## 4 Fatigue Infrared Thermal Imaging Test
### Experimental Results  
Three specimens are selected from each group for the static strength tensile test. The experimental results are shown in Table 1.  
From Table 1, the larger the void fraction of the specimen is, the lower the static tensile strength is, indicating that the existence of void defects reduces the bearing capacity of the composite structure to a certain degree. The average static strengths of the three groups A, B, C are obtained as 479.14 MPa, 376.72 MPa and 265.09 MPa from the test, respectively.  
The load-number of cycles curve and stress-strain curve during the pull-pull fatigue test of the three groups A, B and C are shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Test Number**} & \multirow{2}{*}{**Void Fraction (\%)**} & \multicolumn{4}{c}{**Static Strength/MPa**} \\ \cline{3-6}  & & \multicolumn{2}{c}{**Experimental Results**} & \multicolumn{1}{c}{**Average Value**} \\ \hline A & 0.5 & 488.44 & 470.90 & 478.09 & 479.14 \\ B & 2.8 & 382.97 & 343.81 & 403.37 & 376.72 \\ C & 5.7 & 253.82 & 264.24 & 277.20 & 265.09 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Static strength tensile experimental results.  
Figure 4: Fatigue experimental results: (**a**) load-number of cycles curve; (**b**) stress-strain curve.  
As shown in Figure 4a, during the loading process, the load-number of cycles curves of the three groups of specimens follow the same trend, that is, they all increase first and then decrease. Additionally, the larger the void fraction is, the lower the fatigue load value is, because the local stress concentration is generated in a field where the void defects are densely distributed, resulting in a decrease in the strength. The stress at this field quickly falls to the limit stress, which is the corresponding stress when each group of specimens is subjected to fracture or failure. The limit stress can be sustained by nearby fibers at best, and then damage is formed. Figure 4b illustrates the stress \(\sigma\) and strain \(\varepsilon\) of the three groups of specimens at any time during the fatigue loading. The value of the internal energy storage \(E\) can be calculated by substituting the stress \(\sigma\) and strain \(\varepsilon\) into Equation (16). However, there is no determination in previous studies of the critical moment at which the defect evolves into actual damage. The defect evolution causes the interior of the material to be irreversibly deformed, and the energy is released in the form of heat dissipation, which is reflected as the temperature change on the infrared thermograph. The surface temperature of the specimen is measured by an infrared camera and formation of actual damage is studied from the perspective of energy dissipation. Taking group C as an example, the infrared thermographs of the defect evolution process monitored in the specimen are shown in Figure 5, and the voids area is marked.  
It can be seen from Figure 5 that in the marked defect area, the surface temperature distribution is not uniform, and as the heat dissipation increases during the defect evolution process, the surface temperature rises, and an obvious regional temperature field appears. Later, this defect evolution area continues to expand and be enhanced until the number of cycles is up to 8076, indicating that the defect would not evolve any longer, which means that the material has undergone tremendous damage. When the defect evolution areas of the infrared thermograph of group A and group B are no longer expanded, the corresponding numbers of cycles are 9501 and 8784, respectively. However, the criterion for the critical moment of damage still needs to be clarified from the theoretical variation of internal energy storage.