A metal foam is a cellular structure consisting of a solid metal (frequently aluminium) with gas-filled pores comprising a large portion of the volume. The pores can be sealed (closed-cell foam) or interconnected (open-cell foam). The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal, making these ultralight materials. The strength of the material is due to the square-cube law
Composite metal foam (CMF) is formed from hollow beads of one metal within a solid matrix of another, such as steel within aluminium, show 5 to 6 times greater strength to density ratio and more than 7 times greater energy absorption than previous metal foams.
Composite metal foam (CMF) is a novel light weight material created at North Carolina State University. The material is made of metallic hollow spheres closely packed together and the empty spaces in between them filled with a metallic matrix through either casting (of a molten metal) or sintering (of a powdered metal).
The hollow spheres are filled with air and provide light-weight and porosities, while the surrounding matrix works to strengthen the bonding between spheres and blunt any potential cracks in the material under load.
Under large amount of compression loading, the spheres collapse and expend the impact energy protecting whatever is behind it. CMF’s strength is further improved under higher speed of impact due to the resistance of air inside its spheres (similar to a bubble wrap performance under pressure, but in a much larger scale). Such extra-ordinary energy absorption capabilities of CMFs inspired their application in ballistic and blast armor systems along with many others.
The product will look like a heavy-duty bubble wrap to protect against larger impacts of train or car crash, blast wave and fragmentation, ballistic and more. The material contains about 30-40% metal and 70-60% air trapped inside its porosities.
Previous work from Rabiei’s group has shown that CMFs, in addition to being lightweight: can reduce armor-piercing bullet penetration; 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. A 100% Steel composite foam shows almost 275% more effectiveness in shielding X-rays compared to Aluminum.
CMF can replace rolled steel armor with the same protection for one-third the weight. It can block fragments and the shock waves that are responsible for brain injuries. Other potential applications include nuclear waste (shielding X-rays, gamma rays and neutron radiation) transfer and thermal insulation for space vehicle atmospheric re-entry, with twice the resistance to fire and heat as the plain metals.
Some of the potential applications of composite metal foams include:
o Light-weight armors with high performance against larger caliber bullets with armor piercing, blast wave and fragments flying with very high speeds (such as 5000ft/s)
o Structural parts for tanks, Humvees and land system vehicles to improve protection against IEDs, improved crashworthiness, and enabling the vehicles to be maneuverable and fuel efficient.
o Helicopter parts to absorb the impact energy upon hard landing while providing a light-weight solution
o Crash energy absorbers in trains, cars and buses
o Mine boots and personal armors and helmets
o Medical devices including implants with good stiffness similar to bone that prevents stress shielding and premature failure of implants.
o Heat protection devices (fire doors, safes, containers,.)
o Radiation shielding parts with light weight and no toxicity
Stainless steel composite metal foam armour offers more protection
New research from North Carolina State University and the U.S. Army’s Aviation Applied Technology Directorate shows that stainless steel composite metal foam (CMF) can block blast pressure and fragmentation at 5,000 feet per second from high explosive incendiary (HEI) rounds that detonate only 18 inches away.
“In short, we found that steel-CMF offers much more protection than all other existing armour materials while lowering the weight remarkably,” says Afsaneh Rabiei, senior author of a paper on the work and a professor of mechanical and aerospace engineering at NC State. “We can provide as much protection as existing steel armor at a fraction of the weight – or provide much more protection at the same weight.
“Many military vehicles use armor made of rolled homogeneous steel, which weighs three times as much as our steel-CMF,” Rabiei says. “Based on tests like these, we believe we can replace that rolled steel with steel-CMF without sacrificing safety, better blocking not only the fragments but also the blast waves that are responsible for trauma such as major brain injuries. That would reduce vehicle weight significantly, improving fuel mileage and vehicle performance.”
For this study, researchers fired a 23×152 millimeter (mm) HEI round – often used in anti-aircraft weapons – into an aluminum strikeplate that was 2.3 mm thick. 10-inch by 10-inch steel-CMF plates – either 9.5 mm or 16.75 mm thick – were placed 18 inches from the aluminum strikeplate. The researchers assessed that the steel-CMF held up against the wave of blast pressure and against the copper and steel fragments created by the exploding round, as well as aluminum from the strikeplate.
“Both thicknesses of steel-CMF stopped the blastwave, and the 16.75 mm steel-CMF stopped all of the fragments from 15 mm2 to over 150 mm2 sizes,” Rabiei says. “The 9.5 mm steel-CMF stopped most, but not all, of the fragments. Based on the results, a 10 mm steel-CMF plate would have stopped all of the frag sizes.”
The researchers also developed computer models of how the steel-CMF plate would perform. When compared to the experimental results, the model matched very closely. The researchers then used the model to predict how aluminum 5083 armor – a type of armor already on the market that has a similar weight and thickness to the 16.75 mm steel-CMF – would perform against HEI rounds.
The model showed that, while aluminum armor of similar weight to the steel-CMF panels would stop all of the frags, the aluminum armor would buckle and allow fragments to penetrate much deeper. This would result in more damage to the panel, transferring large amounts of stress to the soldiers or equipment behind the armor. The steel-CMF, on the other hand, absorbs the energy of the blast wave and flying fragments through local deformation of hollow spheres, leaving the steel-CMF armor under considerably less stress – offering more protection against fragments and blast waves.
Next steps include testing the steel-CMF against improvised explosive devices (IEDs) and high-caliber, mounted ballistics. The researchers have already tested the CMF’s performance against hand-held assault weapons, radiation and extreme heat.
Rabiei’s team is currently at work on three projects that make use of the CMFs:
- A Department of Defense-funded effort to create vehicle armor that addresses threats from small arms, blasts and fragmentation from explosives;
- A Department of Transportation-funded project to develop storage containers for transporting hazardous materials; and
- A NASA-funded project focused on structural applications for airplanes.