Football Helmet Energy Absorption Degradation and Impact Performance Resulting From High Humidity and Temperature

Abstract

The helmet is the primary means for providing head impact protection to adult and youth football players through use of energy absorbing (EA) materials placed in a crush zone located between the head and helmet shell. Ultimate safety performance of the helmet requires uniformly consistent, repeatable and reliable attenuation of the impact energy so as to minimize head injury potential throughout the helmet. However, quasi-static materials tests and dynamic helmet testing results, reported on herein, show that EA materials of current and older helmet designs are susceptible to large levels of EA degradation, or softening, when subjected to a “hot-wet” condition caused by high temperatures and high humidity, such as that produced from the sweat of a player. Depending on the size of the crush zone, and other factors, this condition can lead to increased head impact loads. The standard football helmet certification criteria do not address the issue of “hot-wet” EA degradation. Dynamic helmet testing analyzed in this study consisted of two methods. One method used the standard helmet certification approach where a human responding head form and helmet are dropped vertically, along a twin guide wire set-up, onto a soft rubber pad. The second method employed use of a human responding Hybrid-III head and neck that was incorporated into a free pendulum impact set-up where impact took place on a non-yielding surface and both direct contact impact injury potential and rotational injury aspects of the helmet performance were measured. The dynamic tests were conducted with various size head forms, energy levels, and impact speeds that ranged from the 5.5 m/s level, used in helmet certification, on up to higher speeds of 7.0 m/s that is more consistent with a “5-second 40-yard dash” speed. Based on equal kinetic energy impact comparisons, the two dynamic approaches showed that helmets that were impacted onto the soft elastomeric pad surface produced artificially lower indications of head injury severity than did the helmets tested against the non-yielding surface. The results also showed large variations and inconsistencies of impact attenuation within a specific helmet design, depending on impact location or region being tested. Also, dynamic impact testing was applied with both ambient and 3-hour “hot-wet” soak conditions applied to the EA padding of adult and youth helmets. These results showed that the relatively newer EA pad designs and the older type elastomeric foam EA pads were sensitive to “hot-wet” degradation for soak times as low as 3-hours, which is consistent with game or practice time situations. Finally, as noted above, it was shown that, depending on the size of the crush zone, this EA degradation factor could lead to increased head loads and injury severity measures. The results suggest the need for additional research on the above to enhance helmet safety.