Retrofitting the American Football Helmet with Energy-Absorbing Superelastic Nitinol

J. Sousa
Western Michigan University,
United States

Keywords: American football, concussion, finite element analysis, helmet, hyperworks, impact, nitinol, NOCSAE, optimization, radioss, superelastic


Head injuries sustained while practicing sports are unavoidable, therefore, studying how to mitigate them is crucial for the well-being of players worldwide. Recently, the NFL has used its resources and reach to further develop the study of head injuries sustained in American Football games. Based on the promising preliminary studies developed by Dr. Ari-Gur and her undergraduate students, the implementation of Nitinol on an American Football helmet was found to have reduced the magnitude of the blow to the head. These preliminary studies used Nitinol strips from Johson Mathey Medical Components, to reinforce a Riddell Victor Youth Football Helmet. Five different design options were tested (based on the NOCSAE drop test standard for new American Football Helmets), the best one resulting in a 27.8% acceleration decrease upon impact, while only having a 5.2% helmet weight increase. Nitinol is a Nickel and Titanium metallic alloy, with similar percentages of both components (close to 50%/50%). Nitinol is beneficial for impact mitigation applications is that it can quickly deform under stress, and quickly restore its original shape when the stresses are released (superelastic). The load hysteresis leads to energy absorption, significantly decreases the accelerations felt by football players. In this project, a finite element optimization dynamic analysis and topological optimization following the ram test specifications of the NOCSAE standards and using Altair® Hyperworks®, specifically the Radioss® solver that includes superelastic material models, were conducted. For the construction of the FEA model, the available geometries provided by Biocore LLC and NFL were used both for the helmet and the headform. These resources were developed and made available under the NFL’s "Play Smart, Play Safe" initiative. The simulations were run in an iterative manner, testing different designs and different positions of Nitinol strips within the helmet (between the outer shell and padding). Following the NOCSAE standards, six different impact positions were tested (Side, Rear Boss NC, Rear Boss CG, Rear, Front Boss, Front and Random) with a modeled pneumatic ram impactor head at a speed of 6 m/s. The headform accelerations were measured within Radioss®, and an optimal design was obtained (greatly reduced impact acceleration). Furthermore, experimental tests that followed the same standards, were conducted to verify the computational predictions. Testing was done on a swinging pendulum system (designed and constructed by Professor Alessander D. Santos), that was configured to simulate the Ram test specifications (standard ND081). The optimal Nitinol configuration and distribution within the helmet was experimentally corroborated. The final product developed in this work was a stand-alone insert placed on the inside of the helmet, that houses the Nitinol strips, allowing easy access to the material in case of changes (different playing temperatures will require different types of Nitinols). Overall, the acceleration felt by a player's head in the case of the collision was reduced, mitigating the risk of injury, as initially intended.