Schematic illustration of the laser-induced micro-particle impact test Credit: U.S. Army Research Laboratory



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 where micro-particles penetrated the target and the target material did not bounce back or completely recover."


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 ? and toughness ? the ability to absorb energy and plastically deform prior to fracture.

Contrast between high strain-rate impact response observed in (a) PUU and (b) polydimethylsiloxane elastomers under laser-induced micro-particle impact Credit: U.S. Army Research Laboratory

But from the materials science perspective, these 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, nor the propensity towards a change of their respective physical state during dynamic deformation ? i.e. could elastomers change from rubber-like to glass-like when they are deformed at increasing high rates?

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 , and higher failure strain ? the capability to sustain significantly greater amount of strain before failure ? than most of the plastic materials," he explained.


"This is very exciting." said Dr. Hsieh "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."


Scanning electron microscopy image revealing permanent indent on the surface of polycarbonate in contrast to PUUs where no damage was observed after impact by silica micro-particles. Credit: U.S. Army Research Laboratory






More information: David Veysset et al, Molecular influence in high-strain-rate microparticle impact response of poly(urethane urea) elastomers, Polymer (2017). DOI: 10.1016/j.polymer.2017.06.071

Provided by U.S. Army Research Laboratory