26 G. Corvec et al. Fig. 4.7 Detection of thermal activity of imprint during a Brasilian test equal to room temperature during the test. The thermal emissivity of the material is close to 1 (>0.84 for soda lime glass). The spatial resolution was such that the size of the pixel was equal to 0.015 mm, which is the size of the IR detectors. In order to ensure that the internal temperature of the camera was stabilized before performing the measurements, it was switched on 4 h before the experiment. Then, the Non Uniformity Correction (NUC) was performed by using the manufacturer’s protocole. Once the temperature field at the specimen surface was stabilized, the temperature measurement was started. 4.4.2 Experimental Results Figure 4.7 presents the superposition of temperature variation field obtained for the maximum of the cyclic force applied to the ring with the STFFT filter during first step. Thermal activity was detected along the chips border, not at the tip of the radial cracks. The fact that no thermal activity was detected at the tip of the radial cracks does not mean that no stress variation occurred in these areas. Indeed, temperature variations are due to the variations of the sum of the principal Cauchy stresses, this is the basis of TSA (thermoelastic stress analysis [1]). So, if the mechanical loading induces a variation in the sum of the principal Cauchy stresses at the tip of the radial cracks, temperature variation will be detected. If the Cauchy stress tensor is mainly composed by a deviatoric part, a stress variation exists but does not induce temperature variation. In the present case, the only conclusion is that there is a variation in the sum of the principal Cauchy stresses at the chip’s borders, not at the tip of the radial cracks. 4.5 Conclusion In this study, a denoising methodology has been proposed to improve the spatial resolution of infrared images. The methodology is composed of two steps which consist first in removing the noise of the temporal signal at each pixel and second in shifting the residual offset value, which is different from one pixel to another. Results show that the approach is effective at preserving the spatial resolution of infrared images equal to 1 pixel, contrarily to the traditional spatio-temporal approach. By applying this denoising methodology to an imprint zone at the surface of a soda lime glass submitted to mechanical loading, results showed that thermal activity was detected in the chip zones. Negative and positive temperature variations were measured simultaneously, meaning that at a given time during the mechanical test, chips are submitted to very different loading conditions. The results obtained with this denoising methodology are promising and opennew ways of
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