Fatigue strength estimation of a CF/PEKK composite through self-heating temperature analysis using cyclic bending tests at 20 kHz

Very high cycle fatigue (VHCF) testing at conventional frequencies is highly time-consuming and unrealistic for large sample sizes. Though testing at ultrasonic frequencies can accelerate the testing, any undesirable thermal effect arising from testing at 20 kHz can cause significant heat generation in the specimens, increasing the likelihood of thermally induced failure. When mitigated, the temperature behavior during ultrasonic fatigue testing may conversely provide knowledge of the fatigue behavior of the composite. We study this hypothesis in this work by estimating the fatigue strength of a CF/PEKK composite through its temperature evolution during 3-point bending tests at 20 kHz with different cyclic load amplitudes. The fatigue strength of the composite was estimated using the temperature characteristics from an increasing cyclic amplitude test. This estimation was validated with four constant cyclic amplitude tests. Further, the estimation model was extended to different pulse–pause sequences by two constant cyclic amplitude tests. The results indicate a reliable and rapid method for fatigue life estimation as a material property. Finally, the criticality of the self-heating phenomenon during the non-stationary self-heating condition was evaluated. [Display omitted] •Fatigue strength concept is adapted to VHCF testing.•The fatigue strength value is still a material property for VHCF testing.•Invariance of fatigue strength with different VHCF loading conditions.•Stationary self-heating phenomenon leads to mechanical fatigue.