Fatigue


Gottier


 Training Training Fatigue
Analytical Fatigue Methods
How to determine Structural Fatigue Lives
Software for CG analysis of an antenna installation
Case Study
AFGROW Program

Training Fatigue

  • What is fatigue and what does cause fatigue in an aircraft metallic structure?
  • Fatigue is a phenomenon by which structures fail due to the repeated application of loads, which are smaller than the yield strength of the metallic material.
  • The meaning of the yield strength is the gross stress, which is below material yielding.

Analytical Fatigue Methods

  • What are the major parameters, causing fatigue?
  • Fatigue is a local problem, where local stresses due to applied loads exceed the yield strength of the material.
  • The reason is related to stress concentration and / or material imperfections.
  • Stress concentrations are driven by geometrical changes, such as fastener holes, sharp radii at geometrical changes , notches during manufacturing etc.

How to determine Structural Fatigue Lives

  • What are the methods to determine structural fatigue lives?

  • In order to prevent fatigue damage in a metallic structure different design philosophies are used:

    • Safe Life:

    Crack free design philosophy. The fatigue life is considered as the crack initiation life. Safe-life is also called the crack initiation design philosophy.

    Fail-Safe:

    Cracks are allowed, however the remaining structure must be able to sustain the operational loads. Fail-Safe design philosophie may be considered as a multiple load pass philosophie. S-N Curve Methods is used to calculate the fatigue life.

    * approach and crack initiation approach both combined with the crack growth approach.

    Damage Tolerance:

    Damage Tolerance: Design philosophy with existing cracks in new structure. The fatigue life is considered as the crack growth life. Damage tolerance is also called the crack growth design philosophy. Following methods are used to calculate the fatigue life: Crack growth approach supported by crack initiation approach.

    * Remarks: Structural failures are prevented by scheduled inspections, which are derived from crack grows lives of the most critical structural locations.
    * Remarks: Nowadays, this design philosophie is replaced by the damage tolerance philosophie for aircraft structures.

Software for CG analysis of an antenna installation

    Tool to determine cross section parameters
    Bending moment calculation for different type of a/c
    Mini-Twist spectrum development tool
    Filter and Rainflow Counting tool
    Crack Growth tool (AFGROW)

Case Study

  • Based on an example a complete CG-analysis of an antenna installation will be performed step-by-step. Afterwards the inspection intervals and the zones to be inspected will be defined.

    • Remarks: The same procedure can be used for any kind of braek throughts or repairs in the pressurized fuse-lage.

AFGROW Program

  • Description of AFGROW program features, necessary to performe CG-analyses of antenna installations. This will be limited to the intermediate level.




IBS-AeroTec, T +49 89 84057730, info@IBS-AeroTec.de