Abstract
Fatigue fracture of titanium alloys fabricated by Selective Laser Melting (SLM) in engineering has been studied in recent years, but an underlying understanding of the in-vivo fatigue fracture of SLM-fabricated titanium patient-specific implants (PSIs) is rarely revealed due to the complex in-vivo biomechanical behaviour and biological environment combined. This paper provides a multidimensional and comprehensive analysis of the in-vivo fatigue failure mechanism of an SLM-fabricated Ti6Al4V mandible reconstruction plate, by macro-scaled Finite Element Analysis (FEA) of biomechanical behaviours, static force analysis and micro-scaled metallurgical investigation. FEA indicates maximal tensile stress of 392.6 MPa locally on the implant with high stress gradients around. Although the theoretical stress is far below the fatigue strength of SLM-fabricated Ti6Al4V material, indicating low potential of plate failure, the site with maximal stress and high stress gradients successfully determine the failure route of the plate, which coincides with the factual crack growth mode on the plate. The followed analysis of stress and force conditions theoretically confirms the failure condition on the implant. Metallurgical investigations further show β and α/α’ phases and tiny pores at implant (sub)surface, together with typical features (striations, cleavage facets and dimples) of metal fatigue failure in fractography analysis. Taken together these can conclude that the fatigue crack initiates from the implant surface subjected to surface defects and maximal stress caused by implant design, and propagates radially, showing a combined brittle/ductile fracture mode. The process is synergistically influenced by Ca/P/O deposits and hydrogen embrittlement under the biological environment. Practical measures for the improvement of in-vivo fatigue life of implants are finally provided.
Shi, Qimin