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[Xansys] ANSYS a large displacement analysis of abeam
 
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paris.altidis
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Joined: 21 Oct 2008
Posts: 398
Location: Melrose Park, IL

PostPosted: Thu Sep 03, 2009 6:18 am  Reply with quote

Hi Panagiotis,
If I had to guess: This must be a compressive load.
1) What is the error message you get from Ansys when the 150-step run fails to converge ??
2) Are the results of the auto time stepping and the 1500-substep solutions the same ??
3) What are the relative lengths and X-sections + the loads you are applying to that beam ?? If the loads are compressive indeed and you plot all the substep results up to T=0.66 do you see a constant force from some point onward ?? (in other words: signs of buckling)
4) Any details about the platform (WorkBench, Classic ; Ansys version) ??
5) Other non-linearities in the model (material, contacts) ??

Regards,
Paris Altidis
Belcan Corp.

-----Original Message-----
From: --email address suppressed-- on behalf of Kazantzis
Sent: Thu 9/3/2009 8:13 AM
To: ANSYS User Discussion List
Subject: [Xansys] ANSYS a large displacement analysis of abeam

Dear All



I perform a large displacement analysis (NLGEOM,ON) in a beam which is loaded only by axial forces. This beam has a rectangular section (which represents the bottom flange of an I section and its section varies along his longitudinal axe. This beam consists of seven continuous spans. In order to control load increment I use NSUBST command. When I used a value of 150 load steps (a time step of 1/150) the solution converges. When I use a value of 1500 the solution fail to converge at the time 0.666. I can't understand why. When I use automatic time step of ANSYS the solution converge rapidly. Which is the correct solution?



Thanks in advance



Panagiotis Kazantzis
Computer Control systems SA
94-96, Kifisias Ave.
Athens 15125 - GREECE
Tel. +30-210-8051730
Fax. +30-210-6147121
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carl.mally
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Joined: 21 Oct 2008
Posts: 108

PostPosted: Thu Sep 03, 2009 6:45 am  Reply with quote

>>When I used a value of 150 load steps (a time step of 1/150) the
solution converges. When I use a value of 1500 the solution fail to
converge at the time 0.666. I can't understand why. When I use automatic
time step of ANSYS the solution converge rapidly. Which is the correct
solution?<<

Here are a couple of thoughts on this subject. The first is that if you
have a snap through buckling condition at some point during the
solution, then the solution becomes unstable. Sometimes a larger
timestep will jump past the instability and give a solution. The second
is that with a lot of load steps, like you are using, you may be
applying the load in such small increments that some part of the
solution becomes near zero. For the types of analysis which I do even
150 would be a lot of load steps. 1500 seems like a very large number
of steps.

Carl Mally
Senior Applications Engineer
Centro Inc.
950 North Bend Drive
North Liberty, IA 52317
(319) 626-7472
fax: (319) 626-6920
--email address suppressed--



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panagiotis.kazantzis
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Joined: 29 Jun 2009
Posts: 7

PostPosted: Thu Sep 03, 2009 6:57 am  Reply with quote

Hi Paris

.

Thank you for your response.



The load is indeed a compressive load near the supports and a tensile load
at the mid spans.

The axial load varies along the longitudinal axis.



As you may already understand it is a buckling problem that I am trying to
solve.



In the model there are spring elements (COMBIN14) which are used in order to
model elastic supports at the direction of buckling



The method used is :



- I am performing an Euler buckling (this calculation gives a factor of 15
!!! for the first buckling mode)

- then I use the first buckling mode in order to impose an initial deformed
shape at my structure (ANSYS : upcoord,factor,on)

- then I perform a non linear analysis (NLGEOM,ON)



1) The 150-step model converge without error message

2) The results of the auto time stepping and the 1500-substep solutions are
not the same.

In fact the values of the stresses are much smaller.

The results of the auto time stepping and 150-step model which converge
are the same.

3) I have to check this

4) I am using ANSYS 8.0 structural version

5) There are not other non-linearities.



----- Original Message -----
From: "Altidis, Paraschos C." <--email address suppressed-->
To: "ANSYS User Discussion List" <--email address suppressed-->
Sent: Thursday, September 03, 2009 4:18 PM
Subject: Re: [Xansys] ANSYS a large displacement analysis of abeam


Hi Panagiotis,
If I had to guess: This must be a compressive load.
1) What is the error message you get from Ansys when the 150-step run fails
to converge ??
2) Are the results of the auto time stepping and the 1500-substep solutions
the same ??
3) What are the relative lengths and X-sections + the loads you are applying
to that beam ?? If the loads are compressive indeed and you plot all the
substep results up to T=0.66 do you see a constant force from some point
onward ?? (in other words: signs of buckling)
4) Any details about the platform (WorkBench, Classic ; Ansys version) ??
5) Other non-linearities in the model (material, contacts) ??

Regards,
Paris Altidis
Belcan Corp.

-----Original Message-----
From: --email address suppressed-- on behalf of Kazantzis
Sent: Thu 9/3/2009 8:13 AM
To: ANSYS User Discussion List
Subject: [Xansys] ANSYS a large displacement analysis of abeam

Dear All



I perform a large displacement analysis (NLGEOM,ON) in a beam which is
loaded only by axial forces. This beam has a rectangular section (which
represents the bottom flange of an I section and its section varies along
his longitudinal axe. This beam consists of seven continuous spans. In order
to control load increment I use NSUBST command. When I used a value of 150
load steps (a time step of 1/150) the solution converges. When I use a value
of 1500 the solution fail to converge at the time 0.666. I can't understand
why. When I use automatic time step of ANSYS the solution converge rapidly.
Which is the correct solution?



Thanks in advance



Panagiotis Kazantzis
Computer Control systems SA
94-96, Kifisias Ave.
Athens 15125 - GREECE
Tel. +30-210-8051730
Fax. +30-210-6147121
+-------------------------------------------------------------+
| XANSYS web - www.xansys.org/forum |
| XANSYS blog - xansys.blogspot.com |
| The Online Community for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
| Send administrative requests to --email address suppressed-- |
+-------------------------------------------------------------+



This e-mail transmission contains information that is confidential and may
be
privileged. It is intended only for the addressee(s) named above. If you
receive
this e-mail in error, please do not read, copy or disseminate it in any
manner.
If you are not the intended recipient, any disclosure, copying, distribution
or
use of the contents of this information is prohibited. Please reply to the
message immediately by informing the sender that the message was
misdirected.
After replying, please erase it from your computer system. Your assistance
in
correcting this error is appreciated.




--------------------------------------------------------------------------------


> +-------------------------------------------------------------+
> | XANSYS web - www.xansys.org/forum |
> | XANSYS blog - xansys.blogspot.com |
> | The Online Community for users of ANSYS, Inc. Software |
> | Hosted by PADT - www.padtinc.com |
> | Send administrative requests to --email address suppressed-- |
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panagiotis.kazantzis
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Joined: 29 Jun 2009
Posts: 7

PostPosted: Thu Sep 03, 2009 7:34 am  Reply with quote

I think that the problem is that 1500 is a very small time step and the load
becomes zero at a certain point.

Thanks a lot !


----- Original Message -----
From: "Mally, Carl" <--email address suppressed-->
To: "ANSYS User Discussion List" <--email address suppressed-->
Sent: Thursday, September 03, 2009 4:45 PM
Subject: Re: [Xansys] ANSYS a large displacement analysis of abeam


> >>When I used a value of 150 load steps (a time step of 1/150) the
> solution converges. When I use a value of 1500 the solution fail to
> converge at the time 0.666. I can't understand why. When I use automatic
> time step of ANSYS the solution converge rapidly. Which is the correct
> solution?<<
>
> Here are a couple of thoughts on this subject. The first is that if you
> have a snap through buckling condition at some point during the
> solution, then the solution becomes unstable. Sometimes a larger
> timestep will jump past the instability and give a solution. The second
> is that with a lot of load steps, like you are using, you may be
> applying the load in such small increments that some part of the
> solution becomes near zero. For the types of analysis which I do even
> 150 would be a lot of load steps. 1500 seems like a very large number
> of steps.
>
> Carl Mally
> Senior Applications Engineer
> Centro Inc.
> 950 North Bend Drive
> North Liberty, IA 52317
> (319) 626-7472
> fax: (319) 626-6920
> --email address suppressed--
>
>
>
> --------------------------------------------------------------------
> NOTICE: This e-mail (including attachments) is covered by the Electronics
> Communications Privacy Act, 18 U.S.C. 2510-2521, thus is confidential and
> may be legally privileged. The contents of this e-mail should not be
> forwarded to anyone without consent of the sender. If you are not the
> intended recipient, you are hereby notified that your retention, copying
> or any distribution of this communication is strictly prohibited. Please
> reply to the sender that you have received this e-mail message in error,
> then delete it. Thank you.
>
> +-------------------------------------------------------------+
> | XANSYS web - www.xansys.org/forum |
> | XANSYS blog - xansys.blogspot.com |
> | The Online Community for users of ANSYS, Inc. Software |
> | Hosted by PADT - www.padtinc.com |
> | Send administrative requests to --email address suppressed-- |
> +-------------------------------------------------------------+
>

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fern.thomassy
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PostPosted: Thu Sep 03, 2009 8:33 am  Reply with quote

> From: --email address suppressed-- [mailto:--email address suppressed--] On
> Behalf Of Kazantzis
> ...
> I think that the problem is that 1500 is a very small time step and the
> load becomes zero at a certain point.
>
> Thanks a lot !
>

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