CompositesWorld
Issue link: https://cw.epubxp.com/i/576646
OCTOBER 2015 86 CompositesWorld FEATURE / COMPOSITES IN HEADGEAR "Composites, and in particular Kevlar, allows us to design a more form-ftting helmet and get the same impact absorption as a traditional injec- tion molded ABS," he claims, reporting that comparable interior foam padding for a helmet with an ABS shell needs to be 3-4 mm thicker. Shred Ready also has launched a new model this year called the OSK, for Old School Kool. Te helmet features a shell made entirely of composites, reinforced with fbers supplied by Innegra. Manufactured "under high heat and pressure," the shell sports three inner layers of Innegra-S and an outer layer of Innegra-H carbon hybrid fber. Sherburne says the hybrid fber processes better than the purely hydrophobic olefn fber and provides a better surface fnish — important, because decorative details are applied to the helmet, post-cure. "Te Innegra S fber is equal to Kevlar for impact absorption, and the carbon adds toughness," Sherburne contends, noting an overall gain in cost-efectiveness, compared to other material solutions. A similar construction, involving either or both of the Innegra fbers, might be the "X-factor" in the design and manufacture of a football helmet that could meet all the necessary performance criteria and narrow the price premium between it and football helmets made of polycarbonate. Success in building a next-generation football helmet will no doubt hinge on a similar amount of tweaking and trade-ofs. In a study introducing a new head injury metric, called "the combined probability of concussion," Steve Rowson, research assistant professor, bio-medical engineering, Virginia Tech, writes, "part of the remaining burden of reducing concussion incidence relies on improvement of head protection," noting current football helmets are designed to prevent skull fractures, not to guard against concus- sion. "One of the challenges in designing helmets to account for concussive forces is accurately modeling concussion risk," he writes. One of the original studies attempting to quantify the threshold forces associated with concussions was carried out by researchers at Wayne State University (Detroit, MI, US) in the 1960s (Gurdjian, ES, et. al.). Tis and subsequent studies have focused on concus- sions as a function of two factors: Impact magnitude (accelera- tion measured in G-force) and impact duration (in milliseconds or ms). Research has identifed a range in which for concussion- producing impacts occur in the NFL: between 72G and 125G, with a duration of 14-16 ms. From this quantitative angle, the goal of a composite helmet would be to increase the duration of impact, thus dissipating some of the energy laterally, thereby decreasing the resulting acceleration and G-force to a value substantially below the threshold value of 72G (see the chart on p. 85). But such a helmet must be able to withstand repeated impacts through the course of a game and season, and therein lies the rub. "When you look at the market for helmets, composites are widely accepted for motorcycles, bike racing, whitewater sports," Cates says. But she points out: "What distinguishes a motorcycle helmet Te design solution for energy dissipa- tion has been to pad the interior of the shell with copious amounts of foam. Until now, the possibility that the shell could be a front line of defense against head injuries in football has been constrained by the notion that its frst, and foremost, function is impact resis- tance. "We knew from high-energy impact applications, such as ballistics, that fber-reinforced composites provide great advan- tages in dissipating forces laterally, rather than directly through the material," says Cates. Te second phase of testing, commencing this year, will evaluate a more complex set of fber and resin laminate systems and layups, including toughened resin systems and diferent combinations of commingled yarns (Innegra's H-fber) and woven fabrics. It also will employ a slightly modifed test method that uses an impactor with a larger radius and geometry designed to more closely mimic the curvature of a football helmet. Desjardins reports that the same drop sled used in the frst phase also will be used in the next, although the drop height will be lower due to the greater mass of the impactor. Although basic fber-reinforced materials demonstrated some signifcant potential performance enhancements with respect to the control materials in the frst phase of testing, researchers expect the use of a wider impactor in the next round to change some of the energy dissipation characteristics. In short, "everything is on the table" on the materials side of the equation, says Cates. Further, she notes that greater cognizance of manufacturing cost and processing factors will be necessary as the project moves forward. Helmet optimization: The performance/cost interface Tat is a bridge already crossed by Shred Ready (Auburn, AL, US), a manufacturer of helmets and safety gear for whitewater paddlers. Te company currently makes fve all-composite whitewater helmet models, as well as less-expensive helmets made of injection molded ABS. One of the most common layups for a number of its composite models comprises two layers of glass on the inside ply and a layer of Kevlar as the outside ply. Shred Ready president and founder Tom Sherburne says the aramid fber layer provides excel- lent energy dissipation properties, and enhances design features. Helmet technology transfer Shred Ready (Auburn, AL, US), a manu- facturer of helmets and safety gear for whitewater paddling, has launched a new model this year called the OSK — Old School Kool — featuring a shell made entirely of Innegra Technologies LLC's (Greenville, SC, US) S- and H-fbers. Reportedly, a similar construction could be the "X-factor" for a football helmet that could meet all the necessary performance criteria and narrow the price premium between it and polycarbonate helmets. Source | Innegra