Fatigue behaviour of PEEK fabricated by material extrusion additive manufacturing

• Fatigue life of FFF-PEEK is governed by stress, frequency, and temperature. • 104x life increase achieved by reducing stress from 80 MPa to 75 MPa. • SEM and µCT reveal inter-bead delamination and both ductile and brittle fractures. • Elevated temperature and frequency significantly reduce fatigue life. • Post-fatigue tensile tests show stiffness drop and elongation gain. This study investigates the fatigue behaviour of polyether ether ketone (PEEK) fabricated via material extrusion additive manufacturing—specifically fused filament fabrication (FFF)—with an emphasis on the interactions between stress, frequency, and temperature. Tensile tests conducted across varying strain rates and temperatures reveal that increasing strain rate enhances tensile strength and Young’s modulus but reduces elongation, whereas increasing temperature lowers tensile strength and stiffness while increasing elongation. Guided by these insights, fatigue tests were performed under cyclic tension–tension loading with a stress ratio of 0.5 at a fixed frequency of 5 Hz to establish a stress–life (S–N) curve. Specimens sustained up to 1 million cycles at 75 MPa (88.3 % of tensile strength), with fatigue life decreasing at higher stress amplitudes. Additional tests at varying frequencies and temperatures further demonstrated reductions in fatigue life at higher frequency and elevated temperature. Compared to moulded PEEK, the fatigue performance of FFF-printed PEEK is degraded by process-induced defects that promote viscoelastic losses, interlayer plasticity, and self-heating, accelerating damage and failure. Fractography using scanning electron microscopy (SEM) revealed a combination of brittle and ductile failure modes, while micro-computed tomography (µCT) enabled visualization of inter-bead delamination within the printed microstructure. These results establish quantitative links between processing, microstructure, and fatigue life in 3D-printed PEEK, providing a foundation for component design and qualification in cyclically loaded, high-performance applications.