Fulco (right) and Turner (left), show how they used light to gauge the mechanical stress affected the metamaterial. (Image courtesy of Bella Ciervo)
For years, 3D-printed metamaterials have teased engineers with their wispy, featherweight promise. But in reality, many of them have crumbled under pressure. “Toughness is a limiting factor in not all, but many 3D-printed mechanical metamaterials,” said Kevin Turner, Professor and John Henry Towne Department Chair of Mechanical Engineering and Applied Mechanics (MEAM) at Penn Engineering, in a press release.
Now, Turner and colleagues at Penn Engineering, Penn Arts & Sciences and Aarhus University are extracting strength from chaos. That is, the researchers have developed a metamaterial that’s 2.6 times more crack-resistant by introducing “controlled disorder” into normally repeating lattice structures. By mimicking irregular patterns found in bone and seashells, the researchers accomplished that feat “without changing the material at all — just simply by altering the internal geometry,” Turner said.
‘Controlled disorder’ to the rescue
Mechanical metamaterials derive their unique properties not from their chemical composition but from their precisely engineered internal geometries. Traditionally, these structures feature highly ordered, repeating patterns like honeycombs or triangular lattices. The Penn researchers challenged this convention by introducing “controlled disorder”—strategically moving the connection points (nodes) of lattice structures slightly off their regular positions.
“The samples that performed the best, in which it was most difficult for a crack to grow, did not consist of regular repeating patterns,” explained Sage Fulco, a postdoctoral researcher and the paper’s lead author, in a press release. “They had different geometry in different areas.”
How it works
This approach mimics natural materials that exhibit exceptional toughness. Human bone contains tiny support beams (trabeculae) arranged in seemingly random networks that distribute stress and prevent catastrophic failure. Similarly, nacre (mother-of-pearl) in seashells and mussel attachment threads employ subtle irregularities at the microscale to achieve remarkable resilience.
To test their hypothesis, the researchers performed thousands of computational simulations on triangular lattice structures with varying levels of disorder. They then created physical versions using laser-cut polymethylmethacrylate (PMMA) and subjected them to controlled fracture tests.
“Mechanical metamaterials with engineered failure properties typically rely on periodic unit cell geometries or bespoke microstructures to achieve their unique properties,” the authors write in the paper. “We demonstrate that intelligent use of disorder in metamaterials leads to distributed damage during failure, resulting in enhanced fracture toughness with minimal losses of strength.”
Key findings and visual evidence
In contrast to the more structured design (top), the more disordered one (bottom) cracked less easily. (Image courtesy of Sage Fulco)
Specimens with an optimal level of disorder (approximately 15% deviation from perfect symmetry) were more than twice as tough as their ordered counterparts. This improvement occurred with minimal loss in strength (≤25%) and negligible change in stiffness.
Photoelastic imaging, which is a method that visualizes stress through color patterns when material is viewed through polarized light, provided clues as to what is going on. In ordered lattices, cracks propagate in straight lines. See the top portion of the figure on the right. In disordered lattices, however, cracks must travel through a more complex path, and thus traverse damage across a wider area.
“For the crack to grow through a disordered material, damage has to occur over a much larger area,” notes Fulco. This distributed damage mechanism is clearly visible in side-by-side comparisons of ordered versus disordered specimens under stress.
“At the optimal level of disorder, the toughness is more than 2.6× of an ordered lattice of equivalent density,” the paper reports. “These enhancements in toughness are achieved with minimal losses in strength of ≤25% and negligible changes in stiffness.”
Comparisons to existing toughening methods
Traditional approaches often rely on material-based solutions like fiber reinforcement or specialized coatings that add weight and cost. In contrast, “controlled disorder” achieves improvements through geometric modification alone.
Challenges: The research identifies a key balancing act: “There was a specific level of disorder, so that the patterns we cut into the material looked somewhat regular but not exactly symmetrical, where we were able to achieve the highest level of performance,” notes Fulco. The paper identifies approximately 15% node perturbation as the optimal zone for maximum toughness enhancement.
Looking ahead: The researchers envision exploring multi-scale disorder and combining this approach with other bioinspired design strategies. “Combining different types of materials and adding different geometries at different scales are very exciting opportunities,” says Fulco. “That’s what we see when we look at the highest performing natural materials.”
Potential application could range from aerospace and defense, medical devices, the automotive sector, industrial machinery and beyond.
Three representative designs showing different levels of disorder. The middle design balances order and disorder for maximum toughness. (Image courtesy of Sage Fulco)
Future development
“Disordered systems aren’t often used in engineering because the design is much more complex,” Turner points out. Implementation will require computational tools that can model and optimize irregular patterns while predicting their mechanical behavior.
The approach can be used with current manufacturing methods, including laser cutting, 3D printing and other digital fabrication methods. That is, implementation mainly requires changes to design files rather than to production processes.
Research covering similar ground to the 3D-printed metamaterials described above includes the following studies and preprints:
| Study | Year | Focus | Relevance to Current Research |
|---|---|---|---|
| Disorder Enhances the Fracture Toughness of Mechanical Metamaterials | 2024 | Controlled disorder in 3D-printed metamaterials, 2.6× toughness increase | The researchers announced “a novel strategy to toughen 2D materials through lattice disorder.” |
| Disordered mechanical metamaterials | 2023 | Review of disorder enhancing mechanical performance, inspired by biology | The paper notes:”In this Perspective article, we use examples of biological materials with disordered structures to elucidate how disorder can enhance the mechanical performance of engineered mechanical metamaterials.” |
| Fracture toughness and maximum stress in a disordered lattice system | 2010 | Theoretical model of disorder in 2D elastic networks, fracture strength | Related concept, but not 3D-printed metamaterials. The authors note in the abstract: “We model the disorder-induced increase in fracture strength by applying a two-dimensional elastic network with an initial crack.” |
| Deformation and fracture of 3D printed disordered lattice materials | 2019 | Modeling behavior under tensile load, disordered lattices | Relevant context. It is described as “A method is presented to model deformation and fracture behavior of 3D printed disordered lattice materials under uniaxial tensile load. “ |
| Toughening two dimensional materials through lattice disorder | 2024 | Disorder in 2D materials like graphene, fracture toughness | Relevant abstract quote: “Here, by investigating fracture mechanisms in monolayer amorphous carbon (MAC), we reveal a novel strategy to toughen 2D materials through lattice disorder.” |
| Disorder Unlocks the Strength-Toughness Trade-Off in Metamaterials | 2024 | Disorder in networks, up to 100% toughness increase | Related: “Our analysis revealed two distinct failure mechanisms, with some disordered networks outperforming regular hexagonal honeycombs by up to 20% in strength and 100% in toughness.” |
