CosmosWorks Results Accurate? 4

[PLCKing]

I'm doing a FEA model of stresses in a plate. The plate ASTM A36 steel and dimensions are 140" x 250" x 0.25" thick. The plate is restrained along all 4 0.25" edges. The bottom side has stiffeners that are 0.125" thick and also A36 material and are spaced every 16". The stiffeners are "L" shaped with the bottom of the "L" welded to the bototm of the plate. The long part of the "L" is 9" long and the short part (welded part) is 2" long. The stiffeners are 140" long and run the whole length of the plate. The bottom of the plate is exposed to a 30psig pressure distributed evenly accross the plate.

I'm using cosmosworks to do the analysis with a sold mesh (2" x 0.1"). The results I get is that the von mises stresses in the web of the stiffener are approx. 300 ksi and the max von mises stress at the edges of the plate is approx 450 ksi. These stresses are way over the materials 36 ksi.

What I'm having trouble with is how accurate the results are. I think the result stresses are way to high, but I'm having a hard time proving that they are wrong. Any advice on error checking the analysis would be greately appreciated.

PLC.

 

[PLCKing]

The CosmosWorks version I'm using doesn't have non-linear analysis in it. I understand that the non-linear analysis would give more accurate results (I'd expect lower resulting stresses), but how much more accurate? 10%, 20% 50%?

PKing

 

[GBor]

It's not just a matter of "more accurate", but also of how the stress would actually "spread out". As parts yield, the load path changes. Until the net section yields, the stress doesn't progress on to the ultimate strength of the material.

It's not unlike the boiling point of water. You heat water up to 100 degrees C and it stays there until the water starts to boil. Until it all turns to steam, the temperature of the water stays at 100 C.

How much more accurate...probably MUCH more.

EdDanzer,

I'm not following. The moment of inertia is based upon the distance of the area to the bending axis. The bottom leg of the "L" allows a thinner web and more metal away from the bending axis. Yes, a "T" would work...that helps prevent torque from the load path imbalance, but a flatbar would not be lighter for the same stiffness without penetrating farther into the structure.

 

[EdDanzer]

This link

http://picasaweb.google.com/EdDanzer/FEAPanel

will provide an example of the “L”, “T” and thicker flat idea I posted earlier, and simple FEA results.

PLCKing, the example is not to length scale, but the stiffeners are what I think you specify. If this panel is welded to another structure it might be wise to include some of its geometry in the model as the fixed restraint will show different stress on the panel, panel stiffeners, and weld areas.

 

[johnsmith2]

If we use the shell elements, we should model the geometry (CAD) with mid-surface, but the inner or outer surface, then defining the thickness.

 

[CAEguy]

I believe that the gentleman (PLCKing) who posted the initial message really needs to get formal Training. It is very dangerous to make design decisions based on free advice for such analysis. I would recommend that PLCKing should attend a training class and discuss with an analyst on the best-practices for performing such analysis. In fact, he should first hire the services of a full time consultant who uses CosmosWorks so that he can learn by imitating.

 

[corus]

EdDanzer is correct in saying that a weld does not necessarily infer full restraint on the edges, rather the restraint depends on the stiffness of the adjoining structure, and it is better to include the surrounding structure to a point where the restraints to the limits of the model will have a negligible effect on the region you're interested in. If, on the other hand, you only want to model that region and you're uncertain of what restraint to apply then it's best to choose the worst case, which could be simple (no rotational restraint) supports for maximum bending stresses in the centre.

corus