A new study finds that composite metal foam (CMF) can withstand tremendous force - enough to punch a hole in a railroad tank car - at much lower weight than solid steel. The finding raises the possibility of creating a safer generation of tanker cars for transporting hazardous materials.
The researchers have also developed a computational model that can be used to determine what thickness of CMF is needed in order to provide the desired level of protection necessary for any given application.
"Railroad tank cars are responsible for transporting a wide range of hazardous materials, from acids and chemicals to petroleum and liquefied natural gas," says Afsaneh Rabiei, corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at North Carolina State University. "The safety of these tank cars is important, and the U.S. Department of Transportation has very rigorous testing requirements for any material that might be used to manufacture these tank cars.
"In previous studies, CMF has passed these tests with flying colors, and the next step for us was to see how the material performed in puncture testing," Rabiei says. "The results were outstanding."
CMFs are foams that consist of hollow spheres - made of materials such as stainless steel, nickel, or other metals and alloys - embedded in a metallic matrix. The resulting material is both lightweight and remarkably strong at absorbing compressive forces, with potential applications ranging from aircraft wings to vehicle armor and body armor.
In addition, CMF is better at insulating against high heat, and is stronger when exposed to high heat, than conventional metals and alloys, such as steel. The combination of light weight, strength and thermal insulation means that CMF also holds promise for use in storing and transporting nuclear material, hazardous materials, explosives and other heat-sensitive materials.
For the puncture testing, the researchers used a 300,000 pound ram car that runs on train tracks. The ram car was mounted with an indenter - essentially a steel column with a point that measures six inches square. The ram car was accelerated to 5.2 miles per hour, at which point the indenter would collide with the type of high-quality steel plating used in tank cars. The speed and weight of the ram car generate 368 kilojoules of force, distributed across the six-inch by six-inch end of the indenter.
In the baseline test, the indenter tore a gaping hole in the steel plate. For the experimental test, the researchers placed a piece of CMF that was 30.48 millimeters thick on the end of the indenter. Upon colliding with a steel plate, the CMF absorbed the vast majority of the force, causing the indenter and ram car to bounce off the steel plate, leaving only a small dent. Video of the testing can be seen here.
"The obvious conclusion here is that light-weight CMF can absorb puncture and impact energies more efficiently than solid steel," Rabiei says. "And we have a model that can be used to figure how much CMF is necessary, which maximizes the efficiency of using CMF as we believe that a lower thickness CMF could have performed even better."
The paper, "Numerical Model and Experimental Validation of Composite Metal Foam in Protecting Carbon Steel Against Puncture," is published in Advanced Engineering Materials. First author of the paper is Aman Kaushik, currently a postdoctoral researcher at NC State.
This work was done with support from the Department of Transportation's Pipeline and Hazardous Materials Safety Administration under project number PH95720-0075.
Rabiei is the inventor of composite metal foams. She has assigned related intellectual property to a small business for which she is a shareholder.
