Defend yourself with...sand?

09 December 2016 | Research
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Sand grains dilate and dissipate pressure, resisting continual penetration of the projectile and making it a good alternative building material in the protection of critical infrastructure

A new study by a team of researchers from NUS Engineering shows that you might be safer hiding behind a sand block rather than a steel wall.

While sand has long been used in military fortification, limited literature exists on the mechanism behind its energy absorption capabilities. Hence, a team led by Assistant Professor Darren Chian Siau Chen from NUS Civil and Environmental Engineering (CEE) comprising Associate Professor Vincent Tan from NUS Mechanical Engineering and Adjunct Assistant Professor Anand Sarma from CEE, decided to find out more about the material’s unique ability to resist impact. The researchers experimented by firing projectiles of various shapes and masses at different velocities against a common silica sand block of varying density.

They discovered that the sand block absorbed more than 85 per cent of the energy exerted against it by the impact of a high speed projectile. Further, the resistance offered by the sand increased with the speed at which the projectile travelled. The impact also resulted in an extreme frictional force that could break the projectile into pieces, as the sand grains dilate and dissipate the pressure, resisting continual penetration of the projectile. While heavier and elongated projectiles were more effective at penetrating the target due to higher stress at the point of contact, the compaction of the sand held little influence as the projectile compacted the sand along its path of penetration.

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From left: Adjunct Asst Prof Sarma, Asst Prof Chian and Assoc Prof Tan fired projectiles into a sand block using a gas gun to test the sand’s absorption capabilities

In contrast, the energy absorption capacity of an equivalent steel plate reduces dramatically as the velocity of the projectile increases, resulting in the projectile passing through it without further resistance. This is known as the hydrodynamic effect, where the high speed projectile causes the steel to melt and behave akin to a fluid.

The team said the study points to the wide-ranging potential applications of sand, beyond glass making, building construction and land reclamation. A key application is in modern defence systems, which has yet to be thoroughly explored.

“Steel, which is one of the key materials used in the construction of armour systems, can be partially replaced with sand as a cost-effective, environmentally friendly, and lightweight sacrificial layer, given its superior energy absorption performance. Given the possibility of hostile threats, sand could also be used as a complementary building material to steel to enhance protection of critical infrastructure and household shelters, given its projectile-resisting function,” said Asst Prof Chian. 

The team plans to further investigate the potential of sand through large scale trials, as well as explore the energy absorption capabilities of similar geomaterials such as rock rubble.  

See press release and media coverage.