Posted March 3rd, 2011
When two professional football players collide at full speed, it has been compared to experiencing a head-on car collision at 30 miles per hour. Even with all their padding, players still break bones and wear down over time. It turns out that the speed of the collision is not just bone-breaking on TV but also bone chipping at the microscopic level, according to researchers at New York University (NYU). Nikhil Gupta, associate professor of mechanical and aerospace engineering at the Polytechnic Institute of NYU, and Paulo Coelho, professor of biomaterials and biomimetics at the NYU College of Dentistry, have used a unique bone-crushing machine (the "Kolsky Bar") and scanning electron microscopy to examine how compression speed affects bone and material properties. With support from NSF, the research team recently showed that micro-fractures appear in bones after high-speed collisions. These fractures, which can critically weaken the bone, have gone unnoticed in clinical settings due to limited diagnostic technology. In addition to observing fractures with increasing crushing speed, they noted that direction of the cracks also changes with the compression speed. Building on these results, the team wondered if the speed of compression also affected materials used for military body armor, since they often incur high speed bullets and shrapnel. To their surprise, they discovered that body armor foams substantially stiffen when compressed at higher speeds. To follow up on this work, the team plans to formulate next-generation materials that can reduce bone injury from high-speed compression and hopes to stimulate development of more sensitive clinical imaging instruments so that doctors can see these micro-fractures in the clinic.