Standard For Combining Static & Random Loads - (FEMAP 2306)

[ATNY4D]

Hello All,

I'm looking for assistance in appropriately combining static & random loads or results in FEMAP. I have attempted this using the steps below, however, I'm unsure of the von Mises results of the combination since the RMS component stresses are always positive. Is this a valid concern? Why or why not?
[ol 1]
[li]Perform Static Analysis (SOL 101) using static load and interface constraint[/li]
[li]Perform Modal Analysis (SOL 103) with interface constraint to get solution frequency table[/li]
[li]Perform Random Response Analysis (SOL 111) using random load and PSD driver constraint[/li]
[li]Combine results using Model→Output→Process→Linear Combination (3*Random + Static)[/li]
[/ol]

If this is not an acceptable approach, what is the industry standard or rule of thumb? Please provide references if you have any. If this is an acceptable approach, can this combination von Mises be used for margin of safety calculations as is?

Thanks ahead!

 

[SWComposites]

It is probably way conservative to combine all of those load conditions together. But it highly depends on the specific component and loading environment. This is not an FEA issue, this is a basic analysis issue. How would you approach the analysis if doing it by hand?

It is wrong to simply add Von Mises stresses for multiple load cases. The correct approach is to combine the stress tensors at a given location, and then calculate a Von Mises stress from the combined stress state. However, Von Mises stress is s a yield failure criteria; it is not appropriate for predicting failure in the plastic region. And it is only valid for isotropic materials that have a yield behavior.