable in absolute terms but not in the case where the other source position probabilities are equal to 0.…”

Three-Dimensional Reconstruction of Thermal Volumetric Sources from Surface Temperature Fields Measured by Infrared Thermography

“…For the inversion, several models, including those defined by Equations (3), (4), (5) or (6), can be used. To compare the influence of each model along the z direction, three different configurations of heat sources are tested.…”

“…Then, the study has been extended on 3D problems; several pioneering teams have proposed methods based on thermal and acoustic coupling [2][3][4][5][6] or by introducing the concept of virtual waves [7,8]. These methods of reconstruction that exist today enable reconstructing volume data (sources, initial conditions, etc.)…”

The contribution to the modal analysis using an infrared camera

“…This phenomenon is used, for example, in detecting Luders bands [1], nondestructive material testing [2,3] or detection of plastic areas around stress concentrators. The temperature of the object is then determined after the use of calibration relationships, the relationship between the level of infrared radiation build up and the black body temperature.…”

“…Such a change is, however, at the infrared camera noise level, and its detection may require a specific procedure to increase the signal/noise ratio, e.g. using lock-in method [3,7] to determination of change of temperature. In the case of determination of temperatures it is important to know the emissivity of the tested objects [8].…”