Wave motion may provide novel way to protect warfighters

The wave-motion work of a Worcester Polytechnic Institute (WPI – Worcester, Massachusetts) researcher may lead to bulletproof vests and helmets that can sense the speed, angle of approach, and size of an incoming bullet – allowing the granular materials inside the protective gear to morph their properties to provide greater shock protection at the precise point of impact.

Nikhil Karanjgaokar, an assistant professor of aerospace engineering at WPI, is exploring the mechanical and physical properties of granular materials that can alter their shapes or change their properties to absorb or redirect the force of an incoming bullet or object. These materials show potential for protective gear like bulletproof vests or helmets, but also possibly for protective covers for satellites, spacecraft, or even large objects like buildings, the International Space Station, and underwater missile siloes.

“I want to design materials that can absorb impacts,” explains Karanjgaokar, whose previous research focused on understanding wave motion through granular materials. “People trying to protect themselves from bullets or shrapnel have used sandbags since before the Second World War to absorb impacts. I’m working from the same basic principle: How can we create a versatile material to be a barrier against any impact?”

So he decided to focus on interrupting or diverting wave motion: You can think of the force of a bullet’s impact as surface waves moving across a pond. Impact waves, and the power behind them, move similarly through any collection of unconnected particles that are the same size, shape, and material. If the particles inside protective gear can change their internal structure in the area of an incoming impact, Karanjgaokar says they could disrupt the direction, speed, and force of the wave.

The main idea is to let the forces from an impact die down by the time they reach whatever is being protected, Karanjgaokar adds, noting that a sensor on the vest or covering would detect an incoming projectile and trigger a change in the materials inside it.

To do this, Karanjgaokar starts with granular materials, which start out with the same properties – size, shape, and density. Then he changes the properties of the granules in a specific location with electric or magnetic fields, in an effort to make the materials respond. For instance, electroactive polymers change their size and shape when stimulated by an electric field; when the field is turned off, the material returns to its original state. He is experimenting with making the materials larger, stiffer, or softer, with properties more like rubber than hard plastic. These modified particles act as an obstacle to the wave created by a bullet’s impact, providing shock protection.

Karanjgaokar’s work involves both dry granules and submerged particles, using a variety of methods to alter them. For dry polymer particles, he uses infrared lights on a grid behind the grains in the vest to heat the granules in the specific area where the particles need to be changed. As the granules heat up, they become softer and more like rubber. When the infrared light is switched off, the granules cool and regain their original properties.

To alter granules, Karanjgaokar submerges the particles in a fluid, usually oil. When a magnetic field is applied, the fluid increases the viscosity to the point of becoming a viscoelastic solid, with characteristics of both a liquid and a solid.

A big part of this work involves exploring how changing granules in different patterns affects the way the impact waves are dispersed. How does the wave dispersion differ if the patterns are created in a straight line versus bunched together? Karanjgaokar is trying to determine whether multiple areas of granules need to be modified to absorb or reflect the wave after it’s already been deflected by the first pattern. This insight will help him optimize the system to handle multiple impacts of different speeds or coming from different angles.

Depending on the speed and direction of the incoming bullet, protective gear would need to be able to create different types of patterns. “One pattern can’t protect against all types of incoming projectiles,” he says.

Karanjgaokar plans to collect experimental data, such as information about displacement and velocity of individual grains – all captured using high-speed imaging at 50,000 frames per second. This information will be fed into algorithms to calculate the interparticle forces needed to divert the impact energy and to better understand how energy moves through the system. He’ll also run images taken with a high-speed camera through pattern-recognition software to figure out how structures deform as projectiles hit them, and then analyze the results using data processing.

Eventually – hopefully ­– the technology developed at WPI will make its way to warfighters.