Kutz M. Handbook of materials selection

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3 THE IMPACT OF ADVANCED MATERIALS ON SPORTS PERFORMANCE 1259

Table 2 World Records Set by Joe

Gaetani in the 1992 Paralympics in

Barcelona

Event (m) Time (s)

100 12.23

200 24.82

Bilateral below-knee amputee athlete (Class

A-3).

Table 3 Requirement for a Vaulting Pole

Requirements Possible Materials of Choice

Light (low density) Bamboo

Buckling resistance (stiffness) Aluminum

Strong (resistance to plastic strain) Steel

Minimal twisting (high torsional strain resistance)

Cost

Magnesium

Carbon ﬁber composite

prosthetic device, which features a thin carbon ﬁber/epoxy pylon that provides

the right balance of stiffness and ﬂex at a substantially reduced weight compared

to conventional materials (3 lb compared to 10 lb) such as wood. An energy

storage return aids performance and contributes to the capability to walk or run

greater distances with much less fatigue and discomfort than with traditional

devices. The cost of the Springlite II is about $850.

Born without feet, Tony Volpentest won gold medals in Barcelona in the 100-

m (11.63 s) and the 200-m (23.07 s) events. He runs on carbon–graphite feet

bolted to carbon composite sockets that encase his legs (built by Flex-foot Inc.),

the arrangement acts like a spring-board. With each step, the runner punches the

track, which catapults him forward more efﬁciently than if he were running on

two human feet. And the long-distance runner is not forgotten: This device is

stiff and springey for sprinters but shock-absorbing for the marathoner.

3.2 Pole Vaulting

The requirements for a vault are shown in Table 3. The 1896 Olympics saw a

height of 3.30 m achieved with a bamboo pole in the pole-vault event. Figure 7

shows the winning heights in the pole-vault discipline for all Olympic events

since the ﬁrst. The bamboo pole, which has more spring and is much lighter for

the same stiffness than the hickory pole, was introduced in 1904 by an Olympiad

from my hometown of Moscow, Idaho, A. J. Gilbert (Dan O’Brian is the second

Olympian from our town of 18,000). Initially this change in materials provided

an advantage of about 200 mm in height. Improvements in coaching and tech-

nique allowed a gradual increase in height over the years. By the late 1950s,

however, the gains were starting to level off and lighter weight aluminum poles

were used for a short time. The greatest performance improvement occurred in

1964 with a substantial gain of about 250 mm over the previous Olympics, to

a height of 5.10 m. This improvement was caused by the introduction of glass-

ﬁber composites, which were lightweight and had higher stiffness. Not only was

this new material more efﬁcient, it allowed athletes to change their style by

1260 ADVANCED MATERIALS IN SPORTS EQUIPMENT

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Pole vault height jumped (m)

1896

1900

1904

1908

1912

1916

1920

1924

1928

1932

1936

1940

1944

1948

1952

1956

1960

1964

1968

1972

1976

1980

1984

1988

1992

1996

Fig. 7 Winning heights in pole vaults since ﬁrst Olympic event.

Longitudinal carbon fibers/epoxy

Glass fiber web/epoxy

Rings of glass fibers

Fig. 8 Adding in minimal twisting requirement, carbon ﬁber composite becomes more attrac-

tive with three layers of different ﬁbers being used to optimize the performance. (From Ref. 2.)

turning upside-down and gliding over the bar feet ﬁrst. Improvements in tech-

nique using the new pole once again lead to increases in performance that are

only just beginning to slow down.

If we refer to Table 1, and also add the requirement of durability, the material

of choice is the carbon ﬁber composite, with bamboo not too far behind. If we

add in the minimal twisting requirement, the carbon ﬁber composite becomes

even more attractive (Fig. 8) with three layers of different ﬁbers being used to

optimize the performance. An outer layer of high-strength carbon ﬁber provides

high stiffness, while an intermediate webbing of ﬁbers together with an inner

layer of wound glass ﬁber, builds in resistance to twisting. The glass ﬁber con-

sists of 80% longitudinal and 20% radial ﬁbers. So for the pole vault advanced

materials have a major inﬂuence on performance.

3.3 Bicycling

Cycling is a highly efﬁcient form of transportation, with the energy consumption

lower than for walkers, and much lower than for powered vehicles.

2,7,8

The

Chinese, recognizing this, build over 10 million bicycles per year. The bicycle

3 THE IMPACT OF ADVANCED MATERIALS ON SPORTS PERFORMANCE 1261

Fig. 9 Bicycle diamond frame. (From Ref. 2.)

has been around for almost 200 years maturing from the 1817 Draisienne (walk-

ing device) to the modern-day Rover Safety Bicycle designed by J. K. Starley

in 1885. Nowadays, advanced materials, in combination with aerodynamic con-

siderations, have led to vastly improved bicycles. When Greg LeMond won the

1989 Tour de France (3420 km) by only 8 s, for example, his success was made

possible by a clip-on extension of his handlebars with a padded tube, which

enhanced his aerodynamic shape.

The bicycle can be considered to be a modiﬁed ‘‘space frame’’ such as that

found in bridges, cranes, etc. For the bicycle this is the ‘‘diamond frame’’ (Fig.

9).

A number of advances have contributed to the high efﬁciency of a modern-

day bicycle including the development of spoke wheels, the chain concept, pneu-

matic tires, and such accessories as seats, brake levers, and pedals. The two

major advances are in the frame and wheels, however, and we will now consider

these two components of the advanced bicycle.

Frame

The diamond frame and the alternate structure, the ‘‘cross frame,’’ are both

constructed from thin-walled tubular components that must resist tension, com-

pression, bending, and torsional stresses. The requirements for the materials of

construction of a bicycle are shown in Table 4.

These materials requirements can be simpliﬁed to the minimization of the

bending of a cantilever beam; then the mass is given by

␳

M ⫽ const ⫻

Thus the optimum material is the material with the highest speciﬁc strength

␳

/E. Based upon the data shown in Table 1, carbon ﬁber composites are the

1262 ADVANCED MATERIALS IN SPORTS EQUIPMENT

Table 4 Materials Requirements for a Bicycle and

Possible Materials of Choice

Requirements Possible Materials of Choice

Light (low density) Carbon ﬁber composites

Stiff Aluminum

Strong Titanium

Toughness Magnesium

Fatigue resistance Steel

Corrosion resistance

Cost

Metal matrix composites

Fig. 10 Road bicycle made by Harry Havnoonian with a mix of materials using adhesive bond-

ing for joints, investment-cast stainless steel lugs, Ti-3Al-2.5V head tube, SiC-ﬁber-reinforced

aluminum alloy front triangle, CRFP rear triangle, and CFRP rear wheel.

materials of choice if there is no concern over cost. Lightweight metals—

aluminum, magnesium and titanium—are also attractive, as are metal matrix

composites (Fig. 10

). If cost is a concern, then steel, which is not that far behind

the other materials, is the obvious material for selection.

In addition to the carbon ﬁber reinforced composite frames, recently frames

have been produced from magnesium, aluminum, titanium and metal matrix

composites. In addition hybrid frames such as carbon ﬁber reinforced composite

combined with titanium have been produced.

Wheels

Advances here include wheels with increased stability and rigidity for off-road

bikes constructed from glass ﬁber reinforced nylon, and disc wheels. In the latter

concept discs made of aluminum alloys or carbon ﬁber reinforced composites

replace the spokes in conventional wheels. Developments also include three or

ﬁve spoke wheels for rigidity and crosswind aerodynamics.

The improvements that advanced materials have produced in bicycling can

be gauged in the enhancement in the Olympic pursuit records shown in Table

3 THE IMPACT OF ADVANCED MATERIALS ON SPORTS PERFORMANCE 1263

Table 5 Improvement in Olympic Records for

the Bicycle 4000-m Individual Pursuit

Competitor Year Time

Daler (CZE) 1964 5 min 5 s

Boardman (GB) 1992 3 min 22 s

Fig. 11 Dangerous vibration can lead to tennis elbow. (From Ref. 2.)

3.4 Tennis and Squash

Tennis is a sport in which absolute achievements cannot be compared. Who can

argue with the notion that Bobby Riggs with his 1939 wooden racket would

have no chance against Pete Sampras armed with an oversized composite racket

with an enlarged ‘‘sweet spot.’’ In fact, this confrontation has been framed as

‘‘like ﬁghting against riﬂes with bows and arrows.’’

Until about 25 years ago, tennis rackets were made from wood with ash,

maple, and okume leading the way. In the late 1960s metal frames, generally

fabricated from steel or aluminum, were introduced. At the present time, com-

posite rackets are all the rage, not only from the viewpoint of efﬁciently accel-

erating the ball across the net, but also in terms of damping dangerous vibration,

which can lead to ‘‘tennis elbow’’ (Fig. 11).

The impact force experienced by a player on returning a tennis ball traveling

at 160 km per hour (100 mph) is approximately equivalent to jerk-lifting a

weight of about 75 kg (170 lb). These forces can transmit a high load to the

lateral epicondyle, located on the outer side of the elbow; leading to damage to

the small blood capillaries in the muscles and tendons around the elbow joint.

Better technique can help, but improved rackets can also make a major contri-

bution.

The goal in designing modern tennis rackets is to increase the size of the

sweet spot, the central part of the racket, which leads to little or no shock to

the player and minimal vibration occurs upon impact with the ball. This depends

1264 ADVANCED MATERIALS IN SPORTS EQUIPMENT

upon the stiffness of the frame and the size and shape of the handle and head.

The International Tennis Federation have now imposed an upper limit to the size

of the racket.

Today tennis rackets are produced from monolithic metals, including steel,

aluminum, magnesium, and titanium and metal matrix composites. However, the

high stiffness of carbon-ﬁber-reinforced composites makes them superior to the

metals in imparting high forces to the ball. To reduce the high-frequency vibra-

tion upon impact, racket handles are constructed of multiple ﬁber-reinforced

layers wrapped around a soft inner core, which is often an injected polyurethane

foam or honeycomb construction.

An example of a state-of-the-art tennis racket is the Wilson FPK, which con-

sists of a urethane core, 84% graphite, 12% Kevlar, and 4% ﬁber FP (a pure

form of ceramic aluminum oxide). The graphite provides strength and stiffness,

thereby minimizing head deﬂection and helps to prevent twisting of the racket

head when the ball impacts outside the sweet spot. The Kevlar ﬁbers lead to

additional strength and durability and contribute to damping vibration. The ﬁber

FP produces even greater stiffness and damping to this type of tennis racket.

Squash rackets have shown similar trends to tennis rackets.

Until 1983, the

frame was constructed of wood. Since then, squash rackets have featured hollow

extruded aluminum designs and ﬁber-reinforced plastics, the latter featuring

lightness, strength, toughness, and reduced vibration.

Racket strings have transitioned from ox gut to the modern-day synthetic (e.g.,

nylon) strings. This transition has not been without controversy. In the late

1970s, so-called spaghetti stringing, which resulted in large amounts of spin

being imparted to the ball and an unpredictable bounces, led to a rule requiring

‘‘strings to be interlaced and attachments for durability purposes only, not de-

signed to alter the ﬂight in the ball.’’

11,12

Wheelchairs (Fig. 12) now allow the paraplegic athlete to compete in this

sport, as well as others, including racing events, basketball, and rugby. No heavy

steel frames here—rather bikelike wheels, use of aerospace carbon ﬁbers, and

titanium, as well as computer-aided design of the suspension. Now there is a

chair for each sport: basketball, racing, and even tennis. For example, tennis

chairs are built with sharply slanted back wheels so the athlete can move quickly

from side to side. In basketball, forwards have high seats, guards have more

slant in their chairs in order to turn quickly. Cost? Top wheelchairs are in the

range of $2000–$3300 apiece.

3.5 Cricket

As with tennis rackets, the manufacturers of cricket bats have been concerned

with the size of the sweet spot and the reduction of ﬂexural vibrations.

Three

signiﬁcant modes of ﬂexural vibrations detract from the ideal rigid-body per-

formance, and whereas distributing the weight of the blade to the edges (perim-

eter weighting) does not increase modal frequencies signiﬁcantly, it may increase

the width of the sweet spot.

3.6 Golf

Paralleling the tennis situation, it is very difﬁcult to compare many of the

achievements of the past with those of today in absolute terms. Clubs have

3 THE IMPACT OF ADVANCED MATERIALS ON SPORTS PERFORMANCE 1265

Fig. 12 Steve Meredith of Titanium Sports of Kennewick, WA (left) along with Alistair Godfrey

and Malcolm Ward-Close of the Defense Evaluation and Research Agency (DERA), United King-

dom, are shown inspecting a titanium wheel chair.

evolved tremendously and it is difﬁcult to imagine that Bobby Jones, using

hickory shafts, could compete, at least in length, with Tiger Woods, Ernie Els,

or David Duval armed with a shaft constructed from a graphite-epoxy and an

over-sized hollow titanium head; albeit at a high price.

The materials evolution for the driver is dramatic. The overall weight has

been decreased and the length of the club increased from 100 cm to as much

as 125 cm. The grip and shaft weight has been reduced from 165 grams down

to 115 grams or less. The weight of the head remains the same at about 200 g

but using a hollow titanium (casting) construction, the head is now much bigger,

with the mass concentrated around the outside of the hitting face. The net result

is a club that is claimed to give greater distance (greater club head speed because

of the longer arc) but also a straighter (bigger sweet spot) shot.

A concern of the U.S. Amateur Golf Association (USGA) is that technology

may dominate over skill, i.e., no ‘‘spring-back’’ (trampoline) effect should occur

when the ball is struck by the clubface, which may add distance. When a golf

ball hits any surface it rebounds at a velocity less than that at which it hit the

surface (Physics 101).

Materials with a high elasticity and high strength result

in a high rebound velocity (or high coefﬁcient of restitution [COR]), especially

1266 ADVANCED MATERIALS IN SPORTS EQUIPMENT

when the impacted surface is thin (allowing the ‘‘trampoline’’ effect). Thus the

hitting surface can be tuned to increase the COR. However, this now puts us on

the ragged edge of the mechanical property limits of materials of construction.

Thus you will hear of the ‘‘proprietary heat treatments’’ (simply an aging treat-

ment to increase the strength) to create a beta titanium alloy that is strong and

capable of resisting fracture (most of the time) even though it is thinner than

conventional titanium (e.g., Ti–6Al–4V) hitting faces.

With a tuned (nonconforming) face, the COR will exceed the USGA limit of

0.83 (somewhat arbitrarily set to allow the majority of the drivers at the time

the limit was set to meet the requirement). However, the salient question is: Do

the nonconforming drivers actually lead to increased distance? Tests by Golf

Digest

indicated that the trampoline effect is maximized only in a relatively

small area of the clubface. This means that it requires the impact accuracy of a

robot or a professional to take full advantage of the effect. This has been con-

ﬁrmed in robot testing of a nonconforming club head (COR 0.845) where a 2%

increase in ball speed (over a conﬁrming club with a COR of 0.81) increased

carry distance by 1–2 yards for 220 yards shots and 4–6 yards for 270-yard

drives (the higher the swing speed, the greater the increase in ball speed and

therefore, carry distance). And to the crux of the matter: 40 players (0 to 22

handicap) had essentially the same distance from both the conforming and non-

conforming clubs. Worse: With off-center hits, there are indications that the

nonconforming club hits the ball shorter distances than the conﬁrming club.

The USGA fears that ‘‘trampoline’’ clubs and other ‘‘out there’’ technologies

could add 30 yards to a drive.

With a perfect center hit, a COR of 0.880/0.900

(the maximum likely) and a 109-mph swing (a pro’s swing), the distance gap

between a persimmon wood (COR 0.77) is about 25 yards.

So potentially a

maximum of another 25 yards is possible with a nonconforming driver (COR

0.880/0.900) with an optimally struck approved golf ball. Thus there is a legit-

imate concern with the professionals, probably none for the average golfer.

Driving distance is an absolute criterion, which can be tracked versus the

material of construction, with a caveat that the modern top golfers on the PGA

tour now realize that exercise and body-building help their performance, far from

a common practice in the past. The average PGA tour driving distance is shown

in Fig. 13,

with a spike in 1994 at the time of the introduction of the titanium

driver. A complication here is that balls have also evolved tremendously, with

the most lively being banned because they would obsolete current golf courses.

3.7 Baseball/Softball

Aluminum baseball bats are banned in the U.S. major leagues because they

would make current baseball stadiums obsolete. There would be too many home

runs. However, both new aluminum bat concepts such as the ultralight, with a

double-walled barrel construction, and titanium bats are revolutionizing softball.

These bats have bigger sweet spots and lead to greater velocity off the bat.

However, the Softball Association is concerned with an increase in injuries to

inﬁelders who cannot react quickly enough to this higher velocity. An example

of a softball bat produced from titanium using a powder metallurgy approach is

shown in Fig. 14.

3 THE IMPACT OF ADVANCED MATERIALS ON SPORTS PERFORMANCE 1267

Fig. 13 Average driving distance on the PGA tour. Note spike since the introduction

of the titanium driver in 1994.

Fig. 14 Susan Abkowitz of Dynamet Technology, Inc. holds a softball bat

with a titanium alloy outer shell.

3.8 Boats, Boards, and Wind-Surﬁng Fins

For all watercraft, there are four basic forces to contend with when considering

design: weight, lift, thrust, and drag (Fig. 15).

Materials of construction are

needed that result in lightweight, low skin friction (to give smooth ﬂow and

reduced drag), increased toughness and a high level of safety. Modern craft

consist of a combination of several polymeric composite materials, often incor-

porating cellular (sandwich) concepts.

The construction of a wind-surﬁng board is complex consisting of a core of

extruded foam polyestrene ﬁller enclosed in ﬁberglass. This core is covered

1268 ADVANCED MATERIALS IN SPORTS EQUIPMENT

Fig. 15 Forces acting on a watercraft.

Fig. 16 Method of operation of hydroelastically tailored wind-surﬁng ﬁn.

with graphite and glass ﬁbers embedded in resin matrices, with an outer surface

of glass-ﬁber-reinforced composite.

An interesting materials application involves the design of the wind-surﬁng

ﬁn. The challenge is to design a ﬁn with optimal hydrodynamic shape because

surfers need to perform equally in both directions, requiring a symmetric design.

Innovation relied on a concept known as hydroelastic tailoring.

Through this

process, structural deformation of the cross sections (Fig. 16) is induced by the

hydrodynamic–pressure forces, with the internal structure encased in a ﬂexible

elastomer material covering. As this section moves through the water (at an

incidence angle), a surface pressure loading results, as shown. This surface pres-

sure loading is of insufﬁcient magnitude to deform the supporting internal struc-