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Thu, 2017-06-29 02:04

In reply to Johnson-Holmquist model in Abaqus

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A quick insight

Wed, 2017-06-28 23:55

In reply to What is the physical meaning of Green-Lagrangian strain and Eulerian-Almansi strain measures?

when I thought about this when I took the course - I got this point 

You may think that the strains are mathematically formulated to maintain consistency which is , a rigid body motion or rotaion wont induce any strain in the material. This may have been a reason to start with squares and further go on to get the  strain in form of FF' which chops off the rigid motions.


with repaired links.

Mon, 2017-06-26 21:32

In reply to Planar tension test = Pure shear test for rubber? Explanations

Here is an analysis of the simple shear vs pure shear cases, and here is an explanation of the nomenclature.  Hope this clarifies. 

XFEM and UMAT subroutine

Mon, 2017-06-26 11:45

In reply to How to combine XFEM and UMAT in ABAQUS?

I have UMAT soubroutine for ductile material damage model. When I try to link my subroutine with XFEM for predicting crack iniatiation and crack growth, the simulation is working, no error during running the analysis. But, unfortunately when I want to extract the result from solution dependent variable (SDV), it didn't give me a result. the graph plotted was totally wrong.

Have everybody have a same experience ??

Many thanks.


Sun, 2017-06-25 06:17

In reply to Crack propagation simulations


So far we can classify fracture mechanics theories according to the following main categories:

- brittle fracture (LEFM)

- cohesive fracture (NLFM)

- ductile (elasto-plastic) fracture

NLFM can include LEFM as a limit case when the process zone size tends to zero.

Computationally, if you know the crack path in advance (an interface with adhesive, for instance) then you may use interface elements, which are like the segment-to-segment contact elements with fixed pairing. They have also been extended to node-to-segment or node-to-surface finite elements, which are useful to couple different mesh discretizations at the interface:

If the crack path is unknown a priori, then you may opt for remeshing techniques and singular FEs at the crack tip for LEFM, see FRANC2D by Cornell fracture group, which is freely available and I also used in the past. However, the issue of crack nucleation, branching and similar phenomena are not included in that code. 3D simulations are also complex, not so straightforward for industrial research.

XFEM and phase field are powerful and the phase field, in my opinion, opens new perspectives for 3D simulations, and it can be coupled with interface elements for cohesive fracture. As a limitation, the fine mesh you need to use, which can be problematic for big industrial simulations.

Dynamic fracture deserves a specific discussion, if it is interesting to you.







Dear Professor Paggi, Thank

Sat, 2017-06-24 15:00

In reply to Crack propagation simulations

Dear Professor Paggi, Thank you for the reply. I will check out the link and follow the publications.

The phase field method seems to be used in research more and more now. But if possible can you also shed light on methods to simulate fracture in industry? 

Really appreciate your reply.


Phase field model of fracture

Sat, 2017-06-24 09:15

In reply to Crack propagation simulations

The phase field model of fracture has seen significant progresses since the early stages, and nowadays it becomes a quantitative methodology to simulate crack growth without the a priori knowledge of the crack path, see e.g.:

Large scale computations are still expensive and generalization to dynamic crack growth still in progress. For a better understanding of the role of the length scales involved in the approach, and therefore CPU limitations in the case of large problems, please see a thoroughly discussion in the articles mentioned in my previous blog entry.

As ever,

Marco Paggi

So that is how you find

Sat, 2017-06-24 06:50

In reply to How to find normals stresses in the global x y z direction in ABAQUS?

So that is how you find normals stresses in the global x y z direction in ABAQUS. Been searching for that answer in a while. Thanks

soundcloud music promotion


Thu, 2017-06-22 12:39

In reply to should fortran be installed

Yes, all ABAQUS user subroutines are written in FORTRAN, hence need a FORTRAN compiler to get executed during runtime.

should fortran be installed

Thu, 2017-06-22 12:13

In reply to RE: DLOAD subroutine


I am new to abaqus,and may i know if fortran to be installed to execute dload command.??


Hi Ruobing,

Wed, 2017-06-21 05:54

In reply to They all measure deformation, and provide no extra physics

Hi Ruobing,

Thank you so much for sharing your ideas. It is very much useful. I did have an intuition before of whatever you said. But I haven't convinced myself that my understanding is completely true. What you said totally makes sense in many aspects. Thanks again. 

They all measure deformation, and provide no extra physics

Tue, 2017-06-20 18:24

In reply to What is the physical meaning of Green-Lagrangian strain and Eulerian-Almansi strain measures?

Hi Selvam,

I'm assuming you are talking about the case of finite deformation, since in linear elasticity they would be the same. The straightforward answer is: the only real physical meaning they provide is that they all measure the deformation of a material. Of course such deformation can be represented in different mathematical forms, but these various forms do not provide extra physics. For instance, in addition to the three measures you mentioned, you can also measure deformation using the true strain (or logarithmic strain).

I'd like to further make a point that different strain forms surely are related to different stress forms. For example, you can measure or describe the stress state of a material using the nominal stress, or the true stress etc. Again, they all describe the same stress state of the material, but add to no extra physical meanings (you can argue that nominal stress is the force devided by the area in the reference state while true stress is that devided by the area in the deformed state, but such differentiation is just from definition rather than different physics). As long as you keep consistency when applying the constitutive model in your study, you can use any form of the stress/strain.

Back to the three strain forms you mentioned. If you look at them in tensor forms, the engineering strain (corresponding to F - I in tensor) will not exclude rigid body motions, while the other two will. You can even find more strain measures here:

Minisymposium in 2018 @ WCCM

Tue, 2017-06-20 12:36

In reply to Latest progresses on the phase field model for brittle fracture

We just submitted today a proposal for a minisymposium at the next WCCM 2018 conference in New York on

Open challenges in the phase field approach to fracture mechanics

This minisymposium aims at gathering together experts in computational fracture mechanics to discuss open challenges and generalizations of the phase field approach to fracture (PFAF). Selected original contributions will be invited for being considered for publication into a special issue of an international journal.

Topics of interest are (but are not limited to):
- Generalization of the PFAF to viscoelastic materials;
- Generalization of the PFAF to mechanical and geometrical nonlinear problems;
- Generalization of the PFAF to dynamics;
- Generalization of the PFAF to anisotropic and/or inhomogeneous materials;
- Generalization of the PFAF to interface fracture problems;
- Implementation of the PFAF in finite element technologies;
- Interaction between PFAF and cohesive zone models;
- Comparison between the PFAF and other fracture mechanics/damage mechanics formulations;
- Prediction of size-scale effects;
- Modelling of fatigue crack growth.


If accepted, we will post an update with a link for abstract submission.

It could be a nice opportunity to discuss on common research.

low-speed impact is much fast than nanoindentation

Tue, 2017-06-20 12:32

In reply to Just a thought

Dear Ajay and Guru,

Of course, you’re nanoindentation experts but might not conduct low-speed impact tests for engineering materials. I do both for more than seven years. The key issue is the definition of “low-speed impact by projectile”. The impact of speed of our impact tests on composite materials is around 50 m/s. Of course, nanoindentation cannot do it so nanoindentation is a static test method compared to a low-speed impact problem. Here I find the reviewer didn’t understand nanoindentation tests and didn’t conduct low-speed impact tests too.

Ajay --- can you comment “capture delamination” problem using nanoindentation? Do you know delamination in composites?


Just a thought

Mon, 2017-06-19 18:13

In reply to Seek your input on nano-indentation or basic mechanics of materials

Being in the EU, I can't comment much about the funding & reviewer remarks. But I can share my thought on the feasibility of such a project.

I do not agree with Guru completely here. Nanoindentation is not only to measure material properties - though that is what many have been using it for. It has been my side project over the last 6 years & I have done testing for my group members too.

We have a machine from Agilent (or nor Keysight) & if one is using the standar methods provided by the manufacturer, the number of possibilities are limited. However, one can go and program the back end itself to setup tests that one desires. This opens up a world of possibilities that one can only imagine. At the end of the day, NI is a tool & it is as good as the user. I did a bit of programming on the backend & were able to do fascinating experiments that one would not think about. We are presently writing up on them & would be glad to share in some time.

Though NI is a quasi-static testing setup, by default, one can change the method to actually do low-velocity impact too! So if you would say, you want to do low-velocity impact, I wouldn't say it is impossible. I would only be worried about the lifetime of the tip which are rather costly. But theoretically, such a thing is actually possible.

Nice work!

Mon, 2017-06-19 03:44

the method phase field modelling of fracture

Mon, 2017-06-19 03:28

In reply to Latest progresses on the phase field model for brittle fracture

has been extended to study dynamic fracture, brittle fracture, ductile fracture, fracture in anisotropic materials, plastic materials, porous materials ,hyperelastic materials and viscoplastic materials. ....

It can be coupled with other physical processes such as diffusion, phase transformation and deformation twinning, etc. Its rapid development in recent years is really amazing. The method itself is theoretically elegant and easy

to be implemented.  Worthy to work on! I like it very much. :-)

Biswajit and Ajit, first of

Sun, 2017-06-18 23:48

In reply to Engineer: Career Pathway Conundrum – job or business

Biswajit and Ajit, first of all thank you very much for taking the time to post an answer, I really appreciate it.


I understand iMechanica may not the correct forum but if I haven't posted in this forum, I wouldn't have met you guys and I wouldn't have received such valuable information. Help is available to those who look for it, forum type didn't scare me :) By the way I do have a good research background too, topic for another day.


i am inclined towards a salaried job since I am only starting out and I am inexperienced in terms of engineering. However i am still trying to weigh my options here with SWOT analysis and what not. And also if I had 10K or 100K for consultation, I would have already started my business :) don't take me wrong, those consultations are worth it but only when you are a CEO of Apple wanting to join Microsoft or start a rival company, it's not relevant to us juniors :) but thanks for pointing it out to me.


No i am not telling/selling my ideas to anyone at this stage, sorry.


All points that you guys have mentioned are valid and valuable, I will consider them when taking the final decision, as for now, I am researching.


Have a fantastic day guys and thanks once again for your help!

Phase field modelling of fracture

Sun, 2017-06-18 23:27

In reply to Latest progresses on the phase field model for brittle fracture

<p>One of the most fascinating progresses that have been made in computational fracture mechanics in recent years! Theoretically elegant and useful in many fields! Nice work!&nbsp;<span style="color: #333333; font-family: arial; font-size: 24px; line-height: 24px; background-color: #ffffff;"><br /></span></p>

Re: The [Doing] Business Conundrum

Sat, 2017-06-17 18:02

In reply to Engineer: Career Pathway Conundrum – job or business

It's obvious that you want to go in for business, but can't, for some reason or the other.

Biswajit as often here beat me to it, by listing the # 3) above. Also his # 2).

"Find a customer first and then build the product." That's what he said.

"The purpose of a business is to create a customer" [^]

[Now at least I can find some solace!]

Best [to you; to your business, to your boss in your day-job [hopefully you have one], to your day-job [ditto], to $\dots$. $dots$. You get the idea.]


PS: If I could be your customer, then, please, try to sell me on your idea---on what you could do for me... I mean, more profitably for myself than in your absence, that is. ... Best. ...


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