Inventor Realizes Dream to Create Stronger Metal Foam

Mechanical and aerospace engineering professor Afsaneh Rabiei set out to make a material as light as aluminum and stronger than stainless steel.

Her goal was to create something that could be used in products that would save lives, save energy and eventually save money, all at the same time.

It was quite a demanding engineering puzzle. But Rabiei has taken on many complex tasks. She began her studies of materials science and metallurgy at Sharif University of Technology in Tehran, Iran. After working in industry for a while, she earned her doctorate from the University of Tokyo.

"I received a full scholarship from the Japanese government and that was a starting point of a very interesting stage of my life," Rabiei recalls. "Being a woman in engineering is always challenging, but you wouldn't know how challenging it can be until you work in various countries like Iran and Japan. I really enjoy challenging myself and that was why I can look back and tell you confidently that I will not exchange that experience with anything else in my life."

After the University of Tokyo, she added another world-class university to her resume as a post-doctoral researcher at Harvard.

Now a professor at North Carolina State University in Raleigh, Rabiei has indeed invented a material that's both light and strong. With help from the National Science Foundation (NSF), she created an ultra-high-strength composite metal foam that is capable of making safety devices that can be used in body armor, biomedical implants, car bumpers and even in braces to protect historical buildings against earthquakes.

In some ways "foam" is counter to what people think of when they think "strong."

But by looking at the cellular structure in nature, from leaves to feathers to the human bone, you will notice that the protective structures are mostly made of spongy materials, not solid materials. Even in daily life, delicate objects are often protected by spongy materials such as packing peanuts or bubble wraps.

"For many years, scientists and engineers were trying to remove all the voids and all the air pockets from metals to make them stronger. The concept goes back to solid mechanics and materials, when a simple air pocket can be a source or point for stress concentration. So when you apply a load to the material, it will fail or break around the void area," Rabiei explains.

While there are other metal foams in the market, Rabiei's is unique because she is using uniform hollow metal spheres, combined with a metal matrix. That helps the foam absorb energy much better than similar materials that have uneven cell structures or lack a metallic matrix. The foam she has created absorbs seven to eight times the energy absorbed by other metal foams made from similar materials.

Rabiei's foams can be made from stainless steel and various other metals. For biomedical applications, such as hip and knee implants, her metal foam can be created from titanium or cobalt chromium.

"So, if you put it in the body, the body will like it much better than a solid, heavy material because it is light and its mechanical properties are matching with that of bone," explains Rabiei, who also teaches biomedical engineering.

Rabiei is working with the U.S. military to create both human and vehicle armors using her metal foams. The same energy absorbing capability of these foams could also protect occupants in cars, trucks, trains and airplanes.

"If you are sitting in a car and have an accident, the solid structure of a current car is going to basically transfer all the impact energy to the body of the car and eventually to our bodies," says Rabiei. But, if there is metal foam behind the bumper, it will absorb much of the impact energy and protect the passengers.

"Our comparison showed if you have a car accident at 28 miles per hour, it will feel like 5 miles per hour for the passenger sitting in the car," notes Rabiei.

The applications for composite metal foams are widespread, ranging from moving structures such as cars, trucks, trains and airplanes to stationary structures such as buildings.

While many structures built now in earthquake zones are constructed with special materials to absorb the energy of a quake, older structures are still at risk.

"We can use this material and retrofit it into the current structure and make it safer, so the vibration of the earthquake can be absorbed by this material. This way, a historic building can be protected without taking any risk of digging into the foundation and adding the anti-vibration materials in there," explains Rabiei.

Rabiei says it is sometimes difficult to get industries to adopt something new when the products they now have seem to be working well. It's the "if it ain't broke, don't fix it" mindset. That's why her metal foam can be created with equipment in existing foundries and factories.

"We try to make it very user friendly. You don't need state of the art equipment. A simple furnace, a simple mold, a simple hot press should work fine," she says.

As metal foam products become components in various structures, they will bring cost savings and environmental benefits.

"If you look at the progress that we have made with technology in the past 20 years, from large computers to small laptops, from large telephones to small cell phones, you will see that there is a trend of making things more efficient," says Rabiei. "We know that we have limited resources on Earth and this is another reason for using our resources wisely."

Outside of the lab, Rabiei is just as passionate about educating young people on the exciting opportunities in engineering. She tries to reach young people, especially girls, in elementary school, when she feels they are curious and not as inhibited about exploring.

"I do believe if you go to kindergarten through fifth grade, in elementary school, that's where kids get excited quicker, and you can put a seed down in their mind that can later grow," she explains. "You don't want them to get motivated just for the money or just for the prestige of the job, but for what they do."

"Try until you get it done because that satisfaction is absolutely incomparable," she adds.

It was a school experience that helped lock in Rabiei's career choice. When she was in high school, Rabiei wanted to be a surgeon. But a field trip inspired her to choose engineering instead.

"I was just debating until we had a tour of a factory. I looked at this molten metal... it's hot, it's heavy, and it's under your control," she recalls. "So I was impressed with the field of materials science and engineering. I thought that being a surgeon was interesting, but I thought I could change the world as an engineer."