UCK 353E Aerospace Materials Week10 2015
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Transcript of UCK 353E Aerospace Materials Week10 2015
Aerospace Materials
Week-10
Fatigue of Aerospace Materials
Fatigue
• Deterioration to the structural properties of a material owing to
damage caused by cyclic or fluctuating stresses
• Damage and loss in strength caused by cyclic stresses that are
below the yield strength
• Material does not show a visible sign of damage before it fails
• Fatigue is the most common cause of damage to aircraft
structures and engine components. It is estimated that fatigue
causes over one-half of all metal component failures, and is
responsible for more damage than the combined effects of
corrosion, creep, wear, overloading and all the other failure
sources on aircraft
• Fatigue resistance is the ability of structural materials to
maintain an acceptable level of strength under fluctuating stress
conditions
10.12.2015 UCK 353E-Aersopace Materials-Week9 2
Fatigue Types Cyclic stress fatigue: repeated application of loads to the material
Corrosion fatigue: combined effects of corrosion and cyclic stress loading, affects metallic materials
Fretting fatigue: progressive deterioration of materials by small scale rubbing movements that cause abrasion of mating components
Acoustic fatigue: high frequency fluctuations in stress caused by noise. The pressure waves of the noise impinge on the material thus inducing fatigue effects
Thermal fatigue: fluctuating stresses induced by the thermal expansion and contraction of materials owing to thermal cycling
10.12.2015 UCK 353E-Aersopace Materials-Week9 3
Fatigue stress cyclic loading
Important parameters that
can affect the fatigue
properties:
• Maximum fatigue stress, 𝜎𝑚𝑎𝑥
• Mean fatigue stress,
𝜎𝑚 = (𝜎𝑚𝑎𝑥 + 𝜎𝑚𝑖𝑛) 2
• Fatigue stress ratio,
𝑅 = 𝜎𝑚𝑖𝑛 𝜎𝑚𝑎𝑥
• Stress frequency f, the
number of load cycles per
second
10.12.2015 UCK 353E-Aersopace Materials-Week9 4
Fatigue stress profiles for (a) fully reversed
and (b) repeated stress cycling
Fatigue life (S-N) curves • Curve A: Materials fail
in fatigue with a
sufficient number of
load cycles.
Undesirable because
failure may eventually
occur at low fatigue
stress levels
• Curve B: below the
endurance limit, the
material can endure
an infinite number of
load cycles. Desirable
fatigue because an
infinite life is assured
10.12.2015 UCK 353E-Aersopace Materials-Week9 5
An S–N curve is only valid for a specific set of fatigue
conditions (e.g. R ratio, load frequency, temperature),
and the graph may be different when the conditions are
changed
Fatigue crack growth curves
∆𝑲 = 𝝈𝒎𝒂𝒙 − 𝝈𝒎𝒊𝒏 𝒀 𝝅𝒂
10.12.2015 UCK 353E-Aersopace Materials-Week9 6
∆𝐾, variation in the fatigue stress
𝑎, crack length
𝑌, correction factor
𝒅𝒂
𝒅𝑵= 𝑪∆𝑲𝒎, distance that the
crack propagates in one load
cycle N
𝐶, material constant
𝑚, slope of regime B
Stress Approach
Strain Approach (plastic deformation)
∆𝜺𝒑 = 𝟐𝜺𝒇(𝟐𝑵)𝒄 𝜀𝑝, change in the plastic strain
𝜀𝑓, static failure strain
𝑐, fatigue ductility coefficient (-0.5 to -0.7)
Fatigue of metals
Fatigue life stages of metals:
i) Fatigue crack initiation,
ii) Crack growth under cyclic loading
iii) Final failure
10.12.2015 UCK 353E-Aersopace Materials-Week9 7
Fatigue of metals • Fatigue cracks initiate at pre-
existing defects, such as
voids, large inclusions or
surface flaws that act as stress
raisers
• Under cyclic loading, the metal
near the pre-existing defect is
plastically deformed owing to
the stress concentration and,
eventually, a small crack is
initiated
• Defects that concentrate high
levels of stress, such as
scratches or sudden changes
in section thickness of the
component can reduce the
number of load cycles to
initiate a fatigue crack by
many orders of magnitude 10.12.2015 UCK 353E-Aersopace Materials-Week9 8
Surface analysis of fatigued metals
• Each ripple is a fatigue fracture striation
showing the distance the crack has
advanced in one load cycle.
• The ripples usually radiate outwards
from a single point which is the site of
crack initiation
• Fractured region, the load capacity of
the fatigued metal is reduced to the
maximum fatigue load, then sudden
fracture occurs through the remaining
uncracked region. Ductile tearing has
occurred during the rapid growth of the
crack
10.12.2015 UCK 353E-Aersopace Materials-Week9 9
Fracture surface of fatigued metal: (a) low magnification
view showing fracture surface; (b) ripples caused by
fatigue crack growth
Fatigue of fibre polymer composites
• Fatigue of composites is characterised by a multiplicity of
damage types, which includes cracks in the polymer matrix,
debond cracks between the fibres and matrix, splitting cracks,
delamination cracks, and broken fibres.
• The damage types initiate at different times and grow at
different rates over the fatigue life of the composite material
• The fatigue strains in the high-stress regime approach the
failure strain of the fibres
• Fatigue endurance limit of the composite is determined by the
fatigue limit of the polymer matrix
• Despite the many types of damage, continuous fibre–polymer
composites, such as carbon–epoxy, often exhibit a fatigue life
which is much longer and a fatigue endurance limit which is
higher than aerospace-grade aluminium alloys
10.12.2015 UCK 353E-Aersopace Materials-Week9 10
Fatigue of fibre polymer composites
10.12.2015 UCK 353E-Aersopace Materials-Week9 11
Fatigue life of composites
• The fibre type, fibre volume percent, fibre lay-up pattern, and
matrix properties all influence the fatigue life
• The fatigue resistance of composites generally improves with
their elastic modulus and strength; with materials containing
high stiffness, high-strength carbon fibres having excellent
fatigue resistance
• The fatigue life decreases with a reduction in the percentage of
load-bearing fibres (which in this case are 0° fibres) in the
composite
• Fatigue damage in composites can occur under both tension
and compression loads
• The fatigue life of composites is often reduced when the load
frequency is above about 20 Hz
10.12.2015 UCK 353E-Aersopace Materials-Week9 12
S–N curves for carbon–epoxy composites
with different fibre patterns
10.12.2015 UCK 353E-Aersopace Materials-Week9 13
S–N curves for carbon–epoxy composite under
different cyclic loading conditions
10.12.2015 UCK 353E-Aersopace Materials-Week9 14
Improving the fatigue properties
• Structure have to be free from stress concentrations such as sharp corners and sudden changes in section thickness
• Material should be made thicker around fasteners holes and other cut-outs
• Material must be free from surface scratches, machine marks and other stress raisers
• Metal must be cast, processed and heat treated using processes that avoid the formation of microstructural defects such as voids and large inclusions
• Fine-grained metals generally possess a longer fatigue life than coarse-grained materials
• Surface protective coatings to resist corrosion, erosion
• Increasing stiffness and strength of composites – Maximising the volume pertengate of load bearing fibres
– Using high stiffnes high strength fibres
10.12.2015 UCK 353E-Aersopace Materials-Week9 15