In the Workbench Fatigue Module at Release 11.0, how can the effect of a load alternating about a static load be evaluated?

The fatigue effect of a combined static and varying load can be modeled using a constant amplitude, non-proportional loading case. The static load creates a mean stress and about which the varying load alternates. You will need to create two environments, and use a load combination to combine the effects from each (load combination = E1 + E2). You can then request a fatigue tool for the load combination.

Within the Fatigue Module, the alternating stress is half of the difference between the two load cases (Salt = 0.5*(E1 - E2)), and the mean stress is the average stress between the two load cases (Smean = 0.5*(E1 + E2)); therefore, you need to set up the environments so that the combination is the varying load alternating about the static load. Setup two environments with the following loadings applied.

E1 = 0.5*varying load + static load
E2 = -0.5*varying load + static load

Those environments will produce the following mean and alternating stresses when the E1 is added to E2 in the load case combination.

Smean = 0.5 * (E1 + E2) = 0.5 * [ (vl/2 + sl) + (-vl/2 + sl) ] = static load
Salt = 0.5 * (E1 - E2) = 0.5 * [ (vl/2 + sl) - (-vl/2 + sl) ] = 0.5*varying load

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