## 3 Low Velocity Impact Experiment  
### Low Velocity Impact Experimental System for Variable Thickness Composite Laminates  
As shown in Figure 2, the low velocity impact experimental system consists of variable thickness composite laminates on the wing tank specimen, fiber Bragg grating sensors, impact hammer, SI425 fiber grating demodulator and computer. The material of the variable thickness composite specimen surface is carbon fiber T300/QY8911 (the material parameters are shown in Table 1, the layer structure is shown in Table 2), the fiber type is TG, manufactured using a unidirectional tape. The thickness range of the layer is 45 mm-47.5 mm (different thickness areas are distinguished by different colors, and the thickness value is marked in the upper left corner of the area with the corresponding color), the size is 600 mm \(\times\) 300 mm, the width of the metal solid bracket border is 40 mm. Since the laminates with different properties can be obtained by laying the single-layer panels in different main directions, the skin ply structure in Table 2 is designed according to the specific strength requirements of the wing oil tank. As shown in Figure 3, the region shown by the slash is an impact experimental area with 240 mm \(\times\) 200 mm size, and divided into the grid of 40 mm \(\times\) 40 mm size, as shown in Figure 4. The length of the 6 FBG sensors pasted on the surface is 10 mm, the sensor arrangement shown in Figure 4, the sensor wavelength and position shown in Table 3. The energy of the hand-held impact hammer is third gear adjustable (1 J, 2 J, 3 J), and it can impact on the surface anywhere. The impact hammer head is spherical, because the contact surface and energy are small enough, it is analyzed as a plane. The 1-3 J energy belongs to the lower range of aviation low velocity impact, so the impact process can be regarded as completely elastic impact. The SI425 optical fiber grating demodulator has four optical channels, and it can connect up to 512 fiber Bragg grating sensors through the FC/PC interface for static and dynamic signal measurement; with detection wavelength range of 1520 nm-1570 nm, resolution 1 pm, dynamic range of 25 dB, scanning frequency of 250 Hz. The impact experiment system acquires the impact response signals by monitoring the wavelength shift value of the fiber Bragg grating sensor. When the impact hammer impacts on the specimen, the demodulator transmits the impact signals collected by the 6 sensors to the computer. Based on the vibration test of composite laminates with variable thickness, ply thickness gradients, symmetries, fixations and mechanical analysis, at the same time, in order to detect the strain field changes of the whole variable thickness area, sensors were arranged at six locations of the boundary of the test area. The six sensors were calibrated by four points bending test before they were pasted on the specimen, and the grating strain sensitivity is about 1.198 pm/\(\mu\epsilon\) at room temperature of 20 \({}^{\circ}\)C. Combined with the demodulator resolution of 1 pm, it shows that all grating sensors can monitor the structural strain changes. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Layer** & **Angle** & **Layer** & **Angle** & **Layer** & **Angle** & **Layer** & **Angle** \\
**Number** & **(Degree)** & **Number** & **(Degree)** & **Number** & **(Degree)** & **Number** & **(Degree)** \\ \hline
1 & 45 & 15 & 45 & 29 & \(-\)45 & 43 & 0 \\
2 & 0 & 16 & 0 & 30 & 0 & 44 & \(-\)45 \\
3 & \(-\)45 & 17 & \(-\)45 & 31 & 45 & 45 & 0 \\
4 & 90 & 18 & 0 & 32 & 0 & 46 & 0 \\
5 & 0 & 19 & 0 & 33 & \(-\)45 & 47 & 45 \\
6 & 45 & 20 & 45 & 34 & 0 & 48 & 0 \\
7 & 0 & 21 & 0 & 35 & 90 & 49 & 45 \\
8 & 45 & 22 & 90 & 36 & 0 & 50 & 0 \\
9 & 0 & 23 & 0 & 37 & 45 & 51 & 45 \\
10 & 45 & 24 & \(-\)45 & 38 & 0 & 52 & 0 \\
11 & 0 & 25 & 0 & 39 & 0 & 53 & 90 \\
12 & 0 & 26 & 45 & 40 & \(-\)45 & 54 & \(-\)45 \\
13 & \(-\)45 & 27 & 0 & 41 & 0 & 55 & 0 \\
14 & 0 & 28 & \(-\)45 & 42 & 45 & 56 & 45 \\ \hline \hline \end{tabular}
\end{table}