04 Jul 2023
NIST using laser-launched projectiles plus data help predict target materials’ microscopic properties and behavior.National Institute of Standards and Technology (NIST) have designed a method that uses a high-intensity laser to blast microscale projectiles into a small sample at velocities that approach the speed of sound.
The system analyzes the energy exchange between the particle and the sample of interest at the micro level then uses scaling methods to predict the puncture resistance of the material against larger energetic projectiles, such as bullets encountered in real-world situations. The new method, described in ACS Applied Materials & Interfaces, reduces the need to perform a lengthy series of lab experiments with larger projectiles and bigger samples.
NIST chemist Katherine Evans explained, “When you’re investigating a new material for its protective applications, with our new method we can see earlier if it’s worth looking into it for its protective properties.”
During lab experiments, synthesizing small amounts of a new polymer can be fairly routine. The challenge comes with scaling up to produce kilograms of material to be able to test its puncture resistance. For materials made from new synthetic polymers, scaling up to sufficient quantities is often not possible or practical.
NIST materials research engineer Christopher Soles said, “The problem with ballistic tests is that you must take two steps when making new materials. You need to synthesize a new polymer that you think will be better, and then scale it up to kilogram size. The biggest accomplishment of this work is that we surprisingly show that the micro-ballistic tests can be scaled and linked to real-world large-scale tests.”
During the course of the study, researchers used their method to evaluate several materials, including a widely used compound for bulletproof glass, a novel nanocomposite, and a sample of graphene.
The test is called LIPIT, meaning “laser-induced projectile impact testing”. It uses lasers to launch a microprojectile made of either silica or glass into a thin film of the material of interest. Through a process called laser ablation, the laser creates a high-pressure wave that propels the microprojectile material toward the sample.
The researchers first used the method to analyze a nanocomposite material known as polymer-grafted nanoparticle polymethacrylate (npPMA) composite. It consists of silica nanoparticles that could be useful in a wide range of applications including body armor. The laser propels microprojectiles at velocities from 100 to 400 meters per second at the target material and measures their impact using a video camera.
Researchers connected the results of the microprojectile test to what would happen in larger-scale impacts by combining the measurements they obtained on the npPMA with additional mathematical analysis while incorporating existing data on the material from the research literature. Since npPMA is a novel material and not easy to make, they expanded their analysis to also include a more commonly available compound known as polycarbonate, which is widely used as a bullet-resistant glass.
The combined approach of using literature results, dimensional analysis and LIPIT allowed researchers to show that the puncture resistance of a material is tied to the maximum stress a material can take before it breaks, called failure stress. This challenges the current understanding of ballistic performance, which is typically thought to be related to how pressure waves travel through the material.
Their new approach can identify the strength limits of a material, or how much stress and pressure it can handle, without having to directly measure these properties beforehand, which can help optimize which materials to choose in experiments. This then enabled them to explore materials such as graphene, which showed that multiple film layers of the material can be used in impact resistance applications similarly to high-performance polymers.
The researchers are planning to evaluate the ballistic resistance of additional novel materials and look at the different types and configurations. They will also vary the size of the microprojectiles and expand their velocity range. NIST researchers are also looking to connect LIPIT’s experimental results to two types of simulations. One is finite element analysis (FEA), in which the object of interest is modeled as a group of simpler pieces that are interconnected.