## 2 Materials and Methods
The position and the size of a rectangular area is chosen to include the hottest area, that is, the area where peak temperature occurs. The rectangular area is the same for all steps and the temperature is averaged in this area. Strictly, one pixel should be selected, but this choice is not suitable for two reasons: firstly, it is difficult to individuate exactly the pixel with \(\mathrm{T_{max}}\), and secondly it is not assured that the pixel is the same for all steps. Therefore, a rectangular area has been chosen in order to catch the phenomenon minimizing the scatter. This area as evident from the first cycles, especially as the stress amplitude increases, hosts the first damage and, most likely, the break. The identification of the hot zone is made by examining the heat map in a generic instant of timeframe and determining two opposite vertices of a rectangle that contains it. Figure 4 shows as an example the thermal maps during the last test steps for specimen A4. The non-uniform heating in the net section is due to the non-perfectly symmetric geometry of the specimen (manufacturing tolerances) and to the unavoidable alignment error in the specimen gripping. In Figure 5, the coordinates of the hot zone for the specimen AA02 are shown. It is interesting to note in this case that the specific layup of the specimen [0 90 90 0]\({}_{\mathrm{sd}}\) and the presence of the notch determines that the hot zone is not localized in the net section but in the zone where the 0\({}^{\circ}\) fibers are interrupted immediately before notch. This is a clear example of the complexity arising from the notch effect in composite, which is the result of not only the notch geometry but also and the local orientation of the  
Figure 3: Step test: indication of the load for each step for Panel 1 (**a**) and Panel 2 (**b**). fibres in the high stress zone. The individuation of the stress state that causesthe dissipative phenomena, experimentally measured by IR camera, cannot avoid the physical presence of fibre and matrix adopting a mesoscale modelling [23]. In this particular case, stress acting on 0\({}^{\circ}\) layers must be transferred to 90\({}^{\circ}\) layers through interlaminar shear stress, thus determining a local increase of the stress state, which will constitute the nucleation zone of the fatigue failure. The prediction of the fatigue failure propagation in composite materials is more complex than homogeneous material, since the crack propagation path is governed not only by stress state but also by laminae orientation. Although dissipative phenomena are originated inside the material, the thermal energy is quickly diffused due to the limited thickness and a surface temperature variation is observed. The correlation between the number of cycles and frames can be easily obtained from the acquisition frequency of the thermal camera, which was set at 1 Hz; therefore, 1 frame corresponds to 10 cycles. Parallel to the hot zone, an area of the specimen sufficiently far from the notch is analysed, which is called the cold zone. It is a region that does not resent the notch effect. This area is of fundamental importance for the purposes of the analysis, as it has been shown that the temperature trend of this area perfectly coincides with that of the analogue specimen without a notch [21]. This suggests a way to determine the fatigue notch coefficient. In fact, by examining the hot zone and the cold zone of the same notched specimen, it is possible to obtain the fatigue limit, as well as the fatigue limit of the corresponding smooth specimen, with evident savings in time and costs, having to analyse only one sample. The fatigue notch coefficient can be obtained very simply as the ratio of these two fatigue limits. Figure 4: Thermal maps during the last steps of the test for specimen A4. Figure 5: Identification of the hot zone (specimen AA02). The identification of the cold zone occurs in the same way as the hot zone (see Figure A4 in Appendix A for coordinate details).