Fatigue - Options

The Options tab of the Fatigue dialog box allows you to set options for the active fatigue study.

You can define studies with constant amplitude events or variable amplitude events. You cannot mix constant and variable amplitude events in a study.

• Constant amplitude event interaction. Sets the interaction between constant amplitude fatigue events.

• • Random interaction. The software considers the possibility of mixing peak stresses from different events for the evaluation of alternating stresses. This option is meaningful only if you define more than one fatigue events.

The ASME Boiler and Pressure Vessel recommends this option. It  is more conservative (predicts higher damage) than the No interaction option.

For studies with fatigue events referencing to multiple studies, the program calculates peak stresses based on the Random option even if you select the No interaction option.

• Variable amplitude event options. Sets the options for variable amplitude fatigue events.

• • No. of Bins for rainflow counting. Sets the number of bins for the decomposition of the variable amplitude record. For example, if you enter 32, the program discretizes the loads into 32 equally spaced ranges. The load within each range is constant.

  • Filter load cycles below ... % of maximum range. The program filters out load cycles with ranges smaller than the specified percentage of the maximum range. For example, if you specify 3%, the program ignores cycles with load ranges less than 3% of the maximum range of the load history. Use this parameter to filter out noise from measuring devices.

Using a large number distorts the variable amplitude record and results in underestimating the damage. For accurate damage prediction, the highest alternating stress filtered out should not be larger than the equivalent endurance limit of  any associated S-N curve.

• Computing alternating stress using. Sets the stress type for calculating the equivalent alternating stresses to be used for extracting the number of cycles from the S-N curve(s).

• • Stress intensity (P1-P3)

• Shell Face. Sets the shell face for which fatigue analysis is to be performed.

• • Top. Performs fatigue analysis for the top shell faces.

  • Bottom. Performs fatigue analysis for the bottom shell faces.

• Mean stress correction. Sets the method for the mean stress correction.

• • None. No correction.

  • Goodman method- generally suitable for brittle materials.

  • Gerber method - generally suitable for ductile materials.

  • Soderberg method - generally the most conservative.

These methods are used only if all associated S-N curves are based on fully reversible environment (zero mean). The software calculates the mean stress in addition to the alternating stress for each cycle and then it evaluates the corrected stress using the specified method. The correction becomes significant if the applied fatigue load cycles have large mean stresses compared to the stress range. If you define multiple S-N curves with different load ratios, the program interpolates between the curves linearly to account for mean stresses and no correction method is used. If you define one S-N curve with a stress ratio other than -1 for a material, the curve is used without correction for that material.

• Fatigue strength reduction factor (Kf). Use this factor, between 0 and 1, to account for differences in test environment used to generate the S-N curve and the actual loading environment. The program divides the alternating stress by this factor before reading the corresponding number of cycles from the S-N curve. This is equivalent to reducing the number of cycles that cause failure at a certain alternating stress. Fatigue handbooks suggest numeric values for the fatigue strength reduction factor.

• Infinite life. The number of cycles to be used when the corrected alternating stress is less than the endurance limit. This number is used instead of the number of cycles associated with the last point of an S-N curve.

This value is only used for S-N curves for which the maximum number of cycles is less than the specified number.

• Results folder. Sets the folder for the results of the fatigue study.

Related Topics

Remark Tab

What is Fatigue Analysis...