Design and mechanical analysis of additively manufactured primitive flexures

Abstract

In the design of compliant mechanisms, thin flexural members, consisting of bends and curves, can be used to produce a controlled path of rotation under load, which could benefit the design of cervical artificial disc replacements (c-ADR) for example to improve compliance and avoid adjacent segment degeneration. However, to date, there is little literature to characterize the design and mechanical properties of additively manufactured flexural members, limiting crucial data needed to inform implant design. To address this knowledge gap, the goal of this work is to design and additively manufacture a family of primitive flexures to explore how different design features affect the resulting mechanical response under load. Several flexure primitive design features were varied within a common 3-prong flexure component design, including flexure thickness, overhang angle, and number of bends. The response of these designs was then analyzed through applied loading (non-destructive cyclic bending and compression to failure) based on a targeted application of a compliant mechanism for cervical artificial disc replacements. The ability to realize complex parts with latticed or flexible features has value in improving compliance in orthopaedic applications. The primitive flexure designs were printed using Ti6Al4V on the EOS M290 laser powder bed fusion system. For mechanical testing, the flexures were printed between custom-designed endplates to attach to the AMTI VIVO joint motion simulator. The deformation response was captured using the ARAMIS 3D digital image correlation system. Testing results indicated that thicknesses of at least 1mm were required in Ti6Al4V flexures to replicate the axial compressive stiffness in the cervical spine. Introduction of compliant flexure zones led to a negligible reduction in stiffness, while increasing structure compliance in compression and rotation.