Accelerated estimation of the very high cycle fatigue strength and life of polymer composites under ultrasonic cyclic three-point bending

Ultrasonic fatigue testing (UFT) offers a strong potential to accelerate the testing time of fatigue experiments of materials up to very high cycle (VHCF) and giga-cycle (GC) fatigue regimes. This can especially be useful during the materials selection processes when designing load-bearing structures. In this study, one increasing- and three constant-amplitude fatigue experiments were conducted using a UFT system on three different composites consisting of woven Carbon fibers with Poly-Ether-Ketone-Ketone (CF-PEKK), Polyphenylene Sulfide (CF-PPS), Epoxy (CF-Epoxy) as polymer matrix materials. The surface temperature data obtained from these experiments were used to evaluate the heat dissipation rate and fracture fatigue entropy (FFE) range of the three composite materials. The fatigue strength estimated using this FFE range for CF-PEKK (normal stress), CF-PPS (shear stress), and CF-Epoxy (shear stress) composites were 26.3 MPa, 4.7 MPa, and 15.1 MPa, respectively. The fatigue life comparison between the estimation model adopted in this study and the experimental results found in the literature was in reasonable agreement for all three composites. The FFE concept is adapted to the UFT method to rapidly estimate the VHCF properties of composite materials used in load-bearing structures within three weeks.