Material and Geometric Effects on Mass Reduction and Structural Performance of an Implicitly Morphed Lattice

Abstract

Energy Storage and Return (ESAR) prosthetic blades are designed to meet design objectives of low mass, sufficient structural margin under physiological loading, and manufacturability. This study presents a staged engineering analysis of a prosthetic geometry, structured across four phases to study and analyze the effects of material selection, lattice geometry, and a structural workaround on mass and performance. Starting from an openly available prosthetic-leg geometry, the blade was evaluated as (Phase 1) a solid AISI 304 stainless steel baseline, (Phase 2) a solid composite baseline using the same geometry and load case, (Phase 3) a graded lattice design combining a hexagonal-truss zone, a body-centered-cubic (BCC) zone, and a field-driven transition zone, and (Phase 4) a homogenized-property structural evaluation of that lattice design using Gibson-Ashby cellular-solid scaling. A 700 N physiological load, derived from a 70 kg nominal body mass, was applied as a pressure boundary condition, with an independently calculated contact area and pressure for the heel (2457 mm², 0.285 MPa) and the toe (4432.9 mm², 0.157 MPa), and the shincup interface fixed as the structural constraint. The steel baseline failed under this load (Factor of Safety, FOS, of 0.2 at the heel and 0.3 at the toe), while the composite baseline survived at both locations (FOS 1.6 at the heel and 2.5 at the toe), isolating a clear material effect. Because direct explicit finite-element simulation of the sub-millimeter lattice strut geometry could not be resolved by the meshless solver used in this study, each lattice zone was instead assigned a homogenized material property card, derived by scaling the parent composite's stiffness, density, and yield strength using Gibson-Ashby relationships at the design's relative density, and this homogenized-property body was then structurally analyzed directly. This workaround identified a structurally inadequate lattice region near the shin-cup interface (FOS below 1), which was corrected back to solid material before the final configuration was reported. The final homogenized lattice design achieved a mass of 0.171 kg against the 0.968 kg steel baseline, an overall reduction of approximately 82.3%, of which the majority is attributable to material substitution and a smaller share to lattice geometry and the subsequent cup-region correction. The final lattice configuration returned a minimum FOS of 1.1 at the heel and 2.0 at the toe under the homogenized-property approximation. These results support treating material selection and lattice geometry as separable, additive contributors to mass reduction, and demonstrate a practical, literature-consistent workaround for approximating lattice structural performance when direct strut-level simulation is not feasible.