Strain life

The strain life criterion is typically used for low cycle fatigue. The stress level may be higher and the number of cycles to failure may be lower.

The durability solver can calculate the fatigue life using the following strain life approaches NOTE:

• The maximum principal strain amplitude approach

• The maximum shear strain amplitude approach

Maximum principal strain amplitude

The maximum principal strain amplitude life equation uses the modified Morrow equation as follows:

where:

• is the maximum principal strain amplitude.

• σm is the mean stress of the cycle along the principal axis.

• 2Nf is the number of reversals to failure.

• σ'f is the fatigue strength coefficient material property.

• b is the fatigue strength exponent material property.

• ε'f is the fatigue ductility coefficient material property.

• c is the fatigue ductility exponent material property.

If you choose not to include the mean stress effects in the fatigue evaluation, the mean stress σm is zero.

The maximum principal strain amplitude life equation is used when you select Strain Life Maximum Principal from the Fatigue Life Criterion list in the event dialog box.

See Updating strain life (maximum principal) equations for more information on the maximum principal strain life equation modified for the biaxial damage propagation.

Maximum shear strain amplitude

The maximum shear strain amplitude approach is based on the assumption that the notch shearing strain amplitude will correlate life with the shear strain amplitude in uniaxial test specimens. It uses the shear strain amplitudes on the maximum shear plane in the strain life equation as follows:

where:

• is the shear strain amplitude on maximum shear plane.

• νe is the Poisson's ratio.

• νp = 0.5

This equation does not take into account the mean stress effects.

The maximum principal strain amplitude life equation is used when you select Strain Life Maximum Shear from the Fatigue Life Criterion list in the event dialog box.

See Brown-Miller approach for more information on the maximum shear strain life equation modified for the biaxial damage propagation.

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© 2020 Siemens

Dowling, N. E., “Mechanical Behavior of Materials”, Prentice-Hall, 1993, p. 670.

Tipton, S.M., and Fash, J.W., “Multiaxial Fatigue Life Predictions for the SAE Specimen Using Strain-based Approach”, in Multiaxial Fatigue: Analysis & Experiments, SAE AE-14, 1989, pp. 67-80.

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