The most fundamental challenge of designing armor that can protect the wearer against firearms is balancing the need for protection and stopping power with the need for the clothing to be lightweight and flexible; wearing armor of metal plates thick enough to stop bullets is simply not practical for everyday protection. The revolution in synthetic fiber technology that took place during the first half of the 20th century made it possible to address this challenge: material scientists developed ultra-tough and flexible fibers that could be layered together to create a mat of material strong enough to stop a bullet, but still light enough for panels of this protective material to be worn as a vest or jacket.
In the time since then, further advancement in polymer fiber technology has allowed for the creation of ever stronger and lighter ballistic armor. Now, after rapid advancement of carbon nanotube (CNT) material science in the last decade, CNT fibers may be ready to compete with the current state of the art in ballistic protection.
Ballistic Fiber Properties
To function well, ballistic armor needs to have several characteristics. A bullet impact does damage by putting a lot of kinetic energy into a very small package; the location the bullet hits will experience a high concentration of force. The first task of ballistic armor is to prevent the bullet from passing through the material entirely, and in order do to this the fibers need to have high tensile strength and high total toughness – measurements of how much stress (force on a fiber divided by the cross sectional area of that fiber) and how much total energy input a fiber can withstand without breaking.
Just as importantly, the fibers need to be overlapped or woven in such a way that they reinforce one another to spread the energy of the impact over a larger area. The faster that energy is dissipated along the fiber strands, the less total energy needs to be absorbed by the fibers at the direct point of impact and the less likely it is that those fibers will break. Additionally, the fibers need to be able to absorb all of this energy without stretching too far, or the human body under the armor will take a larger share of the impact. A vest that could absorb all of the energy of a speeding bullet, but which had to stretch several inches like a rubber band in order to absorb it all, would not be very helpful for the person wearing it! The ability of fibers to translate force quickly along their own length and their ability to come under stress without stretching significantly are both governed by a property called the tensile modulus. The tensile modulus is a measure of stiffness; a stiff fiber with a high tensile modulus can sustain a very large amount of stress with only a small amount of stretch.
So, both tensile strength and tensile modulus are important properties for ballistic fibers. Since armor also needs be light enough to wear (and since extra thickness can be added if it is made from a lighter material), low density is also important. Therefore, the properties that are truly critical for a ballistic fiber are the specific tensile strength (also referred to as tenacity) and the specific tensile modulus – these are the tensile strength and modulus normalized by the linear density of the material. Other important prerequisites of ballistic fiber include the ability to resist wear from flexing and friction, resistance to melting and degradation due to heat, resistance to chemicals, and other issues that contribute to wearability and durability.
The materials commonly used for modern ballistic armor are aramid fibers (such as Kevlar and Twaron), and ultra-high molecular weight polyethylene (UHMWPE) fibers (such as Dyneema and Spectra). UHMWPE fibers can be made with a wide variety of formulations and therefore have a range of different mechanical properties; at the high end of their performance they are able to slightly out-perform aramid fibers in tensile strength and tensile modulus. They also have lower density than Aramids, so their tenacity and specific tensile modulus values are superior (see the table below). Aramid fibers, on the other hand, are more durable against exposure to certain chemicals and high temperatures. Aramid fibers can also be more flexible than the highest-performing UHMWPE products. The two types of fiber are sometimes used together to form ballistic armor with an optimal mix of weight, strength, and flexibility.
|Tenacity (mN/tex)||Specific Modulus (N/tex)|
|Aramid Fibers||20801||49 – 721|
|UHMWPE Fibers||1610 – 42502||87 – 1592|
|Rice University CNT Fibers||21003||2603,4|
|Galvorn CNT Fibers||17005||2405|
Representative specific mechanical properties for aramid, UHMWPE, and CNT fibers. References:
Ballistic Carbon Nanotube Fiber?
Like aramids and UHMWPE molecules, CNTs are long-chain molecules with high tensile strength, high stiffness, and low density; this is what has motivated their use for making lightweight high-strength fibers. Given the potential of this material, it makes sense to consider whether CNT fibers could be used to make ballistic armor in the near future. We have performed some "informal" tests in the past that demonstrate the toughness and resistance of Galvorn CNT fiber to cutting impacts...but how do the material properties of CNT fibers really compare to those of state-of-the-art ballistic fibers?
The tensile strength and tensile modulus of a single CNT molecule can attain outstanding values: up to 100 GPa and 1 TPa, respectively; however, the properties of a macroscopic fiber formed by millions of interconnected and overlapping CNTs will depend on the structure into which they are assembled. Alignment, packing, and the length of the interfaces of contact between the molecules within the fiber determine how well they transfer stress to one another and whether neighboring molecules will slip away from each other before they themselves break, which determines the tensile strength. The alignment of the CNTs within the fiber also determines whether there is “slack” in the fiber structure that might allow it to stretch before the CNTs themselves truly come under tension, which affects the modulus.
The earliest CNT fibers attained only a fraction of their potential due to the challenges of achieving good fiber structure; however, the properties of CNT fibers have improved rapidly over the last decade as techniques for optimizing their structure have been developed and refined. Today, commercially available CNT fibers such as Galvorn have achieved over 80% of the tenacity of Kevlar and a specific modulus that is significantly higher than that of even high-stiffness UHMWPE fibers. Academic researchers at Rice University have achieved even better results than this: newly published results have demonstrated production of CNT fibers that have a tenacity equivalent to Kevlar. Researchers have also recently shown that lab-produced CNT fibers exposed to supersonic impact loads exhibit tensile strengths that are even higher than those of aramid fibers exposed to the same type of impact, indicating that they may be even more suitable for ballistic protection applications than their static mechanical properties would imply.
CNT fibers possess some of the other properties that are beneficial for ballistic armor, such as thermal stability and excellent chemical resistance. The property that really sets them apart from existing ballistic fibers is their electrical conductivity - both aramids and UHMWPE fibers are insulators. This could allow for the creation of ballistic vests that are also smart clothing: armor equipped with biometric sensors, heating elements, or other electrical functionality.
The manufacture of CNT fiber and other solid CNT materials is still a very new industrial technology, especially compared with the history of aramid and UHMWPE fibers, each of which have been in production for several decades. The rapid and continued advancement in CNT fiber properties over the last decade, as understanding of manufacturing has deepened and improved, is a hopeful sign that this material has not yet reached its full potential. CNT fibers already have a superior tensile modulus compared to aramid fibers and UHMWPE, and as their tenacity continues to improve, CNT fibers could very well displace the current generation of ballistic polymers and herald a new generation of ultra-lightweight body armor.