FEA in SolidWorks For The Cassis
Hey! I'm running FEA in SolidWorks and as I am going through the list of tests that need to be ran, I'm having an issue determining what joints should be fixed for different tests. The biggest concern I have is for the Front Bulkhead and Bulkhead Supports off axis. It says to have a max deflection of only 25 mm, but there is no way I can get anything close to that. I can't get below 100 mm with 120 kN head on and 10.5 kN sideways. The only joints I have fixed are the engine and suspension mounts on the back. I was reading the rules and noticed the Boundary condition section, can anyone help to decipher this or let me know where you have fixed different joints for theses tests?
Thanks,
thatoneguy
FEA so far does not help. Let's talk about how it could do so.
Hi Cody,
doing a FEA according to AF-Rules with Solid Works could be challenging because of documentation requirements.
Regardeless of that I agree with Adam that it is useless to try to use AF-Rules. The resulting design will be allways heavier than a conventional design. It might be a different storry if alternative boundary conditions according AF5.6 are used. I haven't tried this yet, how ever I judge it unlikely as AF-Rules consider lateral load cases where the conventional rule set does not.
Regarding the improper triangulations: Yes you suffer from it - in top view. However top view is not rule relevant (for reasons I don't understand) so your design could be rule compliant for the conventional rule set. For sure it suffers from a massive lack of bending stiffness around vertical axis. This is also what can be seen from your picture.
Short summary up to now: Your frame FEA so far does not support your efforts in finding a good frame design in any way. Lets talk about what could probably help.
I would define following requirements to a good frame design:
1. Rule compliance
2. low weight
3. sufficient stiffness according to the assumptions made during the suspension design of your car
FEA helps in achieving targets according point 3 while minimizing mass. In absence of targets for stiffness you could also define a mass target and maximize the frame stiffness.
There are two different types of analysis you could perform:
1. Modal analysis of your frame inclding the major unsprung masses
- Stiffness at a given mass distribution results in a certain resonance frequency --> The higher the frequency the higher the stiffness
- masses and inertias of components can be estimated - results have comparative character therefore estimations should be kept consistend
- the mode shape will also allow you to see weak sections in your design - target would be a smoth displacement distribution along the longitudinal axis
- This analysis can be done free-free which avoids mistakes because of boundary conditions. It will not give a stiffness results according to point 3 of the requirements.
2. Static analysis of the wheel to wheel stiffness of your frame design
- The model consits from your frame and stiffening structures like the engine - no mass is required
- Loads could be aplied via displacements, forces are results --> stiffness is calculated.
- This analysis needs loads as applied kinematically correct. In my expirience the easiest way is to include a kinematic model of the suspension and aply loads and restraints at tire contact patches
Side effect of the 2. analysis: If detailed enough you can get results for hidden compliance in support points for a-arms or rockers. However don't get to picky while setting up the model and be careful during result interpretation.
The analysis described can be done with any FEA package I am aware of, including CATIA's in house solution. Therefore I do not see necessity to switch to a dedicated FE-Package. (Sorry Lutz, however Hypermesh would be my favourite preprocessor :-) ) Just use whatever you are most comfortable with.
That's it for the moment. I hope I have given some inputs to improve your design.
Timo