*mat_add_erosion

[Nava Farhadzadeh]

Hi James/Len

I have used *mat_add_erosion in my concrete simulation, I know you both are not fans of this keyword ( neither am I), I am just wondering if you could suggest good publications that have mentioned the disadvantages of this keyword please?

I see there are alot of publications that have used this keyword and have claimed that it works fine, however this is not the case. I need to read good reliable resources that have actually mentioned that this keyword is no good. 

Kind regards

[l...@schwer.net]

Hello Nava –

 

I do not know of a publication that discusses the “evils” of using element erosion.

 

The problem with erosion is users tend to abuse its significant power. I sometimes use this handgun metaphor  – when in the hands of a trained police officer it can save lives, but in the hands of an unknowing child it can inadvertently take lives.

 

So one cannot say *MAT_ADD_EROSION is “no good,” rather users who abuse *MAT_ADD_EROSION are computationally dangerous.

 

                --len

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[James M. Kennedy]

Dear Nava,

 

Perhaps this might be helpful:

 

https://www.lstc.com/classes/fail_frac_damage

 

El­e­ment ero­sion: ad­van­tages & short­com­ings

 

Sincerely,

James M. Kennedy

KBS2 Inc.

November 15, 2021

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/00a301d7da69%24ed838640%24c88a92c0%24%40schwer.net.

[Евгений Калентьев]

I agree with Len. The problem is not with the  *MAT_ADD_EROSION   itself, but with how it is used. In my opinion, most often the logic of reasoning is as follows: let there be some criterion for the destruction of the material and upon reaching it, the material erodes, but in reality the "disappearance" of matter does not occur. For example, if an element erodes when the tensile criterion is reached, this does not mean that in reality the material does not resist compressive or shear loads.

Sincerely,

Eugene Kalentev

вторник, 16 ноября 2021 г. в 03:55:48 UTC+4, jmk:

[sabbir siddique]

But some cases, the elements really erodes. For example, in crash and impact scenario. I request someone volunteer to clarify the Erosion issue in simple words. 

Sabbir Siddique

https://groups.google.com/d/msgid/ls-dyna2/45d02fb5-1620-4dec-9179-523b0c66b5acn%40googlegroups.com

.

[l...@schwer.net]

There are two aspects of erosion that mathematicians dislike in any numerical solution:

 

1\ Non uniqueness – the solution can be obtained in multiple ways. *MAT_ADD_EROSION offers many erosion criteria, and combined with an essentially infinite range of erosion criterion values, allows multiple ways to achieve the same eroded solution.

 

2\ Ad Hoc – solutions that work for one problem but not for a similar problem. If you have an eroded solution, just change the mesh discretization and see if you get the same result.

 

For some odd reason too many “computationally dangerous” users are not at all bothered by these aspects of erosion.

 

We are all “computationally dangerous” to some degree.

The least dangerous among us are those that know they are dangerous.

 

                --len

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/313599897.1076846.1637055893110%40mail.yahoo.com.

[Kağan GENÇ]

Hello everyone,

I want to share my own observations about erosion. I carried out several blast explosion simulations of a RC slab and found out that even if principal strain values in erosion were changed, the result displacement value did not change. I think erosion parameters should not affect results. Otherwise, this might be dangerous as you mentioned. Please take a look at the graph given below. 

As you can see here, even if the principal strain values(both minimum and maximum applied) were changed while all other features maintained the same, the central deflection path did not change and followed the same path. I cut the simulation because it is obvious that both follow the same path. 

This is my observation, if I might be mistaken or you think my angle is problematic, please let me know. 

Kind regards,

Oguz Kagan GENC

<l...@schwer.net> adresine sahip kullanıcı 16 Kas 2021 Sal, 23:36 tarihinde şunu yazdı:

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/001601d7daf7%2449493230%24dbdb9690%24%40schwer.net.

[Nava Farhadzadeh]

Hi Oguz

I actually do agree with your statement. it will not affect the displacement history if the erosion value is not too low to cause premature failure.

Regards

Nava

[l...@schwer.net]

The “algorithm” for *MAT_ADD_EROSION is simple, when any user specified criterion is met, or exceeded, the element is eroded.   --len

 

From: ls-d...@googlegroups.com <ls-d...@googlegroups.com> On Behalf Of Kagan GENÇ
Sent: Wednesday, April 5, 2023 3:59 AM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: Re: [LS-DYNA2] *mat_add_erosion

 

Hi Len and Kennedy,

 

I would like to heat up this topic again since there are a lot of confusion. Some say there is no certain method to estimate these erosion criteria and we need to have a trial-and-error session to validate structural damage. I cannot process this statement in my brain, so I would like to ask you. 

 

Let's think about this. There is an algorithm or a script works for erosion behind the interface. It is already determined I mean the way to delete element by this erosion algorithm. So, if we know how this algorithm deletes elements, I suppose we can estimate these values if we have material properties. At least, we should have known what the limits values(liek max and min) are for each erosion parameters as a result of this estimation. In my opinion, having the algorithm and material properties, we may obtain these erosion values. 

 

So my question is that is there any documents present the scrip or the algorithm of mat_erosion?

 

Kind regards,

 

Kagan

 

17 Kasım 2021 Çarşamba tarihinde saat 20:14:58 UTC+9 itibarıyla Nava Farhadzadeh şunları yazdı:

Hi Oguz

I actually do agree with your statement. it will not affect the displacement history if the erosion value is not too low to cause premature failure.

Regards

Nava

On Wednesday, November 17, 2021 at 6:20:44 AM UTC kagan...@gmail.com wrote:

Hello everyone,

 

I want to share my own observations about erosion. I carried out several blast explosion simulations of a RC slab and found out that even if principal strain values in erosion were changed, the result displacement value did not change. I think erosion parameters should not affect results. Otherwise, this might be dangerous as you mentioned. Please take a look at the graph given below. 

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/ed7abbd6-586a-4570-bc58-c926abc8aad6n%40googlegroups.com.

[l...@schwer.net]

Hello Kagan –

 

Below is a posting I made to the old LS-DYNA forum in June of 2008 on the folly of using erosion. Perhaps this will answer your questions?                --len

 

Many in this forum are familiar with my views on the use of erosion, i.e.

hate it!

 

Erosion is not a material characteristic, it is strictly a numerical artifact. There is no laboratory test to determine erosion anything.

 

The utility of erosion is to allow one continuum to pass through another when using a Lagrange solver. The availability of Eulerian and SPH solvers, for example, has obviated this utility, for the most part.

 

Still users continue with erosion. I can only assume out of a lack of motivation to learn new techniques.

 

But if you insist on using erosion here are three steps I consider

necessary:

 

1. Select an ad hoc erosion criteria; it really matters not what criteria since it is a numerical artifact. [BTW - you might as well change your eroded material to elastic and save plasticity computations, since your result will depend on a numerical artifact and not a material parameter.]

 

2. Perform an erosion criterion value convergence study, i.e. as the value of the selected ad hoc erosion criterion is changed how does the system response of interest change, and most importantly, does it converge to a non-trivial answer?

 

3. Given an ad hoc erosion criterion and converged erosion value, now perform a mesh convergence study. Again, as the mesh is refined how does the system response of interest change, and most importantly, does it converge to a non-trivial answer?

 

Steps 2 and 3 are inter-related and should be performed an iterative manner.

 

After all the above work, there is a fair chance you will discover there is no convergence. But even if you obtained a converged answer, there is no assurance it is correct, since you started with an ad hoc erosion criteria.

 

Some argue you can calibrate an erosion criterion to a given experiment.

That is true, but of what use, since you already know the experimental result; and again there is no unique way to perform the calibration, so what is the expected PREDICTIVE value of such a non-unique calibration.

 

If you like magic, you'll like erosion. But remember magic isn't mechanics.

 

               --len

 

 

From: Kağan GENÇ <kagan...@gmail.com>
Sent: Monday, April 17, 2023 12:40 AM
To: L...@schwer.net
Subject: Re: [LS-DYNA2] *mat_add_erosion

 

Dear Leonard,

 

Thank you so much for your answer. The reason why I am thinking about this is if we know the algorithm it should not be that hard to assign the erosion parameters accordingly after investigating results. We can check max principal strain results and assign the parameters accordingly then. The only thing written everywhere about these erosion criterias is that there should be a trial and error session that needs to be done to assign parameters. So it does not make sense to me at all. We have inputs and outputs. Why does everybody carry out a trial and error session? or There is smt that I have not realized yet. That's all I wanted to debate about. 

Kind regards,

 

Kagan

 

<l...@schwer.net> adresine sahip kullanıcı 12 Nis 2023 Çar, 03:58 tarihinde şunu yazdı:

[SUMAN KUMAR]

Hi everyone,

The current thread on MAT_EROSION card is really interesting and knowledgeable.  Yes, there is disadvantage of erosion criteria in fem. In no event there is loss of mass but using erosion criteria there will be significant loss of mass and results into inaccurate prediction. Such as the rock blasting event were the generated debris may apply impact load to the surrounding area. But due to erosion the impacting mass is not available in the computational domain produces inaccurate results.

Now, I want to ask question like whether it's possible to preserve elements in the domain after fragmentations. I know there are COUPLED Fe methods but I want to use Fem only.

Regards,

Suman. 

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/002c01d9713a%24c38649e0%244a92dda0%24%40schwer.net.

[l...@schwer.net]

By definition, when an element erodes it is removed from the computation. All that remains are the element’s nodes, i.e. representing portions of the element’s mass.

 

When adjacent elements erode, some of their common nodes may become unattached and act as rigid particles traveling with mass and velocity. These nodes may optionally remain active/inactive in the simulation as determined by the *CONTROL_CONTACT parameter ENMASS; see the User Manual Volume I.                             --len

[SUMAN KUMAR]

Thanks for the clarification. I will definitely try this. 

Regards,

Suman 

[James Kennedy]

Dear Suman,

 

A note taken from that may be of interest

 

Jaime, M.C., "Numerical Modeling of Rock Cutting and its Associated Fragmentation Process

Using the Finite Element Method", Ph.D. Thesis, Department of Civil and Environmental Eng-

ineering, University of Pittsburgh, Pittsburg, Pennsylvania, November, 2011.

 

http://d-scholarship.pitt.edu/10611/1/Jaime_Maria_PhD_diss_ETD_Nov30_11.pdf

 

5.3.1.1 Treatment of the Mass of Eroded Elements

The fact that elements are deleted from the finite element model is commonly thought as an

infringement of the basic modeling principle of conservation of mass. Fortunately, LS-DYNA

provides a mechanism to circumvent this violation. It is important to note that within the def-

inition of contact controls (see Section 5.2.2.3), the parameter ENMASS equals 1, which works

by retaining the mass of the eroded nodes in the calculation, and keeping them active in contact.

Consequently, the overall energy calculation of the system is not affected, as its mass is not re-

duced.

 

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

 

Some ENMASS notes:

 

https://ftp.lstc.com/anonymous/outgoing/support/FAQ_docs/contact_shorter.pdf

 

ENMASS - Flag for treatment of eroded nodes in contact. 

 

An eroded node is defined as a node that is no longer attached to any element owing to element

deletion.  To display eroded nodes as particles in LS-PrePost, toggle on “Show Deleted Nodes”. 

ENMASS is not supported when SOFT=2 on Optional Card A of *CONTACT.   ERODING_

NODES_TO_SURFACE is supported in both SMP and MPP.   ERODING_SINGLE_SURFACE

and ERODING_SURFACE_TO_SURFACE are supported only in SMP.

EQ.0:      Eroded nodes are not considered in the contact algorithm.

EQ.1:      Eroded nodes of solid elements remain active in the contact algorithm.

EQ.2:      Eroded nodes of solid and shell elements remain active in the contact algorithm.)

 

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

 

Sincerely,

James M. Kennedy

KBS2 Inc.

April 19, 2023

 

 

From: ls-d...@googlegroups.com [mailto:ls-d...@googlegroups.com] On Behalf Of SUMAN KUMAR

Sent: Wednesday, April 19, 2023 2:03 AM
To: L...@schwer.net

Hello everyone,

 

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