Issue 43

N. Montinaro et alii, Frattura ed Integrità Strutturale, 43 (2018) 231-240; DOI: 10.3221/IGF-ESIS.43.18 237 Figure 8 : Plots of MT values computed over the five ROI versus the ROI position along face C on sample 2; in (a) ROI 4 and 5, in (b) ROI 1, 2 and 3. From the results of this simulation, it is first observed that the ideal position of the ROI is the nearest possible to the laser spot (see Fig. 6), on the side of the laser trail (i.e. behind the advancing laser direction), where the temperature field influenced by the defect stretches over a wider area. In order to increase defect sensitivity, the ROI area should be preferably shaped as to include the direction of highest temperature gradients. In this case, as shown by the temperature map of Fig. 6 the temperature highest gradient is located very close to the heat source. This is confirmed by the plot of Fig. 8 where the ROIs that are closer to the heat source present an enhanced peak-to-peak value. In particular, ROI 5, the closest one, seems to be the best choice to enhance defect sensitivity. It is worth noting that the defect signature of the defect 1/2 has a different shape and this is due to the fact that it is superficial and not embedded in the part. E XPERIMENTAL RESULTS aking into account the results of the FEA, the experiments on the two samples have been performed using a ROI equal to the ones modelled as ROI 5. The chosen ROI for the experiment is a 0.5 x 0.5 mm 2 positioned very close to the heat source, where the temperature gradient is the highest. In order to enhance the thermal resolution of the IR camera, the value of the Integration Time set in the present analysis is rather high (3000  s). This choice determines a reduced maximum temperature at which the received radiation saturates the detector. In the present case, the IR camera has such limit at a rather low temperature of about 40 °C. This determines a rather wide zone around the laser spot where the thermal signal is saturated, and then useless for the analysis. Hence, the choice of the best integration time is a trade-off between the need to obtain a high thermal resolution, and the possibility to place the ROI as close as possible to the laser spot. In Fig. 9 is shown an acquired thermogram where the picked ROI is defined. Figure 9 : Thermogram acquired on sample 2 with characterization of the region of interest. 276 278 280 282 284 286 288 290 0 5 10 15 20 25 30 35 40 45 MT [K] Distance [mm] MT MT_ROI_4 MT_ROI_5 288 290 292 294 296 298 300 0 5 10 15 20 25 30 35 40 45 MT [K] Distance [mm] MT MT_ROI_1 MT_ROI_2 MT_ROI_3 Defect 1/2 Defect 2/2 Defect 3/2 Defect 1/2 Defect 2/2 Defect 3/2 T (a) (b)