Limit Load Analysis using ANSYS

[r420]

I am having one heck of time in trying to solve limit load solution for a simple cylinder with internal pressure using ANSYS. It was so easy wiht ABAQUS. Does any one has any suggestions?

 

[johnhors]

Yes, use Abaqus !







Sorry, couldn't resist, but seriously all FE solvers can do what you ask, I suspect that your trouble lies with using an unfamiliar gui (graphical user interface), I suggest that you look at and try out any example problems that get delivered with the software.

 

[Stringmaker]

Ansys is a very capable analysis program. I would suggest you read in the Ansys Help:

1) Modeling and Meshing Guide
2) Structural Analysis Guide

or use Abaqus as suggested above.

 

[johnsmith2]

What is the limit load analysis? Anyone can tell me, Thanks.

 

[cbrn]

Hi,
it can be anything depending on what you are analyzing. Limit Load is the ultimate load combination that a structure can stand before violating the criteria. By "criteria" I mean both "loosing functionality" (by overcoming limit stress in some points, or by deformating too much wrt the design intent), and "collapsing": these are two mandatory cases to analyze separately. The concept is described in any Norm which uses the Limit-States Analysis instead of the Allowable-Stresses Analysis (e.g. EN13445).
Btw, in some cases the limit load is dictated by buckling, in other cases by structural integrity, so the calculations for determining it are either static structural analyses (non-linear or not, it depends) or buckling ones (here also, eigen-buckling in linear hypothesis or full non-linear buckling, depending on what is imposed by the Norm - in the second case, the analysis type becomes a non-linear static structural analysis with "a lot" of timesteps).

Regards

 

[rb1957]

for us "aero's", limit load is the maximum load that the structure is expected to see in service and must be carried "without significant deformation" (usually interpreted as "no yielding"). as opposed to ultimate load which applies a safety factor to the limit load (usually 1.5) and the strcuture just needs to "hang together" ... permanent deformation is allowed (encouraged?)

in this context, i imagine the OP means a linear analysis (hence no non-linearities).

 

[cbrn]

Hi,
rb1957, interesting... As I can see, really each field has its own rules! Well, I've seen that the O.P. is "structural", so if he was in Europe he'd have to refer to the Eurocodes. For the Eurocodes, allowable stress analysis is not allowed any more, and so is eigenbuckling (eulerian, linear) analysis. So, when you have possibility of buckling, you are obliged to shift to a non-linear analysis.

Regards

 

[rb1957]

interesting ... do you think software salemen were involved in that decision ? or was there a rash of problems (or potential problems) ?

 

[cbrn]

Hi,
rb1957, no, there are "scientific" reasons behind the change in methodology between the new Eurocodes and the previous norms. Basically, the Limit States Analysis (LSA) introduces a semi-probabilistic approach where you introduce "load factors" to all the possible causes of sollicitation on your structure, and you compare the worst loadcase combination to the "service" or "ultimate life" criteria. Other semi-probabilistic factors are taken into account for the material properties' scatter, etc etc...
As for the buckling, I think it's correct to oblige "structurists" to consider second-order effects (eccentric load, ...) now that almost all the FE softwares allow to do non-linear analyses at relatively small expenses of complication and effort (and time). In fact, eulerian buckling and/or eigenbuckling are really too "approximative" in the majority of the designs "pushed to the limit". Of course, if you do a linear buckling and accept to have a load factor of 17, you are about sure that your component is safe, but if the goal is to optimize this component wrt weight, etc, then it becomes critical to take much more factors into consideration, = non-linear buckling.
On the same idea, if you apply "at best" the EN13445, wrt ASME BPVC you can save up to some tens of percents of material, which is not neglectable, without loosing in safety, only because you are using the most up-to-date techniques.

Regards