4/20/2026 Taylor Parks
Written by Taylor Parks
Mechanical engineering PhD candidate Allison Rzepka, alongside advisor and associate professor Kathryn Matlack, recently published findings from their study of energy-absorbing composites in the International Journal of Mechanical Sciences.
Fellow coauthors include MechSE Professors Sameh Tawfick and Bill King as well as PhD mechanical engineering students Hyeongkeun Kim (co-advised by Tawfick and King) and Christopher Conway (advised by King) and University of Colorado Colorado Springs assistant professor Sezer Ozerinc (formerly advised by King).
In reviewing the literature for energy-absorbing composites, or composite materials that have energy-absorbing properties, the team found an interesting gap.
“Granular media had been shown to have beneficial energy-absorbing mechanisms, and so did architected lattices,” Rzepka said. “But no one had combined the two, and doing so would bring together the best mechanisms from both worlds and create the opportunity for interplay between those constituents.”
From a materials design standpoint, constructing a particular lattice geometry can create properties that are separate from those of the material itself. Architected lattices can take on many different configurations and be designed for different scales. Rzepka investigated two well-studied 2D lattice geometries, the bowtie and hexagonal lattice. In particular, the bowtie lattice has a negative Poisson’s ratio, meaning that it will compress closer together under the point of load as opposed to spread out. In contrast, the hexagonal lattice has a positive Poisson’s ratio. The team tested glass beads and crushed glass (i.e., the same material and diameter in different geometric shapes) in each lattice configuration.
“We could test the same size and material to look at differences in shape and aspect ratio,” Rzepka said, noting that the team also experimented with different garnet media. “We iterated through the different geometries and types of media fill and ultimately found that the bowtie lattice is a better structure for activating energy-absorbing mechanisms.”
“The result that auxetic AGMs preferentially activated the embedded granular media was very exciting to us, and we think the mechanics-based understanding gained from the study can spawn much more fundamental work on these new composites,” Matlack said.
To perform the experiments, the team 3D printed the lattice structures that were filled with different granular media. They tested nearly two dozen iterations under compression, finding that the bowtie lattice resulted in increased particle-particle contact among the media. The increased contact caused unique interactions with the lattice that improved its mechanical response.
“We demonstrated these concepts with granular media, but we’re not limited to [the ones we tested],” Rzepka said. “You can create different medias that can have different shapes.”
Furthermore, the team demonstrated control over the fracture behavior of their composites—in other words, the ability to engineer deformation in materials. These findings are significant for a variety of applications—for example, an array of filled lattice structures could be embedded in the ground around a building or other permanent structure to protect against damage from earthquakes or other disasters that can act on buildings through the ground. The energy-absorbing properties also make this new composite a good candidate material for body armor or other protective wearables.
In terms of next steps, the team anticipates an opportunity for the development of computational tools to begin probing the mechanics of their new composite.
“There aren’t many computational tools that combine finite element analysis with discrete element analysis,” Rzepka said, noting that FEA is a more fitting method for lattices while DEA is better for granular media. Furthermore, the ability to investigate mechanical properties in depth would open up new materials design possibilities.
“I’d like to see a focus on designing the particulate media itself,’ she said. “I think you would see some really cool interplay.”
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Kathryn Matlack is a Richard W. Kritzer Faculty Scholar. Sameh Tawfick is a Ralph A. Andersen Faculty Scholar and a member of the Autonomous Material Systems Group at the Beckman Institute. Bill King is the Ralph A. Andersen Endowed Chair and a faculty member of the Carle Illinois College of Medicine.