Single Fatigue Event Based on Multiple Loads

The program calculates the alternating stresses for each node by considering the combination of peaks from different fatigue loads. The fatigue loads can refer to one or more static studies, or to a particular solution step of a nonlinear or time history linear dynamic study.

To set up a fatigue event based on multiple load cases, select Find Cycle Peaks (Add Event (Constant) PropertyManager). You can define up to 40 load cases.

Suppose that you specified fatigue loads using studies A, B, and C with scale factors F_A, F_B, and F_C to define an event. In order to evaluate the alternating stress and the associated stress ratio at a node, the program proceeds as follows:

• It evaluates S_A*F_A, S_B*F_B, and S_C*F_C, where S_A, S_B, and S_C refer to the stress component values at the node for studies A, B, and C, respectively. The program calculates the SX, SY, SZ, TXY, TXZ, and TYZ stress components at each node.
• Evaluates the differences in the stress component values for all possible combinations of stress peaks and determines the combination that produces the largest stress fluctuation.
• Evaluates the alternating stress associated with the event by dividing the stress fluctuation range by 2.
• Calculates the stress ratio based on the calculated stress extremes (S_min and S_max).
• If multiple S-N curves with different stress ratios are defined, the program uses linear interpolation to extract the proper number of cycles for the calculated alternating stress and stress ratio.
• If a single S-N curve with zero mean is defined and a mean stress correction method is selected, the program uses the corrected alternating stress against the S-N curve.