Carbon fibre reinforced PEEK laminated as a material for orthopaedic devices: An experimental and computational investigation

Abstract

The overall objective of this thesis is to investigate carbon fibre reinforced poly-ether-ether-ketone (PEEK) laminates as a material for fracture fixation devices through a combination of experimental testing and computational modelling. Orthopaedic devices using unidirectional carbon fibre reinforced PEEK laminates potentially offer several benefits over metallic implants including: anisotropic material properties; radiolucency and strength to weight ratio. However, despite FDA clearance of PEEK-OPTIMA™ Ultra-Reinforced, no investigation of the mechanical properties or failure mechanisms of a medical grade unidirectional laminate material has been published to date, thus hindering the development of first-generation laminated orthopaedic devices. This study presents the first investigation of the mechanical behaviour and failure mechanisms of PEEK-OPTIMA™ Ultra-Reinforced. This thesis presents a multi-axial suite of experimental tests are presented: 0° and 90° tension and compression, in-plane shear, mode I and mode II fracture toughness, compression of ±45° laminates and flexure of 0°, 90° and ±45° laminates. Three damage mechanisms are uncovered: (1) inter-laminar delamination, (2) intra-laminar cracking and (3) anisotropic plasticity. A computational damage and failure model that incorporates all three damage mechanisms is developed. The model accurately predicts the complex multi-mode failure mechanisms observed experimentally. The ability of a model to predict diverse damage mechanisms under multiple loading directions conditions is critical for the safe design of fibre reinforced laminated orthopaedic devices subjected to complex physiological loading conditions.

Laminated fracture fixation plates can be designed with custom anisotropic material properties, thus enabling the engineer to tailor the overall stiffness of the implant to the specific loading conditions it will experience in vivo. In this work a multi-scale computational investigation of idealised distal radius fracture fixation plate (DRP) is conducted. Physiological loading conditions are applied to macro-scale finite element models of DRPs. The mechanical response is compared for several carbon fibre reinforced PEEK (CF/PEEK) laminate layups to examine the effect of ply layup design. The importance of ply orientation in laminated DRPs is highlighted. A high number of 0° plies near the outer surfaces results in a greater bending strength while the addition of 45° plies increases the torsional strength of the laminates. Intra-laminar transverse tensile failure is predicted as the primary mode of failure. A micro-mechanical analysis of the CF/PEEK microstructure uncovers the precise mechanism under-lying intra-laminar transverse tensile crack to be debonding of the PEEK matrix from carbon fibres. Plastic strains in the matrix material are not sufficiently high to result in ductile failure of the matrix. The findings of this study demonstrate the significant challenge in the design and optimisation of fibre reinforced laminated composites for orthopaedic applications, highlighting the importance of multi-scale modelling for identification of failure mechanisms. A common feature of all such fixation devices is that screws are used to anchor the plates to the fractured bone. However, drilling holes in fibre reinforced laminates has a direct effect on the laminate strength. Therefore, fully characterising the open and filled hole failure mechanics of fibre reinforced laminates is essential. Additionally, it is imperative that the computational modelling framework developed in this thesis can accurately predict the damage and failure progression at screw holes. Experimental testing of both open hole and filled hole configurations in tension and compression reveal extensive intra-laminar cracking initiating on the transverse side of the hole in the open hole tests and on the axial side of the hole in the filled hole experiments. Failure mechanisms for open and filled hole tests are correctly predicted by the computational model.

When holes are machined in fibre reinforced laminates the continuous fibres are broken, promoting inter-laminar delamination and cracking at screw hole sites. An alternative approach is to mould the screw holes into the implant during processing, eliminating the requirement of drilling holes following fabrication of the laminate structure. In this approach carbon fibres are moulded around the holes during processing, so that fibres remain continuous in the laminate without broken exposed fibre ends at the site of the hole. This thesis presents, for the first time, a computational methodology for creating finite element models of moulded holes in continuous fibre reinforced laminated devices. Models of fracture fixation devices with moulded and drilled holes are constructed and their mechanical performance is compared. The drilled implant is predicted to have considerably greater failure strength in compression and bending but a slightly lower failure strength in torsion.

The findings of this thesis have important implications for the development of next-generation laminated fibre reinforced fracture fixation plates. The experimental results represent the first full material characterisation of medical grade unidirectional carbon fibre reinforced PEEK and uncovers the complex failure mechanisms present. The novel damage and failure computational model presented provides the ability to predict diverse damage mechanisms under multiple loading directions conditions is critical for the safe design of fibre reinforced laminated orthopaedic devices.