Researchers have discovered that composite metal foam offers greater protection than traditional armor steel plate at a third of the weight. The discovery has broad implications for armored vehicles, and could result in stronger, lighter vehicles better able to protect occupants from the impact of kinetic weapons, explosive shockwaves, and fires.Scientists at North Carolina State University and the US Army’s Aviation Applied Technology Directorate have invented what they call Composite Metal Foam (CMF). “Metal foam” is exactly what you think it is—metal with sponge-like holes in it. This not only makes CMF lighter than normal metal, but it also makes CMF spongy, allowing it to give slightly under impact, soaking up some of the energy of a collision.
Researchers have demonstrated that vehicle armor using composite metal foam (CMF) can stop ball and armor-piercing .50 caliber rounds as well as conventional steel armor, even though it weighs less than half as much. The finding means that vehicle designers will be able to develop lighter military vehicles without sacrificing safety, or can improve protection without making vehicles heavier. CMF is a foam that consists of hollow, metallic spheres – made of materials such as stainless steel or titanium – embedded in a metallic matrix made of steel, titanium, aluminum or other metallic alloys. In this study, the researchers used steel-steel CMF, meaning that both the spheres and the matrix were made of steel.
For the study, researchers manufactured a hard armor system consisting of a ceramic faceplate, a CMF core and a thin back plate made of aluminum. The armor was tested using .50 caliber ball and armor-piercing rounds. The armor was tested with the rounds being fired at impact velocities from 500 meters per second up to 885 meters per second.
The CMF layer of the armor was able to absorb 72-75% of the kinetic energy of the ball rounds, and 68-78% of the kinetic energy of the armor-piercing rounds.
“The CMF armor was less than half the weight of the rolled homogeneous steel armor needed to achieve the same level of protection,” says Afsaneh Rabiei, corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at North Carolina State University. Rabiei, the inventor of CMF, has spent years developing and testing CMF materials.
“In other words, we were able to achieve significant weight savings – which benefits vehicle performance and fuel efficiency – without sacrificing protection,” Rabiei says.
“This work shows that CMF can offer a significant advantage for vehicle armor, but there is still room for improvement,” Rabiei says. “These findings stem from testing armors we made by simply combining steel-steel CMF with off-the-shelf ceramic face plates, aluminum back plate and adhesive material. We only optimized our CMF material and replaced the steel plate in standard vehicle armor with steel-steel CMF armor. There is additional work we could do to make it even better. For example, we would like to optimize the adhesion and thickness of the ceramic, CMF and aluminum layers, which may lead to even lower total weight and improved efficiency of the final armor.”
In previous work, Rabiei and her collaborators demonstrated that CMF could block blast pressure and fragmentation at 5,000 feet per second from high explosive incendiary rounds detonating only 18 inches away. Her team also showed that CMF could stop a 7.62 x 63 millimeter M2 armor piercing projectile at a total thickness of less than an inch, while the indentation on the back was less than 8 millimeters. For context, the National Institute of Justice standard allows up to 44 millimeters indentation in the back of armor.
In addition, Rabiei’s group has shown that CMFs, in addition to being lightweight, are very effective at shielding X-rays, gamma rays and neutron radiation – and can handle fire and heat twice as well as the plain metals they are made of.
“In short, CMFs hold promise for a variety of applications: from space exploration to shipping nuclear waste, explosives and hazardous materials, to military and security applications and even cars, buses and trains,” Rabiei says.
The new paper, “Ballistic Performance of Composite Metal Foam against Large Caliber Threats,” is published in the journal Composite Structures. First author of the paper is Jacob Marx, a Ph.D. student at NC State. The paper was co-authored by Marc Portanova of the Aviation Development Directorate in the U.S. Army Combat Capabilities Development Command.
The work was done with support from the Joint Aviation Survivability Program, under award number W911W6-15-D-0001-0001.
Note to Editors: The study abstract follows.
“Ballistic Performance of Composite Metal Foam against Large Caliber Threats”
Authors: Jacob Marx and Afsaneh Rabiei, North Carolina State University; Marc Portanova, U.S. Army Combat Capabilities Development Command
Published: May 28, Composite Structures
Abstract: The goal of this study is to investigate the effectiveness of Composite Metal Foam (CMF) armors against 0.50 caliber ballistic threats. A hard armor was manufactured using a sandwich panel construction consisting of a ceramic faceplate, a CMF core, and a thin aluminum back plate. The hard armor system was tested against 0.50 caliber (12.7 x 99 mm) ball and armor piercing (AP) rounds. The CMF armors were tested with a variety of areal densities at impact velocities between 500-885 m/s. The armors stopped the threats at speeds up to 819 m/s without penetration. The CMF layer was found to absorb 72-75% and 68-78% of the kinetic energy of the ball and AP round respectively. When compared to rolled homogeneous steel armor (RHA), the CMF hard armors, in their current unoptimized condition, have a mass efficiency of approximately 2.1. The CMF armor offers a much needed weight savings without sacrificing protection. Finite element analysis was completed using ANSYS/AUTODYN Explicit Dynamics solver to study the material interactions and impact. The results are shown to be in good agreement with the experimental findings.