Researchers have long been working to find new materials that are better protected against 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 target materials, according to an article in ACS Applied Materials & Interfaces, LaserMade.com understands. This is done by using a high-intensity laser to eject a microprojectile at near the speed of sound at a 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 a scaling method is used to predict the material's resistance to puncture by a larger high-energy projectile, such as a bullet. In this way, combining testing with analysis and scaling methods, scientists can 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 study a new material for a protective application, with our new approach, we can get an earlier idea of whether its protective properties are worth studying."
Synthesizing small amounts of a new polymer may be fairly routine in laboratory experiments; the challenge is scaling up the quantity to test its puncture resistance - materials made from new synthetic polymers where 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 in making new materials," said Christopher Soles, a materials research engineer at NIST. You need to synthesize a new polymer that you think is better, and then scale it up to the kilogram level. The big 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 study, the researchers used their method to evaluate several materials, including widely used ballistic glass compounds, new nanocomposites and samples of graphene materials.
The test method is called LIPIT, which stands for "Laser Induced Projectile Impact Test. It uses a laser to fire a microprojectile made of silica or glass into a thin film of the material of interest. Through laser ablation, the laser produces a high-pressure wave that pushes the microprojectile material into the sample.
The researchers first used the method to analyze a nanocomposite called polymer-grafted nanoparticle polymethacrylate (npPMA) composite. It consists of silica nanoparticles and could be used in a wide range of applications, including bulletproof vests. A laser propels the micro-bullets toward the target material at speeds of 100 to 400 meters per second, and a camera is used to measure their impact.
The researchers combined the measurements obtained on npPMA with additional mathematical analysis, along with available data on the material from the research literature, to relate the results of the micro-ballast tests to the impact in a larger impact. Since npPMA is a new material that is not easily fabricated, 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 the material's puncture resistance is related to the maximum stress the material can withstand before fracture (i.e., failure stress). This challenges the current understanding of ballistic performance, which is typically thought to be related to how pressure waves pass through a material.
Their new method can determine the strength limit of a material, or how much stress and pressure it can withstand, without directly measuring these properties beforehand, which helps 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 the next step, the researchers plan to evaluate the ballistic properties of other new materials and study different types and configurations. They will also vary the size of the microbombs and expand their velocity range.
