Researchers from the U.S. Army and MIT have created a unique experimental device to test the durability of high performance and robust polymeric materials that strengthen when they are under attack.
Army Research Laboratory’s Dr. Alex Hsieh, with a research team from the Army’s Institute for Soldier Nanotechnology at MIT, have discovered that when targets made of poly elastomers or PUUs, are impacted at a very high speed by micro-particles made of silica, the PUU targets have hyperelastic behavior. This means they become extremely stiff when deformed at strain rates on the order of 108/s, and the material of the targets deform to half of their original thickness in a very short time -- around one second divided by one hundred million. The PUUs bounce back after the impact.
The tests' device uses a pulsed laser to shoot micrometer-sized bullets at targets made of PUUs. Researchers found “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 can help design matrix materials for composites for the future of Army combat helmets. The Army’s enhanced combat helmet uses high performance, ultra-high molecular weight polyethylene or UHMWPE fibers-based composites. The fibers have high breaking strength per unit cross-section area -- about fifteen times stronger than steel while staying flexible like fabrics.
Traditional armor materials include ceramics, metals and lightweight fiber reinforced composites for soldier and vehicle protection, which are typically based on stiffness. The resistance of a material against deformation and toughness, with the ability to absorb energy and plastically deform prior to fracture, are important parts of a good armor.
From the materials science perspective, the standard bulk metrics alone are not sufficient to quantify how fast molecules in a polymer solid can change their mobility with respect to the rate of deformation. The tendency for a change of the respective physical state during dynamic deformation sparked the research teams' interest.
Hsieh said the team focused on polymers, which are made of a large number of small molecular units strung together to form a long chain that can be well organized or randomly packed. Polymeric materials are strong, for uses like impact-resistant safety glasses or flexible rubbers. Elastomers are a class of manmade rubbers that 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 a significantly greater amount of strain before failure than most of the plastic materials," explained Hsieh.
In order to further study the molecular influence, the team has conducted studies on PUUs, along with a glassy polycarbonate known for its high-fracture toughness and ballistic strength. These PUUs exhibited greater dynamic stiffening during impact at strain rates on the order of 108/s. The resistance against penetration of the microparticle can be optimized.
"This is very exciting," said Dr. Hsieh. "Seeing is believing. New understanding from these research discoveries - the essence of the hyperplastic 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."
PUUs are known to have a complex microstructure, along with a broad range of relaxation times. For PUUs, molecules with longer relaxation time on the order of microseconds at ambient conditions, like slower dynamics, enable dynamic stiffening. Those with nanosecond relaxation times at ambient conditions were capable of providing additional energy absorption towards dynamic strengthening. These viscoelastic characteristics show the elastomers, as well as other polymeric materials, may deform in different fashions, depending on how fast it is being deformed.
The team believes that a cooperative molecular relaxation mechanism resembles a resonance phenomenon of “chainmail-like” molecular motions, each oscillating at specific frequencies to dissipate absorbed energy. These dynamic strengthening and stiffening characteristics could be facilitated by intermolecular hydrogen bonding, present through the physically cross-linked network in PUUs.
In contrast, the microsecond relaxation at ambient conditions is not present in polycarbonate, nor is hydrogen bonding and the corresponding enabling molecular mechanism available at all in polycarbonate, despite its toughness and impact strength. PUUs or high-performance elastomers with multiple relaxation times are key to enabling the dynamic strengthening and dynamic stiffening over the temporal scale from microseconds to nanoseconds.
In addition to combat helmets, other potential applications of robust high-performance elastomers for soldier protection include transparent face shields, mandible face shields, ballistic vests, extremity protective gear and blast-resistant combat boots. Researchers hope to use this research discovery for protection of football players and young athletes against concussions or any other brain-related injuries via collisions.
A paper on this research was published in Polymer.