Improving Armor, Helmets and the Diagnosis

It can mean the difference between life and death or living with a catastrophic injury, and has far-reaching implications for the fields of medicine, automotives and athletics.

Rabbit bone specimen tested at slow compression rate. The specimen is 10 mm high, showing that the crack is very large.

Rabbit bone specimen tested at slow compression rate. The specimen is 10 mm high, showing that the crack is very large.

In a counter-intuitive finding, scientists at New York University (NYU) and Polytechnic Institute of New York University (NYU-Poly) have found that the foam used in helmets and other body armor indeed absorbs damage when compressed slowly, but can cause as much injury as a hard object when hit at high speeds.

The materials scientists also found that bones fracture differently according to loading – another factor that will lead manufacturers to select protective materials according to the speed of impact, whether for sports equipment, military armor, car interiors or submarines. Their findings could change the methods of diagnosis for soldiers and athletes whose injuries are not immediately detectable but whose symptoms evolve over time.

High magnification electron microscope image of rabbit bone fractured at very high compression rate. Such rates can be encountered in blast loading. Notice the multiple small and random fractures, as compared to the bone tested under a slow compression rate.

High magnification electron microscope image of rabbit bone fractured at very high compression rate. Such rates can be encountered in blast loading. Notice the multiple small and random fractures, as compared to the bone tested under a slow compression rate.

Nikhil Gupta, assistant professor in NYU-Poly’s Department of Mechanical and Aerospace Engineering, and Paulo Coelho, assistant professor in NYU’s Department of Biomaterials and Biomimetics, led a team that undertook two recent studies using rabbit femur bones. Gupta became interested in the research after speaking with veterans of the wars in Iraq and Afganistan.

“Discussions with the soldiers about the nature of the IED blast injuries sparked my interest because physicians were unable to detect any injury immediately after the blast,” says Gupta. “The symptoms of serious injury usually evolved over time. My interest in materials characterization provided me with a background to develop techniques that can test bones at high-compression rates, simulate similar effects in the laboratory and conduct scientific investigation of this problem.”

Advanced engineering and medical equipment such as CT-scanners, electron microscopes and high-speed camera systems documented how bones and lightweight composite materials deformed and fractured under high compression rates. Gupta believed it was imperative to work with a doctor to investigate his hypothesis from the medical side and found the perfect complement in Coelho, who holds a PhD in Materials Science as well as a medical degree. “Our connection was easy and instantaneous,” says Gupta.

Slow-motion video of crashed bone.

In addition to helmets and armor, the lightweight foams are widely used in marine structures such as boats. Describing the importance of the findings on foams, Gupta said that “the foam materials that seem soft when slowly compressed can actually become much stiffer as the loading rate is increased. A foam that would crush when slowly compressed can cause injury if punched hard.” In subsequent studies, the team plans to investigate whether the change in the material behavior at high loading rates can actually increase the risk of injuries.

Nikhil Gupta and Paulo Coelho

Nikhil Gupta and Paulo Coelho

The team used a high-speed camera to take about 7,000 images per second when the bones were loaded at high rates, simulating a blow to the body or the impact from a nearby bomb blast, among other conditions. They found that the fracture pattern varied according to the load. The team expects similar results on human bones.

“The CT-scan and electron microscope allowed us to observe that at high loading rates, the fracture starts as numerous hairline cracks and causes substantial damage to the entire specimen,” Coelho said. Lower loading rates generated fewer cracks. “Clinically, it may be very challenging to detect and repair small-scale but widespread damage caused by high loading rates,” he said.

This observation will be useful in analyzing injuries of soldiers who are subjected to blasts or football players who do not display immediate detectable injuries but have their symptoms evolve over time.

“We also hope that our studies will lead to better diagnostic equipment,” says Gupta. “Today’s medical scanning devices are unable to detect the microscopic damage to bones – and we assume such damage also occurs to the brain and other parts of the body. With today’s technology, it could take years to detect the injury, making treatment difficult.”

The papers authored by the team appear in Materials Science and Engineering: A and Journal of Biomechanics. Their studies were supported by a National Science Foundation grant and supplemented by an NYU seed grant designed to foster research collaboration between the two affiliated schools.