Oct 11 2017
A distinctive experimental instrument has been developed by army and MIT Scientists to better investigate the durability of robust and high-performance polymeric materials with the ability to self-strengthen when they are subjected to high impact.
Schematic illustration of the laser-induced micro-particle impact test. CREDIT: Courtesy U.S. Army Research Laboratory.
Dr Alex Hsieh from the Army Research Laboratory collaborated with Professor Keith A. Nelson, Dr David Veysset and Dr Steven Kooi from the Army’s Institute for Soldier Nanotechnology at MIT and found out that when micro-particles formed of silica impact on targets formed of poly(urethane urea) elastomers (i.e. PUUs) at exceptionally higher speeds, the PUU target exhibits hyperelastic properties.
Put differently, the targets are extremely hardened upon being deformed at strain rates of nearly 108/s, indicating that the thickness of the target material is nearly halved within a very short time period equal to one second divided by hundred millions. According to Hsieh, during the impact, the PUUs also rebound.
A pulsed laser is used by the investigative instrument to bombard the PUU targets with micrometer-sized bullets. For the first time, the Scientists discovered that “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.”
According to the Researchers, their findings related to bulk elastomers can assist in developing matrix materials for composites for making next-generation U.S. Army combat helmets. Composites based on high-performance, ultrahigh molecular weight polyethylene (UHMWPE) are used for making the improved combat helmet for the Army. The strength of such fibers, which have greater per unit cross section area as well as high breaking strength, is nearly 15 times more than that of steel. However, they are still flexible like fabrics.
Conventional armor material designs include metals, ceramics and lightweight fiber reinforced composites for vehicle as well as soldier protection which are usually dependent on stiffness, toughness, a material’s resistivity against deformation and the potential to absorb energy and plastically deform before being fractured.
However, from the point of view of materials science, just these typical bulk metrics are not adequate to evaluate the speed with which the mobility of molecules in a polymer solid is changed in relation to deformation rate, as well as the tendency of modification of their respective physical state as part of the dynamic deformation. The question remains, do elastomers get modified from rubber-like to glass-like while being deformed at increasingly higher rates?
According to Hsieh, the focus of the Researchers was on polymers, formed of a greater number of small molecular units linked together and forming very long chains that are either arranged perfectly or are packed randomly. In particular, they focus on flexible polymeric materials such as rubbers and stronger polymeric materials such as impact-resistant safety glasses. Elastomers—a kind of artificial rubbers—can be produced by combining a wide array of polymer chemistries.
For further verification of the molecular effect, the Researchers performed elaborate research on PUUs together with a glassy polycarbonate. Whereas the polycarbonate has high ballistic strength and fracture toughness, the PUUs (in spite of their respective composition) were found to have higher dynamic stiffening upon being impacted at strain rates of 108/s. In addition, Researchers can optimize the resistance to micro-particle penetration, that is, approximately 50% decrease in the average maximum penetration depth was accomplished by just altering the molecular composition of the PUUs.
This is very exciting. 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.
Dr Alex Hsieh, The Army Research Laboratory
It is well known that PUUs have a complex microstructure and also a wide array of relaxation times, which are the properties used to demonstrate the efficiency with which molecules in a polymer chain react to an external impact. Particularly, PUU molecules with longer relaxation times of a few microseconds at ambient conditions (e.g. slower dynamics) allowed dynamic stiffening. In contrast, those with relaxation times of a few nanoseconds at ambient conditions could absorb additional energy to enable dynamic strengthening.
Such viscoelastic properties indicate that elastomers and other polymeric materials might get deformed in disparate ways, which is strongly based on the speed at which it is deformed.
The Researchers have proposed that a cooperative molecular relaxation mechanism is similar to a resonance phenomenon of “chainmail-like” molecular motions, where each motion oscillates at a particular frequency to release the absorbed energy. Such dynamic stiffening and strengthening properties can probably be enabled by intermolecular hydrogen bonding through the entire physically-crosslinked network in PUUs.
On the other hand, the microsecond relaxation at ambient conditions is absent in the case of polycarbonate, and hydrogen bonding and the respective enabling molecular mechanism do not occur in polycarbonate, in spite of its impact strength and toughness. This indicates that PUUs or high-performance elastomers including multiple relaxation times are highly propitious to enable the dynamic stiffening as well as the dynamic strengthening over the temporal scale from microseconds to nanoseconds.
The distinctive findings have been reported in a paper recently published in the journal Polymer, 123 (2017) 30-38, http://dx.doi.org/10.1016/j.polymer.2017.06.071.
In the mean time, materials identical to PUU (such as polyurethane), when used as matrix elastomers, were found to withstand backface deformation observed in lightweight UHMWPE composites. It is typically the buckling of material inside combat helmets that conveys huge forces to the skull and results in blunt impact trauma. According to Hsieh, polyurethanes, PUUs and other such elastomers that enable dynamic strengthening in high-rate deformations and considerably decrease the deformation of the helmet upon being impacted—when combined with ultra-modern fibers—can be highly advantageous for next-generation combat helmets.
Apart from producing combat helmets, high-performance, robust elastomers for soldier protection can be used to make ballistic vests, transparent face shields, extremity protective gear, mandible face shields and blast-resistant combat boots.
The Researchers also perceive that this study on the hyperelastic properties of PUUs, specifically during impact at very high speeds, can also be extended and applied for protecting professional football players and young athletes from concussions or similar brain-related injuries during collisions. According to Hsieh, from the point of view of materials design, robust, high-performance elastomers can be applied for making the outermost layers of the helmet or just as a substitute for the polycarbonate shell.