SolidWorks Analysis - Accurate?

[Manolis]

I started playing around with the FEA in SolidWorks tonight (I built a wheel for my car, and decided to test it). Does anybody know how accurate it is? Hopefully it's a ways off, because if not, i've got some work to do

[ben]

How much FE have you done before?

In my experience the results of FE analysis depend dramatically on how the user specifies the geometry, mesh and boundary conditions.

If a commercial package is spitting out unbelievable numbers I would say it's more likely to be your modelling that's wrong.

Ben

University of Birmingham
www.ubracing.co.uk

[dancin stu]

A computer will always give the right answer, you just have to ask it the right question

[Denny Trimble]

Are you using "cosmos express" or the full COSMOS/Works? You should be using Works.

As for "funny results", a lot of first time users say "My part's going to fail" because the program spits out a deformed plot that's 500x or 1000x, when in reality, deflection is quite low.

Also, if you have any doubts, do some simple verification studies. Cantilevered beam bending, buckling (can be difficult to set up boundary conditions correctly, but it's critical!), torsion, etc. Compare the results with your hand calcs.

I recommend this book as a practical guide to using FEA. My school's copy has lived on my desk for a few months now

University of Washington Formula SAE ('98, '99, '03, '04)

[Erik C]

We are supposed to never start an FE model until all hand calculations are complete. The hand calcs should be a simplified version of what you will be modeling. The results of which should be used as a reality check for the FE output.

As far as FE modeling itself, Remember, in FE, a rigid constraint is just that, rigid. For your wheel model the constraints (lugs) will actually flex slightly to allow a more even distribution of load. Try using degree of freedom spring elements.

For the loads, if you expect large displacements you MUST run the model in a nonlinear fashion. One example I have run across is on a flat pressure bulkhead. When run in a linear static fashion, the maximum Hoffman F.I. was predicted to be 5.5 with a deflection of 4". When the same model was run using a nonlinear analysis, the max F.I. decreased to .75 and the deflection to a more reasonable 1.2". Having done the hand cranks prior to the analysis allowed me to quickly see that the linear static model was unreasonable, and that a nonlinear method should be used. (The hand cranks; using formulas derived from Roark's and Bruhn, were within 10% of the predicted FE results)

[Manolis]

I've never done any FE modeling before, this is my first crack at it. This was done with 'cosmos express' - what does the full version do differently?

I'll go to the library today and see if we've got access to that book! Are there any other recommendations for books on the subject? Simple is good - I'm a first year, and I haven't seen any of this before.

Thanks!
Manolis

Wheel Pics

[Denny Trimble]

Manolis,
Your picture link doesn't seem to work.

Cosmos Express will give you von mises stress plots and factor of safety, but that's it. You can't get deflections out of it, or other types of stress plots. I'd say it has about 10% of the power of COSMOS/Works. Being able to create and probe plots of "x-direction displacement", and discrete tone stress plots, do buckling, thermal, and frequency studies, are all very valuable. Not to mention mesh controls (local refinement), shell elements, assembly contact control (bonded or not). I can go on.

Check to see if your school computers have COSMOS/Works installed- go to Tools/Addins and look for a COSMOS/Works box.

The student edition of SolidWorks is $200, and it comes with COSMOS/Works, COSMOS/Motion (dynamics), and COSMOS/FloWorks (CFD).

University of Washington Formula SAE ('98, '99, '03, '04)

[Manolis]

Wheel Pics

[Sam Zimmerman]

I just attended a conference where the COSMOS representative was giving a "how to" for COSMOS Express. His statement was that, even with correct modeling of boundry conditions, COSMOS Express is only meant to give the user enough information to narrow down the number of prototypes that need to be built and tested. It is not meant to model a part in such a way that multiple prototypes are no longer needed.

Sam Zimmerman
Vandals Racing

[Erik C]

Manolis-if this is your first year in school, I would worry more about understanding the mechanics that go into calculating stresses and not on FE. Very few people who graduate with a ME or AEM degree will use FEM right out of school so a good foundation is important. or at least that is my .02...

As far as using a model as a qualitative tool rather then a quantitative tool. I couldn't agree more. Using FE to compare different concepts is much easier and far less time consuming then relying on FE to calculate real world stresses.