Plastic zone to crack growth: Unraveling thickness-dependent fracture mechanism in PEEK

Polyether-ether-ketone (PEEK) is a high-performance biomaterial increasingly being employed in biomedical implants due to its excellent mechanical properties, radiolucency, and biocompatibility. Despite its clinical effectiveness, PEEK implant components remain prone to fracture under in-service conditions, and systematic research into the role of specimen thickness in determining fracture behaviour and mechanism remains inadequate. This study investigates the thickness-dependent fracture mechanism of PEEK using compact tension (CT) specimens with five thicknesses ranging from approximately 17 to 41 mm. Mode I fracture tests were performed to obtain load–crack opening displacement (COD) curves and conditional fracture toughness (Kq), which were compared with predictions from a phase-field model (PFM). The PFM reproduced load–COD responses with good accuracy across all thicknesses. Fractographic analysis using optical microscopy and scanning electron microscopy revealed that thinner specimens developed comparatively larger plastic zones, with shear dimples and risers indicating significant plastic deformation, which promoted crack branching. In contrast, thicker specimens exhibited smaller plastic zones, single crack paths, and slightly higher conditional fracture toughness, until a brittle-like transition emerged at maximum thickness. These findings demonstrate that PEEK shows a different thickness dependent fracture behavior which does not follow a monotonic toughness-thickness trend.