Helmet shell

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. 

One other helmet shell, the king helmet. Cassis iuberosa, is occasionally used instead of the black helmet. It has the same colouring. 

Figure 10.22 Detail of helmet shell cameo, showit stiiations at r t ai es to each other.

Crack at the side of a drilled hole in a polycarbonate helmet shell-fracture surface and schematic of features. The hole was loaded by impact forces from the chin strap rivet. 

Figure 10.2 Innovative helmet shell configuration with engineered flex points that allow the outer shell to damprai impact force. The photo is of the Riddell Speedflex football helmet. With kind permission from RiddeU.

Engineered high-impact acrylonitrile-butadiene-styrene (ABS) blends (see Figure 10.8) afford a medium degree of impact performance for helmet shells that is sufficient for moderate intensity impact and high-to-medium repetitive impact scenarios. Contemporary sport helmets with ABS outer shells include the youth and amateur levels of American football, baseball, softball, lacrosse, ice hockey, cricket, equestrian, alpine sports, and action adventure sports. Similar to PC helmet... 

Helmet shells requiring protection from moderate intensity, single-impact scenarios are commonly constructed with polyolefin materials such as polyethylene (PE) and polypropylene (PP). Sport helmets with PE and PP outer shells include recreational levels of cricket, ice hockey, equestrian, alpine sports, and action adventure sports. In comparison with PC and ABS shells, the diminished magnitude and frequency of expected impact events for polyolefin shells precludes the use of many additives on a cost-per-helmet basis. Caswell et al. (2007) offer a more thorough review of relative performance selection criteria for helmet shell materials. 

Legislative body/ helmet manufacturer Reconditioning frequency Helmet shell warranty Service-life policy... 

Krzeminski, D.E., Fernando, D., Gould, T.E., Piland, S.G.,Rawlins, J.W., 2012a. Infrared thermography of helmet shell material. Unpubhshed work. 

The quality of the helmet shell in terms of its hardness is a significant factor in the shock absorption capability. Helmets are generally fabricated spherically to block and divert blows. Salvaterra (2006) reported that softer helmet shells sustain more focal impact with a higher peak force. The more rigid a shell, the more diffuse the impact would be with lower peak force. Newer helmet prototypes or models employ thin plastic for decreased weight, with better ventilation (for cooling effect) and material (for improvement in helmet strength). 

Helmets have headbands and suspension webs inside them. They are very important in preventing head injuries. This stmcture distributes forces from a blow to a helmet over a large area of the skull. This structure helps absorb the energy of a blow. Headbands and suspension webs inside a helmet require proper adjustment. There should be a 1 1/4 in. clearance between the helmet shell and the suspension. This prevents the helmet shell from striking the head during a blow. 

Figure 13.17 3D angle-interlock fabric, developed single-piece hehnet shell and finished look of developed single-piece riot helmet shell. 

Zahid, B., Chen, X., 2012. Manufacturing of single-piece textile reinforced riot helmet shell from vacuum bagging. J. Compos. Mater. 47 (19), 2343-2351. 

The matrix material was chosen to be a polyester resin for both the inner and outer layer. The FEA model consisted of four parts namely the helmet shell, the foam liner, the head-form and the hemispherical test anvil. The foam liner was modeled using a set of 8500 solid elements, the woven ply was modeled as an orthotropic material with damage and the glass mat polyester layer was modeled as an isotropic elastic-plastic material with kinematic hardening. 

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