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Turn off Nonlinear effect in linear elastic analysis?

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Mm.Kaiser

Mm.Kaiser

Mechanical
Nov 1, 2011
45
Dear members,
I use Ansys for Elastic analysis of a part according to ASME VIII-2 Part5 and i have 2 questions:

1- Should i turn of "Nonlinear effects" in Geometry material properties or it automatically use linear analysis if we didn't assign a nonlinear properties (such as Tangent modulus) to the material's specification?

2- As you know, CONTACTS convert our analysis to nonlinear. Is there any problem to use contacts in Elastic Analysis and make Stress linearization according to part5?

Thanks.
 
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As far as I know you cannot use full non linear contact in a linear analysis, in linear analysis you may be able to use a welded or bidirectional slide type of contact.

But it may have changed now. As far as nonlinear material goes I do not think you can use it in linear analysis.

It will simply take the slope of the first part of the curve and go along that curve.

It is very uncommon in aerospace in general to use a nonlinear material, we prefer to be in the linear part.



Stressing Stresslessly!
 
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Thus question would better be posed in the ASME forum.
 
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Thanks all although i didn't clearly get my answer.
Maybe it was better to post it in ASME forum.
Regards
 
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In Abaqus it is possible to run nonlinear analysis (contact) with linear material and geometry. In certain cases this makes sense and speeds up the analysis a bit.
 
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A couple of general notes:

a) A stress-strain plot that is NOT a straight line within the elastic limit does NOT automatically mean the stress-strain relationship is NONLINEAR. A straight line is only a special case of linearity.

Nonlinearity has a precise mathematical meaning in that if the superposition principle does NOT apply, then the mapping is nonlinear. So, for instance, if a AND b both are solutions to a linear differential equation, then a (linear) combination of a and b is also a solution to the said differential equation.

b) Nonlinearity does NOT arise only by material relationship being nonlinear. There are multiple sources of nonlinearity (large deformation, contact, and large rotation).

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a) A stress-strain plot that is NOT a straight line within the elastic limit does NOT automatically mean the stress-strain relationship is NONLINEAR. A straight line is only a special case of linearity.

Nonlinearity has a precise mathematical meaning in that if the superposition principle does NOT apply, then the mapping is nonlinear. So, for instance, if a AND b both are solutions to a linear differential equation, then a (linear) combination of a and b is also a solution to the said differential equation.
Click to expand...

Can you provide a practical example where the superposition principle does apply for materials that have a stress-strain relationship that is not a straight line?

Doug Jenkins
Interactive Design Services
 
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Sure, I can think of some soft tissues (ligaments) who non-straight stress-strain relationship scales linearly (within a certain range).
Click to expand...

Are you just saying that parts of a non-straight stress-strain relationship can be straight over a limited range?

If not, can you give more details.

Doug Jenkins
Interactive Design Services
 
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No, that is not what I am saying. What I am saying is - within certain limits of strain rates and other reasonable conditions - some soft tissues can be considered to obey the superposition principle. There is a vast body of literature (experimental, theoretical, and computational) that makes such an assumption.

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I thought I already gave an example. I am unable to sense if you are genuinely curious or pretending to be curious because you find my statements refutable or worse. I would like to believe I am wrong so I recommend going through the first chapter of Mechanical Response of Polymers by Alan Wineman and K. R. Rajagopal. They clearly point out the point that I have been trying to make, unsurprisingly, far more eloquently than I ever can.

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Icebreakersours - I was asking for an example or two from the "vast body of literature (experimental, theoretical, and computational) that makes such an assumption", which you had not given before. I was hoping for a link or two to an article that I could download, rather than buying a book on a subject outside my area of work.

I am trying to understand what your point is, and I was hoping that a technical paper might make it clear, with some examples of how non-linear stress-strain behaviours combine in a linear way, other than as a rough approximation.

Or perhaps you just mean that assuming linear behaviour can often be a useful approximation to highly non-linear behaviour, which (I hope) no-one would dispute.

At the moment, I just don't know.

Doug Jenkins
Interactive Design Services
 
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Fair enough. I will scan a page or two from the text I mentioned above and post the image here, if you wish. However, if I recall correctly, that specific section does NOT mention any examples. I do not know if they discuss examples later on in the text but I can check it for you.

To be sure, I believe we are in agreement.

IceBreakerSours said:
within certain limits of strain rates and other reasonable conditions - some soft tissues can be considered to obey the superposition principle.
Click to expand...
IDS said:
Or perhaps you just mean that assuming linear behaviour can often be a useful approximation to highly non-linear behaviour, which (I hope) no-one would dispute.
Click to expand...



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Thanks IceBreakerSours. Probably I was reading too much into your original post. I would be interested to see the extracts, if you have time.

Doug Jenkins
Interactive Design Services
 
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