# Infrared Thermography for Inline Monitoring of Glass/Epoxy under Impact and Quasi-Static Bending
## 4 Discussion and Concluding Remarks  
This paper was concerned with the damaging of a glass/epoxy composite under mechanical stresses induced in two different ways: abrupt shot, and quasi-static bending. For both cases, owing to the appearance and successive growing of hot spots, it is possible to visualize inline initiation and the  
Figure 10: Photos of a specimen which failed under quasi-static bending; (**a**) top surface; (**b**) bottom surface. Figure 9: A comparison between force distribution (black) and maximum surface temperature (\(\Delta T_{\text{Max}}\)) distribution (red). propagation of damage. In fact, any form of damage, whether cracks with subsequent spreading or fiber breakage, is accompanied by heat release and temperature rise. Conversely, the temperature decreases when stretching the polymer network (thermo-elastic effect). In particular, the cooled zone accounts for the overall affected area; this is mostly evident in the presence of an impact event. The objective of getting information about the material behavior from thermal signatures is successful if, amongst others, the most appropriate infrared imaging device is chosen. In fact, thermal signatures coupled with impact events, even if occurring at low energy/velocity, rapidly evolve (especially cooling effects) and can be captured only with high frame rate and high sensitivity cameras. Instead, the evolution of surface temperatures associated with the gradual quasi-static bending is slow enough to be well visualized with a cheaper camera. As a common aspect, under both types of load, damaging initiates with the formation of cracks, which reveal themselves through the formation of hot spots. Then, following the time evolution of such hot spots, it is possible to get information on the subsequent damage propagation. However, there are some differences between the temperature-time evolution coupled with each of the two types of test. In fact, the impact is accompanied by a sudden appearance of hot spots in the opposite to the impacted one, meaning that the damage starts on the visualized surface. Conversely, quasi-static bending entails the initial appearance of a hot spot in the loaded specimen surface, which much later is followed by hot spots on the bottom opposite surface; this means that the damage initiates on the front surface in direct contact with the loading nose and then propagates through the entire specimen thickness. In other words, the above described differences in the temperature-time evolution well comply with the different damaging ways of glass/epoxy. The impact causes an abrupt deformation with a concavity on the impacted side and a protrusion on the other one; the damage (cracks and/or breakage) initiates on the latter side, which is directly viewed by the infrared camera. At low energy impact, without perforation, once the impact ceases and the impactor moves away, the surface recovers its original unbent configuration. Therefore, through surface temperature variations, the damage initiation is sudden visualized as well its subsequent propagation in plane. The propagation in depth may appear as delayed owing to thermal diffusion through the material thickness; of course, this also has to be accounted for with the decrease of thermal diffusivity induced by the damage. Post-processing of the sequences of images and in-depth analysis of the obtained results allows us to understand more of the material behavior under load and to get quantitative information that can contribute to assess the material performance. In fact, it is possible to evaluate the damage extension to a given impact energy from the warm area (\(A_{\text{h}}\)), the peak contact force from the \(\Delta T_{\text{Min}}\) peak (\(t_{\text{p}}\) ), the duration of the material deformation (\(\Delta t\)), and the importance of the damage given by \(\Delta T_{\text{Max}}\) and \(\Delta t_{\text{D}}\) values (Table 1). In addition, the overall impact on the affected area might be evaluated from the extension of the cooled area.