Materials 5 Golf Club Report

25.11.2010
Golf Club Failure Analysis
Materials 5
1. Executive Summary
The fracture of a composite golf club shaft is investigated to determine the cause and nature of the failure. The golf club specifications, manufacturing techniques, typical loading and component material
failure theory are reviewed in the introductory sections of this report. A Finite Element stress analysis
carried out on the club confirmed that the failure occurred at the structurally weakest point. A bending
deflection test was carried out to determine the shaft stiffness and composite material modulus of elasticity which complied with material suppliers specifications. The fracture surfaces were inspected by
the naked eye, by stereomicroscope and by scanning electron microscope (SEM). Characteristic microscopic features of tensile and compressive failure were identified on opposite sides of the fracture indicating a flexural failure due to bending of the shaft (figure A).
Figure A: Golf Club subject of investigation and stereomicroscope
image showing the fracture surface on the head
The orientation of the fracture surface indicates an abnormal impact direction on the club head as indicated in figure B. The areas of maximum stress concentration as predicted by FE analysis align with the
characteristic failure modes (compressive and tensile regions) identified by SEM. No unexpected material or manufacturing defects were identified during the investigation. Failure of the club is attributable
to a misuse event when the head struck the ground from an overhead held position.
Force
Figure B: FE analysis illustrating direction of applied impact force that lead
to the club failure
Charlie Seviour
Roslyn Shennan
Michael Smailes
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2. Table of Contents
1.
2.
3.
Executive Summary.......................................................................................................... 1
Table of Contents ............................................................................................................. 2
Introduction ...................................................................................................................... 3
3.1
Task Specification…………………………………………………………………3
3.2
Golfing Standards………………………………………………………………….3
3.3
Manufacturing……………………………………………………………………...3
4. Material & Component Failure Theory............................................................................ 4
4.1
Club Loading………………………………………………………………………4
4.2
Failure Mechanisms of Long Fibre Reinforced Polymers…………………………4
5. Investigation Strategy ....................................................................................................... 6
5.1
Visual Examination………………………………………………………………...6
5.2
FE Analysis………………………………………………………………………...6
5.3
Assessment of Bending Stiffness & Material Integrity……………………………6
6. Results .............................................................................................................................. 7
6.1
Naked Eye Examination…………………………………………………………...7
6.2
Study by Stereomicroscope………………………………………………………..7
6.3
Study by Scanning Electron Microscope………………………………………….8
6.4
FE Analysis………………………………………………………………………..9
6.5
Assessment of Bending Stiffness and Material Integrity………………………….9
7. Discussion....................................................................................................................... 10
8. Conclusions .................................................................................................................... 11
9. Recommendations........................................................................................................... 11
10.
References .................................................................................................................. 11
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3. Introduction
3.1
Task Specification
The project investigates the cause and nature of the fracture of a Donnay Evolution men’s regular
graphite flex golf club. The club consists of two main sections; the head and a shaft which is wrapped at
the opposite end by a rubber grip. The club is a 12˚ driver which defines the angle from the perpendicular axis to the club face of the hollow titanium alloy head.
The shaft is a hollow, tapered, cylindrical structure composed of a graphite fibre epoxy matrix composite material, enclosed in a plastic coating.
At the base of the shaft a cylindrical plastic joint, called a ferrule, graduates the club diameter change at
the hosel (where the shaft is attached into the head). The ferrule makes some contribution to reducing
stress raising effects. The plastic coating, ferrule and the base of the shaft at the hosel are all fixed using
an epoxy adhesive.
3.2
Golfing Standards
Although no industry standards have been sourced for clubs, design is regulated by The United States
Golf Association standards1 and the Professional Golf Association. There appears to be no standards
for club stiffness as players may employ an insert to the shaft to customise its flexibility.2
3.3
Manufacturing
Graphite fibres are produced by ‘graphitisation’ of a carbon based polymer such as polyacrylonitrile.
Polymer molecules twisted to create a fibre are heated to temperatures of >2000˚C removing surplus
elements leaving a 99% Carbon strand which prefers the lower energy crystalline structure of graphite3.
This initial fibre is referred to as a filament and is combined with thousands of counterparts to produce
a ‘tow’ which can be subsequently combined to produce a ‘roving’4. The fibres are woven to produce a
graphite fibre cloth and impregnated with epoxy resin to produce a ‘prepreg’ sheet. The hollow golf
club shaft is made by winding this composite material cloth around a mandrel mould in multidirectional
plys as shown in figure 1.5
Figure 1: Diagram depicting the prepreg sheets being wound around the mandrel
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Materials 5
4. Material & Component Failure Theory
4.1
Club Loading
A force of approximately 9kN1 is applied to the face of the club on impact with a golf ball. figure 26
shows the bending moment experienced by the shaft over a 20s stroke. At impact with the ball a sharp
discontinuity can be seen followed by oscillating moments (vibration of the shaft).
Figure 2: Planar bending moment diagram for club shaft during a stroke
4.2
Failure Mechanisms of Long Fibre Reinforced Polymers
Long fibre reinforced polymers have been shown experimentally to behave as ideal brittle solids with a
strength limited by the most severe flaw.7 Research into this area has demonstrated that the dominant
failure mode in long fibre polymatrix composites is microbuckling. This is caused by a plastic shear
instability due to further bending of misaligned fibres when compressed8, illustrated in figure 3. This
‘fibre waviness’, is unavoidable in the manufacturing process9. This failure mode produces a highly
characteristic “kink” in the fibres as shown in figures 4a11&b10 and the presence of compression debris,
figure 4c11.
Figure3: Fibre waviness exploited by compressive loading leading to stress concentrations in corners
Figure 4a: micrograph of a kink
failure due to excessive compressive
force
Figure 4b: micrograph of fibre matrix
surface following a kink failure
Figure 4c: micrograph of characteristic compression debris
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Golf Club Failure Analysis
Materials 5
Other important mechanisms to note include interlaminar shear and tensile failure.10 Interlaminar shear
describes the separation of different layers in the matrix. This delamination has been observed to be
most prominent in plies which are orthogonally aligned, ie. -45o/45o and 0o/90o.12
In tension, individual fibres fail before their theoretical limits due to surface flaws. Since the distribution and severity of these flaws are random the ultimate tensile strength of each fibre is not consistent13.
The failure of one fibre causes the load it supported to be redistributed to neighbouring fibres according
to a load sharing rule, e.g. local load sharing, depicted in figure 5.7
Figure 5: local load sharing rule assumes each fibre
carried equal load prior to failure.
The probability of failure of the neighbouring fibres increases due to the raised applied stress. A critical
number of failed fibres results in a shared load which cannot be sustained by the remaining fibres, leading to the catastrophic failure of the bundle.
Tensile failure varies with the relative stiffness of the matrix. Brittle tensile failure is depicted in figures
6a&b13 in the case of a soft and hard matrix respectively. A soft matrix is unable to transfer high shear
loads between fibres leading to the spider like fracture surface. Composites with a stiffer matrix transfer
the stress concentrations more easily to surrounding fibres leading to a flat fracture surface. Figure 6c11
shows a micrograph of fibre pullout: another characteristic observation of tensile failure.
b.
c.
a.
Figure 6: Brittle fracture of carbon fibre in a. soft matrix and b. stiff matrix
Flexural failure in fibre reinforced polymers is characterised by the presence of compressive and tensile
loading separated by a clear barrier, frequently near the neutral axis of the matrix (See figure 7). In
most cases the initiation of flexural failure is in the compressive region since long fibre reinforced
polymers are weaker in compression11 due to the dominant failure mode of microbuckling. Research in
this area has been focussed on unidirectional layered materials however studies have shown that the
fractographic features are identical in multidirectional layered materials11. Therefore the observations
made here have equal relevance to more complex systems such as the multi-ply golf club shaft.
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Golf Club Failure Analysis
Materials 5
Figure 7: SEM micrographs of flexural failure in carbon composites (compression observed in lower region) [left11, right10]
5. Investigation Strategy
5.1
Visual Examination
Initial observations and sketches of the specimen were made
•
•
•
To identify golf club specifications
Determine materials and manufacturing techniques
To identify areas of interest for further examination
Cuts were made ~1cm below the fracture surfaces at the shaft and head to facilitate their examination
using a stereomicroscope
•
•
To confirm details of the layered material structure as gathered from background research
To develop ideas regarding the failure scenario
A scanning electron microscope was used to complete the visual examination of the fracture surface of
the head by
•
•
5.2
Identifying possible failure initiation points
Providing detailed evidence of the nature of the failure in the material at a microscopic level
FE Analysis
A finite element stress analysis of the structure was carried out to assess areas of greatest stress concentration during normal use and potential abuse of the club.
5.3
Assessment of Bending Stiffness & Material Integrity
As the position of the fracture left the club shaft intact, the bending stiffness of the shaft could be experimentally determined providing an evaluation of the composite material properties. The shaft was
clamped at the grip and a load applied as shown in figure 8 to trace the resultant deflection.
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Materials 5
Figure 8: Bending Stiffness Experiment Set-up
6. Results
6.1
Naked Eye Examination
The head of the golf club fractured from the base of the shaft just below the ferrule. The fracture surface revealed a fibrous internal structure of the shaft which had splintered apart (figures 9a&b).
Figure 9a: Fracture surface at the head
6.2
Figure 9b: Fracture surface at the shaft
Study by Stereomicroscope
Suspected initiation point
Fracture Propagation
Figure 10a&b: Stereomicroscope images of head fracture surface. Left
focussed on lowest point. Right focussed on higher opposite side
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Golf Club Failure Analysis
Materials 5
The section cut from the head reveals a fracture with significantly varying depth and a spiral nature.
figures 10a&b which are focused on the lower and upper levels of the surface respectively, show the
shape of the face and the hypothesised trajectory of crack propagation through the structure.
6.3
Study by Scanning Electron Microscope
In figure 11i. remnants from the epoxy matrix can be seen resting on top of undamaged fibres. This is
commonly referred to as compression debris11 which along with the microbuckling which can be seen in
figure 11ii. is indicative of compressive failure. The fracture surface at the left side on these figures is
largely consistent with the topography of the micrographs shown in figures 11i. & ii. suggesting that
this entire side was in compression.
i
ii
iv
iii
Figure11: SEM micrographs of the club head fracture surface revealing i. compression debris ii. microbuckling iii. bundle pull-out iv. fibre pull-out
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Golf Club Failure Analysis
Materials 5
On the opposite side of the shaft the serrated fracture surface is compliant with a tensile brittle fracture
in a soft matrix. There is also evidence of fibre pull out in figure 11iv. which is congruent with tensile
failure. The combination of compressive and tensile failure mechanisms present at the fracture surface
is consistent with flexural failure.
6.4
FE Analysis
The finite element stress analysis results are illustrated in figures 12a&b which have the orientation of
impact load indicated. The areas of greatest stress concentration are displaced between the two cases by
90⁰. In either case these locations align with the point of component failure where the shaft has tapered
to the smallest diameter.
Force
Force
Figure 12: Stress analysis diagrams of club head & shaft under application of a. normal
use load and b. misuse of the club indicated by differing orientation of impact force
6.5
Assessment of Bending Stiffness and Material Integrity
The shaft’s deflection was plotted as Eq (1)
w = −0.0598 x 2 − 0.023x
Eq (1)
The bending stiffness, EI, of the shaft was determined using Eq (2).
d 2w
M = EI 2
dx
F×r
EI =
0.1196
⇒ EI ≈ 80Nm 2
Eq (2)
Eq (3)
The Young’s Modulus, E, of the composite material could be extracted following calculation of the area
moment of inertia, I, of the shaft. The outer (D) and inner (d) diameters of the tapered hollow cylinder
are taken as average values along the length of the shaft.
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Golf Club Failure Analysis
π
(D4 − d 4 )
64
I = 4 × 10 −10 m 4
I=
Materials 5
Eq (4)
∴E = 2.0×1011Pa
The stiffness, k, of the shaft structure is calculated using Eq 5 and the experimentally derived value for
the Youngs Modulus of Elasticity. Average values for the inner and outer diameters are used to account
for the tapered cross sectional area of the shaft, which has length l.
k=
k=
EA
l
Eπ (D4 − d 4 )
4l
k = 1440 Nm −1
Eq (5)
Eq (6)
7. Discussion
The fracture aligns with the structurally weakest point in the shaft as determined by FE analysis, implying that the club has failed in a location which would be expected following an overloading or misuse
event. The experimentally derived value for the modulus of elasticity is comparable to specification by
suppliers of graphite fibre epoxy composites14,15 (180 – 200 GPa), which implies material integrity and
further supports an argument that the failure of the shaft is not attributable to a material defect.
From the stereomicroscope observations it was proposed that the fracture initiated in the lowest region
of the spiral and propagated upwards as the two surfaces pulled apart. The spiral nature of the fracture
surface is likely to be partly attributable to consecutive plies of composite material being oriented at
45⁰.
The identified features of compressive and tensile failure at opposite sides of the fracture surface indicate a flexural failure as the shaft bent under applied load. Dominant fibre pullout in the tensile region
could suggest inadequate fibre-matrix adhesion which may be expected from the club since graphite
does not generally bond well with other compounds. However these effects are not thought to be a significant factor in this particular failure case. Assessing the orientation of the flexural fracture (parallel to
the club face), it can be seen that the impact loading on the golf head would not have been typical of
normal use, but rotated anticlockwise by 90⁰. It can be hypothesised that the club was struck directly
into the ground from an overhead hold. This conclusion is supported by the FE analysis where it was
shown that in the event of impact loading from this direction the area of greatest stress concentration
aligned with the central points of the compressive/tensile regions located via SEM inspection.
The tensile region mirrors the characteristic form for brittle fracture in a soft matrix which is an expected result as the epoxy resin is designed to produce a golf club shaft with a degree of flexibility.
Since microbuckling is the dominant failure mechanism in long fibre reinforced composites it is reasonable to assume that the failure initiated in the compressive (lower) region of the fracture spiral which
supports the initial speculations made.
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Golf Club Failure Analysis
Materials 5
8. Conclusions
•
•
•
•
•
The club failed at the structurally weakest point at the base of the shaft
The shaft was shown to have an expected bending stiffness implying material integrity and no
other unexpected material defects were found during the investigation
Impact at the head caused the club to bend until a flexural failure of the composite shaft occurred
The orientation of the tensile/compressive regions on the fracture surface implies an impact
scenario which is not conducive with normal use of the club (impact direction as illustrated in
figure 12b)
The head struck the ground as though from a direct overhead hold suggesting that a misuse
event by the user has led to the failure
9. Recommendations
•
•
•
10.
As the failure is directly attributable to misuse of the club there are no relevant design issues
and therefore no recommendations to make to the manufacturer
The user could be warned that the club is not designed to withstand such misuse events
A potential design improvement could be in developing improved fibre-matrix adhesion properties, however, this flaw was not considered to be a significant factor in this particular failure
scenario
References
[1] United States Golf Association. Equipment Rules– http://www.usga.org/Rule-Books/Rules-onClubs-and-Balls/Shafts/#bend
[2] Hodgetts G.W. Golf Club Shaft Tuner (2010), US Patent no. US7,758,446 B2
[3] University of Wisconsin Department of Chemistry. Buckyballs, Diamonds, and Graphite –
http://www.chem.wisc.edu/~newtrad/CurrRef/BDGTopic/BDGtext/BDGGraph.html
[4] Jake Crum. Graphite Fibre Composite Materials Terminology (Aug 23, 2009) –
http://www.suite101.com/content/graphite-fiber-composite-materials-terminology-a141709 - carbon
fibre terminology reference
[5] Shigetoh H. Fuchu, Shaft structure of golf club and production method of the shaft (1992); Ryobi
Limited (JP), United States Patent no. 5,088,735
[6] Horwood G.P. Golf shafts a technical perspective, Science and Golf II: Proceedings of the world
scientific congress of golf (1994); Ti Apollo Limited, 1st ed. p248
[7] Pitkethly M.J. & Bader M.G. Failure Modes of Hybrid Composites Consisting of Carbon Fibre
Bundles Dispersed in a Glass Fibre epoxy Resin Matrix (1987); Journal of Physics D: Applied Physics
Vol.20 p.315-322
[8] Jelf P.M& Fleck N.A. The Failure of Composite Tubes due to Combined Compression and Torsion
(1994); Journal of Materials Science Vol. 29 p.3080-3084.
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Golf Club Failure Analysis
Materials 5
[9] Jumahat A, Soutis C, Jones F.R, Hozic A. Fracture Mechanisms and Failure Analysis of Carbon
Fibre/Toughened Epoxy Composites Subjected to Compressive Loading (2010); Composite Structures
Vol.92 p.295-305
[10] Parry T.V. & Wronski A.S. Kinking and Tensile, Compressive and Interlaminar Shear Failure in
Carbon –Fibre – Reinforced Plastic Beams Tested in Flexure (1981); Journal of Materials Science
Vol.16 p.439-450
[11]Srinivasa V, Shivakumar V, Nayaka V, Jagadeeshaiaih S, Seethram M, Shenoy R, Nafidi A.
Fracture Morphology of Carbon Fibre Reinforced Plastic Composite Laminates (2010); Materials Research Vol.13(3) p.417-424
[
12]Singh S. & Greenhalgh E. Micromechanisms of Interlaminar Fracture in Carbon-Epoxy Composites at Multidirectional Ply Interfaces
[13] Fuwa M, Bunsell A.R, Harris B. Tensile Failure Mechanisms in Carbon Fibre Reinforced Plastics (1975); Journal of Materials Science Vol.10 p.2062-2070
[14] Torayca. M35J Data sheet. – http://www.toraycfa.com/pdfs/M35JDataSheet.pdf
[15] MatWeb Material Property Data. Hexcel property database: M35J –
http://www.matweb.com/search/DataSheet.aspx?MatGUID=65321508c43947ebba1e703deb9a3d38&c
kck=1 -
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