# Infrared Thermography for Inline Monitoring of Glass/Epoxy under Impact and Quasi-Static Bending
## 2 Materials and Methods  
The material considered here is basically a glass fiber reinforced polymer (GFRP) including unidirectional E-glass fibers (300 g/m\({}^{2}\)) and low viscosity epoxy resin (MATES\({}^{\text{\tiny{(\textregistered}}}\) SX10, Italy). More specifically, eight epoxy impregnated E-glass plies are hand lay-up stacked at [0\({}^{\circ}\)2, 90\({}^{\circ}\)2]s to obtain an overall specimen thickness of 2.9 mm; curing is performed under pressure at ambient temperature. The main peculiarity of this GFRP material is to be translucent, allowing eyesight of imperfections for a direct comparison with results supplied by infrared thermography. The dimensions of the specimens are 100 mm \(\times\) 130 mm, or 30 mm \(\times\) 130 mm, depending on the type of test: impact, or quasi-static bending, respectively.  
Impact tests are carried out with a modified Charpy pendulum [20], which is equipped with a special fixture to lodge specimens and allow for the contact with the hammer from one side and optical view (by the infrared camera) from the other one. In particular, a small sample of the same material is positioned over one corner of the viewed window and is used as reference to correct the camera noise. The hammer has a hemispherical shaped nose, 12.7 mm in diameter; values of the impact energy \(E\) are set by suitably adjusting the falling height of the Charpy arm.  
Two infrared cameras are used, the SC6000 and the SC6800 (Flir systems), with images acquired at frame rate of 83 and 960 Hz, respectively. Sequences of thermal images are picked up during the impact event; or better, to allow for a complete visualization of the evolution of thermal effects with respect to the initial ambient temperature, the acquisition would start before the impact and last for some time after. Having two sequences at two different frame rates for each test is appropriate to analyze either the thermo-elastic phase (which is fast), or the plastic one (which is slower), by considering one sequence or the other. More specifically, it is possible to get information on the thermo-elastic phase by relating to images recorded at the higher speed during the first fractions of a second after the impact. Conversely, attention reverts to the sequence recorded at lower speed to analyze the slower evolving thermo-plastic phenomena, without unnecessary overloading of the computer memory. A supplementary video S1 (GFRP-12J) taken with the SC6800 camera of a specimen impacted at 11.7 J is supplied for the online version.  
Quasi-static bending tests are carried out with a standard three-point bending configuration set-up. The specimen is horizontally placed over two supports at a mutual distance of 50 mm and loaded downwards in the middle at a speed of 5 mm/min with a 5 mm radius loading nose. The infrared camera views the specimen at about 45\({}^{\circ}\), so to contemporaneously include both its bottom flat surface and its thickness (side view). Sequences of thermal images are recorded at 30 Hz with the handheld Flir systems T650 sc camera.  
The images are post-processed to extract information about the material thermal behavior. Post-processing is performed by using the ResearchIR software supplied with the infrared cameras and routines specifically developed in the Matlab environment. In particular, the two sets of image sequences acquired during the two types of tests refer to either a static object, or a deforming one, which require a different handling. In particular, from sequences acquired during impact tests, the influence of the ambient temperature is removed by subtracting an image taken at environmental conditions (before starting of impact tests) from all the subsequent images of the sequence; thus a sequence of \(\Delta T\) images is created. Specific post-processing is discussed in the next section.