iMechanica - Fracture Criterion - Comments

Calculation of K values in anysis

Gagandeep Singh — Fri, 14 Mar 2008 12:35:48 -0400

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.

Why is not arrest toughness unique value?

Nguyen Chau Linh — Thu, 06 Mar 2008 23:20:14 -0500

Dear All;

I have drawn the relation between dynamic stress intensity factor and crack tip velocity under mode-I impact loading for brittle material. In my research, some specimens were give nice results and I want to know:

Why is the dynamic stress intensity factor (DSIF) at initiation point not same DSIF at arrested point?

Why are almost the DISFs at arrested point always less than DSIF at initiation point of crack?

Why are DSIFs difference although they are same crack tip velocity?

I am very glad when give your comments;

Thanks

Material Failure

MILOUD SOUIYAH — Thu, 17 Jan 2008 22:39:59 -0500

Well, related to fracture criterion, there is another very common method which is, Displacement Extrapolation Method (DEM). It`s one of the most accurate method used in Linear Elastic Fracture Mechanics (LEFM) as would be applied in Mode I and Mixed mode as well.

I think, as the spread of

De Xie — Wed, 18 Jul 2007 01:27:57 -0400

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.

Dhruv, Thank you so

Arash Karpour — Wed, 18 Jul 2007 00:50:21 -0400

Dhruv,

Thank you so much for your explanations. It is been most helpful for me. I just couldn't relate one criterion to another before. Thanks again

Mastery on others is Force
Mastery on self is Strength

Applicability of Fracture Criteria - text book interpretation

Dhruv Bhate — Tue, 17 Jul 2007 22:41:58 -0400

Arash,

I will take a "crack" at this, with a disclaimer at the outset: my opinions are derived only from the few texts and one class I have taken. If any of my arguments are incorrect or outdated, I would appreciate them being pointed out as such.

For a globally brittle material with small scale yielding, such as you describe, you should not have trouble applying K based criteria, or, as we have been discussing in the other thread, the SED theory. In general, I imagine these are the relevant questions you need to resolve, listed below. Of course after you answer each question, there may be sub-issues to deal with.

1. Is your material considered generally ductile or brittle? This you have answered. If your material is ductile, you cannot apply G or K based criteria.

2. Assuming your material is "generally brittle" and you have conditions of small-scale yielding, you still must ask: Are your cracks self-similar? In other words, is the increment of the crack similar to the pre-existing one? If not, again you may not be able to apply the above criteria and will have to, in all likelihood, resort to another approach - SED may be of use to you here.

3. Finally, generally speaking, if you have significant plasticity, you may want to use the J-integral approach, provided that your loading conditions are monotonic. The J-integral approach is essentially based on a "deformation Plasticity" model, which is essentially a nonlinear elastic model - so no history is kept track of, which may be a problem for materials and loading conditions that need this (e.g. fatigue in viscoplastic materials like solder).

4. If all else fails, you need to enter the domain that some people call "Nonlinear Fracture Mechanics" (I think the J-integral is included in this group when applied to nonlinear materials). Here you can use the CTOA and CTOD methods, but there is a lot of skepticism about their generality, including the issues Prof. Chao mentioned. The more promising approach is that of the Cohesive Zone Models (CZM), which actually has a critical "separation". This approach is almost a back door approach to solving the problem, but a very powerful one and there are several review papers (including one by Ingo Scheider that you can google for).

I hope that sums up the criteria for you. My point really is this: the kind of problem helps you select the criteria. It is a true (and somewhat unsatisfying) fact that almost every theory in fracture seems to come with its cautionary list of limitations. Fortunately, there are several approaches out there and you should find one to fit your need.

With regard to the second question you bring about, regarding the relationship between different criteria, the K-G-J-SED equivalence is well known for elastic materials, Anderson's text can help with the K-G-J equivalence. SED can be written in terms of Ks quite easily too. The problems start to arise for nonlinear materials. Hopefully someone can shed some light on this.

I think it would make sense for us to reach some sort of summary at the end of threads of this nature - perhaps the author of the thread can summarize the points learnt so that a quick read of the summary can help future readers.

Thanks.

Fracture criterion

chao — Tue, 17 Jul 2007 18:15:16 -0400

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).

Dan, Maybe it is

Arash Karpour — Sun, 15 Jul 2007 20:47:35 -0400

Dan,

Maybe it is my lack of understanding of fracture...I found many fracture criterion and I don't know what would be the reason to apply each criterion...what are the general criterion? (that is for brittle material with small yeilding condition)

Mastery on others is Force
Mastery on self is Strength

Henry, I am using brittle

Arash Karpour — Sun, 15 Jul 2007 20:37:54 -0400

Henry,

I am using brittle material with small scale of yelding...(small plastisity at the tip of the crack)

Zhigang, I am reading

Arash Karpour — Sun, 15 Jul 2007 20:35:08 -0400

Zhigang,

I am reading your review aritcle on why K works. It is helping with some of my questions.

Thank you for you help.

Mastery on others is Force
Mastery on self is Strength

Nguyen, Thank you for

Arash Karpour — Sun, 15 Jul 2007 20:32:05 -0400

Nguyen,

Thank you for your response

Mastery on others is Force
Mastery on self is Strength

Two related threads of discussionn on fracture criteria

Zhigang Suo — Sun, 15 Jul 2007 02:59:01 -0400

This thread of discussion is related to another thread.

CTOA as a fracture criterion

Bhawesh Kumar — Sat, 14 Jul 2007 01:06:12 -0400

To add on above comments,

CTOA(Critical tip opening angle) can be used as a fracture criterion for ductile material. In ductile material, SIF is found to be dependent on a/W ratio, but CTOA remains constant. The following paper gives a fair idea about this approach(Look at the Fig.3)

"A review of the CTOA/CTOD fracture criterion" :JC Newman, M James - Engineering Fracture Mechanics, 2003

You can read

Nguyen Chau Linh — Fri, 13 Jul 2007 20:57:20 -0400

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

Good luck,

Propagation criterion

Dan Cojocaru — Fri, 13 Jul 2007 12:48:19 -0400

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.

Fracture Criterion

Arash Karpour — Fri, 13 Jul 2007 02:27:08 -0400

This is either a wage question I am asking or a stupid one...

How many fracture criterion do we have and what are they? some of these criterion that I am reading are related or a form of others...For example CTOD (crack tip opening displacement) criterion is related to energy release rate in

" A CTOD-Based Mixed-Mode Fracture Criterion" by Fashang Ma et al.

Any guid or direct help would be much appreciated.

Thanks for your time

Arash