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Low Cycle Fatigue - Cooler Campaign - June 2021


Low Cycle Fatigue

Low Cycle Fatigue (LCF) Testing is typically performed in the plastic region (i.e. at or beyond yielding) of a material that experiences cyclic load or strain and/or temperature fluctuations in service. LCF is conducted primarily for the aerospace, oil and gas, and automotive industries on carbon steels, stainless steels, nickel and aluminum alloys. One of the most common applications to be evaluated by LCF testing is rotating components in aircraft engines and pumps.

Test specimen preparation is critical for fatigue testing. When machining fatigue test specimens, any operations performed on the specimen should minimize the chances of imparting residual stresses or undesirable machining marks into the specimen surface. Prior to testing smooth fatigue specimens, each specimen is low stress ground and then longitudinally polished to remove all circumferential machining marks. A transverse flaw, such as a machining mark or a scratch, can cause a premature failure in the material.

Testing is conducted in an axial test frame with the capability of applying both tensile and compressive loads or strains. Tests are usually cycled using a triangular wave form pattern and with a frequency of between 0.5 Hz and 2 Hz. The amplitude and loading ratio are prescribed by the customer. Most cases involve creating E-N Curve, which relates applied cyclic strain to number of cycles. This will give design engineers the fatigue coefficients they need for a particular application. The standard information obtained from the analysis of an LCF test is as follows:

  • Total Cycles to Failure (Nf)

  • Cycles to Crack Initiation (Ni)

  • First and Half Life Analysis

  • Loading Modulus

  • Max Stress

  • Min Stress

  • Total Strain Range

  • Plastic Strain Range

Most Common Testing Specifications:

  • ASTM E606/E606M - Standard Test Method for Strain-Controlled Fatigue Testing

  • ISO 12106:2017 - Metallic Materials - Fatigue Testing - Axial Strain Controlled Method

Entry Originally Posted June 2021

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