Robot Structural Analysis

Robot Structural Analysis

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Posts: 135
Registered: ‎10-10-2012
Message 1 of 7 (339 Views)

Plastic ratio

339 Views, 6 Replies
10-10-2012 09:48 AM

I would like to ask a question that concerns the elasto-plastic analysis.

I modeled an element with a standard section, HEA200, having an elastic-perfectly plastic behavior subjected in pure bending. An unjustified result that I noticed is that when the moment at a node of a member reaches the plastic moment resistance of the section (Mpl,Rd), the corresponding plasticity ratio is never equal to 1.0 but is less (much as I increase the discretisation along the element or/and the number of layers in the cross section).

Is this because of  the deficiency of the used formulation (DBE) ?

Thank you.

Product Support
Posts: 3,765
Registered: ‎04-26-2010
Message 2 of 7 (320 Views)

Re: Plastic ratio

10-10-2012 11:36 PM in reply to: Tuctas

Hi

Full plasticity of a section - Value of the plastic ratio equals 1.0

Example:

Rafal Gaweda
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Posts: 135
Registered: ‎10-10-2012
Message 3 of 7 (309 Views)

Re: Plastic ratio

10-11-2012 02:56 AM in reply to: Tuctas

In your example there is no moments at the member with plastic properties. If there is only axial force then the value of plastic ratio is o.k indeed.

Take a look at the attached picture, the plastic resistance moment is reached at the base node of the member but the corresponding plastic ratio isn't equal to 1.00. (I have done anything in order to model the member more precisely: number of cross-section division 25/25, the element is divided in 12parts and each of them has 4 intermediate integration points, load increment number 30..).

Thank you,

Product Support
Posts: 3,765
Registered: ‎04-26-2010
Message 4 of 7 (299 Views)

Re: Plastic ratio

10-11-2012 03:39 AM in reply to: Tuctas

It is more sensitive for bar length than discretization of cross section.

Example attached.

Rafal Gaweda
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Posts: 135
Registered: ‎10-10-2012
Message 5 of 7 (277 Views)

Re: Plastic ratio

10-11-2012 09:15 AM in reply to: Tuctas

When i run your .rtd file (with your settings), the message "instability of type 3" occurs continiously and only if i select "cancel" the calculations end.

But even if we ignore that message i think that it is not rational at all to choose as 0.005m (=5mm...) the max length of member's division for the simpliest of simple cases (just a cantiliver), what would happen if we have a normal structure...?

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Posts: 3,765
Registered: ‎04-26-2010
Message 6 of 7 (241 Views)

Re: Plastic ratio

10-12-2012 12:38 AM in reply to: Tuctas

Either lower results accuracy or much denser discretization (number of bar fes along bar)

Rafal Gaweda
Distinguished Contributor
Posts: 135
Registered: ‎10-10-2012
Message 7 of 7 (236 Views)

Re: Plastic ratio

10-12-2012 01:16 AM in reply to: Tuctas

That is general rule for structural analysis anyway, i am just "worrying" about Robot's accuracy because it is not easy for me to justify such kind of results for a so simple model.

I just wanted to keep in mind the same worry too..

Thank you

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