Bicycle helmets

The high chemical stability of many polymers is both a blessing and a curse. Heat resistance, wear resistance, and long life are valuable characteristics of clothing fibers, bicycle helmets, underground pipes, food wrappers, and many other items. Yet when those items outlive their usefulness, disposal becomes a problem. 

The samples most commonly tested in compression are foams and rubbers, which experience compressive forces during use. Very often, the polymer foams that experience compression are not readily visible to us, even though they are all around. Polymer foams are widely used in carpet underlay, upholstery, shoe insoles, backpack straps, bicycle helmets, and athletic pads. Solid rubbers are much more visible, including automobile and bicycle tires, gaskets and seals, soft keys on calculators, and shoe soles. 

Touch your head you re touching a polymer— your hair Polymers are in plastic milk cartons, bicycle helmets, tires, playground balls, cotton, wood, tennis shoes, and your toothbrush. 

A polymer can be hard or soft, rigid or flexible. To make plastic objects, chemists and engineers first make the type ofpolymer they need based on the properties they re looking for. For example, a bicycle helmet needs a much tougher polymer than a grocery bag. Usually, the polymer is first made into very small pieces, in factories. These pieces are then melted and molded into tubes, sheets, or any other shape desired. 

Helmets for sports such as canoeing or cricket tend to have less severe impact requirements than for bicycle helmets, so may use smaller amounts of LDPE or EVA foam. As ventilation is important, there may be large holes in the foam and in the helmet shell. 

The article describes the process of producing expanded polypropylene foam parts with a solid integral skin on one side, citing a bicycle helmet moulded from BASF s Neopolen EPP bead, as an illustration. The self-skinning EPP process reqnires some modification of standard beadmoulding equipment, needing an extra steam chamber. Automotive interior trim is a potential application for the process. 

Both my kids bring joy to my life, and I find so much inspiration when I look at the world through their perspectives. Clyde wears a mini apron, and we have matching bandanas to cover our hair while cooking. When he couldn t find his "baking bandana once, he came up with the idea to wear his bicycle helmet Baking with... 

Polycarbonates are a class of thermoplastic polymers that are used in the plastic lenses of eyeglasses and in the shells of bicycle helmets. A polycarbonate is made from the reaction of bisphenol A (BPA) with phosgene (COCl2) ... 

Foamed plastics are used in helmets because of their very low density, their ability to crush and absorb energy and the possibility of economical mass production. There are many other applications of polymer foams for injury prevention, for instance the fascia padding in cars. Bicycle helmets are chosen because of the author s research experience, and because the typically 0.3 mm thick outer shell plays a minor part in energy dissipation. Consequently, the design process concentrates on the foam Hner (Fig. 14.11). In contrast, the deformation of the 4-5 mm thick thermoplastic shell of a motorcycle helmet absorbs energy of the order of 40 J, when the helmet hits a hemispherical anvil of radius 50 mm. 

The purpose of bicycle helmets is to reduce head injuries and deaths in accidents . The word accident is a misnomer as there may be culpable parties, whose behaviour could have been modified by training or by the... 

Figure 14.1 I Components of a bicycle helmet, seen in cross section.

The micro-shell of bicycle helmets causes negligible load spreading, so it is ignored. Initially, the foam is assumed to contain no ventilation holes these are allowed for later. Although neither the human skull nor the outer surface of a helmet is exactly spherical, it is a reasonable approximation that the impact site is locally spherical. Both the skull and the road surface are treated as being rigid. The approximately 5 mm thick scalp is so soft and deformable that it plays little part in the impact energy absorption. 

Figure 14.14 Force-deflection curves predicted for a bicycle helmet impacting flat, kerbstone and hemispherical anvils.

Problem 5 Determine the minimum thickness of foam in a bicycle helmet to keep the head acceleration below 200 g for an impact on a flat surface with lOOJ kinetic energy, at a site where the radius of curvature is 100 mm. 

Force-deflection graph for a bicycle helmet impact— the impact energy (area under the loading curve) must be absorbed without exceeding the injury force limit, or the foam... 

Figure 14.16 Large ventilation holes in a 2004 bicycle helmet.

Bicycle helmets can be designed for impacts of up to lOOJ kinetic energy, their mass can be as low as 200 g and they are comfortable to wear. Helmets designed for much higher impact energy levels would be unacceptably large, so it is impossible to protect riders from the most extreme impacts. Compromises are necessary when designing to protect people, who have variable (and unknown) tolerances to impact acceleration, from crashes with variable circumstances. 

Problem 6 According to Eq. (8.20), the density of polypropylene foam must be 59% higher than the density of polystyrene foam, to achieve the same yield stress. Other factors such as materials and processing costs will be important. It was noted that PP foam recovers more than EPS after impact. Therefore, EPP is preferred for multiple-impact helmets, but EPS is preferred for bicycle helmets (single impact, minimum weight). 

Five years into the campaign, the plastics industries fa-vorability rating is now over 60, which is in the band with our competitors. In our most recent tracking polls, 98 percent of Americans agreed that plastics make important safety contributions in products such as tamper-resistant packaging, shatter-resistant bottles, bicycle helmets, and... 

The Consumer Product Safety Commission (CPSC) estimates that there has been about 500,000 nonfatal bicycle-related injuries treated in emergency rooms every year since the early 1970s. About 30 percent of these incidents involved head or face injuries. Additionally, it is estimated that there may be as many as 1000 bicycle-related fatalities annually, and about 90 percent of these incidents involve motor vehicles. The CPSC bicycle standard (16 CFR Part 1512), effective in 1976, set safety requirements for reflectors, wheels, tires, chains, pedals, braking and steering systems, and for the frame and fork structural components. The CPSC Bicycle Helmet Standard (16 CFR 1203) sets forth the requirements for bicycle helmets (Figure B.l). Biohazard... 

CFR, Parts 1101 to 1406, Consumer Products Safety Act of 1972 16 CFR 1203, Consumer Products Safety Commission (CPSC), Bicycle Helmet Standard... 

Sports Safety. Sports-equipment engineering has had an impact on injury prevention at all levels. Helmets have become commonplace for sports such as downhill skiing and bicycling, and in sports such as hockey, a helmet is now required at both the amateur and professional levels. Baseball is another sport in which a helmet is required for certain positions. In some jurisdictions, bicycle helmets are required by law to be worn by riders under the age of eighteen. Many amateur participants will wear helmets to reduce the chance of injury even if they are not mandatory. There is evidence in the peer-reviewed medical literature that helmet use for sports such as bicycling and hockey does reduce injury. Full facial protection has also been shown to reduce injury in hockey players. 

Sunoolor Sunpor,AT 40 Bicycle helmets, sport helmets. ... 

---