Random Interaction between Multiple Fatigue Events

The Simulation application mixes peak stresses from different events to evaluate alternating stresses.

Suppose you specified three fatigue events that reference studies A, B, and C using scale factors F_a, F_b, and F_c. The software calculates the alternating stresses and damage factors as follows:

• For the three reference studies, the software computes the state of stress at each node scaled by the associated factors: S_a * F_a, S_b* F_b, S_c*F_c

  , where S_a, S_b, and S_c refer to the stress component values [SX, SY, SZ, TXY, TXZ, TYZ] at a node for studies A, B, and C, respectively.

• For all possible pairs of load combinations, the software subtracts the six stress components of one load case from the corresponding components of the second load case to yield the component stress range. For n number of loadings, the possible load combinations are (n * (n -1) / 2).
• For each component stress range, the software calculates the alternating stress as one-half of the selected quantity (stress intensity, von Mises, or maximum absolute principal). The software creates a list of the alternating stresses with the corresponding loading pairs for all possible combinations in decreasing order.
• Based on the selected fatigue curve (S-N curve), the software checks the alternating stress list from the highest value to the lowest value.

  The software evaluates the partial damage factor Di for the ith alternating stress in that list by evaluating the cycle ratio ni / Ni (ni is the lower number of cycles remaining from either events; Ni is the allowable number of cycles interpolated from the design fatigue S-N curve.

• After evaluating the partial damage factor, Di, the software updates the alternating stress list by reducing the number of cycles of both events by ni. Consequently, one of the two events will be eliminated (or both if they have the same number of cycles) and the other event will have ni cycles less in the later calculations. Elimination of an event results in elimination of the corresponding loading. Once a loading is eliminated, the corresponding alternating stress (formed by combining that loading with other loadings) is also eliminated from the list.
• After updating the list, the software checks the next alternating stress in the list and evaluates the corresponding partial damage factor, adds that to the cumulative one, and updates the list. This procedure is repeated for the next alternating stress in the list and continues until all stress values are considered.