Experimental and numerical analysis of a CF-PAEK beam under low and ultrasonic cyclic bending frequencies

This work investigates the fatigue damage behavior of a carbon fiber-reinforced Poly-Aryl-Ether-Ketone (CF-PAEK) laminate under monotonic, low frequency, and ultrasonic frequency three-point bending conditions. Strains and stresses are evaluated using digital image correlation (DIC) and compared with finite element analysis (FEA). The numerical results, confirmed by microscopy, successfully predict damage initiation, which occurs as localized fiber kinking in the top 0° layers due to compressive stresses and transverse cracks in off-axis layers (90° and ± 45° plies) triggered by out-of-plane shear. Under low frequency cyclic loading (10 to 50 Hz), self-heating analysis revealed that fatigue life and strength are independent of testing frequency. In contrast, 20 kHz ultrasonic loading condition induces pronounced self-heating, reducing fatigue strength by approximately 12% compared to low frequency fatigue conditions. For CF-PAEK beams, overlapping stress levels between testing systems are successfully achieved, and the S-N diagram shows comparable fatigue lives up to 1 0 9 cycles. While damage initiation mechanisms remain comparable, damage accumulation is altered by the control mode. Force-controlled low frequency experiments cause multiple rapidly coalescing delamination, whereas displacement-controlled ultrasonic testing yields a single propagating delamination leading to resonance loss.