Cyclic deformation and fatigue behavior of polyether ether ketone (PEEK)

In this study, the fatigue behavior of a semi-crystalline thermoplastic polyether ether ketone (PEEK) is investigated. A series of tests at ambient temperature, including uniaxial monotonic (tension and compression) at different strain rates and uniaxial fully-reversed strain-controlled cyclic tests at strain amplitudes ranging from 0.02 mm/mm to 0.04 mm/mm at various frequencies, were conducted. The frequency influence on the strain-controlled fatigue lives of unfilled PEEK specimens was found to be highly dependent on the strain level. A minimal frequency effect was observed at a lower strain amplitude of 0.02 mm/mm, whereas increasing the test frequency at higher strain amplitudes resulted in longer fatigue lives. Additionally, load-controlled cyclic tests that utilized applied loads corresponding to stress responses obtained from the strain-controlled fatigue tests were performed. The cyclic behavior under the two control modes were compared and discussed. Three types of fatigue models, including a strain-based (Coffin-Manson) model, a strain-stress-based (Smith-Watson-Topper) model, and an energy-based model, were employed to correlate the data in this study. Among the three fatigue models, the energy approach was found to better correlate the PEEK experimental data at various frequencies.