iMechanica - ansys - Comments

Re:NLGEOM in ANSYS

shrimad — Fri, 01 Aug 2008 17:08:00 -0400

Biswajit,

Thanks for the reply.

If I consider a small problem; a square as an axisymmetric model (giving rise to a cylinder in 3D). The material model is viscoelastic. If I have a line on the top surface and bottom surface and press the top surface to about half the height with the lower surface fixed (contact analysis, no friction) I get a deformed rectangle. I do the same with ANSYS and with my FEA code in FORTRAN. What happens is:

Without NLGEOM ON, I get an exact match with my code and ANSYS, but the volume is not conserved.

With NLGEON ON, I get a slightly larger rectangle and the volume is conserved in ANSYS in this case.

I want to conserve the volume in my FEA code also, and hence I am trying to understand what modification do I need to do in my FEA code? My FEA code follows Newton Raphson Method and the same algorithm as ANSYS for viscoelastic stress calculations. I am stuck in my research because of this. Please guide me with whatever you know.

Thanks,

Shriram

Research Assistant
Structural and Multidisciplinary Optimization Lab.

University of Florida

Re: NLGEOM in ANSYS

Biswajit Banerjee — Thu, 31 Jul 2008 23:24:37 -0400

The NLGEOM command activates corrections for large rigid body rotations and translations when a small strain constitutive relation is used. It is also activated for large strain material models. For details on the implementation you can go to Chapter 3. Structures with Geometric Nonlinearities of the ANSYS Theory Manual.

Using NLGEOM should have no impact on whether the incompressibility condition is satisfied. Incompressibility is usually satisfied using a Lagrange multiplier approach which can lead only to approximate satisfaction of incompressibility in a numerical code. However, there are are stress update algorithms that satisfy incompressibility exactly, often by using an exponential map - see Weber and Anand, 1988 (or so).

A cut/paste job from the ANSYS manual follows.

-- Biswajit

3.3. Large Rotation

If the rotations are large but the mechanical strains (those that cause stresses) are small, then a large rotation procedure can be used. A large rotation analysis is performed in a static (ANTYPE,STATIC) or transient (ANTYPE,TRANS) analysis while flagging large deformations (NLGEOM,ON) when the appropriate element type is used. Note that all large strain elements also support this capability, since both options account for the large rotations and for small strains, the logarithmic strain measure and the engineering strain measure coincide.

3.3.1. Theory

Large Strain presented the theory for general motion of a material point. Large rotation theory follows a similar development, except that the logarithmic strain measure ((Equation 3–6)) is replaced by the Biot, or small (engineering) strain measure: (3–37)where: [U] = stretch matrix [I] = 3 x 3 identity matrix 3.3.2.

Implementation

A corotational (or convected coordinate) approach is used in solving large rotation/small strain problems (Rankin and Brogan(66)). "Corotational" may be thought of as "rotated with". The nonlinearities are contained in the strain-displacement relationship which for this algorithm takes on the special form:(3–38)where: [Bv] = usual small strain-displacement relationship in the original (virgin) element coordinate system [Tn] = orthogonal transformation relating the original element coordinates to the convected (or rotated) element coordinates

The convected element coordinate frame differs from the original element coordinate frame by the amount of rigid body rotation. Hence [Tn] is computed by separating the rigid body rotation from the total deformation {un} using the polar decomposition theorem, (Equation 3–5). From (Equation 3–38), the element tangent stiffness matrix has the form: (3–39)and the element restoring force is: (3–40)where the elastic strain is computed from: (3–41) is the element deformation which causes straining as described in a subsequent subsection.

The large rotation process can be summarized as a three step process for each element:

Determine the updated transformation matrix [Tn] for the element.

Extract the deformational displacement from the total element displacement {un} for computing the stresses as well as the restoring force .

After the rotational increments in {Δu} are computed, update the node rotations appropriately.

All three steps require the concept of a rotational pseudovector in order to be efficiently implemented (Rankin and Brogan(66), Argyris(67)).

Chandra, Thanks for the

shrimad — Thu, 31 Jul 2008 11:25:47 -0400

Chandra,

Thanks for the reply.

I couldn't find the technical details in the ANSYS help menu. By theory manual do you mean the 'help' section or it is something different? Please let me know.

Thanks once again.

Shriram

It uses a Hencky or

Chandra Veer Singh — Wed, 30 Jul 2008 22:22:52 -0400

It uses a Hencky or logarithmic strain measure, which is ln(U), where U is the right stretch matrix obtained by polar decomposition of the deformation gradient. For calculating Hencky strain, it uses the eigen values and eigen vectors of U. You can read more in ANSYS theory manual, there is a chapter on structural nonlinearity.

Chandra Veer Singh

Thank you I appreciate your

Bernard-Marie Feron — Wed, 16 Jul 2008 10:53:57 -0400

Thank you I appreciate your suggestion.

I've got an excel table showing clearly the comparison, i can transmit it to everyone who wants to.

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Just ASK.

This is just a suggestion.

RoozbehSanaei — Wed, 16 Jul 2008 10:26:36 -0400

Most of users are interested in more popular softwares may be because there is more information about them in the web. also chance of finding person who know that softwares is more. i suggest you to make a comparision table between your software and similar softwares to show you advantages and also disadvantages of this software. what is possible with your software which is not possible with other more popular softwares?

do you need ?

Bernard-Marie Feron — Wed, 16 Jul 2008 09:15:53 -0400

Hi, do you need infos?

Make a comparison table

Roozbeh Sanaei — Wed, 16 Jul 2008 08:05:44 -0400

Roozbeh Sanaei.

Iran University of Science and Technology.

Acoust. Lab.

Yes we are

Bernard-Marie Feron — Wed, 16 Jul 2008 07:15:00 -0400

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This way was chosen to optimize the strong couplings.

We can answer to most of the demands and also more than ANSYS, but we have no industrial solution for High Frequency.

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What about ansys comparison? You seem real competitors

proE — Wed, 16 Jul 2008 07:07:43 -0400

I've been checking on your website, I see that you have a full solution under the same interface, do you have any comparison table between you and ansys?

More further , do you have support and training courses available?

thnx

simple

Bernard-Marie Feron — Tue, 15 Jul 2008 10:53:55 -0400

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regards

where and how much

proE — Tue, 15 Jul 2008 10:50:26 -0400

Hi oofelie, I'm reseracher, don't know about your solution.

How to get it and test it?

Do you have any demo license or academic licenses?

I'm currently facing the

Julio — Wed, 02 Jul 2008 05:52:46 -0400

I'm currently facing the same problem. Can you give me some advise?

APDL can solve your problem by performing an simple code

Pengfei Liu — Fri, 23 May 2008 02:33:16 -0400

Here, I have dealt with your problem successfully. I implement the code using APDL in ANSYS, which is employed as interface to the main program.

So, I suggest you conduct an subroutine using APDL in ANSYS to link the failure criteria to the main program.

I think the function of ANSYS is not strong for simulating the failure and damage of the laminated structures, especially the plastic post-buckling collapse because I have been concentrating on this reseach recently.

Mail

laplace — Sun, 27 Apr 2008 16:09:15 -0400

I send a solution to ur mail "rafsanjani@mecheng.iust.ac.ir"
hi i form shahrood univercity of technologoy and i master in mechanical engineering
I model a moving Load(Mass) in ansys that mooving over the simple suported baem this is very simple
U must aplead a load to first node and solve the problem and delete a load and solve again
and aplead a load to second node and solve again this loop result a good soloution to movung load.
the time inteval betwin the laod is (lenth of beam)/(velocity of load*number of node)

U can contact to me and IF U can talk and write and read Persia that very good
The moving load is the my bachelor proje