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Fatigue Failure

Fatigue Failure Fatigue failure of metal components is by far the  most common type of the failure of aircraft components. Fatigue Failure is defined as the progressive and localized structural damage that occurs when a material is subjected to repeated or fluctuating loads (cyclic loading). The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.

Factors affecting Fatigue Life

  • Number of cycles: Pressurization & flight cycles.
  • Geometry: Notches and variation in cross section throughout a part lead to stress concentrations where fatigue cracks initiate.
  • Surface quality. Surface roughness cause microscopic stress concentrations that lower the fatigue strength. Compressive residual stresses can be introduced in the surface by shot peening to increase fatigue life.
  • Material Type: Fatigue life,  varies widely for different materials,
  • Residual stresses: Machining cutting, casting, and other manufacturing processes involving heat or deformation can produce high levels of tensile residual stress, which decreases the fatigue strength.
  • Size and distribution of internal defects: Casting defects such as gas porosity, non-metallic inclusions and shrinkage voids can significantly reduce fatigue strength.
  • Direction of loading: Grain orientation of the metal
  • Grain size: For most metals, smaller grains yield longer fatigue lives, however, the presence of surface defects or scratches will have a greater influence than in a coarse grained alloy.
  • Environment: Environmental conditions can cause erosion, corrosion, or hydrogen embrittlement  may all affect fatigue life. Corrosion fatigue is a problem encountered in many aggressive environments.
  • Temperature: Higher temperatures generally decrease fatigue strength.

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