## 3 Results and Analysis
### Fatigue Test
#### 3.3.3 Fatigue Limit Through Infrared Thermography and Volumetric Dissipated Energy
However, in composites with fibers aligned to the loading direction, fiber ratcheting due to microfibril alignment would happen, even if the stress applied is lower; therefore, the fatigue limit would be lower, as observed for the \([0^{\circ}]_{\text{s}}\) using the exponential method, which was also reported by [29], where they found that samples loaded along the fiber direction present a lower fatigue limit. Although this is not evident for the \([0^{\circ}/90^{\circ}]_{\text{s}}\), due to half of the laminas being transversely oriented to the loading direction, mixed-mode damage would happen; therefore, a higher fatigue limit is found. For \([90^{\circ}]_{\text{d}}\), due to the high variability of the temperature at low stress applied, it was not possible to apply the thermography method to determine the fatigue limit, while volumetric dissipation energy fits well to the exponential equation. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & & & \\ \cline{2-5} \multirow{2}{*}{**Laminate**} & \multirow{2}{*}{**Exp. Weibull**} & \multirow{2}{*}{**Exp. Basquin**} & \multirow{2}{*}{**Therography**} & \multicolumn{2}{c}{**Volumetric**} \\  & & & & & **Dissipated Energy** \\ \cline{4-5} \multirow{2}{*}{\([0^{\circ}]_{\text{s}}\)} & \multirow{2}{*}{**Exp. Weibull**} & \multirow{2}{*}{**(\(2\times 10^{6}\) Cycles)**} & \multirow{2}{*}{**Bilinear Model**} & \multirow{2}{*}{**Exponential Model**} & \multirow{2}{*}{**Exponential Model**} \\ \hline \([0^{\circ}]_{\text{s}}\) & 0.48 (105.2) & 0.45 (98.7) & 0.44 (96.8) & 0.23 (49.1) & 0.18 (39.6) \\ \([0^{\circ}/90^{\circ}]_{\text{s}}\) & - & 0.35 (47.1) & 0.40 (54.0) & 0.41 (54.9) & 0.43 (57.4) \\ Basket weave warp & - & - & 0.46 (40.5) & 0.49 (43.1) & 0.55 (48.1) \\ Basket weave wewft & - & - & 0.47 (47.6) & 0.44 (44.0) & 0.46 (46.1) \\ \([+45^{\circ}/-45^{\circ}]_{\text{s}}\) & - & - & - & 0.69 (52.0) & 0.65 (48.5) \\ \([90^{\circ}]_{\text{d}}\) & - & - & - & - & 0.43 (14.8) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Fatigue limit determined according to the Weibull, Basquin, bilinear, and exponential models; values in brackets are the fatigue limit in MPa. During the fatigue test, superficial temperature attains stabilization around 2000 cycles for applied stress lower than 0.6 or even 0.7 from \(\sigma_{\text{ut}}\); although an apparent stabilization is reached, this temperature may drop, which may happen due to the environment temperature changes or even with the small airflow around, the inherent variability of the laminate properties notwithstanding. These small changes may have a huge impact on the surface temperature of the tested samples at low stress; therefore, high variation in the result is observed, which was found in this study. Therefore, in the thermography method, tests must be performed under different levels of stress at low stress in order to increase the resolution and reduce the variability, which at the end increases the duration of the measurement and the total cost. On the other hand, using a calibrated extensometer and with a sufficient rate of data acquisition, the volumetric dissipation energy is an alternative, showing low variability even at low stresses.