PSI - Issue 61

Ahmet Çevik et al. / Procedia Structural Integrity 61 (2024) 291–299

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Cevik et al. / Structural Integrity Procedia 00 (2019) 000 – 000

For specimen Fabric 2c, field of view of the high-speed camera (HSC) is set to capture the curved region and part of the vertical arm, and for specimen Fabric 1c, the extent of the vertical arm. The capture speed of the camera is adjusted to 420,000 frames per second (interframe time of 2.38 μs) to record the high speed delamination propagation. The initiation and propagation occur during the almost instantaneous load drop. Figure 6 shows the initiation of the failure at the curved region inside the 6th ply from the inner side and its propagation into the arms for specimen Fabric 2c. The propagation of the crack is seen at t=2.38 μs for the first time. The crack initiates in the inner side of the curved region at approximately 31% of thickness from the inner side. At t=4.76 μs, it can be seen that the crack tips travel in both directions to the arms of the curved beam, at 9.52 μs before the both crack tips go out of our field of view. Figure 7 shows the propagation of the failure into the vertical arm for specimen Fabric 1c, which initiates approximately inside the 11 th ply from the inner side. The crack enters our field of view at t=0 μs in the curved region by propagating on the outer side of the curved region (which is approximately %65 of thickness from the inner side) . After 7.14 μs, it starts to travel through the vertical arm and seems to arrest in the center of the vertical arm at approximately t=19.04 μs.

Fig. 6. High-speed images (interframe time = 2.38 μs) showing the initiation of the failure at the curved region inside the 6 th ply from the inner side and its propagation into the arms for specimen Fabric 2c.

Fig. 7. High-speed images (interframe time = 4.76 μs) showing the propagation of the failure into the vertical arm, which initiates approximately inside the 11 th ply from the inner side for specimen Fabric 1c. Crack tip positions obtained from the experiment of the fabric specimens are shown in Figure 8a. The crack tips propagating through the vertical and horizontal arms are named as lower and upper crack tips, respectively. Since only the vertical arm is monitored in specimen Fabric 1c, the lower crack tip position data is shifted by 4.76 μs which is the approximate time when the crack reaches the location where it is seen in high-speed camera images for the first time. Crack tip speeds, shown in Figure 8b, are calculated by using the central difference method to the crack tip positions, while the last speed data is obtained with the backward difference method. Shear wave speed, C S , and Rayleigh wave speed parallel to the fibers, C R ║ , for this material are calculated as 1767 m/s and 1678 m/s, respectively. In the experiment of specimens Fabric 1c and 2c, the lower crack tips travel in a fluctuating manner. In the experiment of specimen Fabric 1c, the speed history of the lower crack tip is in the sub-Rayleigh wave speed regime and reaches a maximum speed of 1133 m/s. In the experiment of Fabric 2c, vertical crack initiates with an intersonic speed of 2173 m/s. During its travel, it reaches intersonic speeds around 1875 m/s in the time interval 9.52- 11.90 μs. In the experiment of specimen Fabric 3c, the vertical crack tip initiates with a sub-Rayleigh speed of 1038 m/s, and then it reaches a maximum speed of 1383 m/s at t= 4.76 μs. After that time, the crack tip speed gradually decreases to 525 m/s. In the experiment of specimen Fabric 2c, the upper crack tip initiates with an intersonic speed of 1891 m/s, and its speed gradually decreases to 275 m/s. In the experiments, some fluctuations are observed in the crack tip, showing slowing down and speeding up of the crack tip.