fatigue limit

  • tags: #Engineering Plastics
  • Goodman haigh diagram for infinite life
  • Strain life approach For calculating fatigue life in the plastic region of SN curve.
  • SN curve #MIT machine prep
    • SN Curve Seimens
    • An SN-Curve is a plot of the magnitude of alternating stress versus the number of cycles to failure for a given material. Typically both the stress and number of cycles are displayed on logarithmic scales.
    • Given a load time history and a SN-Curve, one can use Miner’s Rule to determine the accumulated damage or fatigue life of a mechanical part.
    • developed by the German scientist August Wöhler (Figure 1) during the resulting investigation of an 1842 train crash in Versailles, France. In this crash, the axle of the train locomotive failed under the repeated “low level” cyclic stress of everyday usage on the railroad.
    • Wöhler discovered that cracks formed and slowly grew on an axle surface. The cracks, after reaching a critical size, would suddenly propagate and the axle would fail. The level of these loads was less than the ultimate strength and/or yield strength of the material used to manufacture the axle.
    • Wöhler developed an apparatus to apply repeated loads to railroad axles and chart the relationship between load level and number of repeated cycles to failure. “Wöhler Curves” plot the relationship of alternating/cyclic stress levels against the number of cycles to failure.
    • the frequency at which the cycles are applied is not considered to be a factor in the number of cycles to failure. It is strictly the number of cycles, and not the rate at which the cycles are applied, that affect the SN-Curve results.
    • In real life, the frequency of the cycles can be a factor, especially if the loading frequency coincides with a natural frequency or resonance of the object which amplifies the magnitude of the cycles.
  • fatigue limit

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