Material fatigue occurs when structures fail due to cyclic loading, even when the stress is below the material's strength. Fatigue is a common cause of failure in aircraft structures due to various cyclic loads experienced during operations. Typical fatigue issues addressed include cyclic, corrosion, fretting, thermal, and sonic fatigue. Aircraft structures are tested for fatigue using visual, dye penetration, magnetic particle, radiographic, ultrasonic, and tap testing to detect cracks or flaws from fatigue damage.

1. Fatigue in Aero
Structures
BY: SHOAIB FARID
REG NO: 200512006
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2. Contents
 Introduction
 Aircraft Materials
 Types of Loading on Aircraft Structures
 Causes of Fatigue Failures
 Typical Fatigue in Aero structures
 Testing of structures under fatigue loading
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3. Introduction
 Material fatigue is a phenomenon where
structures fail when subjected to a cyclic load.
This type of structural damage occurs even
when the experienced stress range is far below
the materials strength. Fatigue is the most
common source behind failures of Aero
structures
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4. Introduction
 Some cyclic loads on aircrafts
• Push back (on landing gear)
• Turning (high stresses on landing gear)
• Taxy
• Take off
• Cabin pressurization
• Landing impact
• Undercarriage loading
• Turbulence
• Gust and maneuvers
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5. Introduction
 Often, machine members are found to have failed under the action of
repeated or fluctuating stresses
 The most distinguishing characteristic of these failures is that the stresses
have been repeated a very large number of times. Hence the failure is
called a fatigue failure.
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6. Introduction 6

7. Introduction 7

8. Aircraft Materials
 Steel
 Titanium
 Aluminum
 Laminate Composites
 Sandwich honeycomb Structures
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9. Aircraft Materials 9

10. Types of Loading on Aero Structure 10

11. Causes of Fatigue Failure
 Pre Manufacturing
a) Oversize hole
b) Porosities
c) Incorrect fastener pitch
d) Alternative fastener type
e) Part out of tolerance
f) False cut
g) Deep countersunk
h) Alternative material used
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12. Causes of Fatigue Failure
 Post manufacturing
a) Corrosion damages
b) Cracks , Dents, Nicks & Scratches
c) Temperature Variations
d) Sonic or Acoustic effects
e) Debonding between skin and core (sandwich)
f) Free edge damages (Delamination's, notches etc.)
g) Impact damages
h) Fretting
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13. Typical Fatigue in Aero structures
 Cyclic Fatigue
Stress, strain and deformation
Induced in a material by cyclic loading.
Aircrafts are in a complex fatigue condition
because the magnitude, frequency and
Duration of some of the loading
changes randomly.
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14. Typical Fatigue in Aero structures
 Corrosion Fatigue
Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical
degradation of a material under the joint action of corrosion and cyclic
loading. It is accelerated by surface corrosion and penetrates inwards.
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15. Typical Fatigue in Aero structures
 Fretting Fatigue
Fretting fatigue is the phenomenon of crack initiation and propagation from
the region of stress concentration in reciprocating frictional contacts.
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16. Typical Fatigue in Aero structures
 Thermal Fatigue
Thermal fatigue is a specific type of fatigue failure mechanism that is induced
by cyclic stresses from repetitive fluctuations in the temperature of
equipment. The degree of damage is affected by the magnitude and
frequency of the temperature swings.
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17. Typical Fatigue in Aero structures
 Sonic Fatigue
Sonic Fatigue is caused by high frequencies noise generally emitted from aero
engines which cause stress fluctuations and vibrations.
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18. Testing of Fatigue loaded structures
 Visual Testing with aids (Borescope)
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19. Testing of Fatigue loaded structures
 Dye Penetration Testing
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20. Testing of Fatigue loaded structures
 Magnetic Particle Testing
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21. Testing of Fatigue loaded structures
 Radiographic Testing
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22. Testing of Fatigue loaded structures
 Ultrasonic Testing
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23. Testing of Fatigue loaded structures
 Tap Testing
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24. Thank You
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