Several studies have been conducted in the Very High Cycle Fatigue (VHCF) regime on Carbon Fiber Reinforced Polymers (CFRP) in search of their fatigue limit beyond their typical service life, which is itself in the order of 108 loading cycles. The ultrasonic fatigue test (UFT) method has been recently gaining attention for conducting fatigue experiments up to 109 loading cycles. This can be attributed to the reduction of testing time, as the testing facility operates at a cyclic frequency of 20 kHz. The fatigue loading in UFT is usually performed in a pulse-pause sequence to avoid specimen heating and undesirable thermal effects. For this study, the pulse-pause combination of the UFT methodology was explored and its influence on the self-heating behavior of the CFRP material was analyzed. This was realized by monitoring the temperature evolution in the CFRP specimens at different pulse-pause combinations and correlating it with their final damage morphologies. From the obtained results, it is concluded that the specimen heating phenomenon depends on several variables such as cyclic loading amplitude, the pulse-pause combination, and the damage state of the material. Finally, it is proposed that the test procedure, as well as the testing time, can be further optimized by designing the experiments based on the self-heating characteristic of the composite and the glass transition temperature (Tg) of the polymer matrix.
Keywords: 20 kHz; carbon fiber reinforced polymers; heat generation; material characterization; online-monitoring; power-ultrasonics; self-heating; ultrasonic fatigue.