Enclosure Bracing Simulation

[Milzie]

As a request by some others, I have created a "quick and dirty" site that outlines some FEA simulations on various bracing methods. If anyone has some feedback or suggestions let me know.

http://www.geocities.com/milzie/Index.htm

Jeff

 

[MJK]

Hi Jeff,

Tried to view your vibration data, but it looks like you have overwhelmed Yahoo. I'll try again later and hopefully get a look at your simulations. Just out of curiousity, what FEM software are you using?

 

[Milzie]

I use catia V5 and unigraphics v18 at work for all solid modelling. I use catia for the fea simulations because its pretty basic and quick to use. It generaaly gives pretty good results as well. For what I have done its just fine.

I also have access to Ansys, Nastran, and Altair at work. I would need some tutorials from the structural engineers at work how to use these though as they are very complicated from what I understand.

 

[pinkmouse]

Jeff

Great stuff, well done

Could you give us a bit more detail on how you did the simulations, i.e. exciting frequency, dimensions of the box, and what material you used for the box wall.

Cheers

 

[LaMa]

The greatest problem is to get proper material parameters, which is difficult because most materials behave isotropic. (in each direction other stresses and strains). How to measure these is another question.

It's great that you have taken the effort to model and insert it in a FEM package, the differences between the braces can be recognised, but the results of the calculation of the box without bracing gives only a maximum displacement of 1.54E-5 mm. That is nothing! Roughly you can say that displacement is cut in half when the box is proper braced.

The force you have applied is 10N/m2 if I read it correctly on your site. How did you came to this value?

I just want to add that if you want realistic output of a calculation the input has to be correct and that is the hardest part. Which parameters does your program need to calculate the displacement?

 

[MJK]

I finally made it into your site and looked at what you have analyzed and some of the plots you present.

You can find a lot of good info on different wood properties at the USDA site. One publication that I recently used at work to design wooden shipping crates is shown below. More info then you probably ever wanted to know!

US Department of Agriculture - Wood Handbook

http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm

If I understand what you have done, the displaced shape plots are for a unit pressure case and the results are only static deflections. While this is interesting data, these plots should not be mistaken for vibration plots that show natural frequencies and mode shapes. I would guess that when you run a modal analysis the deflected shapes will not look like those calculated by the static analysis. The change in the natural frequencies and the mode shapes will be very interesting as the different bracing schemes are applied.

 

[Milzie]

The greatest problem is to get proper material parameters, which is difficult because most materials behave isotropic. (in each direction other stresses and strains). How to measure these is another question.

Thats correct, because most materials behave the same, they will all deflect in a similar manner. I could have used steel and it would not have changed the results, only the value of deflection. I was not trying to determine the actual value of deflection.

It's great that you have taken the effort to model and insert it in a FEM package, the differences between the braces can be recognised, but the results of the calculation of the box without bracing gives only a maximum displacement of 1.54E-5 mm. That is nothing! Roughly you can say that displacement is cut in half when the box is proper braced.

That was in fact the purpose of the study, to determine how bracing effected to displacement of the enclosure walls and the reduction in deflection as a percentage from no braceing.

The force you have applied is 10N/m2 if I read it correctly on your site. How did you came to this value?

I pulled the number out my butt. The pressure is not important. I was not trying to calculate the actual deflection value, only compare the deflection between different styles of bracing. If you convert the deflection values to percentages relative to an unbraced enclosure, that may make the results more easily understood.

I just want to add that if you want realistic output of a calculation the input has to be correct and that is the hardest part. Which parameters does your program need to calculate the displacement?

To calculate displacement, I would need the modulus of elasticity as well as the poisson ration of MDF. I would then have to determine the actual pressure a loudspeaker exerts on the cabinet. This would be useful if I wanted to know how much the box deflected.

It really doesnt matter how much the box deflects in my simulation. What we are looking for is improvements from baseline (no bracing).

If I understand what you have done, the displaced shape plots are for a unit pressure case and the results are only static deflections. While this is interesting data, these plots should not be mistaken for vibration plots that show natural frequencies and mode shapes. I would guess that when you run a modal analysis the deflected shapes will not look like those calculated by the static analysis. The change in the natural frequencies and the mode shapes will be very interesting as the different bracing schemes are applied.

I think you understand exactly what I have done. While this is not a modal analysis, and does not give us the resonant frequency of the enclosure, it does give us the relative stiffness of the enclosure walls and how various forms of bracing effect that stiffness.
I am in the process of building some models of loudspeakers as well as the mdf cabinet. I then plan to export the information to a more powerful FEA software such as Ansys to do some dynamic and modal simulations.


Jeff