Specimen and experiment design for on- and off-axis fatigue and self-heating characterization of a woven CF-PEKK composite at low and ultrasonic frequencies

This work investigates the fatigue behavior of satin fabric-woven carbon fiber-reinforced poly-ether-ketone-ketone (PEKK) laminates under low (20 Hz) and ultrasonic (20 kHz) testing frequencies using identical specimen geometries. Specimen designs across all orientations were based on modal, harmonic, static-structural, and buckling analyses to ensure comparable results. This design enables uniaxial tension-compression loading of woven carbon-fiber reinforced polymers (CFRPs) to fail in the gauge section without global buckling and ensures overlapping stress amplitudes between the two test systems. The maximum possible stress amplitudes of ultrasonic fatigue testing (UFT) are higher than the lowest stress amplitudes that cause failure in the conventional servo-hydraulic (SH) system. By using the anisotropy of composite laminates, this design was validated through tension-compression experiments on dogbone-shaped specimens with four fiber orientations: 0°, 15°, 30°, and 45°, using SH and UFT systems. Results comparing high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) behavior of angle-ply laminates indicate a strong dependence on fiber orientation. A comparison of self-heating and microscopic analysis between the two systems demonstrates the applicability of UFT for off-axis VHCF characterization of woven composites. Finally, shear stress-induced damage initiation in 0°fiber-oriented dog-bone-shaped specimens, as observed in this work and reported in the literature, is addressed as a multi-axial stress state problem by incorporating the resultant normal, transverse, and shear stresses into the Tsai-Wu formulation.