VON MISSES STRESS
VON MISSES STRESS
(OP)
THE VON MISSES STRESS IN A PLATE IS 12 KSI.
FOR A36 STEEL IS MY SAFETY FACTOR 36/12 = 3
OR 58/12 = 4.8
IS VON MISS BASED ON YIELD OR ULTIMATE ?
FOR A36 STEEL IS MY SAFETY FACTOR 36/12 = 3
OR 58/12 = 4.8
IS VON MISS BASED ON YIELD OR ULTIMATE ?





RE: VON MISSES STRESS
The Von Mises stress is not based on yield or ultimate stresses, which are material properties. The Von Mises stress is purely a function of the stress field at a point in the material. It is equal to the octahedral effective stress and is used to characterise failure in ductile materials, i.e. if the VM stress is greater than the material yield stress plastic strain is assumed to occur. See books like Dowling: Mechanical Behavior of Materials.
Andries
RE: VON MISSES STRESS
it is better to use it as a limit stress, since it is based on elastic theory.
but from your post if your steel as a yield of 36 ksi and ftu of 58 ksi, then you have a limit (or proof load) MS = 2 (or RF = 3) and an ultimate MS of 3.8 (or RF of 4.8).
RE: VON MISSES STRESS
RE: VON MISSES STRESS
Also, keep in mind that for A36, the minimum yield is 36 ksi. You might be able to demonstrate (through testing) that the ACTUAL yield is considerably higher. I'm not sure how much higher it would be for plate, but for rolled shapes it is generally expected to be about 54 ksi!!
RE: VON MISSES STRESS
RE: VON MISSES STRESS
I wouldn't worry too much if the material is ductile. What you could ask the structural engineer is to show that the gusset will not buckle if the free edge is under compression.
Andries
RE: VON MISSES STRESS
RE: VON MISSES STRESS
it looks like a TET mesh ... TET4s are known to be overly stiff, maybe remesh with TET10s ?
it also looks like a single element centroid stress. i'd like to see a section through the web, to see if it is highly localised (and so redistributed by yielding) all trhu thickness (which reduces the redistribution).
if you're relying on von Mises (which is relying on the thru-thickness stress), how many elements do you have thru the thickness ? 3? (the pic looks like 1).
comments about the free edge of the web being in compression are good.
RE: VON MISSES STRESS
Is there fatigue requirements, as is usually the case on ships? This would mean the peak stress will need to be considered for cyclic failure using ABS or similar.
What about a check of the welds? The model assumes full penetration but most of the time gussets are only fillet welded. This will also be a critical concern if fatigue is an issue. Did they also supply hand calculations for the welds?
Peak stress aside, you also need to consider stress classification. Highly localised stress can 'shakedown' during the first few applications of load though local yielding. However, it cannot be known if this is the case or if it is just peak stress because the mesh is too coarse.
RE: VON MISSES STRESS
My boss thought the mesh was too coarse also. What would be the 'optimum' size of the elements for these plates. The designer said the elements are 3x3x3" on average. We will ask him to remesh the gussets and 1" base plate. Should we have the pipe remeshed smaller also ?
RE: VON MISSES STRESS
Ultimately all you have to do is ask them to show you proof of convergence of their results. This will require them to refine their mesh until the stress stops increasing (assuming no singularities in the region where the measurements are taken).
RE: VON MISSES STRESS
if that mesh is 3" x 3", this thing is much bigger than i thought it was.
be aware, remeshing is certain to increase the peak stress, albeit over a smaller area, but this is FE detecting something which wasn't detectable with hand calc methods. convince yourself that the high stress is very localised and in the real world there'd be a tiny amount of plasticity.
if the gusset is welded, what are the properties post-welding ? maybe that's why you have such a low allowable ??
if you're still worried, you could test it later ??