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Block Verification

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In the Block Verification example - www.comsol.com/showroom/gallery/9759/

Why must you have all 3 roller boundary conditions (RBC) to get a correct solution? If you just keep the RBC opposite the prescribed displacement boundary, and turn the others to free, you don't get a correct result (forget the boundary loads and plasticity, just a simple linear block). Doesn't make sense, with all 4 sides free the block should't rotate or asymmetrically move under a uniaxial and uniform displacement, but it does.

What am I missing?

~C

2 Replies Last Post 10 juil. 2012, 13:08 UTC−4
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Hello Chris Layman

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Nagi Elabbasi Facebook Reality Labs

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Posted: 1 decade ago 10 juil. 2012, 11:31 UTC−4
Hi Chris,

In a stationary structures problem you need to have enough boundary conditions to remove all rigid body modes even if your loading does not trigger these rigid body modes. Without proper constraints the stiffness matrix is singular and there is no unique solution since you can add a rigid body displacement and stiff satisfy equilibrium. If you pose this as a transient problem you should get a unique solution.

Nagi Elabbasi
Veryst Engineering
Hi Chris, In a stationary structures problem you need to have enough boundary conditions to remove all rigid body modes even if your loading does not trigger these rigid body modes. Without proper constraints the stiffness matrix is singular and there is no unique solution since you can add a rigid body displacement and stiff satisfy equilibrium. If you pose this as a transient problem you should get a unique solution. Nagi Elabbasi Veryst Engineering

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Posted: 1 decade ago 10 juil. 2012, 13:08 UTC−4
Great! Thanks for the tip!
Great! Thanks for the tip!

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