## 2 Materials and Methods
Table 2 shows the dimensional characteristics and the theoretical notch coefficient \(\text{k}_{\text{t}}\), which is a measure of the severity of the notch itself. Figure 1: Specimens obtained from panel 1. Figure 2: Specimens obtained from panel 2. Fatigue tests were performed applying several increasing load steps on the same specimen [21]. During each step, the temperature trend has been recorded to reveal both stationary temperature variation for each load block and to monitor the crack rise and evolution. The specimen areas with damage accumulation are always characterized by dissipative phenomena clearly evident in temperature maps. To carry out the _stepped loading procedure_, the test characteristic parameters must be chosen to obtain a sufficient number of steps, which effectively describe the thermal response of the specimen, and single-step duration such as to avoid the accumulation of fatigue damage. In this procedure, there are basically two problems, namely choosing a value of the load increase between the steps, to obtain a sufficient number of steps that can appropriately describe the thermal response of the specimen and set an appropriate number of cycles for each step. The optimal duration of the test has two conflicting needs: on the one hand, it must be high enough to stabilize the thermal response and to obtain the limit temperature; on the other hand, it must be short enough to neglect the damage accumulation effect on the subsequent steps. The choice of the initial load level depends on the mechanical properties of the material and must be lower than the fatigue limit to obtain a sufficient number of steps (about eight/nine steps). Obviously, this value is not known a priori. There are several experimental formulas designed to determine the initial load level according to the material; the simplest one consists of choosing a starting level equal to a fraction of the ultimate tensile strength with increments per step equal to \(5\div 10\%\) of the same. A stress ratio equal to R = 0.1 was imposed. Then, an appropriate loading sequence was chosen, making sure that the increase in amplitude \(\sigma_{\mathrm{a}}\) guaranteed an adequate number of steps. The load blocks sequence, whose choice was derived from the static tensile stress [20,22], is shown in Figure 3, for panel 1 and 2. The duration of every single step has been set at 5000 cycles, based on previous experience. This compromise value avoids the accumulation of damage on the subsequent steps while still allowing the extrapolation of the stabilization temperature of the specimen. As regards the load frequency, the 10 Hz value was chosen to make heat exchange more evident. Fatigue tests were carried out using a universal servo-hydraulic testing machine (MTS 810) with a 100 kN load cell. Monitoring the tests employing a thermal camera, it is possible to create a thermal map of the specimen throughout the test. An infrared thermal imaging FLIR camera, model SC 7500M, was used to acquire the thermal images. The thermal camera was equipped with a Focal Plane Array sensor, cooled using a Stirling cycle, having a NETD resolution of 25 mK; the detector acquires 320 \(\times\) 256 pixels, being able to reach an image acquisition frequency (frame rate) of 383 Hz (see Figure A3 in Appendix A for set-up details). \begin{table}
\begin{tabular}{c c c c c} \hline
**Specimen** & **h [mm]** & **b [mm]** & **Net Section [mm2]** & \(\mathbf{k_{t}}\) \\ \hline _AA01_ & _11.004_ & _2.253_ & _24.79_ & _2.41_ \\ \hline _AA02_ & _9.103_ & _2.211_ & _20.13_ & _1.71_ \\ \hline _AA03_ & _10.117_ & _2.197_ & _22.23_ & _1.46_ \\ \hline _A4_ & _14.271_ & _4.025_ & _57.44_ & _2.35_ \\ \hline _A11_ & _13.296_ & _4.126_ & _54.86_ & _1.48_ \\ \hline \end{tabular}
\end{table} 
Table 2: Dimensional characteristics of the specimens. During the test, there will be an area, called the hot zone, in which an increase in temperature will develop, easily identifiable in the case of a notched specimen and coincident with the root of the notch.