User-defined S-N curve

A user-defined S-N curve is a stress-based life criterion where you specify a table that relates maximum principal stress amplitudes, , to a corresponding number of reversals to failure, 2Nf. For isotropic materials, you define only one S-N curve. For orthotropic materials, you define 3 S-N curves: curve one for stress amplitude in direction 1, curve two for stress amplitude in direction 2, and curve three for shear stress amplitude.

No mean stress effects

When you do not include mean stress effects in the damage calculation, the maximum principal stress amplitude, , is read from the user-defined S-N curve table for a given number of reversals to failure, 2Nf. You specify the user-defined S-N curve for the material. The specified S-N curve can be written as follows:

Mean stress effects

When you include the mean stress effect in the damage calculation, the software uses one of the following methods to update the user-specified S-N curve for isotropic materials.

• Goodman method

• Soderberg method

• Gerber method

• Morrow method

where:

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

• Su is the ultimate tensile strength material property.

• Sy is the yield strength material property.

• σ'f is the fatigue strength coefficient.σ'f is computed by curve fitting the following equation in which b is the fatigue strength exponent.

See Updating stress-based life equations for more information on the stress life equations modified for the biaxial damage propagation.

Stress amplitude range limit

Make the last point the endurance limit and make sure that it is smaller than 1020 cycles. By default, the lowest stress amplitude data point on the user-specified S-N curve is set to the endurance limit. If the endurance limit is greater than 1020 cycles, then the S-N curve flatlines at 1020.

Stress values above the highest stress data point on the user-defined S-N curve are truncated to the highest stress data point.

How do I

Define an S-N curve for isotropic materials

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Static events

Transient events

Durability damage evaluation

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User-defined E-N curve

Orthotropic material fatigue analysis

Plate thickness correction

Von Mises effective amplitude approach

Material properties for durability analysis

Understanding cyclic stress-strain behavior

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User-defined S-N curve, Simcenter 3D 2021.1 Series

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