Fatigue: FEA (fatigue analysis at every node) 3

[KnacKeN]

thread727-156008

forgive my english, regarding using principal streses for fatigue analysis.

If I have two FE cases, and suppose FEA based fatigue analysis will be caried out by cycling between these two cases.

How would principal stresses be used? Does it matter that the principal vectors may be in different directions between the two cases?

 

[rb1957]

well actually he now has several (re post on 28 Jun 11:56).

also, do you Really think he'd run a zero loadcase through the FEM.

 

[kellnerp]

There isn't a post in this thread on 28 Jun 11:56. It is the next day at 10:34.

If he was calculating in his post processor he would need a zero load case if that was important.

You simply can't assume that what he is analyzing would have one load go to zero before applying the next. It is not uncommon in the real world for loads to not pass through zero and on to the next load case. Consider for example the loads on the spokes in a bicycle wheel. They smoothly change during rotation. The same is true for loads on a bridge. They smoothly change as the vehicle passes over the bridge.



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[rb1957]

ok it was on the 29th (not 28th).

 

[johnhors]

Surely it is up to the OP to decide whether a zero case is inserted between load cases or not, to realistically model the correct sequence of events. Any spectrum presented to a rainflow routine must of course incorporate zero loads where relevant, otherwise the pairings will most likely be incorrect. This should not be an issue.

maybe you load up 2 whilst maintaining 1, then once load 2 is fully applied you relax 1; in which case the cycle would be 1-2-1.

If load case 1 and load case 2 are applied together as you suggest here as a possibility, then that is a third load case (case 1 + case 2) which should be incorporated into the spectrum.

 

[kellnerp]

Yup, you can't make assumptions unless the OP states it.

A zero crossing is not the same as a zero load to an RF count.

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