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Auxetics, new materials for lightweight body and vehicle armor that strengthen when a bullet strikes

What do a flea and an eagle have in common? They can store energy in their feet without having to continuously contract their muscles to then jump high or hold on to prey. Now scientists Now scientists at Queen Mary University of London and University of Cambridge have created materials that can store energy this way, be squeezed repeatedly without damage, and even change shape if necessary. These kinds of materials are called auxetics and behave quite differently from regular materials. Instead of bulging out when squeezed, they collapse in all directions, storing the energy inside.


Current auxetic material designs have sharp corners which enable them to fold onto themselves, achieving higher density. This is a property that has been recognised recently in lightweight armour designs, where the material can collapse in front of a bullet upon impact. This is important because mass in front of a bullet is the biggest factor in armour effectiveness. The sharp corners also concentrate forces and cause the material to fracture if squeezed multiple times, which is not a problem for armour as it is only designed to be used once.


In this study, published in Frontiers in Materials, the team of scientists redesigned the materials with smooth curves which distribute the forces and make repeated deformations possible for other applications where energy storing and shape-changing material properties are required.


Auxetics are materials that have a negative Poisson’s ratio. When stretched, they become thicker, (instead of becoming thinner) perpendicular to the applied force. This occurs due to their particular internal structure and the way this deforms when the sample is uniaxially loaded. When compressed, the material becomes thinner.


When the material contracts, it becomes more dense, increasing indentation resistance . It is this property that is of interest from a defence point of view. These materials are predicted to have exceptional resistance to blast and ballistic threats, while not contributing to excessive weight compared to conventional materials


Auxetics can be single molecules, crystals, or a particular structure of macroscopic matter. Such materials and structures are expected to have mechanical properties such as high energy absorption and fracture resistance. In rigid dome-shaped helmets and protective pads, auxetics offer lightweight, stiff and strong materials solutions. In flexible protectors, the ability to conform to convex surfaces, ensures close fit of the equipment to the body for maximum comfort and performance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.


Some examples of auxetic materials include: Auxetic polyurethane foam, Certain rocks and minerals, Graphene, which can be made auxetic through the introduction of vacancy defects, Folded sheet materials such as zigzag-based folded sheets, Living bone tissue, Tendons within their normal range of motion and Specific variants of polytetrafluorethylene polymers such as Gore-Tex.


Paper, all types. If a paper is stretched in an in-plane direction it will expand in its thickness direction due to its network structure and tailored structures designed to exhibit special designed Poisson’s ratios. Chain organic molecules. Recent researches revealed that organic crystals like n-paraffins and similar to them may demonstrate an auxetic behavior. Processed needle-punched nonwoven fabrics. Due to the network structure of such fabrics, a processing protocol using heat and pressure can convert ordinary (not auxetic) nonwovens into auxetic ones.


Microporous auxetic materials work because their porous structure allows sufficient space for the “hinges” to fold or the “nodules” to spread apart. Unfortunately this same porous structure also leads to lower stiffness and are lower density, unsuitable for load-bearing applications. A further limitation with microporous auxetic materials is the inability to produce them reliably and cost-effectively using techniques which are suitable for large-scale commercialisation. One approach to overcome the problem with production scale is to develop molecular auxetics, however they have their own limitations. One such limitation lies in synthesizing the highly symmetric, network-like structures.


Defence Applications

During military operations, personnel and vehicles are often exposed to ballistic and blast threats. Lightweight armor systems are used in situations where there are weight restrictions such as, personal protection, helicopters, patrol boats, and transportable shelters. The ideal material for use in an armour system must absorb energy locally and be able to spread the energy out fast and effciently.


Negative Poisson’s ratio (auxetic) materials have shown increased shear modulus, increased fracture toughness, and improved impact resistance. All properties beneficial for armour systems which has been predicted to contribute to improved toughness , resilience, and shear resistance, as well as improved acoustic properties associated with vibration. Exploration into the unusual properties of these materials have beand ven supported by NASA, Boeing , and the US Office of Naval Research.


These materials have the potential to revolutionize defence personal protective equipment. Current protective materials are stiff and heavy. Auxetic materials may be able to provide the same protection while being thinner and consequently lighter. The synclastic curvature available from auxetic materials makes it easier to manufacture curved surfaces that conform to the human body (e.g., knee-pads, helmets).


When an auxetic helmet is impacted, material flows into the area to provide reinforcement. This allows less deflection on the interior of the helmet, allowing for less injury to the human wearing the helmet. Recently, the US Office of Naval Research (ONR) has even solicited proposals for “Applications of Auxetic Textiles to Military Protective Clothing” under the Small Business Technology Transfer (STTR) program makes them a good candidate for this application.


Auxetix Ltd. Has developed a helical auxetic yarn which it is marketing as a potential material to make blast curtains. Auxetic materials are not limited to protection applications in their usefulness. Because auxetic materials will expand perpendicular to a force, they will make ideal press-fit fasteners and rivets. An auxetic fastener, when inserted into place under compression will contract, making installation easier. It will then expand when put under tension, thus sealing itself more effectively into the hole. The contraction while compressed also lends to improved projectile materials. As a projectile moves down the barrel, the thrusting force would potentially result in a reduction in lateral expansion.


Similar to making good fasteners, auxetic fibres will also make good reinforced composites. Fibre pull-out is a major failure mechanism in conventional fibre reinforces composites. With auxetic fibres, pull-out is resisted because the fibres expand perpendicular to the pull-out force. This could help to resist potential failure mechanisms in the composite, such as crack growth.


It has also been suggested that auxetic materials could be used in the design of hydrophones and other sensors because their low bulk modulus makes them more sensitive to hydrostatic pressure. In 1991 the US Office of Naval Research funded the evaluation of the theoretical performance of auxetic composites for piezocomposite devices .


D3O’ Helmet Systems provide protection to soldiers from IED blasts

Traditional helmets have evolved to offer ever-better ballistic impact protection, but if the head is subject to the blunt trauma or blast waves from an Improvised Explosive Device (IED) blast, traumatic brain injury can result. Impact protection solutions company D3O seeks to address this with its prototype D3O Shock Absorbing Helmet System from its Trauma Reduction and Unrivalled Shock Technology (TRUST) range, which it claims can provide up to twice the impact protection performance against blunt force trauma offered by current helmets.


It was awarded funding from the UK’s innovation agency, the Technology Strategy Board, to develop a new Shock Absorption Helmet System for soldiers to reduce Traumatic Brain Injuries (TBI). These helmets provide sufficient protection to soldiers from blunt trauma protection caused by IED blasts which was the cause of 35% of the US and UK casualties in Iraq and Afghanistan wars.


The helmet system comprises three parts: a shock-absorbing liner to absorb and dissipate the energy released in the collision; an inflatable system which individually fits the solution to a range of head shapes; and a skull cap to provide additional comfort and help with sweat management. The shock absorption liner is encapsulated using D3O’s new Smart Skin technology: a wipe-clean material which provides durability and an anti-microbial barrier.


D3O unique patented technology is based on non-Newtonian principles: in standard conditions, the raw materials molecules flow freely allowing the material to remain soft and flexible, but on impact, they lock together to absorb impact energy and reduce the force transmitted. The molecules instantly return to their flexible state.


The raw material (dilatant fluid) is strain rate sensitive which means the stronger the impact, the more the molecules react, and the better the protection. This non-Newtonian behavior dissipates the impact energy and reduces the force transmitted to the body.


D3O’s uses its unique patented technology to produce a range of material formulations like ST, focussed on impact protection in a range of ambient temperature environments and Shock+ focused on impact protection in a range of extreme temperature environments.


Scientists design material that can store energy like an eagle’s grip

In this study, published in Frontiers in Materials, the team of scientists redesigned the materials with smooth curves which distribute the forces and make repeated deformations possible for other applications where energy storing and shape-changing material properties are required. The work lays the basis for designs of lightweight 3D supports, which also fold in specific ways and store energy which could be released on demand.


Principle investigator Dr Stoyan Smoukov, from Queen Mary University of London, said: “The exciting future of new materials designs is that they can start replacing devices and robots. All the smart functionality is embedded in the material, for example the repeated ability to latch onto objects the way eagles latch onto prey, and keep a vice-like grip without spending any more force or effort.” The team expects its nature-inspired designs could be used in energy-efficient gripping tools required in industry, re-configurable shape-on-demand materials, and even lattices with unique thermal expansion behaviour.


Eesha Khare, a visiting undergraduate student from Harvard University who was instrumental in defining the project, added: “A major problem for materials exposed to harsh conditions, such as high temperature, is their expansion. A material could now be designed so its expansion properties continuously vary to match a gradient of temperature farther and closer to a heat source. This way, it will be able to adjust itself naturally to repeated and severe changes.” The flexible auxetic material designs, which were not possible before, were adapted specifically to be easily 3D-printed, a feature the authors consider essential. Dr Smoukov added: “By growing things layer-by-layer from the bottom up, the possible material structures are mostly limited by imagination, and we can easily take advantage of inspirations we get from nature.”


Adaptive protein crystal’ could form new kind of protective material

Chemists at UC San Diego have created an “adaptive protein crystal” with a counterintuitive and potentially useful property: When stretched in one direction, the material thickens in the perpendicular direction, rather than thinning as familiar materials do. And when squeezed in one dimension, it shrinks in the other rather than expanding, and gets denser in the process.


It’s a property, called ‘auxetic,’ that has been not been previously demonstrated at the molecular level through design,” said Akif Tezcan, a professor of chemistry and biochemistry at UC San Diego who headed a team of researchers that detailed its invention in  Nature. Tezcan’s group created a sheet-like crystal made of proteins connected in a regular, repeating pattern. They chose a protein called RhuA for its square shape and used it like tiles to make their material.


“We found a way to create strong, flexible, reversible bonds to connect the protein tiles at their corners,” Tezcan said. The flexibility allows the tiles to rotate to open spaces for a porous material or to close up in a kind of adaptable sieve.


Stretching or compressing the material in one directions causes the connected protein tiles to rotate in unison, resulting in a corresponding expansion or contraction in the opposite directions. The Poisson ratio, usually a positive measure for normal materials that stretch and shrink in opposition, describes this relationship. Tezcan’s group measured a Poisson ratio of -1 for their material, a value at the thermodynamic limit of what is possible.


The crystals form perfectly with almost no tiles missing or ajar, and the material is self-healing. Protein tiles easily pop into place, given the right chemical conditions. “This is protein design using a highly chemistry-based approach,” Tezcan said, noting that the materials are made via a streamlined, minimalistic design strategy that requires few alterations to the protein building blocks.



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