Issue 50

A. Kostina et alii, Frattura ed Integrità Strutturale, 50 (2019) 667-683; DOI: 10.3221/IGF-ESIS.50.57 674 The smallest defect (size of 2 mm) in the first column has been chosen as the representative of the worst case for the observations. Thus, further investigations will be carried out for it. The heating duration was assumed to be 4 ms due to the acceptable value of the obtained temperature contrast. Fig. 9 (a) illustrates the effect of the coating thickness on the temperature contrast for this defect. It can be seen that the maximum value of the temperature contrast is a non-linear function of the coating thickness while the peak contrast time is nearly linearly dependent on the thickness of the ceramic layer (Fig. 9 (b)). It is also worth noting that the temperature contrast becomes almost insensitive to the coating thickness of more than 1 mm. In general, the obtained results show that pulsed thermography can be successfully applied to the defect identification in ceramic coatings because typical values of the thickness lies in the range from 0.1 mm to 0.6 mm [14]. For these values of the thickness, the maximum temperature contrast is reasonably high and is reached for the short period of time. For values of more than 1 mm the contrast is poor and the defect can be lost in noise. (a) (b) Figure 8 : Evolution of the temperature contrast for defects of the smallest thickness (0.14 mm) obtained for the square pulse of the following duration: (a) t =1 ms, (b) t =4 ms. (a) (b) Figure 9 : (a) Effect of the coating thickness on temperature contrast. (b) Effect of coating thickness on peak contrast time. Fig. 10 illustrates the effect of the shape of the heating pulse on the value of C . The reason for the difference arises from the fact that the heating source can gain the maximum power gradually and cool down in the same manner. In the limiting case, this shape tends to be triangular. A comparative study of three shapes of heating pulses (square pulse without smoothing, square pulse with the smoothed descending branch and completely smoothed pulse close to the triangle) with the same duration of the heating by the maximum power (which is equal to 4 s) demonstrates that the difference between the obtained results is considerable. The temperature contrast obtained by the smoothed pulse is three times higher than the contrast achieved by the heating with the non-smoothed pulse. Hence, accurate description of the temperature contrast requires specific information on the inertia of the heating source.