Sound waves could be the secret to better armour

An exhaustive search involving thousands of experiments has revealed that a material’s ability to absorb and dissipate energy when impacted is determined principally by the behaviour of sound waves¹. The research, published in the journal Glass Physics and Chemistry, could lead the way to material structures with significantly higher impact protection.

When a material is impacted, kinetic energy is transferred from one object to the other. What happens next is complex, but ultimately if the impacted material can absorb all the transferred energy without being destroyed, it has protected whatever it was shielding. This is of particular interest in the design of new glasses and ceramics, but also the principle behind, for example, vehicle design, where panels are intended to crumple and collapse without complete destruction to protect occupants.

Vladimir Shevchenko from the Grebenshchikov Institute of Silicate Chemistry in St Petersburg, Russia, and colleagues, have been systematically exploring the microstructural behaviour of ceramics and other materials under impact to tease out underlying mechanisms that could give insight into the design of greater impact resistance.

“Our goal was to obtain relatively simple expressions between the loading energy and energy dissipation in a material using fundamental physical properties,” says Shevchenko. “Despite thousands of papers on this problem, little attention has been paid to assessing the ability of a material structure to dissipate and absorb impact energy.”

The study started from two important observations set out by the researchers: that the total energy absorbable by a solid is proportional to the density and speed of sound in the material; and that in ceramics, the ratio of the speed of sound to the loading rate will determine how much brittle versus plastic deformation will occur.

The team then embarked on thousands of experiments using steel or tungsten alloy indenters and various ceramics with different compositions and structures, measuring the impact site and conducting microstructural analyses to understand impact dynamics.

“We found that the underlying mechanism of fracture of brittle materials is the rate of weakening of the load, which determines the failure process,” says Shevchenko. “This means that the speed of sound in the material is identical to the speed of elastic interaction, that is, compression, reflection, shear and Rayleigh waves.”

Interestingly, the researchers also observed ‘reverse’ impact cones, suggesting that optimized structural design could involve a larger volume of material in energy dissipation. Based on these these results, the team designed and tested a diamond/silicon-carbide ceramic with very high speed of sound, which showed excellent impact resistance properties.