# Infrared Thermography for Inline Monitoring of Glass/Epoxy under Impact and Quasi-Static Bending
## 3 Results  
### Impact Tests  
Some \(\Delta T\) images taken with the SC6800 camera (at the higher frame rate) of a specimen impacted at \(E=8.3\) J are shown in Figure 1. As can be seen, initially (before impact), the specimen surface is at almost uniform \(\Delta T=0\) (Figure 1a) while, suddenly at the impact, a dark zone appears, accounting for material bending (Figure 1b). Such a dark zone enlarges with time as the surface bending proceeds and gets dotted with lighter points (Figure 1c-e), which most probably indicates the presence of porosity. The skin over a pore thins with the increasing of the surface curvature and breaks once the thinning limit is reached; this happens first in two central dots (hot spots of Figure 1f) which are the mostly stressed and mark the begginning of the damage. Afterward, the dark zone enlarges, as also the two hot spots do, while another hot spot appears (Figure 1g). Later on, the dark zone starts to shrink while the area around the hot spots gets warmer (Figure 1h-k); finally, the dark zone is replaced by a warm zone (Figure 1l). To better display the above described effects, \(\Delta T\)-time plots in four key points A, B, C and D are shown in Figure 2 and, to highlight the fibers involved in the breakage, a \(\Delta T\) image taken with the SC6000 camera at higher spatial resolution (viewing a reduced area) is reported in Figure 3. It is possible to clearly see the fiber misalignment and the location of hot spots over one bundle of fibers, or at the junction of misaligned fibers. Figure 1: Some \(\Delta T\) images, taken at different time instants with the SC6800 camera, of a specimen impacted at \(E=8.3\) J.  
Figure 2: \(\Delta T\)-time plots in four points A, B, C and D of a specimen impacted at \(E=8.3\) J.  
Minima and maxima (resp. \(\Delta T_{\rm Min}\) and \(\Delta T_{\rm Max}\)) time plots are also extracted from \(\Delta T\) images recorded at the highest frame rate with the SC6800 camera and reported in Figure 4. By analysing these plots, more details about the material damaging under impact can be derived. In particular, \(\Delta T_{\rm Min}\) (negative) values describe a concavity, having a width \(\Delta t\) along the x axis and a peak (height, \(t_{\rm p}\)) along the y axis; \(t_{\rm p}\) identifies the instant the impactor reaches its strongest pushing force, or better the peak contact force [11; 21; 22]. Besides this, the appearance of the first hot spot (\(\Delta T>0\)) is generally concurrent with the formation of micro-cracks in the matrix, and this may occur before the surface reaches its maximum curvature (\(t_{\rm p}\)) for a given E value. The \(\Delta T_{\rm Max}\) peak value increases with \(E\). \(\Delta T_{\rm Max}\) and \(\Delta T_{\rm Min}\) are searched for in a region wide enough to include either the zone with cracks in progress or the zone which continues to bend leading to the \(\Delta T_{\rm Min}\) peak before the end of impact. Figure 3: A higher spatial resolution \(\Delta T\) image taken at \(t\) = 0.036 s with the SC6000 camera of a specimen impacted at \(E\) = 8.3 J. It is worth noting that a \(\Delta t_{\rm D}\) value, even a small one, is always present, as this investigation is focused on low velocity/energy impact; instead, at high velocity/energy with material breakage suddenly at impact, the heating phase may practically start almost with the cooling one, i.e., \(\Delta t_{\rm D}\approx 0\). As the impactor moves back, the surface tends to restraighten and, as it recovers its unbent configuration, any cooling effect disappears (\(\Delta T_{\rm Min}\)\(\rightarrow\)0). The two sides of the concavity--descent and ascent--are almost equal with respect to the \(t_{\rm p}\) value, with the ascent one slightly longer. By comparing \(\Delta T_{\rm Min}\) and \(\Delta T_{\rm Max}\) distributions for \(E=8.3\) J, it is possible to see a correspondence between \(\Delta t_{\rm D}\) and the knee in \(\Delta T_{\rm Min}\), as well the first \(\Delta T_{\rm Max}\) peak and \(t_{\rm p}\).