SUPERMAN SUIT: FUTURISTIC BODY ARMORMarch 31st 2008.

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Emma Lecours, Megan Swain, Jonathan Boulanger, Sarah Xu

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Bullet-proof or ballistic vests are a type of body armor worn on the torso to absorb the impact of projectile objects. Originally crudely made from wood or various types of metal, these became an ineffective form of protection with the invention of firearms.

CURRENT BULLET-PROOF VESTS

Current bullet-proof vests are made of layers of fibrous materials. Upon impact the material absorbs the energy of the bullet and disperses it throughout the material. This slows down the bullet and stops it from penetrating the body.

There are three main materials currently used to produce bullet-proof vests: Kevlar, Twaron and Dyneema. Kevlar is a synthetic polymer fibre with a structure as seen in Figure 1. It isa useful material due to its very high strength to weight ratio. Twaron was later developed and has a nearly identical chemical structure. Finally Dyneema is a thermoplastic material which consists of extremely long polymer chains thatallowsthe material to effectively transfer its load, giving it a high impact strength.

FIGURE 1: The molecular structure of Kevlar

The disadvantage to this type of armor is that the wearer of the vest is still forced to absorb all of the energy possessed by the projectile, resulting in blunt force trauma, possibly causing injury to the wearer. The effects of this can vary from the wind being knocked out of the wearer, bruising of the skin or even fatal injuries to the internal organs.

A superior bullet-proof vest would instead deflect both the bullet and the majority of the energy possessed by the bullet away from the wearer reducing the possibility of blunt force trauma.

BULLET REPELLENT

Carbon nanotubes (CNT) as a bulletproof vest material are ideal due to the combination of high elastic modulus and high yield strain. CNT have been measured to have a Young’s Modulus of ~1 TPa [2]. Furthermore, up to 40% strain to yield in tensile tests has been measured. Consequently, CNT are capable ofelastically storing an extreme amount of energy. The capacity to absorb so much energy while not permanently deforming or failing is the reason CNT make ideal bulletproof vests.

The high strength and elasticity of CNT can be explained by its deformation modes. CNT can be either single or multi-walled. In this application, single walled nanotubes are preferred for their superior mechanical properties. A single walled nanotube (SWNT) is composed of a single rolled graphene sheet. This sheet may be rolled in several ways, indicated by an index (n,n). SWNT used in this experiment were rolled in a zig-zag structure (n,0), thus all tubes were metallic [1,2]. All carbon atoms in the 2D lattice are bonded by sp2 bonding, which is a critical factor in CNT’s remarkable thermal, mechanical, and electrical properties. In particular, these bonds allow elongation of the hexagonal structure when under tensile stress up to extremely high strains. Furthermore, additional elastic deformation modes consist of highly mobile defects, stepwise tube diameter reduction, and tube collapse/bending [2]. These deformation modes are fully reversible after the removal of the applied stress. The final deformation mode of a CNT is necking, which is irreversible. In Figure 2 TEM and computer simulated deformation of SWNT are shown.

FIGURE 2: Tube collapse and diameter reduction reversible deformations

EXPERIMENTAL RESULTS

The team from the University of Sydney investigated the dynamic properties of carbon nanotubes through the use of a computer simulation. For their simulation a very small diamond tip (3.56 x 3.56 x 0.71 nm)was used as the bullet. To replicate the real situation, the width of the projectile’s tip was much larger than the width of the flattened nanotube.

FIGURE 3:Model showing the diamond ‘bullet’

Their simulation used an empirical bond order potential that incorporates bond energies, lengths, and force constants for hydrocarbon molecules, as well as elastic properties, interstitial defect energies, and surface energies for diamond. For the atomic interactions inside the nanotubes, the three-body Tersoff-Brenner potential was used while for the interaction between the bullet and the nanotube, the two-body Tersoff-Brenner potential was used.

They performed a simulation that shot the diamond at 400m/s (arbitrary speed) at a nanotube with two fixed ends to determine its maximum absorption energy, that is to say the energy difference of the CNT before its interaction with the bullet and just before the onset of the fracture. From this experiment, they estimated the initial bullet speed that would not break the bonds of the nanotubes. These speeds (between 1000 and 3500 m/s were used to simulate diamond being fired at a distance of 0.1nm for the nanotube. They investigated the effects of parameters such as height of hit, radius of nanotube, length of nanotube, and initial speed of bullet. Figure 4 demonstrates that the maximum energy absorption occurs at the center of the nanotube. The effect of radius on the energy absorption is very small. Figure 5 shows that the maximum absorption energy varies linearly with nanotube length. Figure 6 shows that the bullet started to bounce back almost at the same time no matter the initial speed and position of the bullet hitting the nanotube.

FIGURE 4: Variation of relative absorption energy with respect to the position at which the bullet strikes.

FIGURE 5: Maximum energy absorbed with CNT length.

If subsequent impact happens right after the first one, the maximum load bearing speed and the absorption energy decreases, unless the CNTs are given at least 12.5ps to recover.

FIGURE 6: Variation of speed during the bullet impact with the CNT.

EXTENSION TO MACROSCOPIC APPLICATIONS:

Because the nanotube diameter is generally defined during the growth process, the atomic structure of CNT is primarily determined by its length. Therefore, CNTs do not go through the transition from monocrystalline structure to polycrystalline structure, which usually happens in metals. The fact that the scale-effect in CNTs is approximately minimal allows us to apply previous analysis and results to a macroscopic scenario.

Say we weave loose CNT bundles into a body armor material composed of several layers of woven CNT yarns. For a (18, 0) CNT, a 100μm diameter bundle usually contains 5 X 109 nanotubes. From previous calculation, we know that the absorption energy varies almost linearly with the tube length. If we assume nanotubes yearns of 0.9cm length are used to protect a person from a revolver bullet, which typically has a damage area of 0.652 cm2 and energy of 320J—as shown in Figure 7—180 nanotubes yearns will be required. A single nanotube yarn should be able to absorb 0.344J of energy; therefore 6 layers of woven fabric should be sufficient, which corresponds to a thickness of 600μm.

FIGURE 7: A Layer of woven CNT yarn material.

REFERENCES

  1. Ajayan, P. M. (1999). Nanotubes from carbon. Chemical Reviews, 99(7), 1787.
  2. Mylvaganam, K., & Zhang, L. C. (2007). Ballistic resistance capacity of carbon nanotubes. Nanotechnology, 18(47), 475701.

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