Design of an optimally safe football helmet system requires an awareness and evaluation of the factors and variables that can adversely affect the impact attenuating performance of energy absorbing (EA) pad materials needed to minimize transmission of linear and rotational forces applied to the head so that risk of head injury is reduced. For instance, player head sweating can induce high temperatures and moisture within a helmet system (i.e. a Hot-Wet condition) which can result in degradation of helmet EA capacity and cause increased measures of head injury risk levels, which are often used for comparative evaluation of helmet designs. In this study, a “multivariable” experimental method was utilized to demonstrate an efficient means for assessment and comparison of currently representative adult and youth football helmet system designs when subjected to a range of variables that included, among other factors: temperature-moisture effects; impact energy; and, repeat impacts. Both quasi-static (QS) compression testing of commonly used EA materials and dynamic impact testing of full helmet systems were conducted and the results are presented in Tables and graphic form. The EA pad types that were QS tested included: Thermoplastic-Polyurethane (TPU) “waffle shaped” EA pad configurations; load rate sensitive “Gel” foam padding; and, dual and single density elastomeric foam padding. Dynamic helmet repeat impact tests were conducted by using a pendulum impact test device where various helmet designs were mounted to a Hybrid-III head and neck system and impacted against a non-yielding surface at energy levels of 108J and 130J after being subjected to ambient and Hot-Wet conditions. The QS tests showed that a short Hot-Wet soak time of only a few hours’ noticeably diminished EA levels. Also, the dynamic full helmet system testing demonstrated that the “Hot-Wet” condition tended to degrade helmet impact attenuation performance such that, depending on the size and type of EA material provided in the crush zone, head injury risk measures tended to increase. Finally, examples of the use and benefits of a “multivariable” experimental method for helmet injury risk assessment, not reported on previously, are provided.
Vehicle to vehicle rear impact crash tests and sled buck tests were run to evaluate seat system performance related to Hybrid III surrogate response and comparison with NHTSA proposed combined load injury assessment values, as well as standard injury criteria. The crash and sled buck test impact conditions were modeled after actual case study incidents where changes in the rear impacted vehicle speeds ranged from about 25 to 50 kph. With the exception of one baseline vehicle-to-vehicle rear impact test, the dynamic tests provided side-by-side comparisons, and test-to-test evaluations, of surrogate response in conventional yielding front seats versus much stronger seat systems such as the belt integrated seat designs. Head, neck and chest injury criteria were used in the evaluations, including both the proposed NHTSA combined load neck criteria and SAE J 885 injury values. The surrogate response injury levels for the conventional yielding seats correlated well with the actual case study injury results. The seat comparison response generally indicated much reduced head and neck injury potential to surrogates seated in the stronger seat designs. The dynamic tests also demonstrate the importance of testing within the full vehicle interior structure to insure that floor strength is compatible with seat strength, so as to attain optimum occupant protection in stronger seat designs, and to assess injury risk to occupants in yielding or collapsing seat designs, as well as rear seated occupants, such as children. The tests indicate that quasi-static seat strength measurements made with more realistic “torso body block” load devices can provide reasonable estimates on the ultimate failure modes and dynamic load capabilities of the seat systems if the seat systems are properly mounted to the vehicle. Quasi-static seat strength results are presented for a variety of conventional collapsing seat designs and stronger seat systems like the belt integrated designs. One sled buck test was run with a rear-seated child surrogate to demonstrate the hazard of front seat collapse into the rear seat occupant area. The results of these tests further demonstrate the need for dynamic testing to assess total seat system performance and full occupant protection in rear impacts.
Since 1996 the NHTSA has warned of the airbag deployment injury risk to front seated children and infants, during frontal impact, and they have recommended that children be placed in the rear seating areas of motor vehicles. However, during most rear impacts the adult occupied front seats will collapse into the rear occupant area and, as such, pose another potentially serious injury risk to the rear seated children and infants who are located on rear seats that are not likely to collapse. Also, in the case of higher speed rear impacts, intrusion of the occupant compartment may cause the child to be shoved forward into the rearward collapsing front seat occupant thereby increasing impact forces to the trapped child. This study summarizes the results of more than a dozen actual accident cases involving over 2-dozen rear-seated children, where 7 children received fatal injuries, and the others received injuries ranging from severely disabling to minor injury. Types of injuries include, among others: crushed skulls and brain damage; ruptured hearts; broken and bruised legs; and death by post-crash fires when the children became entrapped behind collapsed front seat systems. Several rear-impact crash tests, utilizing sled-bucks and vehicle-to-vehicle tests, are used to examine the effects of front seat strength and various types of child restraint systems, such as booster seats and child restraint seats (both forward and rearward facing), in relation to injury potential of rear seated children and infants. The tests utilized sedan and minivan type vehicles that were subjected to speed changes ranging from about 20 to 50 kph (12 to 30 mph), with an average G level per speed change of about 9 to 15. The results indicate that children and infants seated behind a collapsing driver seat, even in low severity rear impacts of less than 25 kph, encounter a high risk of serious or fatal injury, whether or not rear intrusion takes place. Children seated in other rear seat positions away from significant front seat collapse, such as behind the stronger “belt-integrated” types of front seats or rearward but in between occupied collapsing front seat positions, are less likely to be as seriously injured.
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.
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