2009

November 2009

# Why you need to refine the mesh to get accurate stresses?

Posted by Joe Galliera Nov 16, 2009
When doing the  first training lesson from the SolidWorks Simulation manual, the results summary  table (on page 64 of the 2009 book) is meant to illustrate the impact of the  model discretization, or element size, on the data of interest.  In this  example, the max displacement changes only in the fourth decimal place while the  max von Mises stress changes much more.  In general, this same trend is true: displacement will always be the most accurate value calculated  in a static analysis, thus discretization has a minimal effect, whereas stress is the least accurate value.  If you have ever wondered  why, I will try to explain:

What is being  solved in a structural analysis as the unknown is the displacement, the vector  equation given as: {u}=[K]^-1{f}.  Its accuracy is of the same order as the  order of the element, so a high quality mesh will yield 2nd-order accurate  displacements.  A derivative is taken to calculate the strains, thus lowering  the order by 1, so for high quality elements is 1st-order accurate.  More calculations are done to the strains to scale them by the elastic modulus and  averaging the Gaussian points to nodal values (or element values), so error is introduced in this process, but its order of accuracy is maintained, so for high  quality elements stress is 1st-order accurate.  So you can see that stress values are always the least accurate quantities and why extra care should be  taken in obtaining accurate stress results.

Here is a flowchart  type of order of calculations to illustrate this a little  better:

# Hidden Gems in Simulation 2010

Posted by Joe Galliera Nov 2, 2009

Have you been to a roll-out by your local reseller where they covered what's new in the 2010 SolidWorks release?  There's a lot more than they can share in that short amount of time.  In the new 2010 release of Simulation, there are a lot of great hidden gems that are not immediately obvious.  Here is a short list of some of those gems:

1) A great anhancement has been the handling of bonded contacts for a mixed meshing with beams and shells (I assume the same is true with beams and solids too but I haven't really checked yet, maybe a later update on this).  Someone had sent me a model with about 400 beams making up a large rectangular building and then he had the walls done as shells.  He didn't create any contact definitions besides the global bonded contact, meshed and ran it... AND IT WORKED!  The compatible meshing between beams and shells works like a charm.  To me, I think this shows how the Simulation product team is working to make analysis easier for all to use!

(2) I also like the fact now that you can Edit the definition of results plots without first having to show the plot.  Definitely makes it much faster to make changes.
(2a)- Also in the graphics window for results, you can get rid of the plot details and the legend in the Chart Options.  I don't think you were able to do this before.

(3) Defining shell elements by selecting faces of a solid body is welcomed back.  Thanks Simulation team!  Remember though that the shell definition still assumes that it's defined at its mid-plane.  With composite shells though that is not the case for 2010, you can now define the direction of the stack up or also by setting a positive or negative ratio 0±0.5.

(4) Defining bodies as either Rigid or Fixed in Nonlinear, like in the example of an O-Ring, helps to speed up the analysis.  We're only interested in the rubber gasket so one can make the bottom fixed and the top rigid; then simply move that rigid part down a specified amount to just touch the bottom fixture.  Model and videos: http://bit.ly/1l7l2I (342 kB)

5) NAFEMS Benchmarks... Help > SolidWorks Simulation > Validation > NAFEMS Benchmarks.  NAFEMS is an international organization who promotes the proper usage of computational analysis for engineering.  Models are here: <install directory>\SolidWorks\Simulation\Examples\Verification

6) An geometric entity can be both be used as a selection and a reference when defining a load or restraint.  There are countless number of times that have I wanted to do this in the past.  This enhancement is along the same lines as not having to select a face before selecting the hole wizard tool.

7) Backup after both meshing and solved a study to be able to recover the settings and data should there be a problem during this critical phase of a study.  This can be handy in those crucial times when you just can't lose your work!
Click Options (in the Standard SW toolbar) or Tools > Options.  On the System Options tab, click Backup/Recover and select Save auto-recover info after meshing and after running a simulation study.

8) I've found automatic contact definition works much faster than in the past, especially when you have a gap between bodies.  Another enhancement to contacts is that you can drag contact definitions from static studies to nonlinear studies and vice versa. Note: Make sure that these contact definitions are available to both study types.

9) The units available for displacement plots match the units provided in the SolidWorks application. The units are am, nm, micron, mm, cm, m, micron, mil, in, and ft.

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