Anisotropic Behaviour of Extruded Short Carbon Fibre Reinforced PEEK Under Static and Fatigue Loading

Abstract

The influence of fibre orientation on an extruded short carbon fibre PEEK composite, reinforced with 30% weight fraction of short carbon fibres, was investigated. Specimens were obtained from extruded plates along directions transverse and parallel to the extrusion flow and different types of fibre spatial arrangements were observed. Prior to fatigue testing the elastic modulus of each specimen was measured and the specimens were grouped into homogeneous families, from low to high modulus. Each family exhibited distinct fatigue strength, associated with different progressive damage patterns and failure mechanisms. With increasing alignment of the fibres along the loading axis, a noticeable improvement of fatigue strength could be observed, whereas when the fibres were mostly perpendicular, fatigue properties were even lower than those of unreinforced PEEK. Overall, results suggest that extruded short carbon fibre PEEK products may exhibit a heterogeneous microstructure and a wide range of fatigue properties, making the definition of design allowables a critical task.

High-Cycle Fatigue Behaviour of Polyetheretherketone (PEEK) Produced by Additive Manufacturing

“…As the maximum stress was decreased to 75% of 3D-printed PEEK’s tensile strength, infinite fatigue life (runout) was reached (specimen 8), which corresponds to a fatigue strength of 65 MPa. This stress level for the fatigue strength of 3D-printed PEEK is similar to the stress level of 70–75% at the endurance limit reported for bulk PEEK specimens [ 33 , 39 , 40 , 42 , 44 ]. The experimental results show that the fatigue behaviour of 3D-printed PEEK can be compared to that of bulk extruded or moulded PEEK specimens.…”

“…Figure 7 plots the S-N curve obtained in this work for 3D-printed PEEK along with curves obtained in previous works where neat PEEK specimens were cyclically loaded under tension–tension conditions [ 40 , 51 ]. The plotted S-N curves were obtained from neat PEEK specimens by Avanzini et al [ 40 ] and from differently notched PEEK specimens aged in PBS at 37 °C by Sobieraj et al [ 51 ], in both cases machined from extruded PEEK.…”

“…Figure 7 plots the S-N curve obtained in this work for 3D-printed PEEK along with curves obtained in previous works where neat PEEK specimens were cyclically loaded under tension–tension conditions [ 40 , 51 ]. The plotted S-N curves were obtained from neat PEEK specimens by Avanzini et al [ 40 ] and from differently notched PEEK specimens aged in PBS at 37 °C by Sobieraj et al [ 51 ], in both cases machined from extruded PEEK. The fatigue strength reported by Avanzini et al [ 40 ] of 75 MPa corresponds to about 75% of PEEK’s ultimate tensile strength (UTS), which is the same stress level as the fatigue strength reported in this work.…”

“…The plotted S-N curves were obtained from neat PEEK specimens by Avanzini et al [ 40 ] and from differently notched PEEK specimens aged in PBS at 37 °C by Sobieraj et al [ 51 ], in both cases machined from extruded PEEK. The fatigue strength reported by Avanzini et al [ 40 ] of 75 MPa corresponds to about 75% of PEEK’s ultimate tensile strength (UTS), which is the same stress level as the fatigue strength reported in this work. Considering this, the fatigue behaviour of 3D-printed PEEK resembles that of bulk PEEK when accounting for the lower tensile strength of the 3D-printed specimens.…”

“…The high-cycle tension–tension fatigue behaviour of bulk PEEK has been documented in comparison to surface porous PEEK [ 38 ] and PEEK-based composites like carbon fibre-reinforced PEEK (CF-PEEK) [ 39 , 40 ] and PEEK loaded with hydroxyapatite (PEEK-HA) [ 41 , 42 , 43 ]. In these works, bulk PEEK samples are reported to withstand infinite life with maximum stresses of up to 75% of their tensile strength.…”