Effect of materials and manufacturing on the
bending stiffness of vaulting poles
C L Davis and S N Kukureka
The University of Birmingham, Department of Metallurgy & Materials,
Edgbaston, Birmingham, B15 2TT, UK
ABSTRACT: The increase in the world record height achieved in pole vaulting can be
related to the improved ability of the athletes, in terms of their fitness and technique,
and to the change in materials used to construct the pole. For example in 1960 there
was a change in vaulting pole construction from bamboo to glass-fibre reinforced
polymer (GFRP) composites. The lighter GFRP pole enabled the athletes to have a
faster run-up, resulting in a greater take-off speed, giving them more kinetic energy to
convert into potential energy and hence height. The GFRP poles also have a much
higher failure stress than bamboo, so the poles were engineered to bend under the load
of the athlete thereby storing elastic strain energy that can be released as the pole
straightens, resulting in greater energy efficiency. The bending also allowed athletes
to change their vaulting technique from a style that involved the body remaining
almost upright during the vault to one where the athlete goes over the bar with their
feet upwards. Modern vaulting poles can be made from GFRP and / or carbon-fibre
reinforced polymer composites (CFRP). The addition of carbon fibres maintains the
mechanical properties of the pole, but allows a reduction in the weight. The number
and arrangement of the fibres determines the mechanical properties, in particular the
bending stiffness. The vaulting poles are also designed for an individual athlete to
take into account each athlete's ability and physical characteristics. The poles are
rated by ‘weight’ to allow athletes to select an appropriate pole for their ability. This
paper will review the development of vaulting poles and the requirements to maximize
performance. The properties (bending stiffness and pre-bend) and microstructure
(fibre volume fraction and lay-up) of typical vaulting poles will be discussed.
INTRODUCTION
Pole-vaulting has a long history, but from the late 19th century the sport as we know it
began to be developed seriously with improvements in materials and vaulting
techniques throughout the 20th century (Haake, 2000). Early poles were solid, made
from ash or hickory before hollow bamboo poles became common in the first half of
the 20th century. Whilst being lighter than solid poles they were still essentially rigid.
After the Second World War, steel and aluminium poles were also tried.
Glass-fibre composites were first used in the late 1950’s and by 1961 the world record
was broken with a composite pole with multiple composite sheet layers to achieve the
optimum properties (figure 1). Since then there has been a rapid improvement in
heights achieved and it is clear that the performance improvements directly follow
from materials improvements and design changes (figure 2). However the more recent
availability of carbon-fibre composite poles has not had such a significant effect. The
techniques of vaulting have changed too and the flexibility of composite poles has
been utilised to great effect by athletes as they swing into the jump then rock back
while the pole flexes so that they approach the bar feet first. This maximises the
athlete’s ability to use energy effectively and achieve the greatest possible return for
their kinetic energy and muscle power.
UD Fibre
Filament
wound
core
Plain Weave
Twill
Fig. 1. The filament wound structure of a vaulting pole (Jenkins, 2002)
Bamboo
Glass fibre composites
Solid wood
Fig. 2. Change in Olympic record heights over last 100 years and associated vaulting
pole material changes.
REQUIREMENTS FOR A VAULTING POLE
The International Association of Athletics Federations rules state that ‘The pole may
be of any material or combination of materials and of any length or diameter…….’
(IAAF, 2003). The key requirements are length, stiffness and the appropriate design or
selection for each individual athlete. The versatility of fibre-composites makes all of
these more easily variable through changes in design and manufacture.
Length
Several studies have considered the length and stiffness of the pole and found that
there is an optimum value of both which maximises performance. Ekevad and
Lundberg (1997) modelled the effects of pole length and stiffness on energy
conversion from kinetic energy to potential energy during the vault by finite element
analysis. They found that there was an optimum length that allowed sufficient polebend and time for the vaulter to rock back and rotate appropriately. Relatively short
and stiff poles were found to straighten almost completely and the vault height could
be limited by the pole-length. However, longer poles, when bent completely, may not
be able to provide sufficient force to lift the vaulter to the optimum height.
Stiffness
Pole flexibility and the ability to tailor the properties of fibre-composite poles to
required values are key advantages. Linthorne (2000) found that whereas for a rigid
pole, the vaulter’s trajectory is always concave to the planted pole-end, for a flexible
pole the trajectory changes from convex through take-off and maximum pole bend, to
concave from pole release to peak height. The stiffness of the pole and its ability to
bend both have an effect on the changes in energy through the vault (Schade et al
2000). The total energy of the vaulter decreases from a high point at take-off, when
kinetic energy is high, to a minimum at the time of maximum pole bend as this energy
is stored as potential energy in the elastically-strained pole. This energy is returned to
the vaulter as the pole unbends – first as kinetic and finally as potential energy.
Ekevad and Lundberg (1997) found that as with pole length there was an optimum
stiffness value for effective vaulting. Equality between the weight of the vaulter and
the Euler buckling load gives the minimum pole stiffness. A pole with too low a
stiffness straightens incompletely and the vaulter does not achieve a great height. A
pole which is too stiff straightens too quickly and the vaulter reaches peak height too
early. Burgess (1996) further optimised the design by considering a tapered pole with
greater thickness in the middle where the stresses are greater than at the ends. An
appropriate combination of pole length and pole stiffness, optimised for the individual
athlete is required.
Biomechanics and individual selection of poles
For the same pole geometry a heavier vaulter will require a stiffer pole to achieve the
same height. Also an individual athlete may use different poles at different stages in a
competition. Most elite vaulters will specify a certain length, ‘flex rating’ (ie pole
stiffness) and possibly mandrel size for winding the composite layers (Gill Athletics,
2003).
VAULTING POLE CHARACTERISTICS
Introduction
Glass-fibre composites are light and stiff and have a much greater failure strain than
that of bamboo (Haake, 2000). Carbon fibre-composites generally have a higher
stiffness and this can be a limitation since the vaulter has less time to swing and rotate
before the pole unbends. Carbon fibre laminae also have a higher strength and a
typical laminate of carbon fibres in epoxy at 50 % volume fraction would have a
strength of 1000 MPa compared with 700 MPa for a similar laminate of polyester and
glass fibres (Harris, 1999) Most vaulters still use glass-fibre poles and considerable
experience is necessary to use carbon-fibre poles. Published information on materials
and manufacture indicates multiple layers for bending and torsional stiffness (figure 1)
but there is less detail available on the number and thickness of the layers.
Pre-bend
Vaulting poles often have a ‘pre-bend’ to aid the athlete by making it easier to ‘plant’
the pole and bend it initially. A ‘pre-bend’ is where the vaulting pole is not straight
but has a permanent curvature that is introduced during pole manufacture, for example
by allowing the pole to bend the prescribed amount by self weight during the curing
process. It should be noted that for these poles the stiffness around the circumference
is constant, as will be discussed below. For glass fibre reinforced polymer (GFRP)
composite poles the pre-bend has been found to be 20 – 30 mm. The athletes need to
ensure that the vaulting pole is orientated correctly when planted at the beginning of
the vault otherwise torsional stresses will be introduced in the pole as it twists to the
preferred bending position. A vaulting pole with a significant pre-bend will naturally
orientate itself when the athlete rests the tip of the pole on the ground.
Vaulting poles without a pre-bend may be designed to have variable stiffness around
its circumference, i.e. the pole having a ‘soft’ and ‘stiff’ side, with the athletes having
to orientate the pole with the ‘soft’ side facing them at the plant to aid bending of the
pole during the vault. For these poles the athlete needs to orientate the pole using a
mark on the side of the pole; incorrect orientation may cause injury because of
twisting of the pole during the vault.
Stiffness
The stiffness of a vaulting pole will affect the ease with which the athlete can bend it
during a vault. The stiffness of a pole is related to the stiffness of the material it is
made from (the material’s modulus of elasticity, E) and its shape (diameter and wall
thickness). The relative stiffness of a pole is displayed as a ‘weight rating’, which the
athlete should not exceed. The weight rating is calculated by the manufacturers and is
related to how much the pole bends when loaded in the centre with a 50lb (22.3 kg)
mass. The method of testing may vary between manufacturers (e.g. free or
constrained support of pole ends, length of free span etc.).
For GFRP vaulting poles the material’s modulus of elasticity will be related to the
volume fraction, and orientation, of the fibres (silica glass E ≈ 76 GPa) compared with
the epoxy resin (E = 4 – 5 GPa). It was found that for GFRP vaulting poles of the
same length (4.2 m), outside diameter (32 mm) and wall thickness (2 mm) the bending
stiffness ranged from approximately 900 Nm-1 to 1100 Nm-1 with the mass of the
poles ranging from approximately 1.64 kg to 1.76 kg respectively. The increase in
density, and consequently bending stiffness, reflects an increase in the volume fraction
of glass fibres (density 2.65 Mg m-3) compared with epoxy resin (1.2 Mg m-3), rather
than a change in fibre orientation to change the stiffness. A change in fibre
orientation, for example to be more parallel to the pole axis in order to increase
bending stiffness, would also reduce the torsional stiffness of the pole, which is not
desired.
For a predominantly carbon fibre reinforced polymer (CFRP) vaulting pole labelled as
having a ‘soft’ side it was found that the bending stiffness varied between
approximately 835 Nm-1, for the soft side, to 900 Nm-1 when tested in the other
orientation. It is possible to change the bending stiffness by altering the volume
fraction and / or the fibre orientation around the circumference of the vaulting pole.
For this pole it was found that the difference in bending stiffness was achieved by
changing the wall thickness of the pole, with the ‘soft’ side having a wall thickness of
1.75 mm, over 90o around the circumference, compared with an average wall
thickness of 2.25 mm over the remaining 270o. The use of CFRP rather than GFRP
allows the vaulting pole mass to be reduced due to the higher modulus of elasticity of
carbon fibres (235 GPa) compared with glass fibres (76 GPa).
Microstructure
The microstructure of the GFRP vaulting poles, figure 3, showed the existence of
several distinct composite layers. An inner ply, approximately 200 µm thick, was
observed around the entire circumference of the pole. The majority of the pole crosssection was comprised of bundles of fibres, embedded in the polymer resin matrix,
separated by thin, approximately 10 – 20 µm, plies. In total for the GFRP composite
vaulting poles examined there were five regions of fibre bundles separated by the thin
plies, only two of which are clearly visible in figure 3. For the CFRP vaulting pole
the ‘soft’ side was observed to have three layers of fibre bundles, whereas the ‘stiffer’
side had five layers of fibre bundles and more separating plies. Additionally the
CFRP pole had both inner and outer plies (with fibres oriented at 80-85o for the inner
ply but at approximately 25o for the outer and separating plies).
Closer inspection of the microstructure, figure 4, revealed approximately 300 fibres in
each bundle. Image analysis showed the fibres in the bundles to be orientated at
approximately 45o to the pole length, whilst the fibres in the inner ply and the plies
separating the bundles were at angles of approximately 80-85o to the pole length, i.e.
they run almost perpendicular to the pole length. Fibres at an angle of 45o provide
bending stiffness and torsional stiffness, both requirements for the vaulting pole. The
fibres at 80-85o provide hoop stiffness and also act as crack arrestors. For example in
figure 4, a crack can be observed in the microstructure running from the outer surface
of the pole through two fibre bundles then stopping on reaching a ply with fibres
running around the circumference of the pole.
The majority of the vaulting poles are manufactured by filament winding where the
fibres, and fibre bundles, are wound onto a mandrel in the various orientations
required. The fibres / fibre bundles are pre-coated by pulling them through a bath of
resin.
Resin
Inner ply
Fibre
bundle
200µm
Ply
Fig.3. Optical micrograph of a transverse section through a GFRP vaulting pole
showing the different composite layers.
Individual
fibres
50µm
Fig. 4. Optical micrograph showing individual glass fibres in the fibre bundles
During the vault the pole bends most in the middle part of its length, thus to ensure
sufficient strength (to avoid fracture) in this region additional composite plies could be
used (Burgess 1996). At the ends of the poles, where less strength is required, fewer
plies could be used, thereby reducing the overall weight of the pole. This can be
achieved by using a ‘sail piece’, a trapezoidal pre-preg composite sheet that is
wrapped around the mandrel resulting in an increased number of composite layers in
the middle section of the pole.
Failures
During vaulting poles can fail catastrophically by fracture of the pole, usually half
way along the pole during the swing phase where the bending stresses are greatest.
Fracture of the pole can result in injuries to the athlete, as well as a failed attempt at
clearing the bar, and should therefore be avoided. Cracks can exist in the composite
from manufacturing (fibre damage; incomplete curing causing delamination), or can
occur in service (by fatigue). The fracture surface from a GFRP pole that failed
during use is shown in figure 5. The crack runs through the fibre bundle (right hand
side of image) with little fibre pull out and brittle (flat) fracture of the fibres. When
the crack reaches the ply with fibres at 80 – 85o the fibres generally remain intact with
the crack running between them, having to deviate in path.
Fig. 5. SEM image of the fracture surface from a broken GFRP vaulting pole.
CONCLUSIONS
The design of vaulting poles requires not only a knowledge of mechanics,
biomechanics and the characteristics of the individual athlete but also of materials
properties and manufacturing conditions. Modern glass and carbon fibre vaulting
poles are complex structures in which the multiple layers and components provide
flexibility and energy storage whilst ensuring bending and torsional stiffness. Fibre
bundles and the relative orientation of the layers not only give stiffness and strength
but they can help arrest incipient failures. Future designs utilising the manufacturing
flexibility available with fibre composites will further enhance performance.
ACKNOWLEDGEMENTS
We would like to thank Amy Cleeton and Blake Rayner for their help with this work.
REFERENCES
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Haake, SJ (2000), Physics World, September 2000.
Harris, B (1999), Engineering Composite Materials, 2nd ed, IoM.
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