# Spot Weld Inspections Using Active Thermography
## 3 Results
### Pulse Thermography Using Light Heating  
The second light heating technique investigated in this manuscript is pulse heating. The measurement setup is visualized in Figure 2 and the parameters corresponding to 'L1', 'L2' and 'L3' can be found in Table 2. In order to investigate the influence of the pulse duration, each sample is subjected to experiments using different parameter sets. The amplitude is constant with a value of eight, which is similar to the measurements  
Figure 4: Snapshots taken from fast Fourier analysis for the different samples during three measurements. Experiment (**A**) is performed using an excitation frequency of 0.05 Hz and a Fourier frequency of 0.025 Hz, experiment (**B**) uses an excitation frequency of 0.05 Hz and a Fourier frequency of 0.05 Hz, and during experiment (**C**), the samples are excited using a frequency of 0.4 Hz and analyzed using a Fourier frequency of 0.2 Hz. Only the most interesting images are shown out of the large data sets.  
Figure 5: Images taken from the principle component analysis of the five samples during two experiments. The excitation frequency of experiment (**A**) is 0.05 Hz, and the frequency of (**B**) is 0.4 Hz.  
using lock-in thermography. The parameter sets used during the pulse thermography measurements can be found in Table 6.  
Figure 6 visualizes the results of the FFT analyses of the pulse thermography inspections. Similar results can be observed between experiments using pulse durations of 1 and 3 s for samples 1A, 2A and 3A; however, a longer pulse time leads to better results regarding samples 4A and 5A. Both samples 1A and 2A show a clear visible circle around the spot weld, and a different thermal behaviour between the region inside and outside the weld can be distinguished. The black regions inside the spot weld of sample 1A show a distinct phase shift, while in sample 2A, this phase shift is not visible. Sample 3A has a visible circular shape as well, but there is no phase difference between the region inside the weld and the region around it. This might mean that the weld is not optimal since no change in metallic internal structure has occurred. Similar to the lock-in measurements, sample 4A does not show a contour of the spot weld. In the images of sample 5A, a difference can be seen between the inside and outside of the spot weld, but there is no clear boundary visible. Besides the FFT analyses, principle component analyses were performed on the pulse thermography data. The results of this processing can be seen in Figure 7.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Parameter Set 1** & **Parameter Set 2** \\ \hline
**Amplitude** & 80\% & 80\% \\
**Pulse duration [s]** & 1 & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Parameter combinations used during the pulse thermography measurements. The value of the amplitude is kept constant at 8 in order to ensure an adequate heating intensity.  
Figure 6: Snapshots taken from the fast Fourier transform analysis performed on pulse thermography data. Experiment (**A**) is performed using a pulse duration of 1s and a Fourier frequency of 0.038 Hz. Experiment (**B**) has a pulse duration of 1 s as well and is analysed using a Fourier frequency of 0.076 Hz. A pulse duration of 3 s is used for experiments (**C**,**D**), and the used Fourier frequencies are 0.035 Hz and 0.142 Hz.