CMF armour cuts weight and adds protection

Designers will be able to develop lighter military vehicles without sacrificing safety with a composite metal foam (CMF) armour that stops .50 calibre rounds as well as steel does.

CMF armour
Afsaneh Rabiei examines a sample of composite metal foam

The latest advance in CMF materials from North Carolina State University (NC State) can stop ball and armour-piercing rounds whilst weighing less than half as much as conventional steel armour. Their findings are published in Composite Structures.

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, aluminium or other metallic alloys. In this study, the researchers used steel-steel CMF, so both the spheres and the matrix were made of steel.

For the study, researchers manufactured a hard armour system consisting of a ceramic faceplate, a CMF core and a thin aluminium back plate. The armour was tested using .50 calibre ball and armour-piercing rounds fired at impact velocities from 500m/s up to 885m/s.

According to NC State, the CMF layer of the armour was able to absorb 72-75 per cent of the kinetic energy of the ball rounds, and 68-78 per cent of the kinetic energy of the armour-piercing rounds.

“The CMF armour was less than half the weight of the rolled homogeneous steel armour needed to achieve the same level of protection,” said CMF inventor Afsaneh Rabiei, corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at NC State. “In other words, we were able to achieve significant weight savings – which benefits vehicle performance and fuel efficiency – without sacrificing protection.”

Whilst the latest research shows that CMF can be advantageous for vehicle armour, Rabiei cautioned that there is room for improvement

“These findings stem from testing armours we made by simply combining steel-steel CMF with off-the-shelf ceramic face plates, aluminium back plate and adhesive material,” she said in a statement. “We only optimised our CMF material and replaced the steel plate in standard vehicle armour with steel-steel CMF armour. There is additional work we could do to make it even better. For example, we would like to optimise the adhesion and thickness of the ceramic, CMF and aluminium layers, which may lead to even lower total weight and improved efficiency of the final armour.”

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 18 inches away. Her team also showed that CMF could stop a 7.62 x 63-millimetre M2 armour piercing projectile at a total thickness of less than an inch, while the indentation on the back was less than 8mm.