how to mesh an assembly of 4 thin disks joined together? (hypermesh)

[ab8abhi]

Hi all, I am trying to do a fea of an assembly of 4 thin disks (3 disks of 2 mm thickness and 1 disk of 20 mm thickness ; all 6 inches in dia.; all of different materials) in Hypermesh. When I use solid elements which is not recommended, I get too many elements and it takes huge time to analyze. ( I try to get 3 elements through thickness so each of 0.666mm size so a lot of elements across the disk area). If I use shell elements I get a really large deformation.( the deformed assembly looks like an ice-cream cone).
So I will really appreciate if you guys can help me with this.
The disks are joined on top of one another and the bottom surface of the bottom disk is fixed. Pressure is applied on the top disk.
Also if I choose an elements size of 6.66667 for 20mm thick disk and 0.66667 for 2mm thick disks then will I get good results?
Ultimately I want to optimize the thickness of the two disks in the middle to get minimum pressure on the bottom disk.

 

[rb1957]

"the bottom surface of the bottom disk is fixed" ... so then the structure does nothing ?

geometrically non-linear model ? TET10s or bricks ?

how are the shells joined to gether in real life ? will they deflect together ? if so , a single shell should be ok (combine different materials with "rule of mixtures", modify thickness so that all shells have the same E)

Quando Omni Flunkus Moritati

 

[ab8abhi]

Hi rb1957,
I am trying to do a simple analysis that will be used in a helmet model. This simple static analysis is used to get optimum thicknesses for different materials. The different disks are bonded together so that there is no sliding between them. I used 2D automesh (mixed trias and quads) and 3D automesh (volume tetra mesh) options in hypermesh.
The only BC is that the bottom most disk is fixed at its bottom surface.
I guess we can see this as an Anvil made of 4 different layers of materials under pressure.
I am a new user of FEA softwares. So my understanding is limited.

 

[IRstuff]

Your description sounds a little odd to me. The pressure load cannot be magically absorbed by the two middle disks; the load must ultimately transfer to the bottom disk, assuming you were talking about a static load.

TTFN
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[rb1957]

i understand "The only BC is that the bottom most disk is fixed at its bottom surface" to mean that you have a distributed constraint over the lower surface. if you're applying pressure to the shell, won't this be reacted by the distributed constraint (and so i'd expect that the shell is doing nothing).

maybe you mean that the shell is supported along it's edge ?

if the shells are bonded together then modelling as one doesn't sound too bad. it is more complicated than "rule of mixtures" as i expect that you're using anisotropic materials ... in which case i think you need to use laminate properties to combine the different materials.

Quando Omni Flunkus Moritati

 

[ab8abhi]

thanks Irstuff and RB.
I don't get what is wrong with the distributed constraint on the bottom surface. e.g, if you keep a solid cylinder on a floor on its flat surface and apply a pressure on the other flat surface then it will deform right. I want to choose a combination of these 4 disks which will be the least stressed under the same applied pressure.

 

[rb1957]

"if you keep a solid cylinder on a floor on its flat surface and apply a pressure on the other flat surface then it will deform right." ... a cylinder? ok, that'll react the load like a column; but we're talking about a plate (no? "disk") ... IMO if you apply pressure to one face of a plate, and react it on the other, the plate really isn't doing much (not like if you support the plate at it's edge and apply pressure over the surface, getting a lot of bending)

Quando Omni Flunkus Moritati

 

[ab8abhi]

Ok. I get it now. I will change the BC.
I think I can save a lot of processing time if I do an Axi- symmetric analysis. But I am not sure how to do it. I know if I draw a rectangle and rotate it in 360 degrees I will get a disk. But what BCs should I use for the inner surface (near the centre), sides faces of the rectangles? Do I need to draw a sector of the disk instead?

 

[rb1957]

?? how can you relate a rectangle solution to a disc problem ??

a rectangle is not the axi-symmetric solution for a disc ... a pie-shaped sector is.

IMHO you are Way better off keeping the problem as simple as possible, and simply constrain the edges ... who cares about "processing time" ?

question ... are you modelling the individual shells ? if so, how are you joining them together ??

Quando Omni Flunkus Moritati

 

[ab8abhi]

Hello RB,
I am was confused about the axisymmetric problem. Because if you are modelling a hollow cylinder you will draw a rectangular cross section , if you are modelling a ring ,you will draw a circular cross section. So in this sense a rectangular cross section (one end at the symmetry axis) should represent a solid disk right?

Yes I was trying to model the individual shells and but am not able to join them togehter correctly. I tried contact surfaces and interfaces but they are not working.

 

[missil3]

No axi symmetric for a disc is a like a pizza slice shaped segment. But like Rb1957 says best to keep it simple and do a full analysis. Also I would look into why the contacts are not working , this is a simple contact problem and there should not be any issues with it.