Topology-graded structures: Exploring heterogeneous architected materials for load bearing mechanics
| dc.contributor.author | Choi, Chiara | |
| dc.contributor.supervisor | Yu, Bosco | |
| dc.contributor.supervisor | Giles, Joshua W. | |
| dc.date.accessioned | 2026-04-28T21:53:28Z | |
| dc.date.available | 2026-04-28T21:53:28Z | |
| dc.date.issued | 2026 | |
| dc.degree.department | Department of Mechanical Engineering | |
| dc.degree.level | Master of Applied Science MASc | |
| dc.description.abstract | Heterogeneous architected materials (HAMs) are cellular structures whose topology, geometry, or density varies spatially. They represent an emerging frontier in materials engineering, offering property combinations inaccessible to both conventional monolithic solids and homogeneous lattice architectures. Enabled by advances in additive manufacturing, HAMs can be deliberately engineered to produce locally tailored stiffness, strength, energy absorption, and permeability across a single monolithic component. Despite rapid progress, the structure–property relationships of complex heterogeneous networks remain incompletely understood, and a fundamental gap exists between idealized lattice studies and the design of load-bearing components that exploit spatial heterogeneity as a functional design variable. This thesis addresses this gap through two complementary studies, treating heterogeneous architected materials first as materials, then as elements of an engineered system. The first study investigates the compressive mechanics of additively manufactured hexagonal meta-materials incorporating 5-7 defect arrangements inspired by graphene grain boundary geometries. Three specimen sets, bidomain meta-crystals ("bi"), polydomain meta-crystals ("poly"), tilted meta-crystal ("tilted"), were fabricated via selective laser sintering in Nylon PA2200 across eight misorientation angles ranging from 0° to 30°. Quasi-static in-plane compression experiments and finite element simulations were used to characterize apparent compression modulus, peak stress, densification strain, and energy absorption. Results suggest that by varying the misorientation angle of a pure and 5-7 defect hexagonal system a dependency is observed for the strength of the structure, in contrast to energy absorption and stiffness, which exhibit an isotropic response independent of misorientation angle. The introduction of 5-7 defects in a stacking configuration impedes shear band propagation and promotes localized normal deformation, elevating peak stresses, while staggering configurations encourage controlled intersecting shear bands producing characteristic shear collapse. A key finding is that Hall-Petch-like strengthening in polydomain meta-crystals is not unilateral: the stacking configuration yields a Hall-Petch-like relationship with decreasing domain size, while the staggering configuration produces an inverse relationship, establishing misorientation angle as a factor in determining which strengthening regime applies. Experimental and finite element results for bidomain meta-crystal specimens are in general agreement in trend and magnitude; scatter in experimental data is attributed to process-induced factors inherent to selective laser sintering, such as thermal shrinkage and retained powder, which introduce variability not fully captured by the simulation model. Building on these mechanistic insights, the second study introduces a novel simple cubic–face-centered cubic (SC-FCC) unit cell topologically graded design for load-bearing hip implants. A crystallographic-based design method employing symmetry-parameter coordinates was adapted to mathematically describe and fabricate intermediate architectures spanning the SC-to-FCC transition. Two design groups were developed: a uniform group with constant strut thickness to isolate geometric effects, and a tailored group with varying strut thickness tuned to replicate the stiffness of cortical bone (10–30 GPa). Specimens were fabricated via stereolithography for compression and bending characterization, and via selective laser sintering in C300 maraging steel for implant-level testing. Quasi-static compression experiments confirmed that the tailored group produces a linear progression in apparent compression modulus and peak stress across the SC-FCC transition, while the uniform group exhibits a parabolic profile. Four-point bending tests demonstrated that longer SC-FCC transition zones achieve the highest bending modulus and maximum stress, outperforming abrupt step-change configurations. Implant performance was evaluated using a digital image correlation system on specimens embedded in bone-mimicking foam and loaded to 2300 N. All implant variants produced average foam strains within Frost's Mechanostat ideal remodeling range of 1500–3000 με, and interfacial micromotion remained below the 30 με threshold which has been documented to impact osseointegration across all measurement locations. Pore sizes of 100–2600 μm and bulk porosities of 46–60% confirmed compatibility with both vascularization and bone ingrowth requirements. Collectively, this work establishes that deliberate spatial heterogeneity in unit cell topology, whether through grain-boundary-inspired defect engineering or through continuous SC-FCC grading, provides independent control over mechanical performance that relative density variation alone cannot achieve. The findings advance the mechanistic understanding of heterogeneous architected materials and demonstrate a viable, experimentally validated pathway for their deployment in next-generation additively manufactured load-bearing applications. | |
| dc.description.scholarlevel | Graduate | |
| dc.identifier.uri | https://hdl.handle.net/1828/23753 | |
| dc.language | English | eng |
| dc.language.iso | en | |
| dc.rights | Available to the World Wide Web | |
| dc.subject | architected materials | |
| dc.subject | heterogeneous | |
| dc.subject | load-bearing mechanics | |
| dc.subject | topology | |
| dc.subject | gradient | |
| dc.subject | materials sience | |
| dc.title | Topology-graded structures: Exploring heterogeneous architected materials for load bearing mechanics | |
| dc.type | Thesis |