Bench-top test quickly identifies highly impact-resistant materials

A complex structure of honeycomb-shaped struts and beams could withstand a supersonic impact better than a solid slab of the same material. Additionally, the specific structure is important, with some being more resistant to impacts than others.

That’s what MIT engineers are discovering in experiments with microscopic metamaterials, materials intentionally printed, assembled, or otherwise designed with microscopic architectures that give the entire material exceptional properties.

In a study published today in the Proceedings of the National Academy of Sciencesengineers report a new way to quickly test a range of metamaterial architectures and their resilience to supersonic impacts.

In their experiments, the team suspended tiny arrays of printed metamaterials between microscopic support structures, then fired even smaller particles at the materials, at supersonic speeds. Using high-speed cameras, the team then captured images of each impact and its aftermath, with nanosecond precision.

Their work identified a few metamaterial architectures that are more resilient to supersonic impacts than their fully solid, non-architectural counterparts. The researchers say the results they observed at the microscopic level can be extended to comparable impacts at the macro scale, to predict how new material structures at length scales will withstand real-world impacts.

“What we learn is that the microstructure of your material is important, even at high strain,” says study author Carlos Portela, UK Career Development and Alex d’Arbeloff Professor of Engineering. mechanics at MIT. “We want to identify impact-resistant structures that can be made into coverings or panels for spacecraft, vehicles, helmets and anything else that needs to be lightweight and protected.”

Other authors of the study include Thomas Butruille, first author and MIT graduate student, and Joshua Crone of the military research laboratory DEVCOM.

Pure impact

The team’s new high-speed experiments build on their previous work, in which engineers tested the resilience of an ultralight carbon-based material. This material, thinner than the width of a human hair, was made of tiny carbon struts and beams, which the team printed and placed on a glass slide. They then projected microparticles towards the material, at speeds exceeding the speed of sound.

These supersonic experiments revealed that the microstructured material resisted high-speed impacts, sometimes deflecting the microparticles and sometimes capturing them.

“But there were many questions we couldn’t answer because we were testing the materials on a substrate, which could have affected their behavior,” Portela says.

In their new study, the researchers developed a way to test self-sustaining metamaterials, to observe how the materials resist impacts on their own, without a support or supporting substrate.

In their current configuration, the researchers suspend a metamaterial of interest between two microscopic pillars made of the same base material. Based on the dimensions of the metamaterial being tested, researchers calculate the distance between the pillars in order to support the material at each end while allowing the material to respond to any impact, without any influence from the pillars themselves.

“This way we ensure that we are measuring the material property and not the structural property,” explains Portela.

Once the team chose the pillar support design, they then tested various metamaterial architectures. For each architecture, the researchers first printed the support pillars on a small silicon chip, then continued to print the metamaterial as a layer suspended between the pillars.

“We can print and test hundreds of these structures on a single chip,” says Portela.

Punctures and cracks

The team printed suspended metamaterials that resembled complex honeycomb-shaped cross-sections. Each material was printed with a specific three-dimensional microscopic architecture, such as a precise scaffold of repeating bytes or additional faceted polygons. Each repeated unit measured as small as a red blood cell. The resulting metamaterials were thinner than the width of a human hair.

The researchers then tested the impact resilience of each metamaterial by projecting glass microparticles toward the structures, at speeds of up to 900 meters per second (more than 2,000 miles per hour), well within the supersonic range. They filmed each impact with a camera and studied the resulting images, frame by frame, to see how the projectiles penetrated each material. Then, they examined the materials under a microscope and compared the physical consequences of each impact.

“In architectural materials, we saw this morphology of small cylindrical craters after impact,” Portela explains. “But in solid materials, we saw many radial cracks and larger pieces of material gouged out.”

Overall, the team observed that the fired particles created small perforations in the lattice metamaterials, and the materials nevertheless remained intact. In contrast, when the same particles were thrown at the same speeds into solid, lattice-free materials of equal mass, they created large cracks that propagated quickly, causing the material to crumble. Microstructured materials were therefore more effective in resisting supersonic impacts as well as protecting against multiple impacts. And in particular, documents printed with repetitive bytes seemed the most resilient.

“At the same speed, we find that the architecture of the octet is more difficult to break, meaning that the metamaterial, per unit mass, can withstand impacts up to twice as large as the material in bulk,” Portela explains. “This tells us that there are certain architectures that can make a material stronger and provide better impact protection.”

In the future, the team plans to use the new rapid testing and analysis method to identify new metamaterial designs, hoping to mark architectures that can be extended to protective gear, clothing, stronger and lighter coatings and panels.

“What I’m most excited about is showing that we can do a lot of these extreme experiments on a benchtop,” Portela said. “This will significantly accelerate the speed at which we can validate new, high-performance, resilient materials.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.