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Bending rebar (metal dynamics)

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Aaron Pozzer
Bending rebar (metal dynamics)
on Jun 28, 2013 at 8:51:39 pm

so this is a general 3d question. it is not maya specific. if someone has a suggestion that works in another package, please feel free to contribute that idea as well.

im looking for a good way to make bending/deforming metal rebar for use in a demolition simulation. i have tried cloth/soft dynamics quickly, but was not getting the results i would have expected. granted im not a cloth wizard, so maybe thats to be expected.

either way, im looking for a solution that will allow for cylinders (the rebar) to be bent (ideally via dynamics so they dont have to be hand keyed) and RETAIN their bent shape. my quick tests with cloth just lead me to limp floppy spaghetti. i couldnt get it to start firm, be hit/bend, and retain that shape.

iv looked on google and youtube, and there seems to be an overall lack of any info on how to make deformable metal. something that can accept dynamic deformation, but doesnt try to pop back into shape, or blow away like an old sac when hit.

lookin forward to some suggestions!


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Steve Sayer
Re: Bending rebar (metal dynamics)
on Jun 28, 2013 at 9:59:12 pm

Maya cloth should certainly be able to handle this. The key attributes are Restitution Angle and Deform Resistance.

First set your cloth object to have high deformation resistances: try 200 for Stretch and Compression,, and 100 for Shear and Bend. This should leave you with a fairly stiff object.

Now, if you deform it with a collision, it may still spring back into shape. That's what we address with those attributes I mentioned.

'Restitution Angle' is confusingly named, but you can think of it as "how far can this object be bent before it 'gives up' on springing back?" The default value of 360 means you can bend the cloth as far as you like and it will still exert some force in an attempt to spring back. Set it to 0 instead and it will just accept the new bent position and not exert any force back along the bend.

In a similar way, increasing the Deform Resistance to '10' or higher will make the object strongly resist being deformed from its current formation--even if that current formation is very different from its initial state. This is what lets you hammer an object into a new shape and have it keep that shape. Note that this is different from Rigidity, which will always resist deformation, not from the object's current shape, but from its remembered original shape. Again, this will tend to make it pop back into its original configuration.

One thing to keep in mind: if you use a cylindrical model for your rebar, you will find that it tends to get collapsed into a flattened ribbon shape by collisions on one side. You might experiment with using something like hair curves instead of cloth, since those are optimized for doing the sort of simple thin bending you're after.

Hope that helps.


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Aaron Pozzer
Re: Bending rebar (metal dynamics)
on Jun 28, 2013 at 10:50:18 pm

thanks steve, ill give that a try in the morning!


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Aaron Pozzer
Re: Bending rebar (metal dynamics)
on Jun 29, 2013 at 11:49:43 am

thanks steve, that helped out. i think my mistake yesterday was i was not using nCloth. i fooled with the settings and have something that definitely holds its shape more when struck. however...

what i did was make a simple cyclinder, and do a transform constraint on the cap so it didnt fly away completely when struck. when it IS struck, near the tip (opposite the constraint), the entire bar moves using the constraint as a pivot. so my next question is: is it possible to have the bent area be more localized to the impact area. the collision should turn a straight line into and L, rather than just rotate the whole line 90 deg.... if that makes more sense? currently im just getting complete rotation of the object, id like to be able to bend it. i know i could probably map this with weights, but im wondering if there is a setting for this.

thanks again.


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Steve Sayer
Re: Bending rebar (metal dynamics)
on Jul 5, 2013 at 2:00:51 pm

You can either map it with weights or you can make your constraint larger (i.e. constrain half the bar rather than just the end cap).

In the real world, if you had a long stiff bar that only had the very end attached to a fixed surface, and you pushed the free end sideways, I think you'd find that you would get rotation about the fixed end rather than an L-bend in the middle. Try it with a drinking straw!


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