Development of an axial loading system for fatigue testing of textile-composites at ultrasonic frequencies

Fiber-reinforced polymers are increasingly used in lightweight applications due to their high specific properties. Different polymer materials, and fiber architectures, are commercially available for structural applications. Performing fatigue experiments on all these combinations of fiber-matrix composites up to 109 cycles to mimic their typical lifetime of 15-20 years, at a low testing frequency generally in the range of 5-10 Hz in applications such as the aerospace industry, is expensive and time-consuming. In this study, a test system to investigate the very high cycle fatigue (VHCF) behavior of continuous fiber-reinforced polymers under an axial loading (tension-compression) condition is proposed. To do so, a testing system capable of applying cyclic oscillations at a frequency of 20 kHz and stress ratio, R = -1 was developed. For this investigation, the carbon fiber reinforced in Polyether-ketone-ketone (CF-PEKK) material was chosen. For visualization of the specimen oscillation-mode at 20 kHz, video-imaging using a high-speed camera (HSC) was employed. With the obtained displacement field from the HSC images and with thermography, the desired specimen response due to resonance at 20 kHz is validated.