Generative design for compressive loading avoiding buckling

[Anonymous]

I'll introduce myself here. I'm a retired civil engineer living in London with a longstanding interest in shape selection, of sections, stiffened plate, lattices etc and am working on a project which involves designing, manufacturing and physically testing struts. Each strut gets a score based on its strength divided by the strength of its 'reference strut',  a uniform solid cylindrical rod, having (i) the same material (ii) the same length and (iii) the same mass. A high score requires a strut with a large girth, spindle shaped, and enveloping as much space as possible. There is no theoretical limit to the  score that can be achieved. The score is limited practically by (i) the minimum thickness of the material that can be accurately manufactured and (ii) the quality of design.

How could Fusion 360 help me to design a high scoring strut? I am new to Fusion 360. With my free trial, the one I designed using generative design gave me a solid fully stressed rod which buckles under the design load, which was what I would expect in tension, but this was in compression.

[I_Forge_KC]

One of the pitfalls with the current state of Generative Design is that it doesn't have any eccentricity for the loads applied. This means that an object loaded in pure compression will undoubtedly make a very thin structure that meets the technical requirements but not the practical ones.

In order to more accurately design parts for compression, you can either add side loads (as many as needed in a mirrored or radially symmetric configuration) or you can add a moment axially to the strut. The moment is a simpler way of dealing with this because the result is a side force at a given distance, all around the strut (in any direction). If you clone the load case and then flip the sign/direction on the moment, you've created the mirror condition without having to specify any other information. The tough part here is determining the best magnitude to achieve high stiffness without adding too much material.

[Anonymous]

This does not solve my problem. My design space is an oversize solid
cylinder. I need Fusion 360 to create a tube or a lattice. Okay, I suspect
it can't do that. Does that mean I have to choose a topology myself, one
which cannot be changed by Fusion 360? Maybe I would do better to create my
own design and check it for buckling using simulation?

[Ben-Weiss]

Hi @Anonymous , and welcome to the Fusion forums!

By way of introduction, I'm Ben, I live in Portland, Oregon, and I'm one of the team of engineers and researchers that works on generative design's algorithms.

Kenny's advice above is right on. I can clarify that currently generative design does not support buckling in its simulations. We're acutely aware of this and many other areas where we can make generative more awesome and we're working hard to make it better with each release.

One additional thing you could try is to provide generative design with a starting shape that already has holes in it (but not a completely enclosed void; those aren't supported). That may help generative settle on a more tube-like design.

Good luck,

Ben

[Anonymous]

Hi Ben

Thank you for replying, and I appreciate that I'm in touch with someone
right at the sharp end of the software, and also that you're eager to
innovate. What I suggest is that you consider adding a facility to your
generative design that is able to generate form, a shell or a lattice,
depending on the loadings and the design space. When there are stresses in
the solid design space which are small and compressive and act over a
significant distance in the direction of the stress, then form is required,
and it is possible to deduce what form this should be. Your generative
design will be able to refine it, but you need a sensible form to start
with.

This link https://1drv.ms/b/s!AppoELBOIpLQgYFZpvLRWKeQA-2uTg?e=f2xZ2z
is to my
paper 'Macromaterials' published in 1994 in Proc. Seminar: 'Building the
Future: Innovation in design, materials and construction', Ed. F.K. Garas,
G.S.T. Armer, pub. E & A Spon, London, ISBN: 9780419183808. It explains the
relationship between shape and material, and particularly the importance of
repeating shape, i.e. form, for design. I hope you will find it seminal. It
has always been a matter of regret that these ideas, which got very little
attention, never managed to become better known, or indeed known at all, but
now, 25 years later, the climate has changed, and in my retirement I am
working hard to bring them again to people's attention. I am not writing
papers - life is too short - I am going straight to the innovators.

If you go to my website, www.dilution.xyz , which
is still in a very juvenile state, you will be able to download a copy of my
1984 paper 'Structuring: a process of material dilution'. This is not an
easy read, but demonstrates how macromaterials, or 'equivalent materials'
can be used in design.

There are many highly stressed mechanical engineering components that do not
need this treatment, while for light weight beams and struts it is
essential, and could be extremely rewarding for those components that are in
between.

Do tell me whether you think these ideas relating shape to material are
worth pursuing, and if I can answer any questions I shall be glad to do so.
You can email me through my website.

Be smart!

Graham

[Ben-Weiss]

Hello Graham,

Thanks for your message. You have an interesting concept here, and it's something I'll bear in mind as we consider future directions for generative design.

Best,

Ben

[Anonymous]

Hello Ben

Thank you for your response.

You have wisely removed my post from the forum.

Three further points:

1. Macromaterial compressive strengths take local buckling
into account at any number of hierarchical levels. The material
transformation factors must be worked out accurately for a range of forms,
each from stocky to slender, using a buckling analysis as a check perhaps.
This is pure analysis, not design, and is done just once, outside your
design software. Your design software will access this data as necessary.

2. You may be able to reduce iteration and get reliable solutions
more rapidly.

3. I shall be publishing material on www.dilution.xyz

Don't delay.

Best wishes

Graham

[Anonymous]

Ben

Look at my website dilution.xyz, as I have just added an article 'Strut design' explaining how to design the shape of a strut, both its convex envelope shape and its internal shape, its topology, using material dilution theory. Download it and read it carefully. I would appreciate your feedback.

Regards

Graham