Observing the Failure of Open-Hole CFRP specimens in Tensile Tests
Synchronized Imaging Using Two High-Speed Video Cameras
Offering superior specific strength, even compared to other composite materials, carbon fiber reinforced plastic (CFRP) is used in aircraft and some transport vehicles for the purpose of saving fuel through weight reduction. Composite materials have excellent mechanical properties. However, a general feature of composite materials is that their strength decreases markedly when they are notched. CFRP is no exception, so tests of notched specimens are important.
In this case, testing is performed using specimens notched with a circular hole at the center. In this experiment, tensile tests were performed using CFRP specimens (laminate method [45/0/-45/90]2s) with a total length of 150 mm, a width of 36 mm, and a thickness of 2.5 mm, prepared with a 6 mm circular hole at the center. The failure process of the CFRP specimens was observed during the tensile tests.
In particular, it is important to confirm the failure process of weak regions, such as the periphery of circular holes, for CFRP development and to confirm the validity of CAE analysis. However, since the failure of CFRP is a brittle phenomenon, where failure occurs instantaneously, it cannot be confirmed with the naked eye. For this reason, high-speed video cameras are used to observe the failure. In this experiment, synchronized images were obtained at the front and the side of the specimens using two HPV-X2 high-speed video cameras.
In this experiment, the AG-Xplus precision universal testing machine and two HPV-X2 high-speed video cameras were used. Table 1 shows the instruments used. To observe the specimen failure in a tensile test, a trigger signal synchronized to the failure must be transmitted to the high-speed video cameras. The failure starts on the periphery of the circular hole.
Accordingly, aluminum foil was affixed to the periphery of the circular hole using adhesive, as shown in Fig. 1, so that conduction would be lost when the specimen fails. The failure was observed using this timing to trigger the cameras.
Figure 1: Aluminum foil trigger
Table 1: Testing equipment
Table 2 shows the measurement conditions, and Fig. 2 shows the test configuration. As shown in Fig. 2, the failure of the specimen was recorded from the front by camera (1) and from the side by camera (2).
Table 2: Measurement conditions
Figure 2: Test configuration
Fig. 3 shows the test results from the AG-Xplus. Failure begins where the test force suddenly drops in Fig. 3. Fig. 4 shows the specimen failure observed from the front, and Fig. 5 from the side. Image (2) in Fig. 4 shows that the failure starts on the left side of the circular hole.
Figure 3: Test results
In image (3), a crack also appears on the right side of the circular hole. Subsequently, cracks progressed in an orientation of 45 degrees, the orientation of the fibers in the outer layer.
Figure 4: Images from Camera (1) (5 μs between images)
Figure 5: Images from Camera (2) (5 μs between images)
Further, as the test progressed, multiple cracks were confirmed, as in images (7) and (8). In the observations from the side, no failure was confirmed at the time the failure started, and was only initially confirmed in image (5). This is likely because the cracks started at the periphery of the circular hole reached the side of the specimen in image (5).
Subsequently, failure was confirmed in multiple layers, except for the 0-degree layer, in image (6). Further, in image (7), failure was confirmed in the 0-degree layer, after which the failure progressed toward the outer surface. The final condition of the specimen is shown in Figs. 6 and 7
(Left) Figure 6: Specimen After Failure (front), (Right) Figure 7: Specimen After Failure (side)
The HPV-X2 is equipped with a synchronization function, and with excellent sensitivity, is capable of synchronized recordings at 2 Mfps, as in this case. As a result, failures can be observed in tensile tests of materials like CFRP that fails at high speeds.
Generally, failure observations are often recorded from the front of the specimen. However, adding recording from the side enables confirming the failure process that cannot be observed just from the front. In particular, with CFRP materials with different fiber orientations for each lamination layer, where failure progresses in different manner for each layer, as shown in this article, the failure process can be observed in more detail by recording from two directions.