Why bother with shell meshing? 5

[izax1]

Just recieved a Webinar invitation from MSC "Expediting Shellmeshing.... With todays computer power, I wonder why we are even bothering with preparing a 3D Solid Model to accomodate Shell meshing. That can take quite some time. Why not just take the Solid model "as is" and mesh it with TET's or HEXA's. There is no geometry without thickness!

This is mostly out of curiosity, but it would be nice to get some professional opinions.

Thanks for any input.

 

[rb1957]

there are many fine sheet structures, eg airplane fuselage, wings. i wouldn't want to mesh a fuselage say 50" rad (150" perimeter) 50' long (600") say 0.1" thk with 0.1" size TETs ... more than (way more?) 900000 elements.

and then you're proposing solid meshing stringers (rather than using 1D elements) ? and frames too ??

shells (2D elements) have thickness (as an element property)

another day in paradise, or is paradise one day closer ?

 

[rb1957]

opps 300" diameter ... twice as many elements ...2,000,000 ?!

another day in paradise, or is paradise one day closer ?

 

[ThomasH]

If I model a structure consisting of "shell-like" elements, I use shell elements. They are made for that application.
Beam structure => beam elements.

It is not only a matter of computer power but also a matter of result presentation/interpretation/handling.

Regards

Thomas

 

[rb1957]

and we wouldn't use TET4s, but TET10s.

agree with above (clearly) ... shell elements were created to be used with shell structures, solid elements with solid.

another day in paradise, or is paradise one day closer ?

 

[bkal]

In structural engineering, the design is often based on sectional forces and moments, and not stresses.

 

[rb1957]

i wonder if the OP is meaning extracting shell surfaces out of a chunky fttg ? in which case i'd say "yes" ... the biggest problem being the interface between 2D and 3D elements

another day in paradise, or is paradise one day closer ?

 

[jhardy1]

In my (Structural Engineering) world, working with silos, tanks, pressure vessels, plate box-girder bridges, etc, 80% or more of my modelling uses plate-shell elements, 10% uses beam elements, and 10% uses solid Tet / Brick elements.

There is no way in the world I would be using TET4 elements for these sorts of "shell" structures, and there isn't enough time left before my retirement in a few years to analyse the models if I meshed them with multiple layers of TET10 elements. (I need the results in minutes to hours of run-time , not weeks or months!)

Horses for courses.

http://julianh72.blogspot.com

 

[stressebookllc]

The main point you need to keep in mind is that shell elements are idea for bending, membrane and the coupling of these two for sheet or plate type members. Their internal formulation and interpolation functions etc. are optimized for this behavior. If you use solid elements to capture this behavior, then you'd need at least three element 'thru thickness', which will quickly become impractical for even a small structure.

Also, the nodes on shells have all DOFs, you those to properly transfer bending moments. Solid elements cannot do this unless you mix element types and connect them properly. Choosing the right element type is also a skill that we need to develop as stress guys. We need to understand which element type is suited for which member. At the same time we need to think about the many other factors such as solution time, model size, output file size etc.

http://www.stressebook.com

Stressing Stresslessly!

 

[adfergusson]

Some material models built into FEA programs only work wither certain element types. E.g. if you want to use something like a Hashin damage model for composites then you'd wouldn't be able to use 3D elements (at least not with Abaqus) without having having to write your own user defined material.

 

[trainguy]

Another really useful part of having a plate element model is the ease with which you can try varying thicknesses for design optimization.

tg

 

[MacEd]

I like this question because it reminds us that the future might be easier than the present. I find shells generally a pain that will hopefully go away one-day, or at least be relegated to special applications which are really pushing the boundaries. I see it as more a question of "is my software powerful enough that I can use solids?" not "are solids or shells better for this task?". Most of the limitations of solids are really just limitations in the software which don't always exist, or need to exist. Let me go through some:

Solid meshes are too big - That's what happens with a cheap tet automesher. A single 20 node hex element can often do as well as an 8 node shell even when it's thin and subject to bending. Both have about the same number of DOFs. If you do need more elements through the thickness, 2 or 3 is still only a factor of 2 or 3 more, not huge. Not the difference between minutes and weeks. That does depend somewhat on how thin it is though.

Solid elements can't properly transfer bending moments - Yes they can, they just look more like force couples instead. They go beyond shells and also properly transfer out-of-plane loads that you might not have realized were significant.

Solid elements can't report bending moments and forces - That's a limitation of software, not the elements. It's just harder for the programmer so often they don't do that. You might also see it as a limitation of design codes that ask you to find the more human-friendly and artificial forces and moments instead of the more natural general stress state.

Shells are meant for thin parts - Yes, and typewriters are meant for writing but you wouldn't use a typewriter when you want to write something. Even for thin parts, solid elements are more correct than shells and beams which have extra subtle simplifying assumptions that make them harder to use safely. I've seen people incorrectly using shells and beams because their parts were thin or beam-shaped, but they didn't realize that stresses at the details were wrong or their beams aren't nearly slender enough for the no-shear-stiffness assumption of simple Euler beam elements.

You can't easily change the thickness of solids - With FEA integrated into CAD, and fast meshing there's little or no advantage over shells here. Shells can actually be harder to change the thickness of because that might involve moving the midplane too.

You can't use some material models with solids - Again that's just a limitation of software.

If you don't believe me, have a look at Calculix. It internally converts shells and beams to solids for solving, then converts them back again for output!



FEA software feature chart

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