American football helmets

An item of sports equipment that has been improved greatly with regard to player safety is the crash helmet. American football helmets are polycarbonate or ABS shelled, while impact nylons are often used on motorbike crash helmets. Another item of sports equipment in the United States where safety has been improved is a Reduced Injury Factor baseball that has a novel polyurethane core material. 

In 2003, Schutt Sports, Inc. was the first to introduce TPU cushioning into American football helmets (Schutt Sports, 2014). The mechanical performance of this inner liner system was described previously (Caswell et al., 2007). The sustained popularity of the inner liner is reflected in the newly available VTD helmets that contain a similar TPU cushioning design with varying thickness and durometer hardness. Additional helmet models have recently included a new engineered TPU material that displays viscoelastic and shear-thickening behavior for enhanced impact protection (Schutt Sports, 2014). The non-Newtonian, dilatant material increases in stiffness with the rate of shear strain, such that the stress and strain characteristics are dependent on the rate of loading. 

Figure 10.13 SEM images of the surface of an American football helmet outer shell PC/PET material in the conditions of (a) pristine and (b) after 1 h of immersion in n-butyl acetate solvent.

Figure 10.14 Maximum compression height of a novel American football helmet inner bner component across increasing impact velocities.

The resistance to deformation at the surface can be quantified at the micrometer and nanometer scale. At the micrometer scale, traditional hardness measurements are conducted on a polymer system using a handheld durometer, whereby the penetration tip is varied to properly quantify the Shore hardness. At the nanometer scale, measurements of modulus and depth of penetration are quantified using quasi-static nanoindentation. Increases in stiffiiess at the surface of an American football helmet outer shell material exposed to accelerated weathering (Krzeminski et al., 2014c) and repetitive linear impacts (Krzeminski et al., 2014b) have been quantified using quasi-static nanoindentation. However, Shore A hardness measurements were reported to be overly forceful to quantify shifts in surface mechanical properties of injection-molded American football helmet outer shell materials (Krzeminski et al., 2014b,c). 

Figure 10.18 (a) Full ATR-IR spectra across a representative American football helmet outer shell model from each of the four major helmet manufacturers and (b) American football helmet outer shells fixtured into an ATR-IR spectrometer. 

Sport helmet systems are expected to deteriorate over time. In fact, the known reduction in protective capacity of the American football helmet system has led to an accepted restorative process throughout the industry. The pursuit to maintain the initial performance standard of American football helmets and efforts to mitigate the high-prevalence sports-related concussion have prompted the implementation of a certified reconditioning and recertification process. 

Additional additive technologies also include add-ons that are designed to measure on-field impact biomechanics. Systems that are placed on the inside of the helmet are available through American football helmet manufacturers, such as the Riddell Head Impact Telemetry System (HlTs) and InSite Impact Response System. Aftermarket... 

Krzeminski, D.E., Goetz, J.T., Janisse, A.P., Lippa, N.M., Gould, T.E., Rawlins, J. W., Piland, S. G., 2011. Investigation of linear impact energy management and product claims of a novel American football helmet liner component. Sports Technol. 4, 65-76. 

Krzeminski, D., Fernando, D., Rawlins, J., Gould, T., Piland, S., 2014a. Effects of solvent exposure on material properties and impact performance of an American football helmet outer shell material. lYocedia Eng. 72, 508-514. 

Post, A., Oeur, A., Hoshizaki, B., Gilchrist, M.D., 2013. An examination of American football helmets using brain deformation metrics associated with concussion. Mater. Des. 

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