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Managing Editor  | October 2017

Durable material shows promise for future combat helmets


A team of researchers from the U.S. Army Research Laboratory (Adelphi, Md.) and the Institute for Soldier Nanotechnology at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. has developed a novel experimental device to test the durability of materials composed of high-performance polymers that strengthen when attacked by rapid impact.

 

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Scientists say their discovery on bulk elastomers can help design matrix materials for composites for the future generation of U.S. Army combat helmets. (U.S. Army illustration)

 

According to a report from the laboratory, scientists discovered that targest made from poly(urethane urea) elastomers (PUU) were impacted at high speeds by microparticles of silica the PUU demonstrated hyperelastic behavior.

 

“They become extremely stiff when deformed at strain rates on the order of 108/s, which means roughly that the material of the target deforms to half of its original thickness in an extremely short time equal to one second divided by hundred millions,” the article explained. PUUs also bounce back after the impact.”

 

Lasers were used to shoot micrometer-sized bullets at targets made from PUU. The discovery of this new property in bulk elastomers could lead to the design of composite materials to enhance the helmets worn by soldiers in combat.

 

“Traditional armor material designs include ceramics, metals and lightweight fiber reinforced composites for both soldier and vehicle protection, which are typically based on stiffness, the resistance of a material against deformation, toughness, the ability to absorb energy and plastically deform prior to fracture,” the report said.

 

The researchers focused primarily on polymers that could be well-organized and tightly-packed, especially polymers that have strength similar to impact-resistant safety glasses or flexible like rubber. One researcher indicated that elastomers have “low resistance to elastic deformation under loading and ambient conditions and higher failure strain.”

 

The article added, “While polycarbonate is known for its high fracture toughness and ballistic strength, these PUUs, regardless of their respective composition, exhibited greater dynamic stiffening during impact at strain rates on the order of 108/s. Furthermore, the resistance against penetration of the micro-particle can be optimized, i.e. a ~ 50 percent reduction in the average maximum depth of penetration was achieved by simply varying the molecular composition of PUUs.”

 

Researchers found that PUU with multiple relaxations times, which is how efficiently the molecules in a polymer chain respond to external stimuli, are ideal because they combine dynamic stiffening with additional energy absorption towards dynamic strengthening.

 

The research was recently published in Polymer. The abstract stated:

 

“The dynamic deformation response of select model poly(urethane urea) elastomers (PUU) at high strain rates is investigated via an all-optical laser-induced projectile impact test (LIPIT). LIPIT measurements allow the direct visualization of the impact of micro-projectiles (silica spheres) on substrates and in-situ characterization, including depth of penetration and the extent of rebound of the micro-projectiles.

 

“PUUs are proven to be robust and the silica spheres are observed to rebound from them upon impact. In addition, for PUUs a strong correlation was noted between the coefficient of restitution and the maximum depth of penetration.

 

“Also, the coefficient of restitution data is comparable to that of glassy polycarbonate (PC), which is in great contrast to the comparison of the corresponding ambient storage modulus data obtained via dynamic mechanical analysis at 1 Hz.

 

“We hypothesize that high-rate deformation-induced glass transition is a plausible molecular relaxation mechanism towards macroscopic, dynamic stiffening/strengthening in PUUs.”

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