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Non exactly zero stress at unload (elastic model)?

Non exactly zero stress at unload (elastic model)?

Non exactly zero stress at unload (elastic model)?

(OP)
Hello,
For a linear isotropic elastic material, I applied a load defined by an amplitude (0,0;0.25,1;0.5,0;0.75,-1;1,0) to obtain uniaxial tension on a block.
At steptime 0.25 and 0.75, the stress is OK (constant=to the applied load); at time 0.5 and 1, there is a very small negligible stress (1.10^-13 MPa compared to 10 MPa) distribution in the part. I am very surprised with this result at unload (see attachment) especially in linear elasticity where you should come back to the initial state when you unload. I tried it in ANSYS, and one obtains exactly zero!!!!
Is this normal? Is it a problem known with Abaqus? Is there a way to overcome this? It is perturbing when teaching to students.
Thank you for your help
Rgx

RE: Non exactly zero stress at unload (elastic model)?

Without looking at your model, I can tell you one thing: In about a decade of doing FE work, except in a few rare occasions, Abaqus was correct every time I suspected it was wrong. I would imagine any major general purpose commercial grade code with an established history and customer base to be the same.

Also, as a teacher you should know that there is no such thing as absolute zero in numerics. A computer code does not know or care about it; a developer does. Odds are ANSYS (by default, perhaps) decides to round it off for you and Abaqus does not (1.10^-13 in the context of the model is likely to be as good as zero). What you might want to pay attention to is that ANSYS users tend to rely on units, whereas there is no such thing as a unit in Abaqus - the user is responsible to choose a consistent unit system appropriate for the physics they are simulating.

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RE: Non exactly zero stress at unload (elastic model)?

(OP)
OK. Let's give more details.
Find attached the inp file that was run to obtain the previous result.

According to the Field output lines, the end result is different (10^-13 in the present case).
If you comment the first line and uncomment the second one, the end result is EXACTLY zero.

*Output, field, variable=PRESELECT, number interval=4
***Output, field, variable=PRESELECT

I am just impressed that for elasticity, we obtain two different results according to the number of intervals that are requested.
I agree that 10^-13 is zero but it does not explain why it depends on the number of intervals.
Thank you for your help.
rgx

RE: Non exactly zero stress at unload (elastic model)?

I recommend going through the documentation to understand how post-processing is done in a typical FE code i.e., how you go from displacements to strains and stresses, how the field variables get extrapolated and averaged at nodes, what the meaning of a number you are seeing for a field variable in the user interface is, etc. to get a deeper understanding of what you are seeing.

Absolute zero is only possible when you are doing an analytical analysis (i.e., working with mathematical equations), not a numerical one. Again, in certain situations, a developer may choose to display an exact zero on the screen but there is no exact zero in a computer - it can not be. For instance, it is possible that 10^-20 is an insignificant number in one application whereas the same number can mean a LOT in another, depending on the physical quantity/unit system/ .. Without context, numbers have no meaning.

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http://www.eng-tips.com/faqs.cfm?fid=1083

RE: Non exactly zero stress at unload (elastic model)?

(OP)
So as far as i understand, Abaqus decided to display zero in some cases and not in others.
Zero stress is displayed with some "output field" options and not for others.
Zero stress is displayed for the tensile direction at unload but not for other components.
Of course I am going through the documentation, but so far I did not get a clear answer about the "output field" options and their effect on the result.

Regards


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