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Linear Results?

I took an object and applied a 125lbf force and noted the stress and displacement results. I repeated the process with a 1250lbf force and the results were 10 times that of 125lbf. I repeated the process with other forces and found I continue to get linear results. I am exceeding the material's tensile strength so I question these results…

Any thoughts?

Thanks.
3 REPLIES

Re: Linear Results?

This is exactly what a linear analysis is - displacement is linearly proportional to the applied load.

If you end up with stresses that exceed yield, you have moved into the nonlinear realm and must run a nonlinear analysis to get non-proportional results.

Regards,
Jim
--
Jim Bernard
Advanced Applications Engineer

Siemens PLM Software
2000 Eastman Dr., Milford, OH 45150-2712
www.siemens.com/plm

Re: Linear Results?

Dear Apuzzuoli,
When you solve a linear static analysis you are telling to the FEA code that you understand what you do, that is, relation between force & displacement is linear & constant, and relation between stress & strain is linear & elastic, not matter the level of load prescribed to the model. Well, you have what you deserve. But we are intelligent to see that if vonMises stress exceed the material yield stress then he analysis results are useless at all, i.e., our assumtion that the problem is linear for the material is not valid at all, then is our fault, not the FEA code. Also, if displacements results are large instead small (specially in thin wall models, i.e., when the displacements are at the same level of the element thickness) you may experience a stress stiffening behaviour, ie, the structure becames more & more stiffen, then again the relation between load & displacement is not linear, to capture this behaviour you will need to run a nonlinear analysis. And finally, you may experience a stress softening, i.e., buckling, this is typically a geometric nonlinear situation that happens in Shell models where large compressive membrane stresses reduces the bending stiffnes to zero, that is, the structure buckels, and sureley the stress is well below the yield stress of the material.
In summary, is up to you to know what you are doing: real life is all nonlinear and transient, be aware if the real life loads & deformations and mesh the problem with quality elements, then setup the analysis with knowledge: you can run a nonlinear analysis activating nonlinearities of geometry (large dispalcements), material (plasticy, elasto-plastic models, etc..) & contact and compare solutions with a linear static analysis. If both results agree, then your problem is linear, simply!!. Compare & compare, this is the key in simulation ....
Best regards,
Blas.
--
~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
Edificio Ercilla
Rodríguez Arias 23, 3º - Dpto. 19
48011 BILBAO (SPAIN)
Tel. (+34) 94 410 65 50
Fax. (+34) 94 470 26 34
E-mail: info@iberisa.com
WEB: http://www.iberisa.com
"apuzzuoli" escribió en el mensaje de noticias news:apuzzuoli.4262cz@noreply.bbsnotes.ugs.com...
>
> I took an object and applied a 125lbf force and noted the stress and
> displacement results. I repeated the process with a 1250lbf force and
> the results were 10 times that of 125lbf. I repeated the process with
> other forces and found I continue to get linear results. I am exceeding
> the material's tensile strength so I question these results.
>
> Any thoughts?
>
> Thanks.
>
>
> --
> apuzzuoli
> ------------------------------------------------------------------------
> apuzzuoli's Profile: http://bbsnotes.ugs.com/vbulletin/member.php?userid=191150
> View this thread: http://bbsnotes.ugs.com/vbulletin/showthread.php?t=40496
>

Re: Linear Results?

Just to add to Blas's comments...
I assume you are new to FE or Nastran? It is well worthwhile getting some
training from your local FE support team, as these topics are usually
covered thoroughly.
When you created the analysis, you must have chosen "Linear Static" or
"SOL101" in Nastran language. In this case, everything will be linear
(unless you define some contacts) to any scale. If a 1lb force produces a
3psi stress, then 1e9 lbf will produce exactly 3e9 psi in a linear analysis.
The linear assumption is that your whole model is represented by a
complicated set of absolutely linear springs. Accounting for yield will
require a non-linear analysis (static or transient). In that case, don't
try to apply the 1e9 lbf, as the analysis will fail well before that force
is reached!

"apuzzuoli" wrote in message
news:apuzzuoli.4262cz@noreply.bbsnotes.ugs.com...
>
> I took an object and applied a 125lbf force and noted the stress and
> displacement results. I repeated the process with a 1250lbf force and
> the results were 10 times that of 125lbf. I repeated the process with
> other forces and found I continue to get linear results. I am exceeding
> the material's tensile strength so I question these results.
>
> Any thoughts?
>
> Thanks.
>
>
> --
> apuzzuoli
> ------------------------------------------------------------------------
> apuzzuoli's Profile:
> http://bbsnotes.ugs.com/vbulletin/member.php?userid=191150
> View this thread: http://bbsnotes.ugs.com/vbulletin/showthread.php?t=40496
>