# Infrared Thermography for Inline Monitoring of Glass/Epoxy under Impact and Quasi-Static Bending
## 3 Results
### Quasi-Static Bending Tests  
Some thermal images taken during quasi-static bending tests are shown in Figure 6. It has to be pointed out that, in each image, the upper part (about 15% of the total coloured band height) gives the temperature across the specimen thickness, while, moving down, the temperature distribution on the bottom surface, viewed at 45\({}^{\circ}\), is represented (Figure 7). Each image is labelled with the time at which it is taken; time is counted starting with the load application. An attempt to synchronize the loading machine's startup with image recording was manually performed without triggering. However, synchronization can be assumed as fulfilled considering the frame rate of 30 Hz against the machine speed of 5 mm/min. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(E\) (J) & \(A_{\rm h}\) (mm\({}^{2}\)) & \(E\)/\(A_{\rm h}\) (J/mm\({}^{2}\)) & \(\Delta T_{\rm Min}\) (K) & \(\Delta T_{\rm Max}\) (K) & \(\Delta t\) (s) & \(\Delta t_{D}\) (s) & \(t_{p}\) (s) \\ \hline
2.8 & 206 & 0.0136 & \(-1.9\) & 9 & 0.021 & 0.007 & 0.007 \\
8.3 & 467 & 0.0178 & \(-2.1\) & 16 & 0.019 & 0.003 & 0.008 \\
11.7 & 580 & 0.0207 & \(-2.2\) & 17 & 0.019 & 0.003 & 0.008 \\
15.0 & 743 & 0.0202 & \(-1.8\) & 11 & 0.019 & 0.003 & 0.008 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Some data for varying impact energy. Figure 6: Some thermal images taken at several time instants during quasi-static bending. As can be seen, the first image (Figure 6a) gives an almost uniform temperature (ambient temperature), indicating that the specimen is still in its unbent condition. The darker central zone in the second image (Figure 6b), which shows a cooling-down, bears witness to the specimen deformation; this can also be ascertained by looking at the top border, which starts to appear curved with respect to that of the first image. Of course, the border's curvature increases with time as the pushing nose moves down; in the meantime, cooling down enlarges to practically include the entire viewed surface (Figure 6c,d). It has to be stressed that, since the loading machine speed is relatively low, all detected temperature variations have the time somehow to release and are weaker than the theoretical ones. As the critical curvature is reached, the material breakage initiates. The hot spot of Figure 6e indicates that breakage starts on top, at the left of the pushing nose, and, afterwards, another one appears at the specimen centre (Figure 6f). These hot spots appear on the top of the material thickness, while the specimen undergoes further bending (accounted for by local cooling down). Finally, the bottom surface is also followed by a strong warming-up. The temperature variations with time depict the material failure; in fact, under quasi-static bending, glass/epoxy generally breaks on the pushing side (top surface). The failure initiation is suddenly visualized, looking at the specimen thickness, while any temperature variation on the opposite side appears much later, driven by breakage propagation as well as the material thermal diffusivity. To better show this behaviour, a two-points plot is reported in Figure 8 with a key image replicated from Figure 6.  
Figure 8: A two-points temperature plot: the blue curve refers to a point over the top specimen surface; the red curve refers to a point over the bottom specimen surface. Figure 7: Sketch of quasi-static bending configuration and viewing frame. To establish a correspondence between temperature rise and material modifications, the force curve (black), measured by the loading machine, is reported in Figure 9 superimposed on the \(\Delta T_{\text{Max}}\) distribution (red) visualized by the infrared camera. In particular, the \(\Delta T_{\text{Max}}\) distribution is obtained by extracting the maximum temperature value in each image with respect to the ambient value of the first image. As can be seen, the change in slope of the force curve corresponds to a temperature variation.