Army and MIT researchers advanced a unique experimental device to better test the durability of high performance and robust polymeric materials that appear to strengthen themselves under attack by rapid impact.
The Army Research Laboratory’s Dr. Alex Hsieh, along with Prof. Keith A. Nelson, Dr. David Veysset and Dr. Steven Kooi, from the Army’s Institute for Soldier Nanotechnology at MIT, discovered that when targets made of poly(urethane urea) elastomers, or PUUs, are impacted at very high speed by micro-particles made of silica, the PUU target shows hyperelastic behavior. That is, 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. PUUs also bounce back after the impact, Hsieh said.
The test device uses a pulsed laser to shoot micrometer-sized bullets at targets made of PUUs. Researchers found, for the first time, “behaviors that contrast greatly to the impact response observed in a cross-linked polydimethylsiloxane elastomer where micro-particles penetrated the target and the target material did not bounce back or completely recover.”
Scientists say their discovery on bulk elastomers can help design matrix materials for composites for the future generation of U.S. Army combat helmets. The Army’s enhanced combat helmet uses high performance ultrahigh molecular weight polyethylene, or UHMWPE, fibers based composites. These fibers have high breaking strength, per unit cross section area, about fifteen times stronger than steel but are flexible like fabrics.
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.
Hsieh said the team focused on polymers, which are made up of a very large number of small molecular units that are strung together to form very long chains, which can be well organized or randomly packed. Specifically, polymeric materials that are strong like impact-resistant safety glasses or flexible like rubbers. Elastomers are a class of manmade rubbers, which can be synthesized from a broad range of polymer chemistries.
“They generally have low Young’s modulus which means low resistance to elastic deformation under loading at ambient conditions, and higher failure strain — the capability to sustain significantly greater amount of strain before failure — than most of the plastic materials,” he explained.
To further validate the molecular influence, the team has conducted comprehensive studies on PUUs along with a glassy polycarbonate. 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.
“This is very exciting,” Hsieh said. “Seeing is believing. New understanding from these research discoveries — the essence of hyperelastic phenomenon in bulk elastomers, particularly at the moment of target/impulse interaction, strongly points out to be a plausible pathway key to manipulating failure physics and towards a new design paradigm for robust materials.”
In addition to combat helmets, other potential applications of robust high performance elastomers for soldier protection include but are not limited to transparent face shields, mandible face shields, ballistic vests, extremity protective gear, and blast-resistant combat boots.