Protective applications in extreme environments require materials with superior modulus, strength and specific energy absorption (SEA) at lightweight. They must also have the ability to dampen intense stress waves and absorb kinetic energy from impact while providing thermally stable functionality.

A concussion can be caused by the physical trauma that an impact can cause to the brain. Concussions and other traumatic brain injuries can be mitigated, or perhaps prevented, with helmet materials that better absorb and disperse this kinetic energy before it reaches the brain.

Engineers at the University of Wisconsin-Madison have developed a lightweight, ultra-impact-absorbing foam that could vastly improve helmets designed to protect people from hard impacts.

The new material has a specific energy absorption 18 times higher than the foam currently used in combat helmet liners for the US military and is stronger and stiffer, potentially providing better impact protection.

The researchers’ industry partner, helmet manufacturer Team Wendy, is experimenting with the new material in a prototype helmet liner to investigate its performance in real-world scenarios.

Ramathasan Thevamaran, a UW-Madison engineering physics professor who led the research, said: “This new material has enormous potential for energy absorption and thus impact mitigation, which in turn should significantly reduce the likelihood of brain injury.”

A foam made of vertically oriented carbon nanotubes is a new substance. The basic building blocks the researchers used to construct it were carbon nanotubes, carbon cylinders just one atom thick in each layer.

Because carbon nanotubes already have excellent mechanical capabilities, the researchers created a material with distinctive structural features at different length scales to improve performance. Many micrometre-scale cylindrical structures, made up of carbon nanotubes, form the new architecture of the material.

It took time to determine the ultimate optimal design criteria of the new foam, including cylinder thickness, inner diameter and distance between adjacent cylinders. The researchers experimented methodically, adjusting every parameter and looking at every possible combination.

Thevamaran says, “So we took a few different thicknesses and then tested that with every diameter and every possible gap, and so on. In total, we looked at 60 combinations and ran three tests on each sample, so there were 180 experiments going into this study.”

They discovered a clear winner. The best shock-absorbing foam was created by placing closely spaced cylinders with a thickness of 10 microns or less.

Thevamaran says, “I expected the overall properties to improve due to our interactive architecture, but I was surprised by how dramatically the properties improved when the cylinders were 10 microns thick. It was due to an unusual size effect that showed up in the process structure-property relationships. The effect was very pronounced and it was quite beneficial for the properties we targeted.”

Magazine reference:

  1. Komal Chawla et al. Superior mechanical properties by exploiting size effects and multiscale interactions in hierarchically designed foams. Letters on extreme mechanics. DOI: 10.1016/j.eml.2022.101899