Materials for snow sports

14.8 Materials for snow sports

14.8.1 General requirements

The previously quoted examples of equipment fre- quently share common material requirements and prop- erties, such as bending stiffness, yield strength, tough- ness, fatigue resistance, density and comfort. However,

Figure 14.7 Snowboard binding utilizing: thermoplastic each sport makes its own unique demands on materials.

elastomer (Hytrel)—ankle strap A, spoiler B, ratchet strap In snowboarding and skiing equipment, for instance,

G, nylon (Zytrel)—side frames D and H, base and disc F, additional requirements include toughness at sub zero

top frame J; acetal homopolar (Delrin)—strap buckles C1

° C), low frictional drag

and C2 (courtesy of Fritschi Swiss Bindings AG and Du Pont UK Ltd) .

10 Materials research conducted at Imperial College, London, on behalf of the fencing sword manufacturers, Paul

11 Delrin, Hytrel and Zytel are registered trademarks of Leon Equipment Co. Ltd, London (Baker, 1989).

DuPont.

Materials for sports 417

Figure 14.8 Transverse section showing multi-component structure of a downhill ski (from Easterling, 1990 by permission of the Institute of Materials) .

or paper honeycomb). Polyethylene and polyurethane In the older sport of skiing, the principal items of

14.8.3 Skiing equipment

have been used for the soles of skis. equipment are the boots, bindings, skis and poles.

Modern ski poles are tubular and designed to give External Hytrel–Kevlar components have been used

high specific stiffness and good resistance to impacts. to enhance the stiffness of ski boots; this feature gives

Nowadays, CFRP–GRP hybrids are favoured. The the boots a firmer grip on the skier’s ankles and leads

pointed tips are sometimes made of wear-resistant to better control of the skis. Polymers feature promi-

carbide.

nently in many design of ski bindings. For instance,

in the Fritschi Diamir touring binding, 12 acetal poly-

14.9 Safety helmets

mer (Delrin) is used for the locking bar, heel release lever, heel block and front swivel plate while glass-

14.9.1 Function and form of safety helmets

reinforced nylon (Zytel) is used for the front block and Most sports entail an element of personal risk. The the two base plates.

main function of a safety helmet is to protect the Modern ski designs aim at solving the conflicting

human skull and its fragile contents by absorbing as requirements of (i) longitudinal and torsional stiffness

much as possible of the kinetic energy that is vio- that will distribute the skier’s weight correctly and

lently transferred during a collision. The three principal (ii) flexibility that will enable the ski to conform to

damaging consequences of sudden impact are fracture irregularities in the snow contour (Easterling, 1993).

of the skull, linear acceleration of the brain relative Originally, each ski runner was made from a single

to the skull, and rotational acceleration of the brain. piece of wood, e.g. hickory. Laminated wood skis

Although linear and rotational acceleration may occur appeared in the 1930s. The adoption of polymers for

at the same time, many mechanical testing procedures ski components in the 1950s, combining lightness

for helmets concentrate upon linear acceleration and and resistance to degradation, was followed by the

use it as a criterion of protection in specifications. introduction of metal frames for downhill skis, e.g.

A typical helmet consists of an outer shell and a alloy steel, aluminium alloy. By the 1960s, GRP and

foam liner. The shell is usually made from a strong, CFRP were coming into prominence. The internal

durable and rigid material that is capable of spreading structure of a ski is determined by the type of skiing

and redistributing the impacting forces without suffer- and, as Figure 14.8 shows, often uses a surprising

ing brittle fracture. This reduction in pressure lessens number of different materials. Skis usually have a

the risk of skull fracture. The foam liner has a cel- shock-absorbing, cellular core that is natural (ash,

lular structure that absorbs energy when crushed by hickory) and/or synthetic (aramid, aluminium, titanium

impact. Specialized designs of helmets are used in cycling, horse riding, canoeing, mountaineering, ski-

12 Used by Hans Kammerlander in his 1996 ski descent of ing, skate boarding, ice hockey, etc. Some designs Mount Everest.

are quite rudimentary and offer minimal protection.

418 Modern Physical Metallurgy and Materials Engineering In general, the wearer expects the helmet to be com-

With further increase in stress, cell walls buckle and fortable to wear, lightweight, not restrict peripheral

collapse like overloaded struts; in this second stage, vision unduly and be reasonably compact and/or aero-

energy absorption is much more pronounced and defor- dynamic. Production costs should be low. Increasing

mation can be elastic or plastic, depending upon the the liner thickness is beneficial but, if the use of hel-

particular polymer. If the cells are of the closed type, mets is to be promoted, there are size constraints. Thus,

compression of the contained air makes an addi- for a cricket helmet, acceptable shell and liner thick-

tional and significant contribution to energy absorp- nesses are about 2–3 mm and 15 mm, respectively

tion. Eventually the cell walls touch and stress rises (Knowles et al., 1998).

sharply as the foam densifies. This condition occurs Strong and tough helmet shells have been produced

when a liner of inadequate thickness ‘bottoms out’ from ABS and GRP. The great majority of shock-

against the helmet shell. The design of a helmet liner absorbent foam linings are made from polystyrene

should provide the desired energy absorption with- (Figure 14.9): polypropylene and polyurethane are

out ‘bottoming out’ and at the same time keep peak also used.

stresses below a prescribed limit.

Some polymeric structures can recover their origi- nal form viscoelastically and withstand a number of Polymeric foams provide an extremely useful class

14.9.2 Mechanical behaviour of foams

heavy impacts; with others, a single impact can cause of engineering materials (Gibson & Ashby, 1988;

permanent damage to the cell structure, e.g. expanded Dyson, 1990). They can be readily produced in many

PS. After serious impact, helmets with this type of different structural forms by a wide variety of methods

liner should be destroyed. Although this requirement using either physical or chemical blowing agents. Most

is impracticable in some sporting activities, there are thermoplastic and thermosetting resins can be foamed.

cases where single-impact PS liners are considered to The properties of a foam are a function of (i) the

be adequate.

solid polymer’s characteristics, (ii) the relative density of the foam; that is, the ratio of the foam density to the density of the solid polymer forming the cell

s ⊳ , and (iii) the shape and size of the cells.

14.9.3 Mechanical testing of safety helmets

Relative density is particularly important; a wide range Various British Standards apply to protective helmets is achievable (typically 0.05–0.2). Polymer foams are

and caps for sports such as climbing (BS 4423), horse- often anisotropic. Broadly speaking, two main types

and pony riding (BS EN 1384) and pedal cycling, of structure are available: open-cell foams and closed-

skateboarding and rollerskating (BS EN 1078). These cell foams. Between these two structural extremes lies

activities involve different hazards and accordingly the

a host of intermediate forms. testing procedures and requirements for shock absorp- In the case of safety helmets, the ability of a

tion and resistance to penetration differ. In one typical liner foam to mitigate shock loading depends essen-

form of test, a headform (simulating the mass and tially upon its compression behaviour. Initially, under

shape of the human head) is encased in a helmet compressive stress, polymer foams deform in a lin-

and allowed to fall freely through a certain distance ear–elastic manner as cell walls bend and/or stretch.

against a rigid anvil. Specified headform materials (BS EN 960) depend on the nature of the impact test and extend from laminated hardwood (beech) to alloys with a low resonance frequency (Mg–0.5Zr). A tri- axial accelerometer is affixed to the headform/helmet assembly in the zone of impact. The area beneath the curve of a continuous graphical record of striking force v. local deformation taken during the test provides

a useful measure of the kinetic energy absorbed as the helmet structure is crushed. Specified values for permissible peak acceleration at impact which appear in test procedures vary but generally extend up to about 300 g, where g (acceleration due to gravity) D

9.81 m s . Drop heights range from about 1 to 2.5 m, depending upon the striking force required. Test spec- ifications often include requirements for helmets to be mechanically tested after exposure to extremes of tem-

Figure 14.9 Cell structure of polystyrene foam, as used for perature, ultraviolet radiation and water. On occasions, shock-absorbent packaging: average cell diameter 100 µ m

unsafe and/or inadequate helmets are marketed: natu- (courtesy of Chris Hardy, School of Metallurgy and

rally, closer international collaboration and regulation Materials, University of Birmingham, UK) .

is being sought.

Materials for sports 419

Further reading

Knowles, S., Fletcher, G., Brooks, R. and Mather, J. S. B. (1998). Development of a superior performance cricket Baker, T. J. (1989). Fencing blades—a materials challenge.

helmet, in The Engineering of Sport (ed. S. J. Haake). Metals and Materials , Dec., 715–718, Institute of

Blackwell Science, Oxford.

Materials. Lees, A. W. (ed.) (1989). Adhesives and the Engineer. Blyth, P. H. and Pratt, P. L. (1992). The design and materials

Mechanical Engineering Publications Ltd, London. of the bow/the arrow, Appendices to Longbow: A Social

and Military History , 3rd edn. by Robert Hardy. Patrick McMahon, C. J. and Graham, C. D. (1992). Introduction to Stephens Ltd, Cambridge. Materials: the Bicycle and the Walkman . Merion Books,

Cochran, A. (ed.) (1994). Golf: the Scientific Way. Aston

Philadelphia.

Publ. Group, Hemel Hempstead, Herts. UK. Pearson, R. G. (1990). Engineering Polymers (ed. R. W. Easterling, K. E. (1993). Advanced Materials for Sports

Dyson), Chapter 4 on foams, pp. 76–100, Blackie & Son Equipment . Chapman & Hall Ltd, London.

Ltd, Glasgow and London.

Gibson, L. J. and Ashby, M. J. (1988). Cellular Solids–Stru- Shields, J. (1984). Adhesives Handbook. 3rd edn. Butter- cture and Properties . Pergamon Press.

worths, Oxford.