iMechanica - cohesive zone model - Comments

hello , only i commit an

mouna — Tue, 06 May 2008 10:51:53 -0400

hello ,

only i commit an error, i said i can't signed a section=i can't assigned a section

thank you

finite element modeling

mouna — Tue, 06 May 2008 10:49:46 -0400

hello,

Please could any person help me, i search to model an adhesively bonded lap joint using ABAQUS 2D : i use cohesive section for the adhesive the cohesive with continuum response, and i use beam truss section for adherent in this step i have problem i can't signed a section. In interaction module i use tie constraint with the adherent master and the adhesive slave.

this is my adress email haouasmouna@yahoo.fr, when you can send me some articels who can hepl me

thank you for all

bye

Abaqus error

Ayman Ibrahim — Wed, 30 Apr 2008 19:37:49 -0400

Ayman

Hi all

Please could any one help me to solve the messeage error, every time to run the Abaqus to simulate CPT in sand i got that message (The elements in element set ErrElemZeroALEMass-Step1 have zero or negative mass.

The elements contained in element set ErrElemExcessDistortion-Step1 have distorted excessively.

Abaqus/Explicit Analysis exited with an error - Please see the status file for possible error messages if the file exists.)

Please help me?

Ayman

Re: Modeling R-Curve behaviour using CZM

Zhigang Suo — Wed, 30 Apr 2008 14:41:00 -0400

Here is one such paper:

Tvergaard, V., Hutchinson, J.W.,"
The relation between crack growth resistance and fracture process parameters in elastic-plastic solids." J. Mech. Phys. Solids 40, 1377-1397(1992).

Modeling R-Curve behaviour using CZM

Siva P V Nadimpalli — Wed, 30 Apr 2008 12:57:45 -0400

Hi All,

Thanks for your valuable comments and explanations about this topic. I am relatively new to this (Cohesive Zone Model) field. From a limited amount of reading and discussions with collegues I understood this topic to certain extent. From what I understand, CZM requires a traction separation law for modeling, which is obtained based on fracture energy.

My question is, because fracture energy is not a contstant value i.e., in case of ductile materials we have phenomenon called "R-Curve" behaviour (fracture energy changes with crack length initially), is it justifiable to use only one value of fracture energy (i.e., steady state value) to derive the traction separation law?

Please point me towards any article(s) if exist about modeling "R-Curve" behaviour using CZM.

Thanks a lot for your valuable time and suggestions.

--Siva

define path

Masayuki Wakamatsu — Wed, 23 Apr 2008 22:00:25 -0400

Hello Yujie,

Thank you for taking time to provide me a good suggestion. I tried to take a path (nodes) along cohesive elements, but ABAQUS does not allow to do so on the torn (eliminated) elements. Instead, I took a path (nodes) on Master element side, I still need to spend some time to process data.

I hope there will be a commend that would simply tell us the crack (torn element) length. Anyway, thanks again.

Masa

Pore water pressure effect

Ayman Ibrahim — Wed, 23 Apr 2008 21:00:25 -0400

Hi Sir(s)

I am a new user for ABAQUS and looking for D-P model and i do need to simulate saturated sand to study the liquefaction phenomenon. So can any body tell me how i can use D-P model to simulate pore water pressure.

Thanks

define node or element path along the interface

Yujie Wei — Wed, 23 Apr 2008 10:38:27 -0400

If you define a node or element path along the interface in Abaqus viewer, you can plot the stress or displacement along the path at different time and save them as ASCII files. Whith such information, you should be able to determine where is the crack tip at different time. No extra effort is needed in the input file in this case.

Alternative, you can write properties of cohesive elements by adding a command like "*Element Output, FILE ....." to do the same thing. I can not remember the exact definition but it is easy to find in Abaqus manual.

The convergence of cohesive models

Dong Kong — Mon, 21 Jan 2008 03:26:35 -0500

Thanks very much,

Using Abaqus default cohesive element, I model the peeling test (the height of the adhesive is 150 um and the length is very long). The interaction between the adhesive and the soft (E=3MPa) is Van De Waels force. The length scale of Van De Waels force is very short (10 nm), compared with the adhesive and substrate. The compution cann't be finished even for some increments. But if the length scale of cohesive stress (200 um) becomes very large,it works well.

Crack tip definition

Fred Nilsson — Tue, 08 Jan 2008 04:23:43 -0500

Dong,

In a cohesive zone model there is no clearly defined crack tip. Sometimes the front edge is used as reference, sometimes the trailing edge. Thus you can choose any definition that you prefer. I suppose that with the definition you suggest the crack tip will be at or near the front end.

Fred

How to define the crack tip

Dong Kong — Tue, 08 Jan 2008 02:21:49 -0500

Dear colleagues,

Many thanks for these very useful comments. Using cohesive law in Abaqus, I'm doing a peeling test simulation. Within the cohesive layer, the position of crack tip is defined as the element with the maximum S22 (Normal stresses). Is that reasonable?

Thanks,

Dong

Element size and cohesive zones

Fred Nilsson — Mon, 07 Jan 2008 07:59:53 -0500

Rahul,

What I meant with my last comment is that the cohesive zone must be at least of the size of several elements to accurately resolve it. If the parameters of the cohesive law are such that the zone size for a specific problem is only one or a few elements, there is a risk that a length parameter is introduced that bears no relation to the physical problem. Thus, the physical problem may demand (through the appropriate cohesive law) elements that are so small that the computations may be very expensive or even impossible to perform. It may then happen that the cohesive parameters (bearing in mind that in general very little is known about the appropriate values of the parameters) are adjusted so that a solution with an artificial length scale is obtained.

Fred

Length of cohesive zone

Dhruv Bhate — Fri, 04 Jan 2008 09:39:08 -0500

Rahul,

I know of one paper that may help you appreciate this a bit better (Fred, do correct me if I have misunderstood your statement)

A. Turon, C.G. Da´vila, P.P. Camanho, J. Costa, "An engineering solution for mesh size effects in the simulation of delamination using cohesive zone models," Engineering Fracture Mechanics, 74, 2007, pp. 1665-1682

The paper is relatively easy to grasp, and collects all the various interpretations of length of cohesive zone...the process zone is essentially a highly nonlinear damaging region and several elements are needed to adequately describe the behavior in that zone.

I hope this helps.

Dhruv

cohesive zone will be the size of an element near the crack tip

rahul — Thu, 03 Jan 2008 15:23:50 -0500

Fred,

Thank you very much for your interesting comments. I am a newbie in this area and couldn't completely understood the last comment "Some caution must be observed
when using cohesive models in conjunction with numerical models such as
FEM. The minimum size of the cohesive zone will be the size of an
element near the crack tip. Should this be larger than the size of the
process zone of the physical problem, a length scale has been
introduced that does not exist in the physical problem. This is
frequently the case when using cohesive zone models for analysis of
fatigue crack growth." I would really appreciate if you can elaborate on this or point me to some article where this is not taken into account or has been discussed.

Some remarks about cohesive zone modelling

Fred Nilsson — Thu, 03 Jan 2008 13:02:00 -0500

I have followed the discussion on cohesive modelling and would like to give a few additional remarks,

1) The cohesive zone is basically a model concept that can be useful in certain cases. It can be used for instance when the fracture process zone is too large and a point-sized crack tip model is not adequate. It can also be used when modelling the initiation of a crack from a medium without cracks. The cohesive zone may or may not be a simulacrum of an actual physical process. It can be used to model different types of separation processes such as void growth and coalescence, fibre bridging, atomic separation, separation of adhesive layers such as glue etc. Once the cohesive law has been set the problem formulation is complete and no other fracture criterion is necessary.

2) There are different ways to derive a cohesive law.

a) By experimental measurements on special types of specimens (cf. T. Andersson and U. Stigh, (2004), Int. J. of Solids and Structures, 41, 413-434, B. F. Sørensen and E. K. Jacobsen, (1998), Composites Part A, 29A, 1442-1451. and several others).

b) By modelling (numerical or analytical) of the process that is to be replaced by the cohesive zone model.

c) By using a predefined functional assumption for the cohesive law, for instance as predefined in a numerical code (cf. ABAQUS). The parameters are estimated from experiments or by reasonable guesswork. This is probably the most common way.

3) It is often stated that a cohcsive zone model is equivalent to assuming that crack growth is governed by constant fracture energy. This is not true in general. Here are some situations when the fracture energy is non-constant and problem dependent.

a) A crack tip that is extending under conditions that are not steady-state (with or without inertia effects) (cf. L. B. Freund (1990), Dynamic Fracture Mechanics, pp. 237-238).

b) When large deformation effects are significant, depending on how the cohesive zone law is formulated (cf. F. Nilsson (2005), Int. J. Fract., 136, 133-142).

c) When the cohesive law depends on other quantities from the problem such as constraint, displacement rate etc.

When, as often is the case, setting up a cohesive law using fracture energy measured from experiments, it is thus important that this is a problem independent quantity.

4) Some caution must be observed when using cohesive models in conjunction with numerical models such as FEM. The minimum size of the cohesive zone will be the size of an element near the crack tip. Should this be larger than the size of the process zone of the physical problem, a length scale has been introduced that does not exist in the physical problem. This is frequently the case when using cohesive zone models for analysis of fatigue crack growth.