Fracture Mechanics Discussion

[SOMANATH MOHANTY]

2. The asymptotic solution at crack tip gives singularity near that point then what happened to the solution when higher order terms included in the solution?
3.we want to extend LEFM concepts to rock like material then what is the effect of these higher order terms in finding fracture parameters?

[Mark Barkey]

2. The asymptotic solution at crack tip gives singularity near that point then what happened to the solution when higher order terms included in the solution?
3.we want to extend LEFM concepts to rock like material then what is the effect of these higher order terms in finding fracture parameters?

The higher order terms are more of an influence away from the crack tip.  I don't think it will affect any fracture toughness parameters unless the process zone of the crack extends more than "usual".

In my opinion, anisotropy of the material may be more of an issue than the higher order terms.  However, please note that I am not an expert in Rock Mechanics.

Thanks,
MEB

[SOMANATH MOHANTY]

1- suppose I want to carry out any fracture experiment in any material then it can be modelled by lEFM/EPFM. Is there any strength related perticular parameter that distinguish both of them prior to experiment?

[Mark Barkey]

1- suppose I want to carry out any fracture experiment in any material then it can be modelled by lEFM/EPFM. Is there any strength related perticular parameter that distinguish both of them prior to experiment?

If I understand your question, you want to test a material and know before you test it if the conditions of LEFM or if EPFM conditions prevail?

The answer is no, you cannot.  If you look at the ASTM test procedure (metals) for conducting plane strain fracture toughness testing (KIc) then there are several conditions that must be met that can only be determined after the test has been conducted.

However, you can look at the ASTM test procedure for JIc testing, which assumes EPFM, and use that instead.   

[SOMANATH MOHANTY]

hello, professor sir, I found your experimental video from your channel. Your explanations are awesome and very precise. I have a query on the tension test experiment.

I have a 2mm thickness sheet. I want to find the Poisson's ratio and youngs modulus through experiment. I am following the standard Flat test specimen size as per ASTM  test standard DESIGNATION-E8/E8M. As per standard prescribed thickness range lies within (0.005 ≤ T ≤ 0.75) in. So is there any variation in results if I select a minimum thickness and also variation in the result if I select multiple samples. If variation exists on results then how much variation?

[Mark Barkey]

hello, professor sir, I found your experimental video from your channel. Your explanations are awesome and very precise. I have a query on the tension test experiment.

I have a 2mm thickness sheet. I want to find the Poisson's ratio and youngs modulus through experiment. I am following the standard Flat test specimen size as per ASTM  test standard DESIGNATION-E8/E8M. As per standard prescribed thickness range lies within (0.005 ≤ T ≤ 0.75) in. So is there any variation in results if I select a minimum thickness and also variation in the result if I select multiple samples. If variation exists on results then how much variation?

If it is a metallic material, I would suspect that the actual material variation would be rather low for those two material properties.  Having said that, you may get some variation for very thin sheet vs thicker sheet but I think that variation would be due to the test set up.  Very thin sheet can be very easily distorted when clamping it into the testing machine grips.

2 mm is a nice thickness and I don't think you would have a problem with that for metals.  Poisson's ratio can be tricky if you use strain gages, too, in that it will not be very constant.  I think there is a good reason that Poisson's ratio is usually reported to only two significant digits.

Hope that helps.

[SOMANATH MOHANTY]

Questions
1- what is the physical meaning of stress intensity factor value to be Zero or negative in a centre crack under compression loading or it is not possible to get -ve SIF?
2-in a mixed mode case is j-integral is Path independent?
3- EPFM is applicable to nonlinear material behaviour case but as   the process zone is nonlinear then how superposition principle is applicable to this EPFM plastic models like dugdale.. others?

[SOMANATH MOHANTY]

Hi professor thanks a lot for your videos I learn something in plasticity hope you will provide some videos like how to implement UDF in abaqus and more videos on continuum mechanics.

[Mark Barkey]

Questions
1- what is the physical meaning of stress intensity factor value to be Zero or negative in a centre crack under compression loading or it is not possible to get -ve SIF?
2-in a mixed mode case is j-integral is Path independent?
3- EPFM is applicable to nonlinear material behaviour case but as   the process zone is nonlinear then how superposition principle is applicable to this EPFM plastic models like dugdale.. others?

For a center crack in compression, the crack faces should transmit tractions, so the stress fields (ideally) would be very similar to an uncracked plate in compression.  I do not know of a situation where a negative stress intensity would be physically meaningful or possible.

I believe that the J-integral should still be path independent for mixed mode loadings, but I have not done the math, or if I have it has been forgotten.  I think this should be derived in an advanced fracture mechanics book.

It has been a while since I have looked into the Dugdale model, but it may be the stress fields outside of the process zone for which supperpostition is applied.  I will have to let someone else answer that one!

[SOMANATH MOHANTY]

Thank u sir for your reply
but in question no 2 the path independent of j-integral in mixed mode loading, could you please mention where I can find the derivation in detail?

[SOMANATH MOHANTY]

1- suppose I want to arrest the crack in mode-1 loading then crack surface is no more traction free. In this case j- integral will give path independent result or it depends on path or j concept is not applicable?
2- what will happen when the same scenario applied on mixed mode loading?
3- if from crack tip multiple cracks originates simultaneously then how j-integral concept will be applied there to find the energy release rate for individual cracks?
4- during mixed mode loading once the crack initiation starts it will move at a certain angle. Is this angle Will change during its fracture path or it remains constant?

[Mark Barkey]

Those are great questions, but ones that I cannot answer in a meaningful way.  Best wishes!

[SOMANATH MOHANTY]

Sir I am a beginner learner in Fracture mechanics. Whatever may be the answer at-least i will get the direction of answer from your comments. Please provide answers.