My PhD Research: Creating Stronger Materials through Porosity

I have always wanted to understand how the world around me works, from learning about center of mass—when I asked my dad why there were extra pieces of wood across the bottom of a bedside table—to learning how an entire complicated field of physics is based on the observation that light always travels at the same speed. I knew I wanted to become a scientist, but I wasn’t sure what I wanted to study until I heard about shock physics.

 The word “shock” makes most people imagine situations when something surprising happens. In materials science, we talk about shock whenever two objects collide. Think about a car crashing into a wall. When the car and the wall make contact, there is an immense force that makes the car stop abruptly. Whenever force is applied to an area, it creates pressure. In the case of the car, this pressure creates a shock wave that moves away from the impact spot. This shock wave can change how fast the atoms in a material move, kind of like how a wave in the ocean moves water droplets. When the shock wave moves through a material, it pushes the atoms that make up the material forward, similar to the way an ocean wave moves the water droplets. The shock waves that result from collision resemble ocean waves in another way, too: if multiple waves interact, they can combine or cancel each other, like when a wave rolling towards the beach absorbs a wave rolling away from the beach.

Materials scientists study this shock wave phenomenon by intentionally colliding two objects at high speed. As you may remember from school, Newton’s third law of motion says that when two objects collide, they must apply equal and opposite forces to each other. Think of when you high five someone. You and your partner both feel a slight sting on your hands because an equal and opposite force is applied to both of your hands. This same concept happens when two objects collide, but instead of looking at the tingling feeling from a high five, scientists study the shock wave that moves away from the impact. We call the two objects in this experiment a projectile and a target, and, for our experiments, we make them from the same material.

Initially, the target and projectile move towards each other. Eventually, the target and projectile collide, and equal and opposite shock waves begin to move away from the collision spot. The shock waves in both the projectile and target continue to move at their original speeds and in their original directions until they encounter a new material. So, when the wave enters the open air behind the projectile or target, it’s like when the two objects initially collided because the air in this case functions as a “new material.” At this point, the whole process starts anew: the wave “collides” with the air and rebounds back into the target or projectile. The rebounded waves from each surface are now moving back towards the center of the target and will eventually pass each other. When this happens, the shock waves apply a pulling force (what we call tensile) within the target. If this force is big enough, the target will be pulled apart from the inside out.

My research focuses on disrupting these shock waves. My goal is to make it so that the pulling force is never applied in the center of the material and thus never pulls it apart. Specifically, I use our knowledge of shock behavior to create materials that are stronger and won’t be torn apart in a crash. I do this by adding pores to my material. I use 3-D printers to build my sample material layer by layer. To make the pores, I simply use laser to skip certain areas, leaving a hole about half a millimeter long. When a shock wave comes across one of these pores, it makes the pore collapse, which, in turn, makes the wave slow down. So, if I slow the shock wave enough by putting in enough pores, the rebounding shock waves never interact and so they cannot pull the material apart.

So far, I have found that 0.5 mm pores can slow the shock wave enough that my material withstands impact at low speeds; at higher impact speeds, the material still fails. I have also found that even one 0.5 mm pore will cause enough change in the wave interactions to lessen the impact of a collision.

You can probably guess why I study this. There are many instances when it is helpful to know how much internal tension a material can withstand before it is pulled apart. One example is during a car crash. When two cars collide, each car will experience these shock waves. We want to design the bumper material to be strong enough to not fall apart when it experiences shock to keep the passengers safe during a collision. To find the right material, we need to know how it interacts with shock waves.