Fibres

Chapter V : Composite
materials
Materials Selection Course
J. Lecomte-Beckers
1
Content
• Introduction
• Particles reinforced composites
• Structural composites
• Fibre reinforced composites
• Illustrative example: materials for skis
J. Lecomte-Beckers
2
Origin of composite materials
From Ancient Egypt thousand years ago: enhancement of
mechanical properties of bricks with the addition of short
straw yarns to wet clay
Natural composite materials:
– Wood: cellulose fibres in a lignin matrix
– Bone: complex mixture of collagen and calcium phosphate
J. Lecomte-Beckers
3
What is a composite material ?
Solid state material made up of at least two species
keeping their intrinsic properties and combining them
to exhibit a material with improved global properties
≠ from a metallic alloy or a ceramic blend
J. Lecomte-Beckers
4
Assets of composite materials
• Lightness, improved mechanical and chemical
resistance
• Lowered maintenance, freedom of shapes
• Increase life time of some equipments
• Better toughness and
• resistance to fire  enhancement of safety
• Better thermal, phonic and electric insulation
 Enhanced performance as a basis for innovative
technological solutions
J. Lecomte-Beckers
5
Composition of a composite
Made up of 2 phases :
– Matrix : continuous and envelops the other phase
– Reinforcement, surrounded by the matrix
Properties of a composite are depending on
– Properties and fraction of both (or more) phases
– Geometry of reinforcement
J. Lecomte-Beckers
6
Classification of composites
Composite
Particles
reinforced
Coarse
Dispersed
Fibre
reinforced
Long fibres
(aligned)
Structural
Short
fibres
Aligned
Randomly
oriented
J. Lecomte-Beckers
7
Particles reinforced composites
Dispersion reinforcement
• Presence of small particles preventing dislocation motions
(metallic matrix)
• Enhancement of properties is depending on atomic or molecular
interaction between the matrix and the reinforcement (also
called load)
Coarse particles
• Increasing of mechanical properties depends on the binding
strength between matrix and particles
J. Lecomte-Beckers
8
Fibre reinforced composites
Fibres = most efficient reinforcing materials
= improved mechanical properties when
junction with matrix is effective
Generally ductile matrix spreading material applied
stress to fibres
Influence of position and fibres length between each
other on composite properties
J. Lecomte-Beckers
9
Structural composites
Multi-layer composite
• Made up of several layers of an anisotropic material
• Superposition, intersection and sticking of layers  isotropic
material with load direction
Sandwich composite
• Made up of two layers with high mechanical properties separated
by a core material (e.g.: Al/PP/Al)
• High resistance and stiffness with low density
J. Lecomte-Beckers
10
Current composites
• 99% of composites are with an organic matrix
• Composites with inorganic matrix (metallic or
ceramic): low use
J. Lecomte-Beckers
11
Content
• Introduction
• Particles reinforced composites
• Structural composites
• Fibre reinforced composites
• Illustrative example: materials for skis
J. Lecomte-Beckers
12
Coarse particles composites
• Interactions between particles and matrix are not atomic nor
molecular but are coming from mechanics
• Reinforcement particles are harder and stiffer than matrix
and prevent it to flow around them
• Transfer of stress from matrix to particles
• Reinforcement level is depending on binding force between
particles and matrix
• Made from metals, glasses, ceramics or polymers
J. Lecomte-Beckers
13
Dispersion reinforced composites
• Smaller particles (diameter from 0.01µm to 0.1µm)
• Reinforcement by matrix-load interactions at the atomic or
molecular level
• Metallic Matrix bears applied load and particles are limiting the
motion of dislocations

of plastic deformation and
of yield and tensile strength,
and hardness
J. Lecomte-Beckers
14
Dispersion reinforced composites
• Varied particles geometry BUT same size (and
behaviour) in the 3 main directions
• Particularly improved properties with fine particles
uniformly distributed in matrix
• Mechanical properties are rising with an increase of
volumic fraction of reinforcing phase
J. Lecomte-Beckers
15
Dispersion reinforced composites
sup Ec  EmVm  E pV p
inf Ec 
E m .E p
Vm E p  V p E m
Young’s Modulus (GPa)
Dual phase composite:
Upper limit
Lower limit
Tungsten particles fraction
J. Lecomte-Beckers
16
Cermets
• Coarse particles composites: metallic matrix with high
fraction of ceramic particles
• Most used cermets: cemented carbides
• Very hard particles of refractory ceramics (WC or TiC)
binded with a metallic matrix (Co or Ni)
• Application of cermets : maching tools for hard steels
• In certain cermets, fraction of particles can exceed 90
vol.%  excellent resistance to abrasion
J. Lecomte-Beckers
17
Cemented carbide (WC-Co)
Micrograph of WC-Co
Mechanical properties of WC grains
and Co matrix
Co (10% vol.) in black
J. Lecomte-Beckers
18
Elastomers and plastics
• Frequently particulates reinforced
(black carbon)
• Black carbon: cheap, contains fine
spheroïdal carbon particles
• Tires are containing from 15 to 30
vol.% of black carbon  increase of
fracture toughness, tensile strength
and resistance to abrasion and
laceration
Micrograph of a synthetic rubber
J. Lecomte-Beckers
Carbon particle diameter in the order of 20 to 50
nm
19
Concrete
• Coarse particles composite
• Matrix (cement) and reinforcement (sand and
gravel granulates) = ceramic material
• Concrete ≠ cement
• Concrete = composite material made up of
granulates linked with a binding agent (cement)
used in the solid state
J. Lecomte-Beckers
20
Metal and metallic alloys
• Improvement of strength and hardness with uniform
dispersion of very hard fine particles (few vol.%)
• Dispersed phase = oxide (generally but not the best –
low interaction)
• Effective reinforcement is depending on matrix-load
interactions for preventing motion of dislocations
• Reinforcement is not subject to ageing because of its
inert behaviour with increasing temperature
J. Lecomte-Beckers
21
Content
• Introduction
• Particles reinforced composites
• Structural composites
• Fibre reinforced composites
• Illustrative example: materials for skis
J. Lecomte-Beckers
22
Main characteristics
• Made of an homogenous and a composite material
• Properties are depending of species, shape and size
• Multi-layer composites and sandwich-like structures
J. Lecomte-Beckers
23
Multi-layer composites
• Made of weaves or panels
with anisotropic properties
• Stacked layers which are
sticked together with
alternating orientation of
best resistance direction
(e.g.: 0/45/90° or 0/90°)
Représentation schématique d’un
composite stratifié
J. Lecomte-Beckers
24
Multi-layer composites
• Case of plywood: fibres of a layer are oriented to 90°
with previous layer
• Manufacturing of composites from coton, paper or glass
fibres in a polymeric matrix
• High strength in lots of direction of the same plan
(pretty low inter-layer resistance)
J. Lecomte-Beckers
25
Laminate (Sandwich-like)
structures
• Made of two layer of a tough material with intercalated
core material which is less dense and exhibit lower
strength and stiffness (I profile principle)
• External layers are bearing the majority of stress (higher
stiffness in parallel system) - particularly interesting for
bending forces
• Aluminium based alloys, FRP, Ti based alloys or steels for
external layers
J. Lecomte-Beckers
26
Core
• Two functions:
– Bear perpendicular strains between the two external layers
– Provide stiffness with profile shearing forces
• Core made up of polymeric foams, synthetic rubbers,
inorganic cements and balsa
J. Lecomte-Beckers
27
Sandwich-like structure with
alveolar core
• Core = alveolar structure with foils including hexagonal
cells
• Cells are staying next to each other as in a hive
J. Lecomte-Beckers
28
Sandwich-like structure with
alveolar core
• Now: realisation of very complex sandwich-like
structures such as rotary lifting device of helicopter
Unidirectional
composite
Unidirectional
composite
Unidirectional composite
layers oriented at ±45°
Aluminium
honeycomb
Helicopter rotor blade
J. Lecomte-Beckers
29
Content
• Introduction
• Particles reinforced composites
• Structural composites
• Fibre reinforced composites
• Illustrative example: materials for skis
J. Lecomte-Beckers
30
Wished properties
Strength and stiffness combined to lightness
 frequent characterisation of composites from their
specific properties
Specific strength = tensile strength / density
Speficif stiffness= Young’s modulus / density
Notice: low mechanical properties improvement with
randomly oriented short fibres
J. Lecomte-Beckers
31
Effect of fibre length
• Mechanical properties are related to:
– Properties of fibres
– Importance of load transfer from matrix to fibres
• Load transfer
– Given by adhesion strength between fibres and matrix
– Equal to zero at fibre ends  matrix strain
Matrix
Fibre
J. Lecomte-Beckers
32
Critical length of fibres
Strength and rigidity are increased if fibre has a length higher than a
certain critical size, lc
 *f d
lc 
2 c
For GFRC or CFRC: lc ≈ 1mm (that is to say 20 to 150 times the
diameter of fibre)
If l >15 lc, continuous or long fibres  good reinforcement of
composite material
Otherwise, discontinuous or short fibres
If l < lc, deformation of matrix and low increase of properties
J. Lecomte-Beckers
33
Effect of orientation and
fraction of fibres
• Properties are influenced by fibres disposal in matrix
• Importance of orientation, fraction and distribution of fibres
• 2 different fibre disposals:
– Same direction: case of continuous and discontinuous fibres
– Random orientation: case of discontinuous fibres
• Uniform distribution of fibres  optimal properties
J. Lecomte-Beckers
34
Fibre hardening
J. Lecomte-Beckers
Disposition of fibres as reinforcement:
(a) Aligned long fibres
(b) Aligned short fibres
(c) Random short fibres
35
Composites with continuous
aligned fibres
J. Lecomte-Beckers
36
Behaviour with tension –
Longitudinal load
• Response of composite to an external load is
depending on:
– Behaviour and fraction of both phases (matrix and fibres)
– Direction of applied load
– Anisotropy of composite
J. Lecomte-Beckers
37
Behaviour with tension –
Longitudinal load
• Tensile behaviour of composite
with brittle fibres and ductile
matrix
 *f
= ultimate strength fibre
 m* = ultimate strength matrix
 *f
= fibres strain
 m*
= matrix strain
We admit  *f <  m*
Related stress-strain curve
J. Lecomte-Beckers
38
Behaviour with tension –
Longitudinal load
• Failure of composite when fibres are
*
cracking ( f )
• No brittle failure because:
- Mechanical resistance of fibres is not equal
(intrinsic differences + collapsing effect)
 no simultaneous failure
–
Matrix keeps its properties if
 *f <  m*
 cracked fibres are shorter but continue
to bear the load and promote strength
Composite stress-strain curve
J. Lecomte-Beckers
39
Elastic behaviour – Longitudinal
load
• Assumptions:
– High adherence of fibres to matrix
– Isodeformation (  c   m   f )
Fc  Fm  F f
Then  c S c   m S m   f S f
With division by Sc (transversal cross section area) :
Sf
Sm
c m
 f
Sc
Sc
J. Lecomte-Beckers
40
Elastic behaviour – Longitudinal
load
• When composites, matrix and reinforcement have the
S
same length: m = Vm and
Sc
Sf
Sc
= Vf (volumic fractions)
  c   mVm   f V f
With Isodeformation assumption, we can divide each
term by it respective strain
f
c m

Vm 
Vf

c m
f
J. Lecomte-Beckers
41
Elastic behaviour – Longitudinal
load
• However, if elastic strain:
f
c ,

m
,
Ef 
Ec 
Em 
f
c
m
 Ecl  EmVm  E f V f
Only 2 phases (matrix and reinforcement)  Vm  V f  1
Hence,
J. Lecomte-Beckers
Ecl  Em (1  V f )  E f V f
42
Elastic behaviour – Transversal
load
Load ┴ to direction of fibres alignment
 iso-stress
c  m   f  
 c   mVm   f V f
Strain of composite:
However,




Vm 
 E
Em
cl
E
With division by σ, we get 1

Ecl
So
Ecl 
J. Lecomte-Beckers
Em E f
Vm E f  V f E m



Ef
Vf
1
1
Vm 
Vf
Em
Ef
Em E f
(1  V f ) E f  V f E m
43
Longitudinal tensile strength
• We admit tensile strength = maximum stress
• Maximum corresponds to rupture of fibres and start
damaging of composite
Material
Glass-Polyester
High modulus carbon-epoxide
Kevlar-epoxide
Tensile strength
(MPa)
700
1000
1200
Longitudinal tensile strength of 3 common composites
J. Lecomte-Beckers
44
Longitudinal tensile strength
• Complex failuring phenomena; several modes are
existing
• Mode given by:
– Properties of fibres and matrix
– Nature and strength of matrix-fibres bonds
• If  *f <  m* no cracking of matrix before fibres
• When fibres are cracked load is transfered to matrix
J. Lecomte-Beckers
45
Transverse tensile strength
• Materials are designed to bear load aligned with fibres 
otherwise rupture hazard
• If very low mechanical resistance even lower than
matrix, reinforcement lowers mechanical properties!
Material
Transverse tensile strength
(MPa)
Glass-Polyester
20
High modulus carbon-epoxide 35
Kevlar-epoxide
20
Transverse tensile strength of 3
common composites
J. Lecomte-Beckers
46
Transverse tensile strength
• Transverse tensile strength influence factors:
– Properties of fibres and matrix
– Bonding energy between fibres and matrix
– Defect (e.g. presence of air bubbles, inclusions…)
• Improvement of transverse mechanical properties
with modification of matrix properties
J. Lecomte-Beckers
47
Composites with aligned
discontinuous fibres
J. Lecomte-Beckers
48
Longitudinal tensile strength
• Reinforcement with discontinuous fibres lower than
with continuous fibres
BUT commercial interest of glass, C and aramid short
fibres

• If l > lc,
 cd*   *f V f 1 
• If l < lc,
 cd* ' 
J. Lecomte-Beckers

lc 
   m' (1  V f )
2l 
l c
V f   m' (1  V f )
d
49
Composites with randomly
oriented discontinuous fibres
J. Lecomte-Beckers
50
Characteristics
• Young’s modulus:
Ecd  KE f V f  EmVm
K = efficience coefficient of fibres, depends on Vf and
comprised between 0.1 and 0.6
E
Ef
Em
,
with Vf BUT with random orientation of fibres, E
proportionally to K
Property
(g.cm-3)
No reinforcement
Density
Tensile strength (MPa)
Young’s modulus (GPa)
Ductility (%)
Toughness, Izod test (J)
1.19-1.22
59-62
2.24-2.345
90-115
1.3-1.8
20
1.35
110
5.93
4-6
0.23
With reinforcement (vol.%)
30
40
1.43
1.52
131
159
8.62
11.6
3-5
3-5
0.23
0.28
Properties of polycarbonate polymer
with and without unoriented glass
fibres
J. Lecomte-Beckers
51
Summary
• Composites with aligned fibres:
– Generally anistropic
– Optimal properties in the fibres direction
– In transverse direction, reinforcement is nearly inefficient
and tensile strength is low
– If stress is oriented in another direction strength is comprised
between the two extremes behaviour
J. Lecomte-Beckers
52
Reinforcement fibres: general
information
• Characteristics of brittle materials:
lower strength in the block form than in the low
diameter fibre form
• Elimination of most surface defects during
manufacturing of fibres
 Probability of fibre failure is dropping when its
diameter is diminished  asset for manufacturing
J. Lecomte-Beckers
53
Types of reinforcement fibres
• 3 types: whiskers, fibres and wires
Whiskers:
• Rod-like monocrystals with very high length/diameter ratio
• Nearly perfect crystalline structure
• Exceptional and best tensile strength ever
• Very high cost and difficulties (or impossibility) to add them to a
matrix nearly not used as reinforcement fibres
• Made up of carbides, silicon nitride or aluminium oxide.
J. Lecomte-Beckers
54
Types of reinforcement fibres
Fibres :
• Polycristallyne or amorphous
• Low diameter
• Made from a polymer or a ceramic: aramid, glass, C, B, Al oxide, Si
carbide…
Wires:
• Fine but with large diameter
• Made up of steels, Mo or W
• Applications: tyres with radial carcass, external spacecraft wraps…
J. Lecomte-Beckers
55
Examples of reinforcement
fibres
Material
Density
(g.cm-3)
Ultimate
strength
(GPa)
Graphite
Silicon Nitride
Aluminium Oxide
Silicon Carbide
2.2
3.2
4.0
3.2
20
5-7
10-20
20
Aluminium Oxide
Aramide (Kevlar 49)
Carbon*
Glass E
Boron
Silicon carbide
UHMWPE (Spectra
900)
3.95
1.44
1.78-2.15
2.58
2.57
3.0
0.97
1.38
3.6-4.1
1.5-4.8
3.45
3.6
3.9
2.6
Specific
strength (GPa)
Whiskers
9.1
1.56-2.2
2.5-5.0
6.25
Fibres
0.35
2.5-2.85
0.7-2.7
1.34
1.40
1.30
2.68
Young’s
modulus (GPa)
Specific
modulus (GPa)
700
350-380
700-1500
480
318
109-118
175-375
150
379
131
228-724
72.5
400
400
117
96
91
106-407
28.1
156
133
121
Metallic Wires
High strength steel
7.9
2.39
0.30
210
26.6
Molybdenum
10.2
2.2
0.22
324
31.8
Tungsten
19.3
2.89
0.15
407
21.1
* Carbon term is used (in opposition to graphite) to consider theses fibres because they contain
crystalline graphite areas, non crystalline materials and unaligned crystalline areas
J. Lecomte-Beckers
56
Matrix: general aspects
• Metal, polymer or ceramic
• For ductile matrix: metal or polymer
• Reinforcement increases fracture toughness of ceramic
matrix composites
• ≠ roles of matrix:
– Binding of fibres, distribution of stress and transfer to fibres
– Protection of fibres against abrasion and chemical reactions
– Isolation of fibres between each other and et blocking of cracks
J. Lecomte-Beckers
57
Matrix: general aspects
• Resistance of composite until cracked fibres is lower than
a critical value
• Strong enough fibre-matrix bonds in order to avoid
unjoining of fibres
• Ultimate strength of composite is linked to the strength
of this bond
• Optimal joining maximises load transfer between matrix
(weak) and fibres (strong)
J. Lecomte-Beckers
58
Plan
• Introduction
• Particles reinforced composites
• Structural composites
• Fibre reinforced composites
• Illustrative example: materials for skis
J. Lecomte-Beckers
59
Materials for skis
• Materials for 2000 year ski are similar to the one of
80’s
• Difference : evolution of ski shape modification
of behaviour on snow
• Fundamental form parameter of ski: y-axis
geometry
J. Lecomte-Beckers
60
Ligne de cote du ski
• Curve including the most drawn
Traditional ski
aside points of spatula and heel
and the most centred point of
y-axis line
Trajectory
shoe along the edge
• Control the ability of ski to
Parabolic ski
promote turning
• Higher curvature on y-axis line,
heel
y-axis line
lower turn radius
 easier skiing
J. Lecomte-Beckers
shoe
spatula
Trajectory
y
61
Contact with track
• Important function of ski
• Good contact  better trajectory control
• Ski must be:
• Flexible with bending
• Stiff with torsion
J. Lecomte-Beckers
Contradictory abilities
62
Shape of skis
• Evolution since 10 years
• Made possible with breakthrough of CAM and
numeric use in machining (manufacturing of very
complex moulds)
• Progress of CAD
 Optimisation of ski
J. Lecomte-Beckers
63
Used materials
External VDD
Protection layer
GFR epoxy multi-layer
Aluminium alloy
GFR epoxy multi-layer
Polyurethane foam
core
Insert
Aluminium alloy
Internal VDD
Kevlar FR epoxy multi-layer
GFR epoxy multi-layer
Sole (sintered PE)
Side
J. Lecomte-Beckers
Edge (steel)
Cut of a Rossignol Dualtec ski
64
Used materials
Material
Density
(g.cm-3)
Ultimate
strength
(GPa)
Graphite
Silicon Nitride
Aluminium Oxide
Silicon Carbide
2.2
3.2
4.0
3.2
20
5-7
10-20
20
Aluminium Oxide
Aramide (Kevlar 49)
Carbon*
Glass E
Boron
Silicon carbide
UHMWPE (Spectra
900)
3.95
1.44
1.78-2.15
2.58
2.57
3.0
0.97
1.38
3.6-4.1
1.5-4.8
3.45
3.6
3.9
2.6
Specific
strength (GPa)
Whiskers
9.1
1.56-2.2
2.5-5.0
6.25
Fibres
0.35
2.5-2.85
0.7-2.7
1.34
1.40
1.30
2.68
Young’s
modulus (GPa)
Specific
modulus (GPa)
700
350-380
700-1500
480
318
109-118
175-375
150
379
131
228-724
72.5
400
400
117
96
91
106-407
28.1
156
133
121
Metallic Wires
High strength steel
7.9
2.39
0.30
210
26.6
Molybdenum
10.2
2.2
0.22
324
31.8
Tungsten
19.3
2.89
0.15
407
21.1
* Carbon term is used (in opposition to graphite) to consider theses fibres because they contain
crystalline graphite areas, non crystalline materials and unaligned crystalline areas
J. Lecomte-Beckers
65
Characteristics of skis
• Criterion for materials selection: lightness  use
of a foam for the ski core
• Ski has to bear a bending moment
• Reinforcing materials are located aroung the core
and are loaded with tension in the lower part and
with compression in the upper part.
J. Lecomte-Beckers
66
Fibre reinforcement
• High strain level required with no torsion nor failure
 reinforced materials with high yield strength
• Main reinforcing specy: Glass fibres (GF)
• Interesting Performance / Cost ratio;
• For pretty high lenghtening with high deformation ratio;
• Used in the form of fibre weaves bent on a polyester layer.
J. Lecomte-Beckers
67
Fibre reinforcement
• Longitudinal aromatic polyamid fibres (Kevlar K49)
in the ribbon form in the lower part of ski (low
compressive strength)
• Carbon fibres are rarely used because of frequent
rupture and high cost
J. Lecomte-Beckers
68
Other reinforcement
• Aeronautic aluminium alloy sheet (7000 serie, Zicral)
Typical composition (wt.%) : Al + 6Zn + Mg, (Cu, Mn at lower %)
 obtaining stiffness for torsion and flexibility for
bending
 ability to turn with accuracy
J. Lecomte-Beckers
69
Damping devices
• Presence of vibration damping devices (VDD)
• One is external (sticked on ski)
• Other one is internal (in the core)
• Internal VDD is made up of rubber with high damping
capacity and surrounded by twisted steel wires
• External VDD is composed of Zicral and an viscoelastic
material
• Role of VDD: Damp vibration of ski which are detriment
during the slide
J. Lecomte-Beckers
70
Noyau central
• Mousse de polyuréthane semi-rigide
• Obtenue par injection dans un moule de
constituants liquides réactifs (diisocyanates et
polyols) mélangé avant l’injection
J. Lecomte-Beckers
71
Sole
• Protective material on the lower face of ski
• Main role: ensure sliding of ski
• Used material: HDPE (low abrasion resistance but
behaviour suitable for all types of snow) with very high
molecular weight (2 000 000) and formed by
sintering
• Microporous material
J. Lecomte-Beckers
72
Last components of ski
• Edges: made from a chromium steel with 0.6 wt.%
C (Rockwell Hardness: 49-51)
• Protective layer:
• Modified styrene-acrylonitrile copolymer reinforced with
short glass fibres
• Or multi-layer laminate with impregnated paper with
thermoset phenolic or melamine resin
J. Lecomte-Beckers
73
Assembling of ski
• Components are assembled with an adhesive
(thermohardened epoxy resin)
• High pressure joining: from 8 to 10 kg cm-2 and T° from
100 to 120°C during 15 min
• Then pressurised cooling up to glass transition of
adhesive
• Joining
• Distribution of shear stress
• Obtaining a structure with good fatigue behaviour
J. Lecomte-Beckers
74
Manufacturing of ski
• Other technology:
• Addition of ski components in the mould
• Injection of liquid species to form directly a polyurethane
foam in the mould
• Expanding, hardening
• Foam: Core of the ski and adhesive binding agent for the ski
components
J. Lecomte-Beckers
75
Wood
• Core material for more than 50% of skis
• Natural, low density, pretty cheap and microporous
material
• Good resistance to hot crushing (compressive stress)
 hardening of adhesives
• Main defect: moisturisation
J. Lecomte-Beckers
76
Friction of ski
• Function of ski : ensure sliding
 lowest friction between sole and snow
• Friction mecanism is complex because snow is an
heterogenous material with ice crystals
• For T°>-10°C : friction is lubricated by a water film
(friction 
T°  water film)
• Water film: lubricant with low friction coefficient
(µ=0.02)
J. Lecomte-Beckers
77
Friction of ski
• For T°< -10°C, -15°C : induced
friction heat is evacuated 
Waxed wood
 Ice asperities are sticking to
the sole and µ
for wood and
steel
• For PTFE and PE it is not the cas
and µ remains low under -10°C
J. Lecomte-Beckers
Friction coefficient µ
lubricating film
Temperature
78
Sandwich-like structure
• Ski = nice example of sandwich-like structure
combining several materials
• Each material has its function
• Quality of a ski depends on the choice of an optimal
structure and shape
• Changes in length, width, size and camber 
modulation of flexibility distribution
J. Lecomte-Beckers
79
Cut of a high performance ski
Bidirectional layers. Glass fibres (±45° with ski
axis). Provides torsion stiffness.
Unidirectional layers. Glass fibres
(mainly 0° but also 90°). Provides
longitudinal stiffness.
Core wrap. Bidirectional layers
of glass fibres. Acts as torsion
bar and joing external part of
core.
Bidirectional layer.
Glass fibres (±45°).
Provides torsion
stiffness.
Edge. Hardened steel.
Eases in turns allowing
ski to grip in snow.
J. Lecomte-Beckers
Upper part. ABS with low vitrous
transition temperature. External wrap
and appearance
Side. ABS with low
vitrous transition
temperature.
External wrap and
appearance
Core. Polyurethane. Improve
Filling material.
Damping layer. Polyurethane.
Improve resistance to
chattering.
Unidirectional layers. Glass
fibres (mainly 0° but also 90°).
Provides longitudinal stiffness.
Bidirectional layer. Glass fibres (±45°). Provides
torsion stiffness.
Sole. Compressed carbon (carbon fibres in a
polymer matrix). Sliding surface is hard and
abrasion resistant
80