Material

Chapter VI : Materials
Properties
Materials Selection Course
J. Lecomte-Beckers
1
Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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Introduction
Design of a structure
Choice among a long list of materials
Some objectives are crucial and design mistakes can lead
to a disaster
Example : welded boats during WWII
(fracture toughness problems)
 importance of material selection in relation with
properties
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Introduction
Metals: Relationship between
microstructure and properties with
concerned length scale
• Stiffness and physical properties
are mainly linked to atoms, their
packing and defects
• Yield strength and ductility are
related to grains (Hall-Petch law),
defects and precipitates
(Hornboggen) and crystal structure
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Introduction
• Fracture toughness and fatigue
strength are depending from
nanometric to milimetric phenomena
(from crystal to “mesoscopic”
cracks)
• Friction, wear and corrosion are
function of cracks, roughness and
grains (case of grain boundary
corrosion in stainless steels)
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Introduction
Ceramics: Relationship between
microstructure and properties with
concerned length scale
• As for metals, E and physical
properties are related to the
nanometric (crystals, networks) and
picometric scales (electron clouds...)
• Fracture toughness depends
intrinsically of crystals and grains but
(micro)-cracks weaken the material
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Introduction
• Friction, wear and corrosion are
function of cracks (because of brittle
behaviour)
• Glasses are amorphous (no grains
and no long distance order)
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Introduction
Polymers: Relationship between
microstructure and properties with
concerned length scale
• E and physical properties are
linked to atoms and molecules but
also strongly depends on crystallinity
• Strength and ductility are
depending sub-microscopic
(crosslinking, mobility) and
microscopic (crazing...) phenomena
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Introduction
• Fatigue and fracture toughness are
conditioned by microstructure and its
ability to get damaged (cracks,
crazing...)
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Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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Stress-strain curve
• Elastic regime:
  E.
proportionality coefficient
E=
Young’s modulus
slope of the curve
E is related to the
« resistance » to elastic
strain
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Young’s modulus
• If E is high, stiff (or rigid) material
– Steel, iridium, diamond: very stiff
– Light alloys and lead: medium downto low stiffness
– Polymers and organic materials: low stiffness
• For many mechanical application, strain is undesired 
materials with high Young’s modulus
• Materials with low E are promoted for certain
applications: springs, gaskets…
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Measure of Young’s modulus
• 1st method: easy
– Tensile test with measure of strain and stress
– E calculated with
E


– Low accuracy especially with high E values because ε is low
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Measure of Young’s modulus
• 2nd method: better accuracy
– Measure of the natural frequency of a stem made up of the
concerned material fixed at each ends and centre loaded
with a heavy M mass (to neglect own weight of the stem)
– Oscillating frequency of stem, f (in cycle.s-1)
1
f 
2
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3d
4l 3 M
4

16Ml 3 f e

3d 4
14
Measure of Young’s modulus
• 2nd method:
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Measure of Young’s modulus
• 3rd method: even with more accuracy
– Measure of sound speed in material
– Speed of longitudinal vibrations are depending on density and
Young’s modulus
Vl 
E

– Measure of Vl with the sollicitation of one end of a stem (of
the concerned material) with measure of requested time for
sound to propagate to the other end (with piezoelectric cells)
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Young’s
modulus
of some
materials
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Classified
diagramm of
Young’s
moduli
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Origin of elasticity modulus
• Ceramics and metals: low dispersion of E (from 30 to
300 GPa)
Examples : cement and concrete (45 GPa), Al (69 GPa),
steels (200 GPa)
• All the polymers exhibit a much lower value of E
(sometimes several orders of magnitude)
• Why?
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Atomic bonds
• Atoms are bound with strong or weak forces
• Strong bonds Strong material (not especially tough)
• Bonds are represented by springs (oscillating)
• Equilibrium distance of atoms: a0
• Tensile force can draw aside atoms to distance a0 + δ
• When load is relaxed, atoms recover their initial
position (same behaviour for compression)
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Atomic bonds
• Displacement of δ with F applied load
 stiffness (S) is defined by
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S
F

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Stiffness of main bonds
Bond type
Example
Stiffness
(N/mm)
Young’s
modulus
(GPa)
Covalent
C-C bond
50 – 180
200 – 1000
Metallic
Metals
15 – 75
60 – 300
Ionic
Alumina, Al2O3
8 – 24
32 – 96
Hydrogen
Kevlar
6–3
2 – 12
Van der Waals
Polyethylene
0.5 – 1
1–4
Young’s modulus is a function of bond stiffness
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Covalent bond
• Especially stiff bond
• S = 20-200 N/m
• Diamond: very high elasticity modulus (because of small
size of C atoms)
 high bond density and atoms linked with strong bonds
(S = 200 N/m)
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Intermetallic bond
• Slightly less rigid bond
• S = 15-200 N/m
• Metal atoms are quite close
 High Young’s modulus
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Ionic bond
• Bond providing a level of stiffness similar to metals
 High Young’s modulus
• Mainly in ceramics
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Bonds in polymers
• Polymer = chain made up of C strong covalent bound
atoms
BUT chains are joined with weak H bonds, dipoles or de
Van der Waals (S = 0.5 to 2 N/m)
 déformation aisée des polymères et faible module
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Relation between E and S
S
E
a0
• The weakest bond (S = 0.5 N/m) and the most spaced
atoms (a0 = 4.10-10 m)  E ≈ 1 GPa (= magnitude order
for elasticity modulus of the majority of polymers)
• Metals and ceramics: elasticity modulus is 50-1000 times
higher because of stronger interatomic bonds
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Elastomers
• Young’s modulus < 1 GPa because of long chain
molecules with transverse bonds
• Low binding energy  « molten » state with some
elasticity at room temperature
• Situation: T° > Tg (glass transition temperature)
 Modulus < 1 GPa because very low binding energy.
Elasticity is provided by entropy!
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Case study: telescope mirror
Question : How can we select a material minimizing the
strain of a disc loaded with its own weight?
 direct relationship with Young’s modulus
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Case study: telescope mirror
• The largest telescope in the World: LBT (Large Binocular Telescope)
on Mont Graham in Arizona
• Made of 2 telescopes with 8.4 m mirrors set on a 380 tons structure
• Very light mirrors compared to their diameter (16 tons) thanks to
honeycomb structure (material=glass)
• Cost of a telescope: ± 200.000.000 €
• Cost is proportional to the square of the mirror weight
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Case study: telescope mirror
• Up to last century: polished metal mirror
• Now upper face silvered glass
• Selection criteria: material able to bear a 5 m diameter
mirror:
– Material with low deformation when it is moved
– The lightest material as possible
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Case study: telescope mirror
• Mirror = circular disc with 2a as
diameter and e for mean thickness
keeped on its lateral face
• Horizontaly: bending under own weight
(mg)
• Verticaly: neglectible deformation
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Case study: telescope mirror
• Deflection is low enough not to diminish the mirror
performance
 Deflection in the centre of the mirror must be lower
than light wavelength (fmax ≈ 1µm)
• Compensating system to annihilate the deflection effect
(fmax ≈ 10µm)
• Deflection value at the center of a horizontal disc with
own weight:
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3 Mga 2

4 Ee 3
33
Case study: telescope mirror
• Minimise mass with 2a (5 m) diameter and δ deflection (10 µm) as
constants
• Thickness is function of mass:
3 Mga 2  3 a 6  3
  
4 E
M3
1
and M   3g  a 4   
 E 
 4 
 
2
3
1
M  a 2 e  e 
M
a 2 
2
 independant variables: ρ et E
 In order to minimise M, we have to minimise (ρ³/E)1/2
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Case study: telescope mirror
Material
E (GPa)
Density
(tons m-3)
M (ρ3/E)1/2
Mass (tons)
e (m)
Steel
200
7.8
1.54
158
1.0
Concrete
47
2.5
0.56
56
1.2
Aluminium
69
2.7
0.53
53
1.0
Glass
69
2.5
0.48
48
0.97
Wood
12
0.6
0.13
14
1.2
Polyurethane
foam
0.06
0.1
0.13
13
6.6
CFRP
270
1.5
0.11
11
0.38
Value of (ρ³/E)1/2 for some materials
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Case study: telescope mirror
• Best material: CFRP then polyurethane foam
• Glass is better than steel, Al or concrete
• Other constraints to be taken into account:
– Mirror mass for the different materials
– CFRP and polyurethane foam mirrors are 5 times lighter than
glass mirrors  CFRP mirrors are 25 times cheaper
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Materials
for a 6m
diameter
telescope
mirror
Material
Index (M)
Mass (tons)
Notice
Steel
0.7
158
Very heavy. Origin material!
Concrete
1.4
56
Heavy. Low creep properties
(thermal distorsion problem)
Al alloys
1.5
53
Heavy, high thermal expansion
Glass
1.6
48
Current choice
GFRP
1.7
44
Low dimensional stability. Used for
radiotelescopes
Mg alloys
2.1
38
Lighter than glass but high thermal
expansion
Wood
3.6
14
No dimensional stability
Beryllium
3.65
14
Very expansive. Suitable for small
mirrors
EPS
3.9
13
Very light but no dimensional
stability. Expansed glass?
CFRP
4.3
11
Very light but no dimensional
stability. Used for radiotelescopes
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Case study: telescope mirror
• Mirror thickness: with glass, 1 m but with CFRP, 38 cm
• Solutions given are not realistic BUT incredible drop of
money expanse
• Interest to optimise materials in function of researched
properties  dramatic breakthrough with structure
design
 research on materials
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Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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Yield limit
• Stress value over which materials starts to promote
plastic strain (not reversible)
• Written σy or σel and expressed in MPa or MN/m²
• There exists different behaviour for small deformation
of materials :
– Linear and non-linear elasticity
– Anelastic behaviour
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Linear elastic behaviour
• Follows Hookes’ law
• For all solids with low
deformation (<0.1%)
• Same slope for tension and
compression (= f(E))
Stress/Strain curve for a material
with linear elastic behaviour
(e.g. steel)
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Non-linear elastic behaviour
• Behaviour of rubbers with
very low dissipation
(possibility of huge strain)
• Stored energy is recovered
from load to unload
Stress/Strain curve for a material
with a non-linear elastic
behaviour (e.g. low dissipation
rubber)
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Anelastic behaviour
• All the solids are anelastic
• Even in elastic domain load
curve is different from
unload curve  dissipation
of some energy with each
cycle (hatched area)
Stress/Strain curve of a material
with anelastic behaviour (e.g.
glass)
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Effect of tensile force
• Initially elastic strain and after plastic strain
• With continuation of plastic deformation material is lenghtening with
reduction of cross section (constant volume)
• Sometimes necking effect (local drop of cross section area)
• Finally rupture of sample
• After rupture the length of the two parts is lower than the length of
the sample just before rupture (elastic recover)
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Tensile test
Characteristic values:
– Re or σe = yield strength (or
elastic limit)
– R0.2 = elastic limit with
conventional 0.2% strain
– Rm = tensile strength
(maximum admissible stress in
material)
– AR (%) = relative plastic
lengthening after rupture
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Stress/Strain curve for a ductile
material
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Measure of elastic limit and
tensile strength
• Metals: elastic limit = stress related to a 0.2% strain
• Polymers : elastic limit = stress related to the begin
of non-linear elasticity (ε ≈ 1%)
• Ceramics and glass: different behaviour according to
load geometry
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Measure of elastic limit and
tensile strength
• Metals and composites (most of them): tensile
strength is 1.1 to 5 times higher than elastic limit
• Brittle materials (ceramics, glass and brittle
polymers): tensile strength = ultimate strength
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Elastic limit,
tensile strength
and ductility of
some materials
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Material
Diamond
Glassy silica SiO2
Alumina
Zirconia
Silica glass
Cobalt and alloys
Low alloy steels
Austenitic stainless steels
Nickel alloys
Titanium alloys
Cast irons
Copper alloys
Aluminium alloys
Ferritic stainless steels
PMMA
Epoxides
Nylons
Polystyrene
Lead and alloys
Tin and alloys
PP
PU foams
48
Yield strength of
materials
classified
according to
their type
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Origin of elastic and plastic
deformation
• In ordered crystalline materials: presence of defects
(dislocations) at the origin of tensile behaviour
• Motion of dislocations  plastic deformation
• Motion is prevented by internal resistance of crystal (=
yield strength)
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Case study:
Materials for springs
• Various size and shape
• Generally made of a metal: why?
Material
E (GPa)
σe(MPa)
σe/E 10-3
Brass (cold rolled)
Bronze (cold rolled)
Phosporous bronze
Copper-Beryllium
120
120
120
120
638
640
770
1380
5.32
5.33
6.43
11.5
Spring steel
Stainless steel (cold rolled)
Nimonic (high T spring)
200
200
200
1300
1000
614
6.5
5.0
3.08
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Usual materials
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Case study:
Materials for springs
• Is elasticity modulus the most important material
property ?
• Low variation of E for these 7 common spring materials
and E is not particulaly high
• Avoid plasticity
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Matériaux usuels
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Practical case: leaf spring
• Various shape
• Small elastic beams (with bending load)
Loaded leaf spring and deflection
because of F force
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Practical case: leaf spring
• Stress equal to zero along neutral fibre (in the centre) and
maximum on peripheral area in the middle of the spring
• Max stress:

3FL
2be 2
• Objective of a spring: no permanent deformation
 max stress < yield strength to ensure recovery when stress
relaxes
 e 6e
3Fl
e 
 2
2
E
2be
l
 best springs exhibit high values of σe/E ratio
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Practical case: leaf spring
Other constrains
• Spring aim = store and recover elastic energy
• Resistance to fatigue and corrosion (e.g. cars)
• Thermal resistance (valve springs)
• Damping ability
 Global selection procedure
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Sheet metal rolling
• Forging, deep drawing and rolling = forming processes with
reduction of cross section with a plastic compression strain
• Plate rolling: thickness is decreased from e1 to e2 on a L length
between rolls
Laminage d’une
tôle de largeur b
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Sheet metal rolling
• Metal is growing with rolling direction
 it accelerates at each rolling step
 friction between plate (or bloom) and rolls
• With neglecting friction loss and with perfectly
lubricated rolls:
 l² + (r-x)² = r²
If x = ½ (e1-e2) is small,  l = [r(e1-e2)]½
• Applied force F = σebl
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Sheet metal rolling
• With central load (half length) equivalence of reactive
force  Rolling momentum M = Fl/2 = σebl²/2 = ½
σebr(e1-e2)
• F and M are depending on σe, (e1-e2)
• M
with σe  hot rolling is less energy expanser than
cold rolling
• M
size reduction (e1-e2) and r
 use of rolls with small diameter, in contact with high
diameter rolls (in order to limit deflection)
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Sheet metal rolling
4-5 : Support rolls
3: Work roll
6: Metal sheet
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Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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Rupture
= break of material in 2 (or more) parts under the
action of a stress
• Brittle rupture (>< ductile) characterised by no ductility
 fast propagation of cracks with low energy
dissipation
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Fracture toughness
= resistance of material to crack propagation
(KIC = E1/2.GC [MPa.m1/2] ou [MN/m1/2])
• Generally tough material is not brittler
• Fast fracture when (stress concentration factor)
K   a = KIC (a=equivalent crack length)
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Fatigue rupture
= time dependant damaging of materials with no nakedeye viewable deformation under cycle stress
• Important to study because spontaneous rupture with
stress level lower than yield strength and statistic
distribution of time before rupture (Wöhler diagramm is
probabilistic: 50% risk for material to fail after x service
hour on the specified stress magnitude)
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Measure of fracture toughness
• KIC is given with measure of the force value leading to rupture
under a certain c (known) initial crack length
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Mesure de la ténacité KIC
64
Measure of fracture toughness
• With this method KIC is well known for brittle materials
(glasses , ceramics and brittle polymers)
• For ductile materials formation of a plastic deformed
area around initial crack (>< elasticty theory)
 if plastic area is too big measure is not valid
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Some values of KIC
Material
KIC (MPa m0,5)
Tool steel (HSS)
Mild steel
Titanium alloys
PP
PS
Nylon
Pig iron
PMMA
Epoxides
Silicon nitride Si3N4
Silicon carbide SiC
Alumina Al2O3
Silica glass
50 – 154
140
55 – 115
3
2
3
6 – 20
0,9 – 1,4
0,3 – 0,5
4–5
3
3–5
0,7 – 0,8
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Distribution of
KIC according to
material type
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Measure of fatigue
• Fatigue test = measure of fatigue strength and endurance limit
• Application of stress cycles (between 10 and 100 millions) with
tension, compression or bending
• Fatigue strength (or limit) is the maximum stress the material can
bear after a certain cycle
• Endurance limit is the maximum stress a material can bear with an
infinite cycle
• Generally alternating (+/-) stress amplitude is more detrimental
than same sign oscillations (++ or --)
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Measure of fatigue
• Results are given on a
Wöhler curve
• Cycles to failure vs stress
amplitude
• Statistic! 50%
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Endurance limit for some materials
Material
Endurance limit (MPa)
Nickel superalloy
Stainless steel
Ziconia
Alumina
Pig iron
CFRP
Brass
Vitro-ceramic
Silica glass
Nylon
Lime glass
Epoxide
Polyester
PMMA
PS
Silicon elastomer
150 – 900
175 – 753
300 – 500
200 – 488
180 – 330
150 – 300
150 – 250
60 – 170
43 – 143
36 – 66
30 – 33
22 – 35
17 – 36
15 – 33
14 – 23
2–4
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Fracture toughness and
fatigue origin
• Stress concentration in the vicinity of cracks
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Stress intensity field in a cracked
material
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Fracture toughness and
fatigue origin
• Local stress is higher in the crack plan and not
constant (max. next to the crack)
• In glass and ceramics, stress field > yield limit 
brutal rupture by cleavage because no ductility
• In metals and polymers, formation of a plastic
deformed area around the cracks
 ductile rupture
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Fracture toughness and
fatigue origin
• If no cross section change, roughness, cracks, inclusion
or other defect, material is not submitted to fatigue
phenomenon (+- case of ceramics)
• BUT this material doesn’t exist and each defect induces
stress concentration
• Application of a stress cycle localised plasticity 
formation of microcracks and propagation
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Brittle rupture of a compression
die
Nail shape
crack
die designed for the manufacture of
supraconductive alloys (obtained by
Powder
blending metal powders: green
part) and then sintered and pulled
in yarns
die for powder compression
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Brittle rupture of a compression
die
• Higher pressure in die, higher green part density and
better final properties of sintered product
• Use of a special steel die with heat treatment  high
yield strength
• Idea : take benefit from high YS to work with high
pressure
• BUT rupture at first use at half of service load: why?
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Cause: nail shape
crack initiating
failure from
internal surface
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Brittle rupture of a compression
die
• Problem: high yield strength low
ductility
• Not suitable thermal treatment 
KC is very low
• Solution: another heat treatment
can provide higher KC but with
lower σe OR use a steel alloy for
pressurised tanks (easier)
The higher the yield strength is
the lower the fracture toughness
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Pressurised tanks for a
supersonic wind tunnel
• Wind tunnel of Cambridge University is connected with
20 big cylindrical tanks containing compressed air
• Tanks are slowly filled with pressurised air and are
then instantly emptied
 request of design objective to ensure the
resistance of tanks
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Pressurised tanks for a
supersonic wind tunnel
• 1st objective: no significant plastic deformation
 stress < yield strength
• 2nd objective: no brittle rupture
With internal microcracks with 2a length, stress
amplification factor: K   a < KC
• 3rd objective: no fatigue rupture
Cracks growing up to the critical size must be slow
enough
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Pressurised tanks for a supersonic
wind tunnel
• Stress σt in cylindrical tank
containing a pressurised gas
(pressure p) with a low side
thickness (e<<r):
 t  pr
e
• Plasticity criterion: σ = σe
• Brittle fracture criterion: a  K C
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Crack in a pressurised tank
80
Pressurised tanks for a supersonic
wind tunnel
Steel for press. tank
Criteria for plasticity or
brittle fracture for a
pressurised cylindrical
Al alloy
tank for an aluminium
alloy and specific
purpose steel
Crack length
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Pressurised tanks for a supersonic
wind tunnel
• In steel critical crack size = 9 mm
• In aluminium critical crack size = 1 mm
 Ability to point out defects with ultrasound detection
in steel tanks (non destructive test)
 With aluminium tank  lower safety
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Pressurised tanks for a supersonic
wind tunnel
• Tank swallows cycle stress  crack are growing with
fatigue  danger
• Crack growing rate is provided fatigue test on cracked
samples of the same steel
• Pressurised tank is tested with hydraulic pressure equal
to 1.5 - 2 times service pressure
• If no rupture  Pass
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Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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84
Some reminders
• Creep = physical phenomenon causing irreversible strain
in material under a constant stress during a long time
• Deformation is depending on external parameters: time,
T°, stress, pressure…
• Known example of creep: bended shelf in a permanent
way because of books weight
J. Lecomte-Beckers
85
Some reminders
• For a load lower than yield strength, creep is to be
considered over a certain temperature
• Creep = slow and continuous deformation with time
ε = f(σ, t, T) : high T° behaviour
• Law is different when T° is low (not time dependant)
ε = f(σ) : low T° behaviour
Elastic or elasto-plastic solid
J. Lecomte-Beckers
86
Example : tungsten
• W is used for filaments in incandescent light bulb
• W : very high melting point (> 3000°C)
room T° = very low T°
• As T° is high enough filament creeps ( up to rupture)
• Bulb light are working up to 2000°C (high T° for W)
• Focus on fractured wire: own weight rupture  creep
deformation
J. Lecomte-Beckers
87
Example : tungsten
• Creep
ε = f(σ, t, T)
(Which value?)
• Creep initiating T° is depending on melting point Tm (in
K) of material:
– T > 0.3 à 0.4 Tm for metals
– T > 0.4 à 0.5 Tm for glass and more higher for crystalline
ceramics
J. Lecomte-Beckers
88
Melting
point of
materials
J. Lecomte-Beckers
89
Melting (or softening) temperature of some
materials
Material
/K
Material
/K
Diamond, graphite
Tungsten
Tantalium
Silicon Carbide,
SiC
Magnesia, MgO
Molybdenium
Beryllia, BeO
Alumina, Al2O3
Silicon nitride
4000
3680
3250
3110
3073
2880
2740
2700
2323
2173
Silica glass
Aluminium
Magnesium
Lime glass
Zinc
Polyimide
Lead
Tin
Melamine
Polyester
1100
933
923
700 – 900
692
580 – 630
600
505
400 – 480
450 – 480
J. Lecomte-Beckers
90
Melting (or softening) temperature of some
materials
Material
/K
Material
/K
Chromium
Zirconium
Platinum
Titanium
Iron
Cobalt
Nickel
Cermets
Silicon
Alcali halogenides
Uranium
Copper
Gold
Silver
2148
2125
2042
1943
1809
1768
1726
1700
1683
800 – 1600
1405
1356
1336
1234
Polycarbonate
HDPE
LDPE
Rigid polymeric foams
Epoxide
PS
Nylon
PU
Acrylics
GFRP
CFRP
PP
Ice
Mercury
400
300
360
300
340 – 380
370 – 380
340 – 380
365
350
340
340
330
273
235
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91
Some reminders
• Metals: high melting point (most of them)
 creep when T°> T° ambiante
 creep is less known than elasticity or plasticity
• On the other hand melting point of Pb = 600 K
 Room temperature is rather of for Pb  Room
temperature creep
J. Lecomte-Beckers
92
Some reminders
• Polymers : creep with room temperature (most of them)
: Tg (glass transition) = important T°
If T°> Tg, rubbery state and easy creep
If T°< Tg, vitreous state (harder) and limited creep
J. Lecomte-Beckers
93
Creep test
• Need of rigorous T° control
• Standard method: sample loaded with tension or
compression (with constant stress) under constant T°
(in oven)
• Measure : deformation according to time
• Characteristic creep curve for metals, polymers and
ceramics
J. Lecomte-Beckers
94
Creep test
Tertiary
creep
Resistor
Strain ε
Secondary
creep
Initial elastic
strain
Primary creep
Time t
Creep test and curve
J. Lecomte-Beckers
95
Creep test
• Initial strain and primary creep are neglectible (short time, can be
considered as elastic strain)
• Secondary (or steady-state) creep: deformation is increasing
continuously with time (most serious aspect)
• Damaging with internal cavities during creep
• Macroscopic damage appears at the beginning of tertiary creep:
cavities are dramatically growing with damage accumulation
• When cavities are sizing up cross section is decreasing and so stress
is increasing with constant load
J. Lecomte-Beckers
96
Creep test
• For lot of high resistance alloys creep damage is occuring very soon
 rupture with low deformation (1%)
• Design requests for parts which have to be used with high T°:
– Low creep deformation
– Creep ductility to be high enough to face early creep deformation
– Safety with estimated lifetime
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97
Origin of creep:
metals and ceramics
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98
Creep with dislocation motion
• Déformation plastique si contrainte appliquée suffisante
pour que dislocations plus arrêtées :
– Par friction intrinsèque du réseau
– Par obstacles intrinsèques (précipités, atomes dissous, …)
• Diffusion atomes  déblocage dislocations  fluage par
dislocations
• Force mécanique  diffusion atomes loin de dislocation
dans champ de contraintes = montée (pour T°> 0.3 Tf)
J. Lecomte-Beckers
99
Creep with dislocation motion
Unlock with dislocation climb phenomenon anchored on precipitates
 dislocation slipping long to other obstacles  cyclic phenomenon
 continous and smooth nature of creep
 creep rate depends on T° (diffusion))
Climb
Slip
Slip
Climb
Climb
Precipitate
J. Lecomte-Beckers
How to increase creep strength?
Amount of obstacles (precipitates)
100
Diffusion creep
- high temperature
• If stress lowers, creep rate goes down
BUT another creep mechanism can occur with high T°
• Applied stress is relaxed with diffusion along grain boundaries
Grain boundary
diffusion
Diffusion in the
crystal volume
How to increase creep strength?
Increase grain size
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101
Creep origin:
polymers
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102
Creep of polymers
• Major design problem
• Tg is close to room T° (most of them)
• When T°< Tg, polymer ≈ glass (brittle elastic solid)
• When T°> Tg, polymer = rubber or viscous liquid
• Example : thermoplastics (elasticity under Tg and
Newtonian liquids over Tg)
J. Lecomte-Beckers
103
Creep of polymers
• Service T° is close to Tg  nor elastic solid nor viscous
liquid BUT visco-elastic solids
• If elasticity = spring, viscosity = dashpot so visco-
elasticity = assembly of spring(s) and dashpot(s)
• // assembly: When load is applied creep occurs but
with decreasing rate (because energy is stored in
spring)
• // assembly: When load is relaxed, inverse slow creep
occurs because of spring force
J. Lecomte-Beckers
104
Creep of polymers
Spring
Dashpot
Polymer creep modelisation (with
Kelvin model)
J. Lecomte-Beckers
105
Case study: turbine blade
• Ideal yield (from thermodynamics) of a steam machine:
T1  T2
T2
 1
T1
T1
T1 and T2 = T° of hot and cold fluids
• Higher T1, higher yield
• In practice real yield < theoretical yield BUT if burning
temperature is increased  higher yield
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106
Specific fuel consumption (in kg) for one
service hour and one Newton given (that is to
say by provided power unit)
Case study: turbine blade
J. Lecomte-Beckers
Gas turbine yield according to service temperature
(viewed with fuel specific consumption)
107
Case study: turbine blade
• 1950 : service temperature is 700°C
 economic interest to rise service T° (see curve)
• 1975 : RB211 model works at 1350°C
 50% cut in fuel consumption
• BUT, curve asymptocity over 1400 °C doesn’t justify the
use of new materials (for reducing fuel consumption)
unless fuel becomes very expensive
 Other factor: engine power
J. Lecomte-Beckers
108
Case study: turbine blade
proportionaly with T°
 an increase in
service T° can
improve
power/weight ratio
 cheaper
kW needed to move one kilogram
air by second (i.e. engine power,
other parameters keep constant
Power increase
Power of a reactor according to service T°
J. Lecomte-Beckers
109
Case study: turbine blade
• Conditions for new materials for high temperature
turbine blade:
– Creep and corrosion resistance with high T°
 microstructure stability with high T°
– Fracture toughness
– Thermal fatigue resistance and stability
– Low density (centrifugal forces)
– Resistance to fatigue and shocks
 drastic limitation of suitable materials
J. Lecomte-Beckers
110
Case study: turbine blade
• Ceramics: low density and high softening temperature
but too brittle  to be excluded (at the moment)
• Cermets : softening T° is too low for metal matrix  no
advantage
• Nickel superalloys: best known materials
J. Lecomte-Beckers
111
Case study: turbine blade
• Take off: turbine blade support
250 MPa stress
• Design specification: stress
resistance during 30h at 850 °C
with no irreversible creep
deformation higher than 0.1 %
 development of nickel
superalloys
Typical composition of a blade
which is resistant to creep
J. Lecomte-Beckers
112
Superalloys
• Incredible materials (use up to 850°C) with 1280 °C
melting point!
• High creep resistance
• Very hard not machinable with conventional methods
• Precision casting to final shape (lost-wax casting)
• Obtained blades are very expensive and grain size is very
small
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113
Lost-wax casting
 Fine grains
Liquid
alloy
Cavities
With time and
high temperature
Grain are growing from
liquid metal
Turbine blade mould
Turbine blade made by lost-wax casting
J. Lecomte-Beckers
114
Superalloys
• Alloying element strengthening (solid solution and
precipitates)  limits creep through dislocation motion
• BUT at 0.72Tm, diffusion creep
• Solution: increase grain size or monocrystal blades
• Damage is cumulating at grain boundaries
 delay rupture with grain boundary elimination or by
aligning them in parallel to applied load
J. Lecomte-Beckers
115
Superalloys
• Directional solidification (DS)  long lengthened gain in parallel
to applied load
 reduce diffusion creep
Resistors
Melted
alloy
Solidified
alloy
No shear
stress at grain
boundaries
(no slipping).
No stress on
grain
boundaries
perpendicular
to tension axis
(no cavities)
Grain boundaries
Mould is slowly get out of oven
Directional solidification for turbine blades
J. Lecomte-Beckers
116
Superalloys
Improvement of cooling
With cooling
Flame maximum
temperature (depends
on fuel and inlet gas)
limiting line
Niobium and alloys
Directional eutectics
Internal
T°
Nickel base alloys
(cast)
Directional Ni
alloys
Year
Evolution T°fonctionnement et matériaux utilisés
pour les aubes de turbine
J. Lecomte-Beckers
117
Superalloys
•  1960 : Inlet T°in turbine = metal T°
• After 1960 : divergence
Inlet T°> turbine blade T
 Need to cool (internal) blades with air
 increase to inlet T° of 100 °C without changing
materials properties
• Improvement: cold layer cooling
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118
Air cooled turbine blade
Interal side of cooling
pores
Cooling pores
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119
Development cost and profitability
Validation
Tests
1:1 scale test
Improvement: medium
profits expected
Validation
Cost (106 USD)
Cost (106 USD)
Innovation: high
profits expected
Tests
1:1 scale test
Years for program start
J. Lecomte-Beckers
120
Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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121
Dry corrosion
• Essential properties for materials used with high T°:
resistance to high temperature reactivity of gas and
oxidation
• Service oxidation of turbine blades and reaction with
H2S, SO2 and other flue gases.
• Resistance to oxidation at 500 °C = use of a material at
this temperature in air or oxidising atmosphere
without apparition of cracks or cross section reduction
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122
Wet corrosion
• Wet atmosphere or liquid
• Example : mild steel exposed to oxygen and water (or
moisture) is rusting at room temperature  metal loss
 some precaution to be considered
• Aqueous corrosion ≠ according to the surrounding:
neutral water, sea water, acids and bases (weak or
strong) and organic solvents
J. Lecomte-Beckers
123
Corrosion in neutral or sea water
= damage of metal because of electrochemical
reaction with environment
• When 2 different metals are joined together in water
(even pure): metal with the lowest potential is
oxidised with hydrolysis reaction
• In sea (or salted) water, solvent is conducer: ionisation
reaction and damage for metal with the lowest
potential
• Notice: hydrolysis occurs even with only one metal
J. Lecomte-Beckers
124
Aqueous corrosion
Acid or base solvent
= electrochemical reactions induce damage to metal
• BUT reaction is depending on pH and wet species with formation
or not of a passive layer on metal (can inhibit corrosion)
Organic solvent
• Common solvents: lubricates, mazout, cooking oils…
• Materials are immunised (most) againt organic solvents and are
ranked in 5 classes (from very low (1) to excellent (5) resistance)
J. Lecomte-Beckers
125
Measure and values related dry
corrosion
• Oxidation is present because Earth’s atomosphere is
oxide promoter
 Earth’s crust contains oxides, silicates, aluminates…
• Oxides  stable
• Metals are not stable unless gold (and some platinoids)
at the pure state. Gold stands metallic at each
temperature
J. Lecomte-Beckers
126
Measure and values related dry
corrosion
• Quantification of tendency of oxygen to react with
metal and measure of reaction energy:
Material + oxygen  oxide of material+ energy
• If energy < 0, stable material (dG>0)
• Otherwise: oxidation
J. Lecomte-Beckers
127
Dry corrosion
Oxide formation
energy at 273 K
(kJ.mol-1 d’O2)
J. Lecomte-Beckers
128
Dry corrosion
Free energy of oxide formation at 273 K
Material
(oxide)
Energy (kJ.mol-1 O2)
Material
(oxide)
Energy (kJ.mol-1 O2)
Be
Mg
Al
Zr
U
Ti
Si
Ta
Nb
Cr
Zn
Si nitride
Si carbide
Mo
W
Fe
Sn
Ni
(BeO)
(MgO)
(Al2O3)
(ZrO2)
(U3O8)
(TiO)
(SiO2)
(Ta2O5)
(Nb2O5)
(Cr2O3)
ZnO)
(3SiO2+2N2)
(SiO2+CO2)
(MoO2)
(Fe3O4)
(SnO)
(NiO)
-1182
-1162
-1045
-1028
-1000
-848
-836
-764
-757
-701
-636
-629
-580
-534
-510
-508
-500
-439
Co
Wood, polymers
Diamont, WC cermet
Pb
Cu
GFRP
Pt
Ag
PTFE
Alcali halogenides
Magnesia, MgO
Silica, SiO2
Alumina, Al2O3
Berylla, BeO
(CoO)
(CO2)
(WO3+CO2)
(Pb3O4)
(CuO)
(PtO2)
(Ag2O)
(Au2O3)
-422
-400
-389
-349
-309
-254
-200
-160
-5
0
+80
+400 - +1400
Very high
Very high
J. Lecomte-Beckers
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129
Oxidation rate
• Rate (speed) is not proportional to free energy of reaction
• From last table we could suppose oxidation of Al is 25 times faster
than for Fe. In this case it is completely the reverse ratio (ox.
1:25 mm Al:Fe)
• Cause: formation of oxide layer on surface  protective barrier
and lowers oxidation rate
• BUS this layer is more or less efficient according to porosity,
adhesion to metal and diffusion coefficient of oxygen in it
J. Lecomte-Beckers
130
Oxidation rate
• Case of iron: layer is permeable to O2 and with low adhesion
 oxidation is faster than for Al (imperméable oxide layer)
• Oxidation is given by the addition of oxygen atoms on metal
surface  increase of material weight with proportion to oxidised
matter quantity
• Measure of Δm can be followed according to time measure of
oxidation rate
• Notice: for a very long corrosion time no correlation between
free energy of reaction and oxidation rate
J. Lecomte-Beckers
131
Time (in h) to
oxidise 0.1
mm thickness
of metal at
0.7Tm in air
J. Lecomte-Beckers
132
Measure and relative
measure to corrosion
• moving e- in a conductor (easy to measure)
 corrosion tendency in aqueous environment is given
by reduction potential table
J. Lecomte-Beckers
133
J. Lecomte-Beckers
Corrosion generaly occurs
Corrosionalmost
never occurs
Wet corrosion
potential (at 300 K)
134
Aqueous corrosion rate
related to thermodynamics
≠
• Corrosion rate = cinetics of reaction
Metal loss (mm/yr)
• Corrosion (or reduction) potential =
Example of corrosion rate of some
metals in pure water
J. Lecomte-Beckers
135
Aqueous corrosion rate
• In aluminium low wet corrosion rate because formation
of a Al2O3 film at the surface
• In sea water corrosion is quite fast in cause of chloride
ions breaking Al2O3 protective film Al2Cl3
• ≠ corrosion rate because of interaction with other ions
J. Lecomte-Beckers
136
Stainless alloys (dry oxidation)
• Low C steel (excellent as structural material) :
– Cheap
– Easy forming
– Good mechanical strength
– BUT rust with low T° and oxidise at high T°
• However significant demand for steel with corrosion
resistance
 development of steel and ferrous alloys with
stainless ability
J. Lecomte-Beckers
137
Stainless alloys (dry oxidation)
• Low C steel with hot air fast oxidation to form FeO
(unstable with low T°) or Fe2O3 or Fe3O4
• BUT if presence of an element with higher oxidation
free energy in solution in steel: preferential oxidation
of the alloying element and formation of
another oxide layers
• Nature of some protective layers: Cr2O3, Al2O3, SiO2 or
BeO
 steel is protected
J. Lecomte-Beckers
138
Stainless alloys (dry oxidation)
• BUT need of important quantity of alloying element to
ensure a good protection
• Most efficient alloying element: chromium (min 12 wt.%)
• Other elements: Al2O3 and SiO2 protective layers
• Example : 5 wt.% Al divides 30x oxidation rate
5 wt.% Si divides 20x oxidation rate
• Same principle used in other metals: Ni, Co, Cu (cupro-
aluminium), Ag (sulphur + Al or Si)…
J. Lecomte-Beckers
139
Stainless alloys (dry oxidation)
• Advantage of protection with alloying element compared
to coatings:
self-healing material
• If protective film is scratched or worn metal becomes
« nude » and Cr (or Al or Si) are oxidising immediately
immédiatement  healing of the damaged area (a scar is
remaining)
J. Lecomte-Beckers
140
Turbine blades protection (dry
oxidation)
• Current materials: Mainly made up of Ni + addition elements for creep
properties
• Used around 950 °C (close to 0.7 Tm of Ni)
• With this T° Ni is 0.1 mm thinner each 600 h service
• BUT total thickness is ± 1 mm  in 600 h, loss of ± 10% of cross section
area
 serious attack to mechanical integrity and no consideration of
preferential oxidation direction or area (pitting corrosion)
• However, change of blading is very expansive  lifetime > 5000 h is
asked
(over 0.29mm loss for thickness  not admitted)
J. Lecomte-Beckers
141
Turbine blades protection (dry
oxidation)
• Used alloys: high Cr addition, in solid solution in Ni
matrix
• Formation of Cr2O3 gives more energy(701 kJ/mole O2)
than NiO (439 kJ/mole O2)  preferential formation of
Cr2O3 on the surface
• the higher the Cr, the higher the tendency to form Cr2O3
(higher chromium activity)
• With 20 wt.% Cr, enough Cr2O3 to exhibit a similar
behaviour as pure Cr
J. Lecomte-Beckers
142
Turbine blades protection (dry
oxidation)
• Chromium : loss of 0.1 mm in 1600 h at 0.7 Tf (1213°C)
 at 935°C, loss of 0.1 mm in 1,04.106 h
• BUT only 20 wt.% Cr  partial protection with Cr2O3
• Experience shows 20 wt.% Cr only increases 166 times
thickness loss  0.1 mm in 6000 h and not 106 h
• BUT better than pure Ni (10x)
J. Lecomte-Beckers
143
Time (in h) to
oxidise
materials of
0.1 mm at 0.7
Tm in air
Material Time
Tm (K)
Material
Time
Tm (K)
Au
Ag
Al
Si3N4
SiC
Sn
Si
Be
Pt
Mg
Zn
Cr
Na
K
1336
1234
933
2173
3110
505
1683
1557
2042
923
692
2148
371
337
Ni
Cu
Fe
Co
Ti
600
25
24
7
<6
<5
<<0.5
0.2
Very short
Very short
Very short
Very short
Very short
1726
1356
1809
1765
1943
1700
983
2125
3250
2740
1405
2880
3680
J. Lecomte-Beckers
Infinite
Very long
Very long
Very long
Very long
Very long
2 E6
E6
1.8 E5
E5
E4
1600
>1000
>1000
WC based cermet
Ba
Zr
Ta
Nb
U
Mo
W
144
Turbine blades protection (dry
oxidation)
• Performance is not satisfactory at the moment
• Further to increase creep strength wt.% chromium is
lowered to 10%
 lower protection oxide film
 obvious solution: coat turbine blades with another
material
J. Lecomte-Beckers
145
Turbine blades protection (dry
oxidation)
• Coating with pulverisation of melted Al droplets 
formation of a continuous Al layer (some µm)
• Then blade is heated to make aluminium diffusion in nickel
substrate  formation of some species such as AlNi, good
corrosion barrier and Al2O3
• Asset of AlNi : low thermal conductivity  thermoinsulation of blade metal and so service temperature can be
increased (more power, less fuel consumption)
J. Lecomte-Beckers
146
Turbine blades protection (dry
oxidation)
After
pulverisation
Ni alloy
After diffusion
anneal
Ni alloy
Protection des aubes de turbine par pulvérisation d’Al
J. Lecomte-Beckers
147
Turbine blades protection (dry
oxidation)
• BUT AlNi is brittle risk of chipping protection
• Other coatings are more difficult to obtain but exhibit
improved properties
• Example : weld with diffusion of a Ni-Cr-Al alloy on blade
surface (pulverisation then heating)  ductile coating =
very protective oxide film
J. Lecomte-Beckers
148
Turbine blades protection (dry
oxidation)
• Drawbacks of oxide films:
– Oxides are very brittle with high T°  rupture hazard because of
T° variation on blade and effect of thermal stress (differential
thermal coefficient of expansion for Ni alloy and oxide coating)
– Microcracks = initiating area for thermal fatigue cracks. However
good adhesion between coating and substrate  risk of
propagation of cracks in the Ni alloy
 Importance of oxide film properties
for corrosion resistance
J. Lecomte-Beckers
149
Protection of pipelines (aqueous
corrosion)
• Pipelines in the underground for transportation of oil,
natural gas…  corrosion is a problem with wet ground and
low depth (presence of oxygen)
• Reduction of water (O2): O2  2H 2O  4e  4OH
• Corrosion reaction:

Fe  Fe   2e
 corrosion of pipes
J. Lecomte-Beckers
150
Protection of pipelines (aqueous
corrosion)
• Protection : coat pipelines with an inert material to insulate it from
water and air
• Example : thick foil of PE fixed with butylic sticking agent
• Ends of pipes are coated on site after welding
• Low protection risk of brutal handling and rupture of protective
films  metal could be attacked
J. Lecomte-Beckers
151
Protection of pipelines (aqueous
corrosion)
• If pipe is linked to a metal plate with higher reduction
potential, an electrolytic cell is made
 more electronegative material = cathode and is
protected
Sacrificial protection of
pipelines (Mg=anode)
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152
Protection of pipelines (aqueous
corrosion)
• Used for pipelines
• Most frequent alloys: Mg (very low oxidising
potential), Al and Zn
• Protection  dissolution of anodes (spare parts, to be
replaced)
 to minimise anodic weight loss, important to coat
the efficiently as possible the pipelines
J. Lecomte-Beckers
153
Car exhaust systems (wet corrosion)
• Lifetime (common car) : 2 years
• Used material: low C steel
 bad corrosion protection
• If internal part of tube is not painted spontaneous
corrosion in contact with flue gas (contain H2O)
• On external face paint layer is only decorative with low
adhésion  rust, worsen by chloride ions on the road
J. Lecomte-Beckers
154
Car exhaust systems (wet corrosion)
• 1st solution: galvanise steel
BUT problems if coated metal has to be welded (Zn is melted at
420 °C  no coating around welding area)
Same problems with higher melting points coatings
• Sometimes chrome plated exhaust system but only
aesthetic:
If plated before welding, joints are not protected  oxidation
If plated after welding, inside part is not protected 
corrosion
J. Lecomte-Beckers
155
Car exhaust systems (wet corrosion)
• Best method for corrosion protection of car exhaust
systems: use of stainless steel
 addition of alloying elements promoting stable oxide
films  good corrosion barrier
J. Lecomte-Beckers
156
Content
• Introduction
• Elasticity modulus
• Yield and ultimate strength
• Rupture, fracture toughness and fatigue
• Creep
• Oxidation and corrosion
• Friction, abrasion and wear
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157
Friction, abrasion, usure:
• Slipping of a material on another with the action of a
friction force on the opposite direction
Stopped: µs
Moving: µk
Static and dynamic friction coefficients
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158
Friction, abrasion, usure:
• Force Fs = force with just enough intensity to initiate
motion
Fs = µsP
With P = normal force to contact surface
and µs = static friction coefficient
• When slipping is started, friction force is decreasing
Fk = µkP
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Friction, abrasion, usure:
• Même si surfaces protégées par film d’oxyde ou de
lubrifiant, présence contacts solides là où rupture de la
couche suite aux contraintes mécaniques ou là où
absorption lubrifiant médiocre
 contact intime  usure
• 2 types d’usure : adhésive et abrasive
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Adhesive wear
• Good adhesion between A and B atoms  wear
fragments are expelled from the softest material
• If A and B are the same, wear on both sides
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Abrasive wear
• Detachment of particles from surface roughness induced by slipping
of 2 counter faces
• However in the case of oxygen presence (oxide layer)  oxidation of
detached particles abrasive!
Abrasive wear of A material by harder B material
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Smooth bushing design
• Friction and wear properties have no influence because
front surfaces are separated with a thin film of
pressurized oil
Hydrodynamic lubrication
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Smooth bushing design
• Load on pivot is pushing shaft to one side of the bushing
 localized backlash on the opposite side
• Oil viscosity  rotating shaft induces oil motion
• Convergence of oil flow to bringing together area of the
two surfaces 
pressure on oil film (from 10 to 100
atm)  shaft is lifted because of this pressure
• If oil viscosity is high enough, film is thicker and contact
surfaces are totally separated (no dry friction)
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Smooth bushing design
• In ideal hydrodynamic conditions, no contact and no
wear
• Further surface are slipping over an oil layer
• Friction coefficient with hydrodynamic lubrication is
comprised between 0.001 to 0.005 (very low)
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Smooth bushing design
• Hydrodynamic lubrication is perfect in optimal conditions
• BUT presence of stain (very hard silica) et and pig iron
dust (machining residues)
 If particles are coarser than oil film, abrasive wear
takes place
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Smooth bushing design
• 2 solutions :
– Make contact surface harder than extrinsic particles
Cemented bushing with heat and chemical treatments: high
surface hardness
– Pillows: metal soft enough to fix particles (encrusting ability)
• Plastic containment is used when soft alloys is coated on
pillow (layer is thick enough to fix particles but not too much to
support the pivot forces)
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Smooth bushing design
• Important role of soft material of the pillow in case of
inappropriate lubrication
• Lubrication failure  friction heating  increasing T° 
emphasized metal-metal contact
• Pillow surface: soft material with low melting point. It
support shear and is locally melted  pivot protection
and limit parts failure
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Smooth bushing design
• Other advantage of soft material for the pillow:
conformability
• Absorbtion of aligning defects of bushings with plastic
deformation of pillows metal
• BUT compromise between ability to face stress and to be
conformable
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High friction coefficient rubber
• For lots of applications: need of a maximum of friction:
tyres, bands…
• Friction behaviour of rubber is different from metals
• Elastic domain of rubber  high strain level
• If rubber is in contact with a surface: Elastic strain around
contact points
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High friction coefficient rubber
• Elastic forces are collapsing polymer segments on each
other along contact area  adhesion on this surface and
shear is occuring in case of slipping
 Good dry adhesion of tyres
 In wet conditions, other friction system: appear of a
water (or mud) lubricating film between rubber and road 
shear with lower stress and dangerous consequences
• Another phenomenon can be helpful to avoid slipping
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High friction coefficient rubber
• Road surface is pretty rough  asperities are inserted in
tyre  high elastic strain level
• If tyre is slipping, it slides forward with asperities
 deformed area is relaxing and another is compressed
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High friction coefficient rubber
• Anelasticity of rubbers
Compression : storage of energy
(upper curve on diagramm)
When stress is relaxing, energy is not
completely restored and is partially
heat dissipated (hatched area)
Elastic cycle of rubber
(hysteresis)
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High friction coefficient rubber
 tyre slipping on a rough road dissipates energy even with good
lubrication
• Development of special rubber with high dissipation called « high
hysteresis rubber » (excellent adhesion even with wet conditions) in
order to limit slipping
• Bad point: in normal operating conditions, important elastic strain of
tyre sides with significant heating
• Solution: low dissipation tyre with high dissipation rubber on contact
surface (tread): composite structure
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High friction coefficient rubber
Anti-slip tyres, with high dissipation rubber tread and low
dissipation rubber sides
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