## 3 Experimental Results and Discussing  
### Specim and Test Configuration  
The load of a wind turbine blade was extremely complicated during operation. The main direction of the laminated fibers should be consistent with the direction of tension and compression of the component, due to the high strength and stiffness of the fibers in the axial direction. Laminates of 0\({}^{\circ}\), \(\pm\)45\({}^{\circ}\), and 90\({}^{\circ}\) could sustain axial, shear, and lateral loads, respectively. Therefore, the most commonly angles of wind turbine blade were [45]/[-45]s and [0/90]s. The crack opening displacements, as well as the temperature variation at the critical moment, were also affected by the ply angles.  
Four glass fiber reinforced polymer (GFRP) specimens were constructed with representatives of blade materials and construction processes though scale were reduced. The glass fiber cloth was woven roving. The epoxy resin was used as a matrix. The cure method of specimen was vacuum formed with a vacuum hot press. The specimen was fabricated by hand-laminated paste. Air bubbles were directly introduced into the resin during the mixing process prior to infusion with gel balls. Gel ball was a hollow water soaked ball whose surface could be easily vaporized by heating. The volatility of gel ball can be used to fabricate the defective specimen. When the specimen was cured, the gel ball reduced to a tiny size leaving the remaining size as bubbles. The size and location of bubble in the specimen was almost identical to that of gel ball before heating [29]. Gel ball with diameter of 1 mm was inserted in the second layer of specimen and stay at the center of cross section. The specimens were manufactured with fiber directions of both [0/90]s and [45]/[-45]s. Table 1 lists the Lay-up angle, properties of bubbles, and serial number of specimens. The specimens with 30 layers were cut to approximtely 30 mm wide by 200 mm long. The thickness of each fiber layer was 0.5 mm. Table 2 lists the physical properties of glass fibers and bubbles.  
As shown in Figure 1, matte black varnish with high thermal emissivity was painted on the surface of specimen A1 and A2 to improve the accuracy of temperature measurement. An infrared thermal camera InfRecR300, whose accuracy was 0.1 \({}^{\circ}\)C, was used to record the surface temperature of the specimen. The sampling frequency was at as 0.3 Hz.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Lay-Up Angle** & **Type of Defect** & **Location of the Defect** \\ \hline A1 & [45]/[-45]s & Bubble (r = 1 mm) & Second layer \\ A2 & [0/90]s & Bubble (r = 1 mm) & Second layer \\ A3 & [45]s & None & - \\ A4 & [0/90]s & None & - \\ \hline \hline \end{tabular}
\end{table}
Table 1: Serial number of specimens and their defects.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Density (kg/m\({}^{3}\))** & \
\begin{tabular}{c} **Coefficient of Heat** \\ **Conductivity (W/(m\(\cdot\)K))** \\ \end{tabular} &
\begin{tabular}{c} **Specific Heat Capacity** \\ **(J/(kg\(\cdot\)K))** \\ \end{tabular} \\ \hline Resin & 0.96 & 0.2103 & 550 \\ Glass fiber & 2250 & 1.0 & 1920 \\ Air & 1.2 & 0.0259 & 1005 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical properties of glass fibers and bubbles.  
In Figure 2, a servo hydraulic material testing machine MTS810, whose max dynamic load was 250 KN and max static load was 333 KN, carried out the tensile fracture tes. The frequency of fatigue load was set as 8 Hz and the stress ratio was set as 0.1. Two reinforced sheets were installed at both sides of the specimen and a special pattern was manufactured on the fixture to prevent the relative sliding between the fixture and the specimen.