Fatigue: FEA (fatigue analysis at every node)
Fatigue: FEA (fatigue analysis at every node)
(OP)
thread727-156008: FEA fatigue
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?
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?





RE: Fatigue: FEA (fatigue analysis at every node)
corus
RE: Fatigue: FEA (fatigue analysis at every node)
If you want a really accurate answer consider DeltaSnn and use multi-axial fatigue formulas.
You will find that for most geometries, for which fatigue is important, the stress field is 2D and often 1D because it is at a surface or edge. In these cases taking the stress range of the components Sxx, Syy etc often reveals only one stress component to be dominant anyway.
RE: Fatigue: FEA (fatigue analysis at every node)
The most conservative choice is signed Tresca. Sign is defined by the sign of max or min principal stress. Signed Von Mises is not a good choice for your application.
Ref. MSC.Fatigue Manual
A.A.Y.
RE: Fatigue: FEA (fatigue analysis at every node)
I may also looked at a number of cases, like maybe 10 instead of just 2. That is, my spectrum might contain 10 different load case. I guess I would need to just use Principal stresses becasue taking the difference might be impractical.
RE: Fatigue: FEA (fatigue analysis at every node)
RE: Fatigue: FEA (fatigue analysis at every node)
use principal stresses;
after that you have to understand your loading spectrum; like john says above, you need to pair stresses together (don't forget the null stress, before the loading is applied; does the structure return to null state between repetitions of the spectrum ?)
wiki/google "rainflow fatigue" to get more info on rainflow analysis of a fatigue stress spectrum.
RE: Fatigue: FEA (fatigue analysis at every node)
I think I know how to rainflow to find the reversals, and add the damages together for each pair, but as above i was wondering how correct it would be if the streses is in different directions.
RE: Fatigue: FEA (fatigue analysis at every node)
This is the methodology followed by many Codes and Standards, including the ASME Boiler and Pressure Vessel Code.
RE: Fatigue: FEA (fatigue analysis at every node)
RE: Fatigue: FEA (fatigue analysis at every node)
There are Matlab algorithms for doing RF.
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RE: Fatigue: FEA (fatigue analysis at every node)
Gurmeet
RE: Fatigue: FEA (fatigue analysis at every node)
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RE: Fatigue: FEA (fatigue analysis at every node)
take the principal stress from each load case, conservatively neglect that the direction changes from case to case.
RE: Fatigue: FEA (fatigue analysis at every node)
Yes thats looking likely, I will take the "dominant" principal stress.
RE: Fatigue: FEA (fatigue analysis at every node)
Imagine a thin square plate.
Starting at the top edge label the edges A, B, C and D going clockwise.
The x axis is edge C, positive to the right.
The y axis is edge D, positive up.
Load Case 1
tensile load on edges A and C of 1.
no load on edges B and D
Load Case 2
tensile load on edges B and D of 1.
no load on edges A and C
Using the efunda principal stress calculator:
Taking the difference of Principal stresses:
LC1 P1 = 1, P2= 0 and theta = 0 tau.max=.5
LC2 P1 = 1, P2= 0 and theta = 90deg. tau.max=.5
delta P1 = 0, delta P2= 0, delta tau.max = 0 and there would be no fatigue predicted, clearly non-conservative.
Using the difference of stress components to calculate principal stresses:
LC1 syy = 1, sxx = 0, sxy = .5
LC2 syy = 0, sxx = 1, sxy = .5
delta syy = 1, delta sxx = -1, delta sxy = 0
P1 = 1, P2 = -1 and theta = 135deg tau.max = 1 so there would be fatigue predicted and Corus' answer would be more conservative. If a crack were present it would not be Mode I, but would probably grow by shearing.
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RE: Fatigue: FEA (fatigue analysis at every node)
I suggest you use Corus' and rb1957's methods which have a sound formal code-based methodology according to TGS4, or if you absolutely need a more accurate answer use my individual component range method (plus rainflow, now that you reveal multiple load cases) if you have biaxial fatigue data. I repeat, if you look at what all the components are doing it WILL condense to a 2D or 1D stress field because of the geometry.
The rest of these comments are IMHO just partially informed babble which just confuse you and do not solve your problem. Anyone who honestly doesn't know what they are talking about, please don't post.
Regards,
Gwolf
RE: Fatigue: FEA (fatigue analysis at every node)
"it WILL condense to a 2D or 1D stress field because of the geometry"
You will probably have to transform the stresses using a button in your post-processor to the local geometry system - e.g. cylindrical at a hole, to make the off-axis stresses dissapear and make it 2D or 1D.
Have fun, I enjoy it.
gwolf
RE: Fatigue: FEA (fatigue analysis at every node)
how does the loading change from case 1 to case 2 ? does it unload between them ? in which case the cycle would be 1-0-1. 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.
RE: Fatigue: FEA (fatigue analysis at every node)
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RE: Fatigue: FEA (fatigue analysis at every node)
also, do you Really think he'd run a zero loadcase through the FEM.
RE: Fatigue: FEA (fatigue analysis at every node)
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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RE: Fatigue: FEA (fatigue analysis at every node)
RE: Fatigue: FEA (fatigue analysis at every node)
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.
RE: Fatigue: FEA (fatigue analysis at every node)
A zero crossing is not the same as a zero load to an RF count.
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