# Depth Retrieval Procedures in Pulsed Thermography: Remarks in Time and Frequency Domain Analyses
## 3 Results and Discussion
### Depth Retrieval with Thermal-Contrast Peak Slope Time
For instance, while Series A defects had the same peak contrast time, Series B and C defects produced a peak time range of approximately 0.1 s, and Series D and E produced a temporal range of approximately 0.37 s and 0.15 s, respectively. Contrarily, these time differences were remarkably reduced by studying the correspondent contrast slope variations. For instance, Series C defects produced a peak slope time range from 0.40 s to 0.48 s, while the ranges for Series D and E defects were from 0.66 s to 0.76 s and from 0.91 s to 0.99 s, respectively. Additionally, comparing the plots retrieved from each individual series investigation, it can  
Figure 5: Temporal evolution of: (**a**) the thermal contrast curves; and (**b**) the correspondent slope of the thermal contrasts for the detected insertions on the trapezoid surface located to the depth of 0.4 mm (Series B). be seen that the high noise levels produced for increasing the inspection depth, did not have an impact on the determination of the two characteristic time points. Moreover, for the deeper defects no clear observations can be done regarding the correlation of square size and the 3D conduction phenomenon around it. This can be explained by the fact that a reduced contrast was produced mainly for the defects on Series D and E (Figure 3) [40], indicating that the thermal wave did not penetrate efficiently enough to these locations and a weak thermal effect was produced from these heat flow barriers. As a result, the reduced amount of heat reached these areas, was not able to produce sufficient lateral heat diffusion around these defects. The above observation was mainly detected on defect Series D and E, as for instance the smaller detected defects D3 and E2 presented a slightly greater peak contrast time value with respect to the time of the largest defects D5 and E5. Figure 6: Temporal evolution of: (**a**) the thermal contrast curves; and (**b**) the correspondent slope of the thermal contrasts for the detected insertions on the trapezoid surface located to the depth of 0.6 mm (Series C). Figure 7: Temporal evolution of: (**a**) the thermal contrast curves; and (**b**) the correspondent slope of the thermal contrasts for the detected insertions on the trapezoid surface located to the depth of 0.8 mm (Series D). Through the above discussed procedure, the depths of all the detected artificial defects into the CFRP specimen were quantitatively estimated, correlating the measured peak slope time with the depth. This method for depth calculations was selected, as the peak contrast time was proven to be seriously affected by the lateral dimensions of the detected features, whereas peak slope time appeared to remain more constant regardless these size variations. Table 1 summarises the experimental values of peak slopes and peak slope times along with the produced depth results for all the detected defects. In general, peak slope time provided an efficient estimation of the Teflon depths; however, in some instances, increased errors were produced. From the summary of the depth results, it can be seen that the more accurate results were produced for the deeper insertions (despite the noise exhibited in this case) and the larger errors were observed on the defects Series B and C. A possible explanation about these large deviations from the actual depth on Series B and C defects, can be attributed to the fact that these inserts were more strongly affected from the 3D heat conduction, contrary to the shallowest and deepest insertions which seem to be unaffected from the above phenomenon for different reasons as discussed above. On the other hand, a factor that may produce deviations between the measured and actual depth could be the uncertainty on the thermal properties which might contribute to error as well. In particular, the actual value of the thermal diffusivity might differ in anisotropic materials from the value used in this study.