# Spot Weld Inspections Using Active Thermography
## 3 Results
### Inductive Thermography  
The five samples of the A-series are subjected to induction thermography measurements. The inspections are performed both in front and back heating, and for each heating method, three heating times are compared. The heating times can be found in Table 3.  
The thermal data captured during the experiments of sample series A, using induction heating are only processed using PCA since the FFT processing did not lead to useful images. Snapshots taken from the PCA on the data captured during front heating induction are shown in Figure 8.  
Besides front heating, the back of each sample is heated during three experiments. The induction times for each experiment are noted in Table 3. The thermal data are only processed using PCA since FFT did not result in useful images and can be seen in Figure 9.  
Since induction thermography is a fast technique and it results in qualitative images, only this technique is used for the inspection of the remaining four B samples. The optimal parameters found during the experiments of series A can be found in Table 7. These parameters are used in the inspection of the samples of the B series.In contrast to the processing of the A series, the thermal data of the B series are processed using FFT and PCA. Both techniques resulted in decent quality images for this series. The results of the FFT processing of the front heating of the samples can be seen in Figure 10, and the PCA results can be found in Figure 11. The processed images related to back heating the sample can be seen in Figures 12 and 13.  
Figure 7: Results of the PCA processing of the pulse thermography data. Experiment (**A**) is conducted using an amplitude of 8 and a pulse duration of 1 s. Experiment (**B**) is conducted using the same amplitude and a pulse duration of 3 s. There are no data for sample 5A since the processing did not result in useful information. Sample shows a similar result to previous results, namely, no closed boundary can be detected.  
Figure 8: Images taken from the principle component analysis performed on the thermal data captured using induction heating of the front side of the sample. Experiment (**A**) is performed using an induction time of 5 s, experiment (**B**) has an induction time of 10 s, and the sample was heated for 19 s in experiment (**C**).  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **Front Induction Heating** & **Back Induction Heating** \\ \hline
**Excitation time [s]** & 19 & 15 \\
**Distance coil—sample [m]** & 0.03 & 0.04 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Best parameters found during the measurements on the A-series samples. The samples of the B series are inspected using these parameters.  
Figure 9: Processed data images from experiments performed using back heating. During experiment (**A**) an induction time of 4 s is used, during experiment (**B**) this is 8 s, and experiment (**C**) is performed using 15 s of heating.  
Figure 11: Snapshots taken from the principle component analysis of the thermal data of front induction heating.  
Figure 12: FFT results of the B series samples heated from the back during 15 s. Row (**A**) shows the phase images and row (**B**) the amplitude images. The phase images are not sufficient enough to make a conclusion about the spot welds, but the amplitude images show a clear difference between the regions inside and outside the spot weld.  
Figure 10: Results from the FFT processing of the thermal data captured using induction heating from the front side of the B-series samples. Row (**A**) shows the phase images of the measurements using an excitation duration of 19 s, and row (**B**) visualizes the amplitude images.