Getting a Grip: Designing Stress-Resistant and Energy-Storing Materials

The lead hero of Marvel Studios’s “Black Panther,” T’Challa, is known for his iconic black full-body suit. One of its signature properties is the ability to store and later release energy from an enemy’s physical blows. A new microstructure designed by a team led by Eesha Khare at the University of Cambridge in London holds the potential for developing similar energy release mechanisms fit for Wakanda. The new design, named the s-hinge, makes materials better able to overcome the stress from applied forces. By modifying the s-hinge slightly, the researchers also developed a latching mechanism that can store and release energy more efficiently.

The s-hinge design is an improvement to auxetics, a class of super-strong materials that are defined by how they respond to an applied force. Unlike most materials, auxetics become thicker when stretched and thinner when compressed as opposed to a rubber band, for example, which does the opposite. Nature has many instances of and uses for this class of materials. For example, the carotid arteries in cows are auxetic, which seems to make them more resistant to breakage and injury. Materials science researchers are currently developing auxetic materials for uses from bulletproof armor and shock-absorbent sports shoes to stronger prosthetics and hip implants.

Khare’s team was initially pushed to create a new design because of several flaws in auxetic material design, primarily a vulnerability to damage from the stress of withstanding force. The new s-hinge design improves upon the standard honeycomb structure: the honeycomb’s sharp corners gather stress into small areas, making the structure more vulnerable to weakening over repeated cycles of stress from applied force, a process called fatigue. Auxetic and stress-resistant structures found in nature, like the muscular grip of eagles and the locking joints in fleas’ feet, showed a specific hinge structure similar to a tessellation of heart patterns, which inspired the researchers to develop their own s-hinge.

The s-hinge’s main new properties are increased elastic deformation, or a structure’s ability to return to its original shape after being put under stress, and tunable Poisson ratios, a measure of how much a material expands perpendicular to an applied force. Auxetic materials are defined by having negative Poisson ratios, which is why they become thicker when they are stretched and thinner when compressed.

The s-hinge’s tunable Poisson ratios allows it to switch between a negative and a positive Poisson value. Essentially, the s-hinge structure can go from being a regular material to being an auxetic material, which makes it especially flexible and resistant to damage from stress. This property is particularly important for future practical applications, where a material’s performance depends on its resistance to repeated stress.

To make their new design, the researchers created a computerized simulation on ABAQUS software and modeled different aspects of auxetic materials’ behavior under applied stresses. Designing accurate simulations is important to materials science because if successful, they allow future researchers to build off the original design idea. This study’s simulation compared the properties of the common honeycomb design to those of the new s-hinge design. After running simulations, the researchers 3D printed both an s-hinge and a honeycomb auxetic structure and experimentally tested them according to the simulation. During 3D printing, another advantage of the s-hinge design was made apparent: structures like the honeycomb are vulnerable to defects in the 3D printing process that the s-hinge design avoids, such as rounded corners or badly connected edges. 

Following examples of smooth hinge geometry in nature, such as the Venus flytrap, the researchers improved that s-hinge’s stress distribution by designing the hinge to bear stress throughout its entire length, not disproportionately in the corners. The s-hinge structure distributes stress by replacing the straight edges of the honeycomb with carefully designed arcs that are both flexible and capable of being made on a 3D printer.

Next, the researchers compared their predictions from the simulation to their experimental results from the physical 3D printed models. They found that their simulation’s predicted results matched their experimental data. This suggested  not only that the s-hinge design was durable and successful, but also that the modeling simulation was reliable and could be used to design new auxetic material structures.

In the experimental conditions, the researchers proved that the s-hinge was far better at distributing stress and recovering from damage compared to the honeycomb model. The s-hinge’s increased flexibility will allow materials that were formerly too weak or fragile to be used for auxetic structures, such as glass and ceramic, which will ultimately expand auxetics’ range. In particular, the s-hinge design outperformed the honeycomb in a repeated cyclability test in which forces were applied periodically. This cyclical stress imitates what the structures would have to withstand in practical applications.

The honeycomb and s-hinge’s different responses to stress stem from the two categories of how a material can react to applied force: plastic deformation or elastic deformation. Elastic deformation, as previously described, is the amount of force a material can withstand before irreparable damage is created. Plastic deformation, on the other hand, happens when the force applied surpasses the threshold for irreparable damage. When the force is removed, the structure will have permanent damage and be unable to recover completely. Compared to the honeycomb, the s-hinge was more durable because it has a higher range of elastic deformation.

To exhibit further the s-hinge’s ability to recover from strain, the researchers altered a 3D printed Batman logo. They adjusted the angles of the arcs to turn the previously positive Poisson ratio into a negative one, which changed the amount of stress the material could withstand from that of a normal material to that of a stronger, auxetic material. The researchers envision this property extending new possibilities to materials science, such as designing structures to change shape when they are compressed.

Inspired by the s-hinge’s resistance to stress, the researchers also designed a latching mechanism based off the general s-hinge design. The latch works to gather and save the structure’s elastic energy, or the potential energy created when the material is under stress. Similar ways of latching or storing elastic energy are very common in nature, and they are used in a variety of ways. The Venus flytrap is a common example, as are muscle structures of birds of prey. Their tendons can pull their talons shut with extreme force, and because of their latching properties, the birds do not have to repeatedly contract their muscles in order to hold their grip.

The talons of birds of prey catch our interest because they can exert force without having to continually reapply it. But most natural structures function by reapplying force cyclically, and the researchers designed the materials with this in mind. Like an eagle’s talons, the s-hinge was modeled to distribute stress more efficiently, and its tunable Poisson ratio allowed the hinge’s structure to change, becoming stronger as force is applied. Additionally, the accuracy of the researchers’ computer modeling simulation will make designing future auxetic materials far easier. Ultimately, the researchers envision designing a new class of smart materials, capable of reacting with a Venus flytrap’s adaptability, and that could lead to a new field of material robotics. Robots built with ‘smart materials’ could store energy without a continual application of force, and release that energy in reaction to many different kinds of stimuli. Perhaps a material like T’Challa’s suit isn’t as far out of reach as we think.