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SN Curve

You define SN curves for use in a fatigue study. An SN curve for a material defines alternating stress values versus the number of cycles required to cause failure at a given stress ratio. A typical S-N curve is shown in the figure. The Y- axis represents the alternating stress (S) and the X-axis represents the number of cycles (N). An SN curve is based on a stress ratio or mean stress. You can define multiple SN curves with different stress ratios for a material. The software uses linear interpolation to extract data when you define multiple SN curves for a material.

SN curves are based on mean fatigue life or a given probability of failure. Generating an SN curve for a material requires many tests to statistically vary the alternating stress, mean stress (or stress ratio), and count the number of cycles.

Tests to generate SN curves are performed under a controlled loading environment. Uniaxial loading is commonly used. Since the actual loading environment is generally multi-axial, a correction may be required. The software provides the Fatigue strength reduction factor in the Fatigue properties dialog to account for this discrepancy.

When one SN curve with stress ratio -1 (fully reversible or zero mean) is used to define the fatigue properties of a material, you can select a correction method to account for the effects of non-zero mean stresses.

Usually the base 10 logarithm of N is used instead of N due to the typical large range of N values.

Defining fatigue SN curves

SN curves are defined in the fatigue study, after you define a fatigue event. You can define SN curves:

  1. In the Material dialog, on the Fatigue SN Curves tab, or

  2. Right-clicking the top document icon and selecting Define Function Curves.

To create a new SN curve:

  1. In a Fatigue study, add a fatigue event.

  2. In the Material dialog, on the Fatigue SN Curves tab, in the Source box do the following:

    1. For Interpolate, set the scheme for interpolating alternating stresses against the number of cycles of S-N curve.

    1. Select Define and select a curve from the ten curves that appear in the menu.

  3. In the Table data box, do the following:

    1. In the Stress ratio (R) box, enter the stress ratio associated with the curve.

    2. Set the unit of stress.

    3. Populate the curve data by entering Alternating Stress values versus number of Cycles, or click File to import a Curve Data Points File (*.dat).

Note: To add a new row, double-click in cell under the Points column.

The format of the Curve Data Points File (*.dat) should have two columns: number of Cycles (first column) and Alternating Stress (second column).

    1. Click View to graph the data or Save to save the curve to a library.

Preview box shows the curve as it is defined.

    1. To delete a row, highlight it and click the Delete key or right-click it and select Delete.

    2. Type a source of reference for the SN curve data in Source.

    1. Click Apply.

To define an SN curve for a material:

Some materials in the material library have SN curves defined. These material are identified by a (SN) attached to the end of their names in the list box. Materials with stress-strain curves are identified by (SS), and materials with both SN curves and stress-strain curves are identified by (SS,SN).

  1. In the Material dialog, on the Fatigue SN Curves tab, in the Source box do the following:

    1. Set the scheme for interpolating alternating stresses against the number of cycles of SN curve for Interpolate.

    2. Select Derive from material Elastic Modulus and select Based on ASME Austentic Steel curves or Based on ASME Carbon Steel curves.

  2. Click Apply.

Related Topics

Fatigue Events

Performing Fatigue Analysis

Theory of Accumulated Damage

Fatigue Plots

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