Heat dissipation and entropy accumulation of CF-PEKK composite under low and ultrasonic frequency loading

This study investigates the heat dissipation mechanisms and the entropy accumulation in a carbon-fiber-reinforced poly-ether-ketone-ketone (CF-PEKK) composite subjected to cyclic three-point bending loading conditions at low and ultrasonic frequencies. The heat dissipation rates of the fatigue experiments were compared with the results of finite element analysis (FEA) to identify the dominant dissipation mechanisms under different testing conditions. Conduction was found to be the main heat dissipation mechanism at low frequencies, while forced convection dominated at ultrasonic frequencies. Increasing-amplitude fatigue experiments showed that the load level and the loading frequency have a clear influence on the total heat dissipation rate, as well as the heat dissipation rates due to damage. Therefore, a pulse-pause sequence was used to capture the change in heat dissipation rate under constant-amplitude fatigue (CAF) experiments until failure. The CAF experiments showed that the heat dissipation rate increases in the presence of damage, and the heat dissipation rate in the event of failure was comparable between the low and ultrasonic frequencies. Finally, the total accumulated entropy at failure, also known as fracture fatigue entropy (FFE), increased from high-cycle fatigue to the very high-cycle fatigue regime for the CF-PEKK composite and was independent of the loading frequency. Therefore, this work encourages the use of entropy-based fatigue damage assessment across a wide range of cyclic frequencies.