Stress and strain calculation method for orthotropic polymer composites under axial and bending ultrasonic fatigue loads.

Accelerated fatigue testing is one potential solution to evaluate the very high cycle fatigue behavior of composite materials within a reasonable amount of time. The ultrasonic fatigue testing methodology can be adopted to realize fatigue experiments up to 10 cycles at 20 kHz, compared to conventional fatigue experiments usually carried out between 5-50 Hz. The determination of cyclic stresses during ultrasonic loading remains to be one of the major challenges. The cyclic stresses during ultrasonic fatigue loading were investigated for a carbon fiber 5H satin fabric reinforced in Polyetherketoneketone (CF-PEKK) composite material. Two experimental setups were developed to perform ultrasonic testing under uni-axial and three-point bending loading conditions. A 3D-Scanning Laser Doppler Vibrometer (3D-SLDV) and a single-point Laser Doppler Vibrometer (LDV) were integrated into the test systems to measure the oscillation displacement of the CF-PEKK specimens during ultrasonic cyclic loading. These displacement measurements were used to calculate the resulting strains and stresses under elastic loading conditions. The experimental results were found to be in good agreement with those obtained from finite element models, providing evidence for applying the proposed method.