Comparability of fatigue strength and life estimation of a CF‐PEKK composite under low and ultrasonic frequencies using time‐temperature‐based approaches

Predicting very high cycle fatigue (VHCF) life in composites is challenging due to the lengthy testing times required by traditional methods. Ensuring compatibility between VHCF and high cycle fatigue (HCF) data presents an additional challenge. This investigation examines CF‐PEKK fatigue behavior under low and ultrasonic cyclic frequencies using a thermographic approach and the fracture fatigue entropy (FFE) concept. Constant‐amplitude fatigue (CAF) and increasing‐amplitude fatigue (IAF) tests were conducted, with self‐heating behavior analyzed for different loading conditions. Heat dissipation rates from IAF experiments and fatigue data from three CAF experiments at 40 Hz and three at 20 kHz were used to calculate FFE values. Results showed that the self‐heating response of CF‐PEKK specimens helps achieve comparable fatigue data across frequencies, with consistent FFE ranges identified between HCF and VHCF regimes. This consistency highlights that FFE can provide reliable fatigue life estimation for composites.