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[Xansys] [struct] Stress-Strain Curve with negative slope
 
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harish.radhakrishnan
User


Joined: 10 Dec 2013
Posts: 34

PostPosted: Tue Apr 15, 2014 7:08 am  Reply with quote

I don't believe that the user was right when he claimed that he had input a
negative slope. Please note that the slope of the uniaxial stress-strain
curve can be negative if engineering stress-strain curves are plotted.
However once you convert to true stress-strain, the slopes are always
positive.

For information on why you can not have a negative slop, please see the
Drucker's stability postulate.
http://en.wikipedia.org/wiki/Drucker_stability

Harish

Harish Radhakrishnan
Senior Technical Services Engineer
ANSYS Inc
15915 Katy Freeway, Suite 550
Houston, TX 77094
Office: 281-676-7013
Mobile: 509-715-9038
Help-Line:(800) 711-7199
harish.radhakrishnan@ansys.com
http://www1.ansys.com/customer/



On Tue, Apr 15, 2014 at 9:03 AM, Ayoola Brimmo <abrimmo@masdar.ac.ae> wrote:

Quote:
Hi,

In the regional ANSYS conference held in Dubai last year, one of the
presenters claimed to have modeled a material with a strain-stress curve
which included the necking region (negative slope). According to the
materials reference, "No segment on the stress-strain curve can have a
slope less than zero" and I pointed this out to him during his presentation
but he was adamant that the negative slope was modeled on ANSYS. In order
not to drag it for too long, we agreed to meet after the session but I
never saw this guy again. Unfortunately, this presentation wasn't in the
original schedule and he was presenting just because someone else drop out
at the last minute so I have no means of tracking him.

So my questions:
- Is there a material property model that can handle a negative
stress-strain slope?
- If yes, has anyone successfully used it?
- Though a long shot, did anyone on this list make such presentation?

Best,
Ayo Brimmo
Masdar, UAE


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ayo.brimmo
User


Joined: 01 Jul 2013
Posts: 38
Location: Masdar City, Abu Dhabi

PostPosted: Tue Apr 15, 2014 7:21 am  Reply with quote

Harish,

Thanks for the reply. Another reason to believe he wasn't paying attention to his student's explanation of what was actually modeled.

Best,
Ayo Brimmo
Masadr, UAE



-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of Harish Radhakrishnan
Sent: Tuesday, April 15, 2014 6:08 PM
To: ANSYS User Discussion List
Subject: Re: [Xansys] [struct] Stress-Strain Curve with negative slope

I don't believe that the user was right when he claimed that he had input a negative slope. Please note that the slope of the uniaxial stress-strain curve can be negative if engineering stress-strain curves are plotted.
However once you convert to true stress-strain, the slopes are always positive.

For information on why you can not have a negative slop, please see the Drucker's stability postulate.
http://en.wikipedia.org/wiki/Drucker_stability

Harish

Harish Radhakrishnan
Senior Technical Services Engineer
ANSYS Inc
15915 Katy Freeway, Suite 550
Houston, TX 77094
Office: 281-676-7013
Mobile: 509-715-9038
Help-Line:(800) 711-7199
harish.radhakrishnan@ansys.com
http://www1.ansys.com/customer/



On Tue, Apr 15, 2014 at 9:03 AM, Ayoola Brimmo <abrimmo@masdar.ac.ae> wrote:

Quote:
Hi,

In the regional ANSYS conference held in Dubai last year, one of the
presenters claimed to have modeled a material with a strain-stress
curve which included the necking region (negative slope). According to
the materials reference, "No segment on the stress-strain curve can
have a slope less than zero" and I pointed this out to him during his
presentation but he was adamant that the negative slope was modeled on
ANSYS. In order not to drag it for too long, we agreed to meet after
the session but I never saw this guy again. Unfortunately, this
presentation wasn't in the original schedule and he was presenting
just because someone else drop out at the last minute so I have no means of tracking him.

So my questions:
- Is there a material property model that can handle a negative
stress-strain slope?
- If yes, has anyone successfully used it?
- Though a long shot, did anyone on this list make such presentation?

Best,
Ayo Brimmo
Masdar, UAE


+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+

+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+
+-------------------------------------------------------------+
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| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+

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_________________
Ayo Brimmo
Research Engineer
Masdar Institute of Science and Technology
UAE.
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rick.fischer
User


Joined: 31 Aug 2011
Posts: 198

PostPosted: Tue Apr 15, 2014 9:29 am  Reply with quote

I had a PHB-driven need for a negative slope once long ago and it was suggested that a Mooney-Rivlin model with more than two terms could model it. I was dealing with a polymer, so this wasn't too far of a stretch, although for other materials this sounds like a real kludge-job. I never tried it. Don't remember where the suggestion came from but it was likely either the Ansys tech support in place for me at the time (KIT Corp or Silverado) or xansys. The context was that this had been tried successfully in the past.

Rick Fischer
Principal Engineer
Argonne National Laboratory

-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of Ayoola Brimmo
Sent: Tuesday, April 15, 2014 9:21 AM
To: ANSYS User Discussion List
Subject: Re: [Xansys] [struct] Stress-Strain Curve with negative slope

Harish,

Thanks for the reply. Another reason to believe he wasn't paying attention to his student's explanation of what was actually modeled.

Best,
Ayo Brimmo
Masadr, UAE



-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of Harish Radhakrishnan
Sent: Tuesday, April 15, 2014 6:08 PM
To: ANSYS User Discussion List
Subject: Re: [Xansys] [struct] Stress-Strain Curve with negative slope

I don't believe that the user was right when he claimed that he had input a negative slope. Please note that the slope of the uniaxial stress-strain curve can be negative if engineering stress-strain curves are plotted.
However once you convert to true stress-strain, the slopes are always positive.

For information on why you can not have a negative slop, please see the Drucker's stability postulate.
http://en.wikipedia.org/wiki/Drucker_stability

Harish

Harish Radhakrishnan
Senior Technical Services Engineer
ANSYS Inc
15915 Katy Freeway, Suite 550
Houston, TX 77094
Office: 281-676-7013
Mobile: 509-715-9038
Help-Line:(800) 711-7199
harish.radhakrishnan@ansys.com
http://www1.ansys.com/customer/



On Tue, Apr 15, 2014 at 9:03 AM, Ayoola Brimmo <abrimmo@masdar.ac.ae> wrote:

Quote:
Hi,

In the regional ANSYS conference held in Dubai last year, one of the
presenters claimed to have modeled a material with a strain-stress
curve which included the necking region (negative slope). According to
the materials reference, "No segment on the stress-strain curve can
have a slope less than zero" and I pointed this out to him during his
presentation but he was adamant that the negative slope was modeled on
ANSYS. In order not to drag it for too long, we agreed to meet after
the session but I never saw this guy again. Unfortunately, this
presentation wasn't in the original schedule and he was presenting
just because someone else drop out at the last minute so I have no means of tracking him.

So my questions:
- Is there a material property model that can handle a negative
stress-strain slope?
- If yes, has anyone successfully used it?
- Though a long shot, did anyone on this list make such presentation?

Best,
Ayo Brimmo
Masdar, UAE


+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+

+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+
+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+
+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+

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_________________
Rick Fischer
Principal Engineer
Argonne National Laboratory
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siva.ganesh
User


Joined: 14 Apr 2014
Posts: 32
Location: hyderabad

PostPosted: Tue Apr 15, 2014 9:43 am  Reply with quote

Dear All

Greetings to you, let me introduce my self. I am S.Sivaganesh,Currently in
the Ph.D Programme at Indian Institute of Technology Hyderabad on Design of
steel structures.


I'm doing a nonlinear buckling analysis of cold formed steel column....2400
mm length
i'm a beginner to ansys...

Brief about my work and doubt

Element used-Shell 181 0.5 mm thickness

It's a built up column, which means that a two section will be connected
together at the un stiffened portion or edge

I have done a imperfect model which means that including geometrical
imperfections or manufacturing tolerances.
There is no problem about the model.

Problem is about the loading and non linear analysis
i have used Rigid beam element RBE3 at the centroid of the section to apply
the loading

The definition of RBE3 is Distributes the force/moment applied at the
master node to a set of slave nodes, taking into account the geometry of
the slave nodes as well as weighting factors.

Earlier i have applied 2000 newtons through RBE3 element which you can see
which checking my coding.

When i given 2000 newtons of load (2KN) to the RBE3 element which is in the
top of the column and connected to the loaded nodes, i'm getting an error
says that

Reason for Termination..................Error in Element formulation
Can you please look at the way i did it and clear the error.

And also the Load-Deflection curve for the analysis should go up and once
the member reached the ultimate load it has to come down..but i dont know
why i'm not getting that kind of curve.

And also please tell me what is meant by SOLCONTROL command in ANSYS
I tried with OFF mode and ON mode but the load steps are same.

Please suggest me a good way to do it

i have to submit my report my end of this month

please find the attached file for more clarification of my work

thanks for your time


On Tue, Apr 15, 2014 at 9:58 PM, Fischer, Richard <rfischer@anl.gov> wrote:

Quote:
I had a PHB-driven need for a negative slope once long ago and it was
suggested that a Mooney-Rivlin model with more than two terms could model
it. I was dealing with a polymer, so this wasn't too far of a stretch,
although for other materials this sounds like a real kludge-job. I never
tried it. Don't remember where the suggestion came from but it was likely
either the Ansys tech support in place for me at the time (KIT Corp or
Silverado) or xansys. The context was that this had been tried
successfully in the past.

Rick Fischer
Principal Engineer
Argonne National Laboratory

-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of Ayoola Brimmo
Sent: Tuesday, April 15, 2014 9:21 AM
To: ANSYS User Discussion List
Subject: Re: [Xansys] [struct] Stress-Strain Curve with negative slope

Harish,

Thanks for the reply. Another reason to believe he wasn't paying attention
to his student's explanation of what was actually modeled.

Best,
Ayo Brimmo
Masadr, UAE



-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of Harish
Radhakrishnan
Sent: Tuesday, April 15, 2014 6:08 PM
To: ANSYS User Discussion List
Subject: Re: [Xansys] [struct] Stress-Strain Curve with negative slope

I don't believe that the user was right when he claimed that he had input
a negative slope. Please note that the slope of the uniaxial stress-strain
curve can be negative if engineering stress-strain curves are plotted.
However once you convert to true stress-strain, the slopes are always
positive.

For information on why you can not have a negative slop, please see the
Drucker's stability postulate.
http://en.wikipedia.org/wiki/Drucker_stability

Harish

Harish Radhakrishnan
Senior Technical Services Engineer
ANSYS Inc
15915 Katy Freeway, Suite 550
Houston, TX 77094
Office: 281-676-7013
Mobile: 509-715-9038
Help-Line:(800) 711-7199
harish.radhakrishnan@ansys.com
http://www1.ansys.com/customer/



On Tue, Apr 15, 2014 at 9:03 AM, Ayoola Brimmo <abrimmo@masdar.ac.ae>
wrote:

Quote:
Hi,

In the regional ANSYS conference held in Dubai last year, one of the
presenters claimed to have modeled a material with a strain-stress
curve which included the necking region (negative slope). According to
the materials reference, "No segment on the stress-strain curve can
have a slope less than zero" and I pointed this out to him during his
presentation but he was adamant that the negative slope was modeled on
ANSYS. In order not to drag it for too long, we agreed to meet after
the session but I never saw this guy again. Unfortunately, this
presentation wasn't in the original schedule and he was presenting
just because someone else drop out at the last minute so I have no means
of tracking him.
Quote:

So my questions:
- Is there a material property model that can handle a negative
stress-strain slope?
- If yes, has anyone successfully used it?
- Though a long shot, did anyone on this list make such presentation?

Best,
Ayo Brimmo
Masdar, UAE


+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+

+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+
+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+
+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+




--
*S.Sivaganesh*

+-------------------------------------------------------------+
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| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+

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