Mechanics of Hierarchical Architecture

Current Projects: Past Projects:

Lightweight Nanoarchitected Materials for Impact Energy Absorption

Personnel: Kevin Nakahara (Ph.D. student in Mechanical Engineering)

Materials capable of absorbing and dissipating energy under impact loading are of particular interest for applications ranging from automobiles to body armor. In these instances, metals like aluminum or steel and composite layups like Kevlar are commonly used to absorb the impact. These materials, while good at energy absorption, are heavy and in mobile applications, weight limitations pose a design challenge. Previously, our group has shown that nanoarchitected structures exhibit impact energy absorption levels comparable or exceeding homogeneous materials, and high-performance engineering materials (Kevlar and Spectra composites). These nanoarchitected energy absorbing materials, however, can achieve high energy absorption at a fraction of the weight as demonstrated by the laser induced projectile impact test (LIPIT) results in Portela et. al. 2021. Building upon these initial results, the Greer Group's present work centers around designing, modeling, and characterizing nanoarchitected materials which outperform all presently available materials using innovative fabrication techniques, finite element analysis, and a combination of mechanical experiments. By leveraging developments in scalable nanoarchitected fabrication, our nanoarchitected energy absorbing materials will seek to demonstrate high impact performance at unprecedented macroscopic scales for nanomaterials. Additionally, our works seeks to understand how deformation mechanisms uniquely available to architected geometries can offer additional performance benefits.

The work on LIPIT testing was in collaboration with Prof. Carlos Portela's group (MIT)

Mechanics of 3D-architected Metal-coordinated Soft Materials

Personnel: Seola Lee (Ph.D. student in Mechanical Engineering)

3D-architected soft materials have been proposed as promising candidates to achieve unusual material properties and impact a wide range of real-world applications, such as soft robotics, impact/shock absorbers, and flexible electronics. Taking the benefits of extended design space for material selections and complicated geometry from advancements in additive manufacturing (AM), we study the synergetic interplay between the viscoelasticity of soft and highly deformable metal-coordinated polymer and the nonlinear mechanical response of a flexible and compliant structure they comprise to realize new energy absorption mechanisms of soft mechanical metamaterials. Exploiting the unique reversible electrostatic interactions through the formation and dissociation of dynamic physical bonds, we aim to (i) develop a constituent material with controlled versatile mechanical and viscoelastic properties, (ii) integrate this material into an optimal topology to give rise to a wide range of dynamic mechanical properties such as mechanical resilience, deformability, and energy absorption, and (iii) study the mechanics of 3D-printed soft architectures using a combination of mechanical experiments and finite element analysis.

Mechanics of Node-less Materials

Personnel: Widi Moestopo (Ph.D. student in Mechanical Engineering)

Architectural hierarchy in natural and synthetic materials has been shown to grant these architected materials properties unattainable independently by their constituent materials. While exceptional mechanical properties such as high stiffness- and strength-to-density ratios as well as high shape-recoverability have been realized in many human-made three-dimensional (3D) architected materials using beam-, plate-, and shell-based architectures, stress concentrations and constraints induced by the nodes limit their mechanical performance. At the Greer group, we have recently (i) developed a new hierarchical architecture in which fibers are interwoven to construct effective beams (termed woven lattices), and (ii) explored the mechanics of potentially mass-reproducible smooth, doubly curved, shell-based geometries (termed nanolabyrinthine materials).


We study the mechanics of these novel hierarchical architectures using innovative fabrication techniques, finite element analysis, and a combination of mechanical experiments. For woven lattices, we have shown that these lattices possess extreme deformability and a high level of compliance-two properties that are appealing for flexible electronics and biomedical implants. We are currently exploring the new design spaces opened by woven geometries that are unachievable by similarly configured monolithic-beam architectures. For nanolabyrinthine materials, we have shown that the smooth, doubly curved, shell-based geometries can surpass truss-based architectures in terms of energy absorption, stiffness-to-density response, and especially, mechanical resilience through an unprecedented combination of material size effects and optimal topology.

The work on woven architectures was in collaboration with Prof. Carlos Portela's group (MIT), and the work on nanolabyrinthine materials is in collaboration with Prof. Dennis Kochmann's group (ETH).

Dynamic Properties of Nano-Architected Materials

Relevant Personnel: Seola Lee (Ph.D. candidate in Mechanical Engineering), Dr. Carlos Portela (alumnus)

The use of architecture in materials has been reported to enable novel combinations of mechanical properties such as high stiffness- and strength-to-density ratios. When the material feature size reaches a small enough length scale, interesting size effects can be observed such as in nanometer-sized pyrolytic carbon struts, which have recently been shown to exhibit a rubber-like response. While this size effect has been explored in the quasi-static response of 3D beam-based architected materials, no work has explored its implications in the dynamic response. Exploiting these size effects in the dynamic regime has the potential to enable ultralightweight impact-resistant materials for applications such as ballistic impact, blast loading, and micrometeoroid shielding in space.


Using state-of-the-art fabrication and experimental techniques, we fabricate nano-architected carbon lattice materials and perform ballistic impact experiments using micro-sized projectiles. We are analyzing the effects of relative density (i.e., fill fraction), architecture, material strength, and varying projectile energies on the impact response, and we are developing models to predict the impact response of these 3D nano-architected materials.

This work was pioneered by Greer group alum Prof. Carlos Portela (MIT) and is performed in collaboration with Prof. Keith Nelson's group (MIT).

Relevant publications:
  1. Portela, Carlos M.; Edwards, Bryce W.; Veyesset, David; Sun, Yuchen; Nelson, Keith A.; Kochmann, Dennis M.; Greer, Julia R. (2021) Supersonic impact resilience of nanoarchitected carbon. Nature Materials, 20. pp.1491-1497. doi: 10.1038/s41563-021-01033-z

Past Projects

Fracture Mechanics of Micro- and Nano-Architected Materials

Personnel: Dr. Bryce Edwards (alumnus)

Incorporating three-dimensional (3D) architecture into material design across multiple length scales has led to the creation of advanced materials with unique combinations of mechanical properties such as high stiffness- and strength-to-density ratios, damage tolerance, enhanced deformability, and negative Poisson's ratio, to name a few. While a plethora of studies have been conducted on the compressive strength and toughness of these materials, much is still not known on their resistance to crack propagation (i.e. fracture toughness).

We began our exploration by fabricating and testing hollow-beam nanolattices with center-notched tension geometry, and we are currently developing new versatile methods to study fracture properties of micro- and nano-architected materials.

The current work is in collaboration with Prof. Filippo Berto's group (NTNU).

Microstructural Complexity of Biomaterials and Bio-Mimicking

Relevant Personnel: Dr. Haolu (Jane) Zhang (alumna)

The multifunctionality in Biomaterials, often realized through hierarchical ordering of simple building blocks such as collagen or chitin, make them the inspiration for many additively manufactured synthetic materials. Although usually light and porous, their complex microstructures provide various strategies towards resisting mechanical failure at different relevant length-scales and the potential of novel properties such as self-healing and crack bridging. Here at the Greer group, we study a variety of biomaterials, ranging from human bone to Jellyfish Mesoglea, with a focus on decoding the mysterious link between microstructure and overall mechanical behavior.

The unique microstructure of scorpion pincer cuticles render their mechanical properties both fascinating and complicated. Part of the reason why it has been challenging to fully understand the structure and properties of scorpion exoskeleton is because of its multi-scale nature: components range in size from some nanometers to microns to millimeters and centimeters. Isolating a particular feature has been very challenging. We use the Focused Ion Beam (FIB), nanoindenters, x-ray diffraction, combined with FEM to excise micron-sized site-specific sections of dried cuticle shells to characterize its microstructure and to conduct in-situ nanomechanical experiments on these tiny bone samples to determine their strength, failure, and fracture toughness.

In addition to studying human bone, our group investigates the in-solution behavior of biological tissue, including that of jellyfish and brains. The precise mechanisms that lead to tissue healing without cell regeneration are not well understood; it is believed that the viscoelastic properties with specifically tuned relaxation times contributes to the recovering of these complex materials. We use custom-built experimental set-up to mimic their physiological environments while probing their time-dependent and frequency-dependent material response via in-solution indentation.