## 4 Fatigue Infrared Thermal Imaging Test  
### Test Methods  
For this study, the GFRP materials which are widely used in the 1.5 MW wind turbine blade are chosen to fabricate the specimen. ECW600-1270 unidirectional glass fiber and 920W epoxy resin provided by Taishan Fiberglass Inc. (Taishan, China). were used to produce specimens. The fiber cloth used in the experiment was unidirectional prepreg glass cloth, and the ratio of epoxy resin was 42%. The laminates were hand-laid and the fiber layup angle was 0\({}^{\circ}\)[20]. The vacuum bag-autoclave process was used to cure the GFRP composite. The pressure of the vacuum bag is 0.06 MPa. The void fractions can be varied by applying different autoclave pressures [31]. In order to obtain different void fractions, specimens were prepared respectively under three different autoclave curing pressures of 0.1 MPa, 0.3 MPa and 0.4 MPa. According to the ASTM D3039 standard, the specimens were cut with a diamond cutting machine. Ten specimens were produced under each of the three curing pressures with the size of 250 mm \(\times\) 25 mm \(\times\) 10 mm. Then the cutting surfaces of the specimens were polished and the allowable parallelism error of the edges should be less than 0.25 mm. It is necessary to bond the bracing sheets at both ends of each specimen to prevent it from slipping off the test machine during the loading process, which may reduce the reliability of data. The material of the tab was a continuous E glass fiber-reinforced polymer with the length of 55 mm and the thickness of 2 mm. The schematic of the specimen is shown in Figure 2.  
Figure 2: Schematic diagram of the specimen. The cured laminates were separately ultrasonically scanned to evaluate the different void fractions under each curing pressure. The regions with uniformed distributed void defects were chosen to be observed by a microscope according to the scanning results. Then a micrograph was obtained, the void fraction of which was analyzed by an image analysis software. Then the void defects area was marked on the surface of each specimen. The analysis results showed that when curing pressures were 0.4 MPa, 0.3 MPa and 0.1 MPa, the average void fractions of each group were 0.5%, 2.8% and 5.7%, correspondingly. The three groups of specimens were marked as A, B, C.  
The specimens A, B and C were tested in a static tensile test and the pull-pull fatigue test under the condition of room temperature and air medium. An infrared camera NEC R300 was used to measure the surface temperature accurately. As shown in Figure 3, an electro-hydraulic servo universal fatigue testing machine MTS810 was used in these tests. The maximum static load was 333 kN, the maximum dynamic load was 250 kN and the frequency range was 0-30 Hz. The operating wavelength of NEC R300 thermal imaging camera was 8-12 \(\upmu\)m, the measurement accuracy was \(\pm\)0.1 \({}^{\circ}\)C and the frame frequency range was from 8.5 Hz to 60 Hz. In order to obtain a high and uniform surface emissivity of the specimen, a thin oxide layer on the surface of the specimen was previously removed with fine-grained sandpaper. Then a thin layer of black paint with a high emissivity was applied to the surface of the specimen. The specimen in the same environment was placed with the experiment equipment 24 h before the test. The specimen was clamped tightly, and a strain gauge was attached to the center of the back of the specimen, then an external strain meter was attached to the strain gauge. The temperature and humidity of the environment should be constant during the entire experimental process to avoid the influence of changing temperature in the infrared thermograph. The infrared camera was calibrated and automatically focused before being used. During the test, it was necessary to ensure that the infrared camera was on the same horizontal line with the specimen, so as to ensure that the infrared thermograph was clearly displayed at the center of the screen.