Evaluating CFRP fatigue performance with ultrasonic fatigue testing

Carbon fiber reinforced plastic (CFRP) is widely used across various industries due to its notable specific stiffness and strength. To ensure its effectiveness as a structural material, evaluating its fatigue performance is crucial, as fatigue is a primary cause of structural failure.

Traditional fatigue testing methods can take several months to complete a 10^9 cycle fatigue test. However, the Shimadzu USF-2000A ultrasonic fatigue testing system offers a significant reduction in testing time by operating at a frequency of 20 kHz, completing the same test in approximately 14 hours. This method is practical for assessing the long-term durability and reliability of CFRP. The following case study illustrates a three-point bending ultrasonic fatigue test of CFRP using the USF-2000A.

The USF-2000A system facilitates fatigue testing at 20 kHz, enabling a 10^9 cycle fatigue test to be conducted in about 14 hours. A simple three-point bending test of CFRP can be executed using this ultrasonic system. Additionally, a high-speed video camera can capture the behavior of specimens vibrating at 20 kHz.

The experimental setup is depicted in Figure 1, which shows the use of a three-point bending indenter and a prototype jig for ultrasonic fatigue testing, based on the USF-2000A’s average stress loading mechanism. The CFRP specimen prepared for testing measured 40 mm in length, 7 mm in width, and 3.5 mm in thickness. For ultrasonic fatigue testing, achieving resonance at 20 kHz is essential, and in this study, the span was set to 16.5 mm. The instrument settings are detailed in Table 1.

Before commencing the main ultrasonic fatigue test, an HPV-X2 high-speed video camera was employed to observe the specimen’s behavior. Figure 2 illustrates the high-speed recording setup. A random pattern for Digital Image Correlation (DIC) analysis was applied to the specimen, and the recording frame rate was set to 1 million frames per second. Figure 3 shows an example frame from the high-speed recording.

Figure 4 presents an example of DIC analysis results. The graph demonstrates displacement in the test axial direction directly below the ultrasonic indenter, confirming satisfactory loading with a sine wave pattern.

Static three-point bending test results are shown in Figure 5, where the maximum test force recorded was 3479 N.

The arrangement for the three-point bending ultrasonic fatigue test is shown in Figure 6. To mitigate heat build-up in the test samples, cooling was applied using an air cooler, and interval operation was implemented, cycling the oscillation on and off. A radiation thermometer measured the surface temperature of the samples. The test conditions are outlined in Table 2. The applied test force in the ultrasonic fatigue test ranged from 65 to 85% of the peak force from the static test.

Additional tests using a traditional fatigue testing machine were conducted under a higher test force, resulting in a lower cycle count before fracture. Figure 7 compares this data with the results from the USF-2000A. The results from both testing methods were broadly consistent, demonstrating the accuracy of the USF-2000A.

The USF-2000A ultrasonic fatigue testing system was utilized for a three-point bending ultrasonic fatigue test, and the results aligned closely with those from traditional fatigue testing. This indicates the potential of the USF-2000A for efficient fatigue testing of CFRP, significantly reducing the time required for such evaluations.