Modelling perforated sheet in FEA

[gcoopermax]

I typically represent perforated sheet using a solid sheet with equivalent elasticity and Poisson's ratio which can either be obtained from IPA handbook or from a simple FEA on a periodic section of perforated plate. However often there are times where the open area of the perforated sheet is so high that equivalent Poisson's ratio ends up being more than 0.5, which is unphysical and not allowed by the FEA software I use. How do I approach this? One approach could be to "adjust" the elasticity and thickness keeping Poisson's ratio to under 0.5 so as to obtain same strain and stress in a periodic model and then use these values in the FEA simulation. However I am trying to understand how I go about doing this.

 

[WARose]

I'm confused.....why would you model a sheet/plate and try to (if I understand this correctly) represent the openings by making some sort of adjustment in material properties? Why not just model the openings? (Especially since you have FEA software.)

I admit I have (for example) accounted for the cracked section properties of concrete (i.e. the reduced stiffness) by using a reduced modulus of elasticity.....but I always model the openings.

 

[gcoopermax]

The perforated sheets are used in large products (imagine a 40" conical strainer that is 100" long) and often have hundreds or thousands of holes. Modelling them in CAD itself is cumbersome(if possible at all for large models), let alone meshing the perforated sheets with these holes. Computational cost for this would be prohibitive. Representing this as a solid sheet with equivalent (smaller) elastic modulus and (larger) Poisson's ratio is a reasonable simplification. Values of these can be found in publicly available literature and this reduces the computational cost substantially as there is no need to mesh the ligaments between the holes with adequate density. Trouble is what to do when this equivalent Poisson's ratio exceeds 0.5, as FEA software doesn't allow this.

 

[r13]

You should first to check the program's user/technical manual regarding limit set on the poison ratio, then contact the vendor. Or maybe repost this thread on the FEA forum, if it is a well known program.

 

[WARose]

The perforated sheets are used in large products (imagine a 40" conical strainer that is 100" long) and often have hundreds or thousands of holes. Modelling them in CAD itself is cumbersome(if possible at all for large models), let alone meshing the perforated sheets with these holes.

At least in the software I use (i.e. STAAD), if the pattern is constant, copying it and making a large model is no issue. You may want to check if this is possible in what you've got.

Trouble is what to do when this equivalent Poisson's ratio exceeds 0.5, as FEA software doesn't allow this.

I can't think of any program that does. In fact, I'm not sure a Poisson's ratio greater than 0.5 is even possible. (I've heard of a negative Poisson's ratio before....but not that.)

I'd contact the software provider and ask them if it is possible. A additional (possible) resource might be a FEA text. I remember Bathe's book (i.e. Finite Element Procedures) had a couple of options when Poisson's ratio approached 0.5: different element types and/or interpolation functions to prevent "locking". Not sure if that would help here but might be worth a look.

 

[3DDave]

Poisson's ratio above 0.5 is possible for composites where 2D properties can dominate and for certain polymer foams; not sure how it can happen for metals that are perforated.