(Nanowerk News) Bullets that penetrate the armor of first responders, jellyfish sting swimmers, micrometeorites that hit satellites: High-velocity projectiles that pierce material appear in many forms. Researchers are constantly aiming to identify new materials that can better withstand these high-speed puncture events, but it’s hard to relate the microscopic details of these promising new materials to their actual behavior in real-world situations.
To solve this problem, researchers at the National Institute of Standards and Technology (NIST) have devised a method that uses a high-intensity laser to blast micro-scale projectiles into tiny samples at speeds close to the speed of sound. The system analyzes the energy exchange between the particle and the sample of interest at the micro level and then uses a scaling method to predict the material’s puncture resistance against larger energetic projectiles, such as bullets encountered in real-world situations.
This new method, described in the journal ACS Applied Materials & Interfaces (“Projectile Perforation Resistance of Materials: Scaling Thin Film Impact Resistance to Macro-Scale Materials”), reducing the need to perform long series of lab experiments with larger projectiles and larger samples.
“When you are investigating a new material for its protective application, you don’t want to waste time, money, and energy improving your test if the material doesn’t perform well. With our new method, we can see earlier whether the material is feasible to look at for its protective properties,” said NIST chemist Katherine Evans.
During laboratory experiments, synthesizing small amounts of a new polymer — for example, a few milligrams from glassware the size of a coffee cup — can be fairly routine. The challenge came with increasing to produce kilograms of material to be able to test its puncture resistance. For materials made from new synthetic polymers, increasing sufficient amounts is often not possible or impractical.
“The problem with ballistic testing is that you have to take two steps when creating a new material. You need to synthesize a new polymer that you think will be better, then scale it down to kilograms. That is a big jump. The greatest achievement of this work is that we surprisingly demonstrated that micro-ballistic tests can be scaled and correlated with large-scale real-world tests,” said NIST materials research engineer Christopher Soles.
During the study, the researchers used their method to evaluate several materials, including compounds widely used for bulletproof glass, new nanocomposites, and a strong all-carbon material known as graphene.
The test is called LIPIT, which stands for laser-induced projectile impact testing. It uses a laser to launch microprojectiles made of silica or glass into a thin film of the desired material. Through a process called laser ablation, the laser creates high-pressure pulses that push the microprojectile material toward the sample.
The researchers first used a method to analyze nanocomposite materials known as polymer-grafted polymethacrylate nanoparticles (npPMA) composites. It consists of silica nanoparticles which can be useful in a variety of applications including body armor. The laser propels the micro-projectile at 100 to 400 meters per second at the target material and measures the impact using a video camera.
The researchers linked the results of the microprojectile test to what would happen in larger-scale impacts by combining their measurements of npPMA with additional mathematical analysis while incorporating existing data on material from the research literature. Because npPMA is a new material and not easy to manufacture, they expanded their analysis to include a more commonly available compound known as polycarbonate, which is widely used as bullet-resistant glass.
This combined approach using literature, dimensional analysis and LIPIT allowed researchers to show that the puncture resistance of a material is related to the maximum stress that the material can take before breaking, which is called the failure stress. This challenges current understanding of ballistic performance, which is usually thought to be related to how pressure waves propagate through materials.
Their new approach can identify the strength limits of a material, or how much stress and strain it can handle, without having to measure these properties directly beforehand, which can help optimize which materials to choose in experiments. This then allowed them to explore materials such as graphene, indicating that multiple film layers of the material could be used in impact resistance applications similar to high performance polymers.
This new paradigm provides us with new experimental tools to evaluate the hype of some of these graphene and other 2D materials that are predicted to have excellent ballistic properties. We have the potential to experimentally verify whether this material will outperform classic ballistic resistant materials such as polycarbonate, even without increasing the synthesis of new 2D materials, which will be very expensive,” said Soles.
Their methods can help identify new materials for many applications such as additive manufacturing, spacecraft protection, better protection against animal bites, and even drug delivery. Researchers are looking for ways to develop needleless injections in which a high-velocity stream of fluid known as a liquid jet punctures the skin. While many applications aim to avoid punctures, LIPIT can provide insight in this regard to the most effective ways to penetrate the skin using liquid jets as projectiles.
As for next steps, the researchers are pursuing several avenues. They plan to evaluate the ballistic resistance of additional new materials and look at different types and configurations. They will also vary the size of the microprojectiles and expand their range of speeds.
The NIST researchers also wanted to relate the results of the LIPIT experiment to two types of simulations. One is finite element analysis (FEA), in which an object of interest is modeled as a group of simple interconnected pieces. FEA is traditionally used to simulate the mechanical deformation of the entire sample. Sometimes researchers can perform FEA simulations faster than laboratory experiments. Ultimately, though, the simulations have to match experimental data on real materials, says NIST materials science engineer Edwin Chan.
The second simulation approach is called molecular dynamics (MD). This is a much smaller scale type of simulation, looking at behavior at the molecular level of materials such as polymers. MD can explore how polymeric components such as molecular chains change shape after a projectile strikes the material.
“Because we don’t have the ability to see directly what polymer chains are doing, MD is very insightful because it gives us a better idea of why certain polymers are better for impact resistance,” says Chan.
The researchers hope their methodology opens up many new possibilities for investigating material behavior.
“With this approach, we can ask, ‘What else in the system can we change, or how can we improve the material for a particular application?’ Instead of changing the material’s composition, you can change its geometry. Or you can study matter from nature and see how it behaves,” said Evans.