Fatigue analysis maxi principal stress
Fatigue analysis maxi principal stress
(OP)
HI, I'm Alfonso.
I need explanation on a fatigue analysis using Max principal method.
I've applied 8 fatigue load condition on a fem model, with a static analysis.
I've found the critical location on the model, and allowable delta sigma from en-1993.1.9 .
Now I've 8 max pirncipal and 8 min principal stresses in this location, but I don't know how use those.
Is it correct subtract the maximum MaxPrinc and minimum MaxPrinc and compare the result with the allowable (do the same with MinPrinc)?
I've read that there are a lot of different approach.
What are the most popular methods?
thanks.
I need explanation on a fatigue analysis using Max principal method.
I've applied 8 fatigue load condition on a fem model, with a static analysis.
I've found the critical location on the model, and allowable delta sigma from en-1993.1.9 .
Now I've 8 max pirncipal and 8 min principal stresses in this location, but I don't know how use those.
Is it correct subtract the maximum MaxPrinc and minimum MaxPrinc and compare the result with the allowable (do the same with MinPrinc)?
I've read that there are a lot of different approach.
What are the most popular methods?
thanks.





RE: Fatigue analysis maxi principal stress
RE: Fatigue analysis maxi principal stress
is best to create a plot of their values versus
their angle. As Corus recommends one must do this for both tensile and compression
principal stresses. The 2D plot of stress vs angle will usually show two
large critical angles. One for each principal stress (assuming planar problems).
One needs to pick load cases that give both large tensile principals and
large compressive principals.
When there are multiple channels of load inputs scan each channel
for max and min values, and at those times also record the values of the
other channels. i.e.: things may be out of phase. Then plot the principals
for each of these "simultaneous max mins". To save FEA time it is probably
best to run an FEA for a unit of load for each channel independently. Then use
superposition to recombine the loads for each set of simultaneous max mins.
After picking the critical direction from the stress vs angle
plot run all the load cases with stress output for this critical direction
and then rainflow count the result. This assumes no multiaxial stress conditions
where the principals and angles are all over the place.
RE: Fatigue analysis maxi principal stress
Is it possible use von mises stress insted of max princ,if you have a multiaxial stress?
RE: Fatigue analysis maxi principal stress
you say you have 8 stress cases. do your 8 cases happen sequentially ? are the cases assumed to cycle, +ve to -ve, when they occur ? does the stress state reduce to zero between loads ? would this be a conservative assumption ?? someone has mention "rainflow", look it up (as it applies to fatigue analysis). you need to understand how the max. principal stress can cycle at the critical location. It is not simple and difficult to write (in a blog like this).
another day in paradise, or is paradise one day closer ?
RE: Fatigue analysis maxi principal stress
max/min principal vs angle plot, one cannot use von Mises or just a principal.
Best method is to divide the possible angles into 10 or 20 representative
cuts and then resolve the element stress history to stresses
perpendicular (plus shear) to each cut. Then Rainflow these resulting stress
"on a cut" histories to find the critical angle. Probably also best to use
a damage parameter that includes shear. The one by Findley is probably ok, but
there are a number of others.
RE: Fatigue analysis maxi principal stress
another day in paradise, or is paradise one day closer ?
RE: Fatigue analysis maxi principal stress
But its expensive per seat for the commercial codes,
and the process isn't really that difficult for a few elements.
I'm tempted to write an open source version.
RE: Fatigue analysis maxi principal stress
but load sequence and cycles is vital (I mean I'd use the max principal stress for each load case as being conservative and worry about the possible load cycles before I tried to "perfect" the stress story by saying in this particular direction I have max principal stress for case 1, some normal stress and some shear stress for case 2, etc).
another day in paradise, or is paradise one day closer ?