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


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


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


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


∆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


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


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


Fatigue of fibre polymer composites


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


S–N curves for carbon–epoxy composites

with different fibre patterns


S–N curves for carbon–epoxy composite under

different cyclic loading conditions


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


uck 353e aerospace materials week10 2015

Documents

fatigue properties

fatigue stress ratio

desirable fatigue

maximum fatigue stress

mean fatigue stress

fatigue stress profiles

material corrosion fatigue

fatigue crack initiation