How to Stop a Bullet—a Big, Mean One
High explosive incendiary (HEI) rounds are designed to unleash a deadly combination of metal fragments and blast pressure upon detonation. For the soldiers inside vehicles targeted by these lethal rounds, this means a high risk of bodily harm, brain trauma, or worse. Researchers from North Carolina State University and the U.S. Army’s Aviation Applied Technology Directorate have been working to develop a material for use in armored vehicles that would provide soldiers with more protection against this class of explosive ammunition. In a study published last week in Journal of Composite Structures, the team found that a stainless-steel composite metal foam (CMF) offers shielding that is superior to any substance currently in use by the military.
Gauging the Stopping Power of Stainless Steel CMF Plate
The engineers took a two-pronged approach to studying the effectiveness of steel CMF. In the first phase, they set up a physical test in which a live HEI round was fired into an aluminum strikeplate. Plates 10 inch by 10 inch in size made entirely of the composite material were set up at a distance 18 inches away from the explosive round’s point of impact. At thicknesses of roughly 3/8 and 5/8 inches, the CMF plates tested by the team were medium-gauge thickness relative to most vehicle armor. Both gauges of the steel CMF completely blocked the blastwave. The 5/8-inch panel also blocked all the fragments associated with the detonation, while the 3/8-inch barrier stopped the vast majority—more on that later.
The second phase of testing involved computer modeling to demonstrate how the steel-CMF would perform under various real-life circumstances. The results of this modeling compared very favorably with empirical observations from the physical test. Having thus corroborated their findings, the research team moved forward in seeking the answer to another important question: what advantages might their new material have over the existing metals widely used for vehicle armor?
Steel CMF Versus Traditional Vehicle Armor
The researchers turned to the same computer model they used to test steel CMF to determine how aluminum 5083, a widely employed military-grade armor, would respond to similar stresses. The virtual analysis showed that aluminum 5083 of the same weight and thickness as the 5/8-inch steel CMF would stop all the blast fragments, but they would penetrate much deeper into the material. This would result in a much higher level of stress transferred to the other side of the armor than the energy allowed by the new composite foam.
While the steel CMF comes out of this side-by-side comparison looking good, the real kicker is this: the team found that a thinner, lighter wall of CMF could provide the same protection as a thicker version. In the ballistic test, the 3/8-inch CMF plate didn’t stop every fragment, but adding a mere 1/64 of an inch achieved the 100 percent threshold. In summary, while 5/8-inch aluminum 5083 was completely necessary for full blockage, the same level of protection could be achieved by using steel CMF that was almost one-quarter inch thinner. Extrapolated over the surface area of a large vehicle—think a tank—that difference could amount to thousands of pounds of weight. The military implications of that decrease on cost, vehicle handling and fuel economy could portend to be significant, to say the least.
For more on the ways engineers are working to create better protective gear for the military, check out Elastometic Military Vehicle Frame Guards Against Brain Trauma.