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Updated: 12 hours 41 min ago


Fri, 2014-10-24 12:33

In reply to Professor Huajian Gao to Receive 2015 William Prager Medal

Congratulations to Prof. Gao for the leadership in applied mechanics!

material nonlinearity

Fri, 2014-10-24 10:01

In reply to Hi, There are several arc


In my case there is a material nonlinearity problem, I can not properly impose arc length in nonlinear material. Do you have any experience in it?



Thu, 2014-10-23 18:26

In reply to Strain hardening

Dear Professor Sue,


Firstly, thanks a lot for your very instructive notes. This is very generous of you to share them on iMechanica.

I was windering if I can ask a question regarding the strain hardening model. Is it possible to add the Bauschinger's effect to this strain-hardening model? In that case, the compressive yield stress \sigma_YC will be 2Y-\sigma_YT, where Y is the first Yield stress in tension, and \sigma_YT denotes the new yield stress in tension?

Thanks so much





initial void ratio for soil

Thu, 2014-10-23 03:27

In reply to initial void ratio for soil

I have the same problem. Do you solve that error? If you did please give me some informations!

All the best 

cyclic loading

Wed, 2014-10-22 14:15

In reply to T = 1/10 = 0.1s.  So for


i am modelling a composite plate subjected to a cyclic loading

i wnat to model a 1000 cycle with 10 hz frequency.

 i have some question from you.

is it possible to use static,general as step?

on the other hand , i use  umat subroutine for egradation of properties, i want to have a degraded properties,in each cycle.

is it true that i assme  that every increment is 1 cycle?

and at last, i dont understand what exactly,do you mean in this sentences "Recommendation will be to load in one step and unloading in another step"

can you explain it?



gaussian distribution or Goldak heat source model in ANSYS?

Wed, 2014-10-22 01:33


dear friends.. i am learning FEA on ANSYS for Arc Welding. can anyone tell me how to model Gaussian or Goldak heat source model in ANSYS?

Inquiry about your available PHD positions

Mon, 2014-10-20 04:37

In reply to PhD studentship at URV, Tarragona, Spain

<p>Dar Prof. Huera,</p>
<p>My research fields exactly coincides with your research areas. I sent an email to you yesterday with my CV attached. I will appreciate you if you&nbsp; take a look at my CV.</p>
<p>Thanks in advance</p>
<p>Morteza Alinia</p>
<p>Email: <a href=""></a></p>

hello Mr Jahanshahi

Mon, 2014-10-20 02:54

In reply to Displacement Controlled Analysis

hello Mr Jahanshahi

thank you so much for your attention. 

I will do what you said.


hello   peto24ap

Sun, 2014-10-19 23:22

In reply to Geometric nonlinearity

hello   peto24ap

Why do you use line 4 method for computing internal force? I think this is the problem. Use ordinary method for internal force, Just like the    method introduced in  " introduction to finite elements in engineering" ,by chandrupatla or every other fnite element book, to make your internal force vector.

look for this article "COMPOSITE MATERIALS IN  COMPRESSION " by  Jens Lycke Wind, In this thechnical report you can find an algorithm for Newton raphson.

I used it before for load base problem but i could not write a code for material nonlinearity which is under displacement BC, and displacement control.

Ask me if there is some thing to ask, our ways are similar. maybe we can help each other in the modeling


Hi, There are several arc

Sat, 2014-10-18 14:34

In reply to arc length in delamination

Hi, There are several arc-length methods that could be used. But depending on the level of non-linearity, they may or may not be able to trace snap-back exactly.

Geometrical nonlinearity simple arc-length method should be sufficient. But in case of severe strain localization, you would need a method like the one suggested by

Crisfield arc-length method. Try this one, incase this one does not work, I may have other suggestions.



Displacement Controlled Analysis

Sat, 2014-10-18 11:52

In reply to displacement control method

Dear Mr. Jafarzadeh,

I already have answered your question. The answer lies in equation (2) above. In the fisrt iteration of each load step (which is displacement controlled) you apply an incremental displacement. Of course your stiffness matrix should be comouted in each iteration due to material nonlinerity. In subsequent iterations the second term on the right hand side of equation (2) is set to zero and iterations continue to equilibrate out of balance force. The iterative procedure is exactly similar to force-controlled except for equation (20). If you pay a good attention to what I siad you should be able to proceed without any problem.

Good Luck

displacement control method

Sat, 2014-10-18 08:37

In reply to Displacement Controlled Analysis

hello Mr Jahanshahi

thank you for your quick answer

I know how to find reaction forces for the case but my problem is how to use displacement control method for displacement BC case, in which the stiffness matrix in each increment and iteration change. In displacement BC by changing stiffness matrix the total force changes too. I should apply a unique force for each increment to use it in all the iteration.



Displacement Controlled Analysis

Sat, 2014-10-18 06:31

In reply to N-R method

Consider the following partitioned matrix equation (sorry if I could not show it clearly):

|Knn ¦ Knu||un|    |R|

|.......¦.......||....| = |...|              (1)

|Kun ¦ Kuu||uu|    |F|

In this equation un is the vector of specified (and thus known) displacements. This is the specified displacement at certain degrees of freedom in displacement controlled analysis. R is the vector of corresponding reactions at these degrees of freedom. The vector uu is the vector of unknown displacements at other degrees of freedom where no displacement boundary condition is specified. F is the vector of nodal forces at these latter degrees of freedom. From the second row of this equation we can write:

[Kuu][uu]=[F]-[Kun][un]                (2)

which is the equation that should be solved in displacement controlled analysis. Based on this equation, the vector of forces at unconstrained degrees of freedom should be modified by the second term on the right hand side of equation (2). The rest of the analysis is exactly the same as force controlled analysis. Having calculated uu from preceding equation, the vector of reactions at constrained degrees of freedom can be calculated from the first row of equation (1) as

[R]=[Knn][un]+[Knu][uu]          (3)

For more information you can refer to page 187 of the following reference:

 K.J. Bathe, Finite Element Procedures, Prentice-Hall, 1996.

Helpful tips really nice

Sat, 2014-10-18 02:54

In reply to Essays and books on writing well

Very useful information. Thanks all of you.

D&DT course - Part II on Fatigue & Damage Tolerance

Fri, 2014-10-17 17:21

In reply to Course on Design Principles, Fatigue & Damage Tolerance for Aeronautical Engineers

Dear iMechanicians,

Part I of the D&DT courese on Design Principles for Aeronautical Engineers starts just after this weekend. Those, who would be more interested in Fatigue & Damage Tolerance topics, please note this part of the course will be held from November 11 till November 13. The applications, if possible, should be filled in till October 27. If interested, see the details on

Best regards


N-R method

Fri, 2014-10-17 14:28

In reply to Nonlinear Algorithms

Hello Mr Jahanshahi

I wrote a code by N-R method for my problem (a double cantilever beam which has interface elements between two layer for simulating delamination ), every thing was good, I saw the softening of the material, but there was some problem.

For verifying my answers, the Reaction-displacement of the total system should be plotted,for this reason i should impose displacement boundary condition instead of force.

As you know in the material nonlinearity problem the stiffness matrix (D and after that K) will change in every increment and iteration corresponding to displacement (in the geometrical nonlinearity the displacement changes alter the geometry and after that the total stiffness matrix changes). by changing in the stiffness matrix also the force value will change , because of displacement BC formulation in finite element method.

but in the N-R method the at the beginnig a load apply to the system and then in each increment 1 part of it will impose. In my case the force change with K.

now here is my question: Can i use N-R method for displacement BC problem or I must use the displacement control method?

I try displacement control before but I didnt succeed. In this method I have same problems. In displacement control method a first force (not displacement) will introduce, and then a part of force impose in increments. But the base is displacement change. Also In this method the K matrix changes and it is main problem. 

I want a method appropriate for displacement BC problems.

Would you please help me in this topic too?



Thanks for your help! A

Fri, 2014-10-17 07:30

In reply to This problemlooks like

Thanks for your help! A follow up question is that when import a model to Solidworks or AutoCAD, Abaqus can export multiple file extension, such as ACIS (.sat), IGES (.igs)and STEP (.stp). Do you know which is better from 3D reconstruction. By the way, our model is a cylindrical shell. 

Postdoc looking for a job

Fri, 2014-10-17 05:54

In reply to Research Associate/ Postdoctoral Research Associate position in Magnesium Alloy Development

الأخ المحترم الدكتور بلال منصور

السلام عليكم ورحمة الله وبركاته

أود الاستفسار عن فرصة العمل المذكورة اعلاه إذا لا تزال متوفرة أم لا؟

أنا ابحث عن فرصة عمل في مجال دراسة سبائك الماغنيسيوم وطرق تحسين خواص التآكل بها.

أنا خريج جامعة مانشستر لهذه السنة ولدي درجة دكتوراة في هذا المجال وكذلك درجة ماجستير في علوم المواد والتآكل.

ساقوم بتزويدكم بسيرتي الذاتية في حال كانت هذه الوظيفة مازالت شاغرة.


تقبلوا تحياتي مع جزيل الشكر


والسلام عليكم ورحمة الله وبركاته


د. أحمد الشيخي


These are a number of research papers and textbooks

Fri, 2014-10-17 04:05

In reply to These are a number of research papers and textbooks

yes, These are a number of research papers and textbooks by Prof. W. Noll. I suppose they definitely deserve to be here.

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