# Infrared Thermography for Inline Monitoring of Glass/Epoxy under Impact and Quasi-Static Bending
## 3 Results
### Impact Tests
At the lowest 2.8 J impact energy, \(\Delta t_{\rm D}\) is almost equal to \(t_{\rm p}\). Generally, the first \(\Delta T_{\rm Max}\) peak lies in correspondence with the \(\Delta T_{\rm Min}\) peak, while secondary \(\Delta T_{\rm Max}\) peaks occur later when \(\Delta T_{\rm Min}\) is in its ascent phase. The first \(\Delta T_{\rm Max}\) peak is surely to be ascribed to breakage of the material when it is under tensile stress, while secondary peaks may also be caused by frictions at the broken borders when the surface tends to recover its undeformed shape. The \(\Delta T\) images recorded at the lower frame rate with the SC6000 camera are exploited for the evaluation of the warm area extension, which also bears witness to the extension of the delaminated area. The warm area is measured by applying the noise correction reference (NCR) method, which roughly consists in comparing a \(\Delta T\) image taken after impact with one taken before. Actually, the method is more complex, requiring correction (\(\Delta T_{\rm C}\)) for temporal noise. In particular, the \(\Delta T_{\rm CR}\) image (before impact) is the average over 100 \(\Delta T_{\rm C}\) images and their standard deviation \(\sigma\) is evaluated, while \(\Delta T_{\rm CW}\) (after impact) is the average over \(\Delta T_{\rm C}\) images collected in 0.602 s, by discarding images with cold (below ambient) zones. Then, a \(\Delta T_{\rm CW}\) image is transformed into a black/white binary image in which any pixel can assume a value equal to either 0, or 1, i.e., whether it is cold (black), or warm (white), respectively, according to the following rule:  
\[\left\{ \begin{array}{ll}\Delta T_{\rm CW}(i,j)>\Delta T_{\rm CR}(i,j)+3 \sigma({\rm i},{\rm j}){\rm pixel}=1\\ \Delta T_{\rm CW}(i,j)\leq\Delta T_{\rm CR}(i,j)+3\sigma({\rm i},{\rm j}){ \rm pixel}=0\end{array} \right.. \tag{1}\]  
More information can be found in ref [23]. The warm area perimeter is also extracted with the _buperim_ Matlab function and superimposed on a \(\Delta T_{\rm CW}\) image. An example of a contoured \(\Delta T_{\rm CW}\) image is shown in Figure 5 with also a photo of the impacted specimen. Epoxy resin being translucent, it is possible to see as the impact damage footprint practically matches the contoured warm area. Once contoured, the warm area \(A_{\rm h}\) is obtained by counting the enclosed number of hot pixels \({\rm P_{h}}\) and considering the camera spatial resolution \(s_{\rm r}\) through the relationship:  
\[A_{\rm h}=P_{h}s_{\rm r}^{2}. \tag{2}\]  
Figure 5: Comparison between contoured image (**a**) and photo of the rear to impact specimen surface (**b**) for \(E=8.3\) J. \(A_{\rm h}\) values are collected in Table 1 with the absolute maximum values of \(\Delta T_{\rm Max}\) and \(\Delta T_{\rm Min}\) and the above defined parameters. As can be seen, excluding the value for \(E\) = 2.8 J, the ratio between the impact energy and \(A_{\rm h}\) assumes an almost constant value. The \(A_{\rm h}\) value of the specimen impacted at the lowest energy may appear as excessive, but it can be explained considering that the extension of delamination depends mostly on the conditions of the impacted surface and not only on the impact energy. Of great importance is the presence of defects in the material such as fiber misalignement, poor, or rich, resin zones, pores, etc. which may affect the interplay cohesive force. Therefore, the presence of defects may entail also differences in \(\Delta T_{\rm Max}\) values leading, for example, to a lower value in response to a higher \(E\) value since the impact energy, which is dissipated as internal energy, and is spent in the formation of matrix microcracks, delamination and rupture of fibres. As a general indication, light delamination is accompanied by small temperature rise, in general a \(\Delta T\) not greater than 2 K; higher \(\Delta T\) values are indicative of more important damage.