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College of Engineering and Computing

  • Mechanical Engineering Professor Subramani Sockalingam

Protecting soldiers against ballistic impacts

CEC Professor aims to develop lightweight composite materials for improving U.S. Army body armor

Surviving enemy gunshots to the chest, protection against hot metal fragments from explosions and preventing blunt force trauma are some of the life-saving applications of body armor worn by U.S. Army soldiers. The armor must also be made of material that allows for effective movability and speed. Mechanical Engineering Assistant Professor Subramani Sockalingam is currently working on research that aims to better understand structure-property relationships of armor materials by experimenting with a novel lightweight composite material.

Sockalingam’s five-year research project began in August 2020 and is a $380,000 grant-funded award from the U.S. Army Research Office and Department of Defense. The goal is to determine and better understand material properties and behavior that significantly affect ballistic impact performance in armor systems.

“The Army wanted to look at a new type of material and understand how it will respond to impact loads. We’re always interested in understanding the behavior of a new material, which is one of the reasons why we wanted to get involved,” Sockalingam says.

Developing ultra-lightweight body armor for the U.S. Army will make soldiers and units more lethal and safer. While current body armor consists of ultrahigh molecular weight polyethylene (UHMWPE) fiber-based polymer composites, film-based composites have shown promising ballistic performance. They are thin, 2D materials which are relatively new and have the potential to mitigate impact loads and injuries sustained by soldiers at high strain rates. 

“The film composite is less expensive to manufacture and has the potential to perform equally or slightly better than the fiber based. That's why there’s interest in understanding how these materials will respond and fail at ballistic impact loads,” Sockalingam says. 

Interlaminar shear diagram

The lack of understanding of the film composite processing on properties and mechanisms during impact is the biggest issue to enhancing ballistic performance. Sockalingam’s research team aims to gain a fundamental knowledge of the film-based composite’s interlaminar shear behavior at high strain rates. This is essential to gain insights into strength and energy absorbing mechanisms, while providing a guide for the film composite manufacturing process. Previous research has shown that interlaminar shear cracks in-between material layers is a significant factor in ballistic performance. 

“These [film composites] are basically layers, which are built to make the structure. Then there’s bonding between each layer, so its performance is crucial to the overall performance,” Sockalingam says. 

The first two years of the research has focused on understanding the interlaminar shear response. Since one of the concerns is how will the bonding of the film composites perform at different loading rates, Sockalingam’s team is currently focused on slow-speed loading by manufacturing composite panels and testing them at slow speeds. Observing how the bonding failed also helped to determine properties. The bond behavior is characterized specifically in the shear direction at slow rates of loading. With this understanding, the team is currently designing and developing an experimental setup for high-speed loading, which will be performed by Sockalingam’s graduate research assistants early next year at the Army Research Lab in Aberdeen, Maryland. 

“Since this is for ballistic impact, the behavior at higher rates is largely unknown. We want this procedure to not only work at a slow rate, but one that can also be adapted to a high strain rate and closer to the kind of loads you would see in a real ballistic impact,” says Frank Thomas, one of Sockalingam’s graduate research assistants. “We're working on finalizing our slow rate approach and getting the best quality data to have a solid basis for the high strain rate tests.” 

Characterizing heterogenous materials, which have spatially different mechanical properties, is another focus of the project. This aspect is led by Mechanical Engineering Distinguished Professor Michael Sutton. Bone is one example of a complex, heterogenous material. 

“The Army is interested in developing models to understand how an impact load affects the skull (bone) since it can cause traumatic brain injury or blunt trauma. To develop those types of models, we must characterize the bone property accurately so that we can use that as input in the model,” Sockalingam says. 

Sockalingam and Sutton plan to combine two techniques since materials are complex and characterizing their properties can be difficult. The first technique is digital image correlation (DIC), a mapping and measurement tool to determine the shape, motion and deformation of solid objects, which Sutton was a pioneer in developing. DIC will be combined with the finite element method, which creates a solution for determining complex material properties. 

Sockalingam has enjoyed working with the U.S. Army Research Lab, whom he has previously collaborated with since his graduate studies. Once his team develops and validates film-based composite results, he is excited to transfer those techniques to the army’s lab for further testing by his students. 

“It’s not like we are working on our own and just reporting to the U.S. Army Research Lab. They are helping us design and do some initial experiments,” Sockalingam says. “The continued collaboration and strengthening of relationships between the army and us, and having our students get employed at the Army Research Lab would be two successful outcomes of the research.”

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