iMechanica - Forum topic - Comments

Hi Zheng & Yazri I

mohammedyahya — Wed, 08 Feb 2012 07:03:27 -0500

Hi Zheng & Yazri

I am also new to XFEM in Abaqus and i am facing exactly the same problem.

Zheng, would you be able to send that pdf file on how to improve the convergence in XFEM ? You can contact me at mohammed20074@yahoo.com

mohammed

Hi Zheng & Yazri I

mohammedyahya — Wed, 08 Feb 2012 07:02:39 -0500

Hi Zheng & Yazri

I am also new to XFEM in Abaqus and i am facing exactly the same problem.

Zheng, would you be able to send that pdf file on how to improve the convergence in XFEM ? You can contact me at mohammed20074@yahoo.com

mohammed

Hello Roger,

ravi_gutti — Wed, 08 Feb 2012 00:09:39 -0500

Hello Roger,

Natural frequency is directly proportional to stiffness of the body. This stiffness inturn is directly proportional to the internal strain energy of the body.

This means when the internal strain energy increases the natural frequency also increases and vice vers.

Anybody correct me if i am wrong...

ravi

You can change the

muzamil72003 — Tue, 07 Feb 2012 22:39:33 -0500

You can change the frame in field output under step module.(in interval box)

Thanks for sharing

It is in fact a warning and

pirs — Tue, 07 Feb 2012 07:28:20 -0500

It is in fact a warning and Abaqus/Explicit is able to run the entire analysis.

However, the warning is still a bit weird, since I have clearly defined the boundary conditions on the reference node, which of course does belong to the rigid body. Nevertheless, my latest results have improved and seem much closer to what I expected, which makes me think that this issue probably isn't causing much trouble on the analysis or the final results.

Anyway, thank you very much for your input.

Software Implementation of xFEM

hhayrullahh — Sun, 05 Feb 2012 04:46:12 -0500

Hello,

Is there anybody who can provide me with educational materials related to ANSYS or Abaqus, analysis using XFEM (especially for solution of multiphase flows). I want to learn how to use Xfem in these kind of softwares. Thank you!

Re: Johnson-Cook

Biswajit Banerjee — Sun, 05 Feb 2012 00:33:05 -0500

Answers to some FAQs:

1) Non-linear analysis involves a series of load increments- will the effective plastic strain be different at every load increment?

Non-linear finite element analyis of solids is usually displacement driven (see Chapter 6 of Belytschko et al, Nonlinear Finite Elements on how one such algorithm works and how the displacement increment is estimated from the applied load.)

If the displacement increases monotonically (or is constant) and the stress state is on the yield surface, the plastic strain will increase. If the displacement decreases so that the stress state goes back inside the surface, the plastic strain will remain constant.

Whether this reflects the physical world is another question altogether.

2) Suppose we do not include the model for fracture then when will the analysis terminate? We mean there has to be limiting strain or limiting stress?

The analysis terminates when your have reached your applied load. You can break your specimen in a finite element calculation only if you explictly specify when it breaks and what happens after that. The JC damage model can be used to decide when something will break. But each element will have a different damage state unless the deformation is homogeneous.

3) We know that true stress involves area of the deformed specimen and nominal stress involves area of the undeformed specimen. Given the nominal stress and strain only (in case of tension specimen as you see in the charts above), how do we get corresponding true stress and true strain for tension specimen?

Assume incompressiblity during plastic deformation to get the deformed area. Ideally you should measure the deformed area if you can. Just because people don't doesn't mean that it is not neceesary to measue the deformed area as a function of load.

4) Elastic indicates that the specimen regains its original shape/length on removal of load, how to get the elastic part from the stress strain curve?

Using the elastic modulus calculate the strain for a given stress.

Properties of cohesive zone models and their parameters

marco.paggi — Sat, 04 Feb 2012 15:24:16 -0500

Dear All,

in this recent article you may find how to select the properties of the cohesive zone model (stiffness, and so on) according to physical arguments:

M. Paggi, P. Wriggers: "Stiffness
and strength of hierarchical polycrystalline
materials with imperfect interfaces", Journal of the Mechanics and
Physics of Solids, in press, doi:10.1016/j.jmps.2012.01.009

This has been possible by proposing a new CZM for finite thickness interfaces:

M. Paggi, P. Wriggers: "A nonlocal
cohesive zone model for finite thickness interfaces – Part I:
mathematical formulation and validation with molecular dynamics", Computational Materials Science, Vol. 50 (5), 1625-1633, 2011.doi:10.1016/j.commatsci.2010.12.024

Here you may find its implementation in the finite element code FEAP:

M. Paggi, P. Wriggers: "A nonlocal
cohesive zone model for finite thickness interfaces – Part II: FE
implementation and application to polycrystalline materials", Computational
Materials Science, Vol. 50 (5), 1634-1643, 2011. doi:10.1016/j.commatsci.2010.12.021

Best regards,

Marco Paggi

Enjoying great moments

rodrigoengmec — Sat, 04 Feb 2012 15:06:17 -0500

You probably are not using the definition of kinetic energy...

Forget the right side ... and derive the kinetic energy and use that statement you gave to you.

When you do that, magic happens: balance of mass, incompressibility and Divergence Theorem.

Take care of definitions and all tools that math offer.

I think "GURTIN, M.E. - An Introduction to Continuum Mechanics " a excellent book.

Rodrigo

I have implemented the EFG

J.T.B. Overvelde — Fri, 03 Feb 2012 08:27:24 -0500

I have implemented the EFG method for solving the PDE momentum equations. You can find a Matlab implementation here. If you like to know more, feel free to ask!

Dear Sir, Sorry for being

kajalschopra — Thu, 02 Feb 2012 21:38:36 -0500

Dear Sir,

Sorry for being repetitive..I'm actually from the Structural Engineering stream -not Mechanical and am thus not confortable with questions you asked.

Yes,I shall read the book you mentioned.I have read mostly Elements of Strength of Materials by Timoshnko,Mechanics of Structures by Timoshinko and use the e-book by Alan Bower as a reference.

Sorry again

Re: yield surface

Biswajit Banerjee — Thu, 02 Feb 2012 18:01:56 -0500

I though I had answered that question in my previous comment.

Let me ask you a question.

When you roll a billet of steel into a sheet, does the billet undergo plastic deformation? Has the material yielded? When you cut a sample out of the rolled sheet for tensile testing is the material still elastic?

I think you should read a book on mechanics of materials before you get into all these issues. Boresi et al. "Advanced mechanics of materials" comes to mind.

-- Biswajit

You mentioned an error, and

Marcus Rademacher — Thu, 02 Feb 2012 13:06:05 -0500

You mentioned an error, and you only showed a warning. Is Abaqus/Explicit able to run, or does it exit? If the latter, what error is reported.

The warning you are seeing is because you have boundary conditions applied to nodes on a rigid body. Rigid bodies only have 6 DOF for the whole part, and those are defined on a reference point. So no nodes on a rigid body should have a *Boundary card applied to it, except on the reference point node specified on your *Rigid Body card.

I don't know that this issue would give you poor results, since Abaqus would probably just ignore the BCs applied to nodes other than the reference point, but give it a shot and see if it fixes your issue.

My Company - Red Cedar Technology

Surface based cohesive behavior

SAJIB — Thu, 02 Feb 2012 08:53:35 -0500

Hi Rajnish,

I am interested to learn about friction based simulation between Concrete and FRP. I am modeling a FRP strengthened RC beam. FRP is in tension face.

I have simulated the behavior using surface based simulation and got good results with delamination. As there is considerations (Knn) in surface based model for constraining normal behavior between FRP and concrete there is no need for constraining the FRP and concerte in normal direction. But what should i do in case of Friction based analysis instead of cohesive analysis?

I have used the following keywords:

*CONTACT PAIR,INTERACTION=Friction,adjust=0.5,small sliding,TYPE=node TO SURFACE
FRP_top,bottom_concrete_surface
*surface interaction,name=friction
*friction,TAUMAX=4.11,ELASTIC SLIP=0.005
0.018,
*SURFACE BEHAVIOR,pressure-overclosure=HARD

but I didnt apply any constrain between FRP top and concrete bottom. Do i need to constrain ??

using friction based simulation i dont find any effectiveness of FRP. The load carrying capacity of beam should be greater than control beam. but i dont get it using the friction based simulation. can you help me at this poin?? I am stuck here for a long time.

Thanking You
Sajib

Surface-based cohesive behavior

leo — Thu, 02 Feb 2012 06:44:37 -0500

Hi Rajnish,

I'm using surface-based cohesive behavior to predict delamination in compoistes. In my model, two surfeces with idential meshes contact with each other, but I found the delamination areas given by these two surfaces were different. Do you have any idea on why this occurs? thanks!

Leo