Researchers have long been working to find new materials that are better at resisting high-speed punctures, but it's hard to connect the microscopic details of promising new materials to their actual behavior in the real world.
To address this problem, researchers at the National Institute of Standards and Technology (NIST) have devised a new method that uses laser-emitted projectiles and data to help predict the microscopic properties and behavior of a target material, as shown in an ACS Applied Materials & Interfaces article. This is done by using a high-intensity laser to shoot micro-projectiles at speeds close to the speed of sound at the target material, which in this case is a polymer film representing the puncture-resistant material to be tested.
The energy exchange between the particles and the tested material sample is analyzed at the microscopic level, and then scaling methods are used to predict the material's resistance to puncture by a larger, high-energy projectile, such as a bullet. Just like that, combining testing with analysis and scaling methods allows scientists to discover new puncture-resistant materials. The new program reduces the need for a lengthy series of laboratory experiments using larger projectiles and larger samples.
NIST chemist Katherine Evans explains, "When you're investigating a new material for a protective application, with our new approach we can learn earlier whether its protective properties are worth investigating."
Synthesizing small amounts of a new polymer can be fairly routine in laboratory experiments; the challenge is scaling up the quantity to test its puncture resistance - materials made from new synthetic polymers, scaling up to sufficient quantities is often impossible or impractical.
The problem with ballistic testing is that there are two steps you have to take when making a new material," says Christopher Soles, a materials research engineer at NIST. You need to first synthesize a new polymer that you think is better, and then scale it up to the kilogram level. The great achievement of this work is that we surprisingly found that microballistic testing can be scaled up and linked to real-world, large-scale testing."
During the course of the study, the researchers evaluated several materials using their methodology, including samples of widely used ballistic glass compounds, new nanocomposites, and graphene materials.
The test method is called LIPIT, which stands for "laser-induced projectile impact testing. It uses a laser to fire microprojectiles made of silica or glass into thin films of the material of interest. Through laser ablation, the laser generates a high-pressure wave that pushes the projectile material into the sample.
The researchers first used the method to analyze a type of nanocomposite called a polymer-grafted nanoparticle polymethacrylate (npPMA) composite. It consists of silica nanoparticles that can be used in a wide range of applications, including bulletproof vests. Lasers propel the micro-bombs toward the target material at speeds of 100 to 400 meters per second, and a video camera is used to measure their impact.
The researchers combined the measurements obtained on npPMA with additional mathematical analyses, along with existing data on the material from the research literature, to relate the results of the micro-bomb tests to the impacts in larger impacts. Since npPMA is a new material that is not easily manufactured, they expanded their analysis to include a more commonly used compound (polycarbonate), which is widely used as bulletproof glass.
Using a combination of literature results, dimensional analysis, and LIPIT's methodology, the researchers were able to demonstrate that a material's puncture resistance correlates with the maximum stress that the material can withstand before fracture (i.e., the failure stress). This challenges the current understanding of ballistic performance, which is often thought to be related to how a pressure wave passes through a material.
Their new method allows them to determine the strength limit of a material, or how much stress and pressure it can withstand, without having to directly measure these properties beforehand, which helps to optimize which materials to choose in an experiment. This allowed them to explore materials such as graphene, which suggests that multiple thin film layers of the material could be used for impact-resistant applications, similar to high-performance polymers.
For their next steps, the researchers plan to evaluate the ballistic properties of other novel materials and investigate different types and configurations. They will also vary the size of the microbullets and expand their velocity range.
