Foam in helmets, armor can cause injury

In a counter-intuitive finding, scientists at New York University (NYU) and Polytechnic Institute of New York University (NYU-Poly) report 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 themselves 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 football players whose injuries are not immediately detectable but whose symptoms evolve over time.

Nikhil Gupta of NYU-Poly's Department of Mechanical Engineering and Paulo Coelho of NYU's Department of Biomaterials and Biomimetics led the team that undertook two recent studies. They were conducted on rabbit femur bones but the team expects similar results on human bones. 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 loading rates.

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, Dr. 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.

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 the fracture pattern varied according to the load.

"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 their symptoms evolve over time.