Fracture Criterion

What kind of criterion to use depends on what kind material you are fracturing?

I think that (hypothetically) any quantity involved in describing the response of a given structure under a certain set of loading conditions can be used as a propagation criterion as long as a relation between the variation of that quantity and the crack propagation can be established.

You can read some papers of Prof. Nishioka (Kobe University, Japan) on some Journal webs, you can get your answer.

Good luck,

This thread of discussion is related to another thread.

Hi, this is my 2nd visit to iMechanics and found it interesting.  Prof. Suo’s effort in making this happen should be commended.

Let’s get back to fracture criterion.  In the early development of fracture mechanics, a critical stress or strain at the crack tip was used as fracture criterion similar to those used in strength of material for judging failure w/o cracks.  The problem associated with this criterion is the difficulty to “measure” the critical stress or strain.   Irwin proposed the K (which is the strength of the crack tip stress) so a “global” parameter can then be used to quantify the fracture event.  The critical K then became the fracture toughness of the material.   Similarly, J is used for ductile materials.  However, one needs to remember all theories have limitations, e.g. path-dependence of J in crack propagation.  While you are reading Suo’s article on “why K works”, you need to be aware that there are situations where K does not work, e.g. crack curving under mode I conditions..

The advantage of the “critical CTOA/CTOD criterion” is that they directly address what’s happening right near the crack tip.  The disadvantage of it is that it may not be a “material property”, e.g. it is thickness dependent.    Furthermore, it may not work for thick plate (see recent article by Lam, et al in EFM).

I think, as the spread of virtual crack closure technique (VCCT) and cohesivze zone model(CZM), CTOD will finally be aborted. The energy related criteria (e.g. strain enery released rate, G) will predominate the fracture mechanics.

The reasons are
(1) G can be very easily computed by VCCT in conjuction with FEA.
(2) no singular element nor collapsed element is required to achieve accurate results
(3) not mesh sensitive, even coarse mesh can work.
(4) physical meaning is clear: energy to create new surfaces. It combines both force and displacement opening.

So, engineers can easily use G to do some facture analysis at structral level with FEA.

CTOD requires very fine mesh around the crack tip and only displacement opening is taken into acount,no force is considered. Therefore, its physical meaning is not strong.

The only advantage is its easy computation. However, by VCCT, G can be compuated in a manner just as simple as CTOD if not simpler, then how can CTOD survive?

CZM is the nonlinear version of VCCT. Or VCCT is the linear version of CZM.

Now, more and more criteria (fracture and fatigue) are being developed directly based on G.

This is my prediction.

I am trying to calculate K values for a double tip crack orientated at 45 deg in a rectangular plate in ansys.I am stuck with a strange problem. When i give the Kcal command it says the crack face is not parallel to the active x axis. But i have checked the cordinates of the keypoints that i have used to create the crack. The crack is perfectly symetric about the active cordinate system  about the crack tip.Can somebody help me please.

Hello friends

I am doing my research in fracture mechanics. Presently I am working on crack propagation FE model using ABAQUS software. I am new user for the software. Can any body tell me that where I can find the interactive tutorials for crack propagation problems or any idea, how to proceed for the same using ABAQUS software? I heard that there is a node release technique in ABAQUS software to solve such problems, but not able to use it. 

Hi,

I find this topic pretty interesting. A broad
literature has been devoted to it and to a similar one (see
http://imechanica.org/node/1692). Myself as well gave a small
contribution recently published in Journal of the Mechanics and
Physics of Solids, 56 (2008)
2092–2116. A few mistakes and some further developments have been
considered in another note, submitted presently to Engineering Fracture
Mechanics. My conclusion is that most criteria for crack propagation in
the small scale yielding range, and SED among them, have several
weaknesses and should be carefully considered. But of course I
can be wrong.

Any comment on this is sincerely appreciated.

Hope it helps!
Alberto

I have prepared FE model for crack growth analysis in ABAQUS. I want to know that how can we get out put results? e.g. CTOD or J- intergral values. I am getting LLD. Plz help me.

Dear All,

I am using ABAQUS/EXPLICIT for analysing a cracked body.Is there any way to calculate crack growth in EXPLICIT.? somewhere I have seen that " priciple stress/strain element failure approach" can be used. if it is so how to implement it..?

Can anybody help me in this regard.

 MVN

 In order to optimize the blanking processes, it is important to
identify the conditions within the deforming workpiece which may lead
to fracture initiation and propagation. Within this framework,
numerical simulations are widely used in industries to optimize sheet
metal forming processes. However, in order to have a confidence in the
results of such simulations, an accurate material model is required.
The accuracy of a material model is affected by the constitutive
equations and the values of the material parameters. In order to reduce
the danger of fracture of metal parts during manufacturing processes,
advanced optimal design requires knowledge of critical values of some
fracture criteria of the material used.
There are some good reads in pdf on this topic on the web. Might be useful for
the readers.
http://www.pdfok.com/fracture-criterion

Dear friends,

I am writing an XFEM code fordynamic crack propagation and interested in using the Interaction Integral method(as in Rethore et al. (2005)) for computation of Stress Intensity Factors(SIFs). I wonder if anyone knows what kind of dynamic asymptotic crack tip auxiliaryfields could be applied to the method for computation of SIFs for a dynamicallypropagating crack.

Regards

Matin

Hello all,

I am trying to model concrete as an elastic material in ABAQUS. The
model can be any simple structure (cube, beam or column) but the main
feature is that concrete behaviour is elastic up to a limiting strain
(St. Venant's theory). Which of the  Failure criteria in ABAQUS i should
be using (Elasticity or brittle cracking)? if neither would you mind
explaining other options?

Thanks

 Hussam

hi,

i am a new learner of using abaqus.i would like to know whether there is a possibility of simulating a brittle material until cracks appear? i have read that we need to introduce an initial crack?are we not possible to simulate a solid brittle material without having initial crack?

i hope someone out there can advice me on this matter. your help is much appreciated.

K