Steel foam armour stops bullets

Materials World magazine
1 Jul 2019

Composite metal foams are proving highly effective as armour for use in military vehicle protection. Ceri Jones reports.

Ballistic tests have shown that armour made of composite metal foam (CMF) is as effective as steel at half the weight. A team of researchers at North Carolina State University, USA, set out to investigate the CMFs in comparison with the rolled steel constructions currently in use.

The CMF armour concept was invented by the university’s Professor of Mechanical and Aerospace Engineering, Afsaneh Rabiei. Her team previously proved its protection against gunfire, blasts, X-rays, gamma rays and neutron radiation, thermal and fire attack. This latest research examined its efficacy against large calibre threats.

‘We were able to achieve significant weight savings, which benefits vehicle performance and fuel efficiency, without sacrificing protection,’ Rabiei said. By adopting this high-strength, lightweight protection, vehicles would benefit from lower fuel consumption, and for defence purposes, it would support greater manoeuvrability and stealth during military operations.

Bullet testing

Hard armour was produced in a sandwich structure based on typical designs – a ceramic faceplate to blunt the initial impact of a projectile and a ductile aluminium backplate with the plasticity to absorb its residual energy. However, this had a core layer of CMF, consisting of 2mm-wide, hollow, stainless-steel spheres sintered together with stainless-steel powder. The three layers were joined with an aerospace-grade adhesive.

Bullets used were 0.50 caliber (12.7 x 99mm) ball and armour piercing (AP) rounds. From a 5m distance, these were fired at panels of CMF and rolled homogenous steel armours at impact velocities of between 500-885m/s, and finite elements analysis was used to determine the energy absorption rates.

Analysis revealed the strain rate equivalent of the CMF under ballistic impact was 10 times greater than base level because, according to the team, ‘it cushions the bullet’s impact and begins to compress and densify in localised regions’. It successfully stopped rounds fired at 819m/s without any degree penetration, and absorbed 72-75% of the ball and 68-78% of the AP bullets’ kinetic energy.

‘At impact velocities above 800m/s, the CMF layer consistently absorbs up to 78% of the impact energy. As the impact velocity increases, so does the effective strength of the CMF layer due to the strain rate sensitivity of the material,’ the team said. ‘The variation in energy absorption is due to the volume of shattered ceramic and the resulting volume of CMF that is compressed underneath during impact.'

For AP rounds, steel matrix successfully stopped those fired at up to 514m/s velocity. This prompted the team to use a thicker ceramic and CMF layer to stop impacts at higher speeds, which proved successful against APs at up to 800m/s.

Improving protection

This stage of testing set out to prove the efficacy of the CMF sandwich structure so employed standard materials, adhesive and assembly methods. As such, the team is aware of multiple opportunities for development to address performance and delamination.

‘These findings stem from testing armours we made by simply combining steel-steel CMF with off-the-shelf ceramic face plates, aluminium backplate and adhesive material. 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,’ Rabiei said.

According to the team, the total CMF armour has yet to be optimised to ensure the maximum amount of erosion within the ceramic, maximum energy absorption of the matrix, optimal support provided by the aluminium backing and adhesive bonds between each layer while minimising the overall weight.

The ball rounds indicated that creating a faceplate from multiple ceramic tiles helped to isolate layer fracturing far more effectively than those made with a single piece. Also, a structural epoxy was used as the adhesive and suffered a degree of delamination, but this could be improved by a switch to elastomeric-based adhesives.

'Elastomeric adhesives allow for more isolated cracking and failure of the ceramic faceplate and limit the in-service loads experienced by the armour when applied to the exterior of a vehicle,’ the team said.

Full results were reported in the paper, Ballistic performance of composite metal foam against large calibre threats, published in Composite Structures, here: