## 3 Results and Discussion  
The main results of the first phase tests, the monotonic fatigue tests and the specimens' breaks, are shown in Figure 5 which gives the Wohler curve. The fatigue limit was defined as \(\Delta\sigma=40.5\) MPa (180-1800 N) as all the analysed variables were stabilized after \(2\times 10^{6}\) cycles. Figure 6 shows the evolution of E\({}_{\rm T}\) as a function of the number of cycles for the different stress ranges. Figure 4: Explanatory scheme for the determination of individual net damage (D\({}_{\rm NI}\)). Figure 5: Wöhler curve obtained from monotonic fatigue tests. Figure 7 shows that in the thermal fatigue processes, with higher stresses, \(\rm E_{T}\) has a clear increasing trend, while in terms of mechanical fatigue, the energy parameters tend to stabilize or even decrease in some cases. Figure 8 shows the evolution of total damage for the different stress levels. Figure 6: Total energy evolution as function of the number of cycles and the stress range or the monotonic fatigue tests. Figure 7: Evolution of total damage for the monotonic fatigue tests. It can be seen that for the fatigue limit, \(\Delta\sigma\) = 40.5 MPa, both \(\rm E_{T}\) and \(\rm T_{D}\) are fully stabilized after 2 \(\times\) 10\({}^{6}\) cycles. The damage in all cases followed the same pattern, with an initial rapid growth with a decreasing slope, zone D-I, then a linear growth of the damage, zone D-II, and finally an accelerated growth until break, zone D-III, as can be seen in Figure 8. Both the growth rate, \(\rm\Delta T_{D}/\Delta N\), and the orderly one at the origin of the adjustment of the section D-II, \(\rm D_{0}\), and the critical damage, real and theoretical, \(\rm D_{C}\) and \(\rm D_{CT}\), are increasing with the stress level. The second test phase is formed by fatigue tests with breaks and monotonic load values. By analyzing the results of monotonous fatigue with interruptions, see Figure 9, it can be seen that a part of the damage caused during a load step (25,000 cycles) is recovered during the resting period, so that, when the next step is started, the initial damage is less than the damage that ended the previous step. This means that the damage would be composed of an elastic component, which recovers when the cyclic stress is released, and another plastic component, which is the one that remains after the stress and the one that actually causes the material to deteriorate. The elastic component of the damage will be identified with the one ordered at the origin of the section D-II, \(D_{0}\), or initial damage, and therefore, a plastic or net damage, \(D_{N}\), can be defined from Equation (6):  
\[D_{N}=D_{T}-D_{0} \tag{6}\]  
Figure 10 shows the evolution of net damage for all levels of monotonous fatigue without breaks. In contrast to the results obtained for total damage, where the critical damage values depended on the stress level, increasing as the stress increased, for net damage, it can be seen that in the case of thermal fatigue, the theoretical critical net damage values, \(\rm D_{NCT}\), are stabilised at around 15%, regardless of the stress. In the case of mechanical fatigue, the value of critical damage could not be verified because, as mentioned, the breakage took place through the clamp, thus not reaching the number of theoretical breakage cycles. Figure 8: Evolution of total damage (\(\Delta\sigma\) = 54 MPa) for the monotonic fatigue tests. Figure 11 shows that in the case of fatigue breaks, the number of cycles is higher than in the monotonous one of the same levels, but the theoretical net damage also reaches values close to 15%. The third experimental phase corresponds with the Locati tests. Analyzing the evolution of the total damage, \(D_{T}\), in the Locati tests, there was no indication detected that allows it to be established as to which of the steps corresponds to the fatigue limit.