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Updated: 6 hours 45 min ago

a previous curious theory in fatigue comes from Kassapoglou

Thu, 2020-12-10 02:07

In reply to A Griffith theory for fatigue from Delft group?

PS There was another theory from Delft in the past from Kassapoglou in fatigue which was surprising and perhaps interesting. Kassapoglou's approach was so elementary that I got curious, he claimed that you don't need to do fatigue testing, but simply get the fatigue curve from a statistic of static tests!!!! These papers were peer reviewed by good journals, but clearly they cannot be correct. If you are interested about that, see my paper which was peer reviewed here http://casopisi.junis.ni.ac.rs/index.php/FUMechEng/article/view/3768PPS: It did attract independent investigation by the FAA (Tomblin and Seneviratne, 2011 [5]), and others, who found that the theory was --- unconservative for the vast majority of data (10 sets), of which 5 perhaps by 1-2 orders of magnitude, 3 by 2-3 orders of magnitude, and 2 by 4-5 orders of magnitude. But nobody, except me, discussed why the theory was obviously theoretically misleading and not simply wrong. The theory has disappeared and will not resist the test of time. It will be forgotten.I am happy that Kassapoglou nevertheless is a good teacher, and for example has written an excellent book.So the main point is not "peer review" which is just the judgement of 2-3 people sometimes made in 5 minutes, and not "funding", which depends on ability to attract money, but the test of time, and validation by the large scientific community. This is at least what I believe.

Dear Professor Ståhle,

Mon, 2020-12-07 12:45

In reply to Discussion of fracture paper #27 - Phase-field modelling of cracks and interfaces

Dear Professor Ståhle,

Thank you for considering our paper [1] in your blog entry and your
interesting thoughts!

At first, it seems not trivial to compare our simulations to the
considerations of Zak and Williams [2]. In phase-field models for
fracture, there is no real stress singularity anymore due to the
regularization of the crack surface with the length scale l_c.
Furthermore, the phase-field method comprises the specification of the
fracture toughness, which is a key ingredient in controlling the onset
of fracture. In other words, the value of the energy release rate at
which crack growth takes places is à priori prescribed. In addition to
this, we investigate cases with heterogeneous fracture toughnesses
compared to the publication of He and Hutchinson [3].

Nevertheless, we try to answer your expectation

"The expected outcome I think would be that the crack growth energy
release rate increases for a crack in a stiffer material and decreases
for a crack in a weaker material."

Let us reformulate your expectation a bit and replace "crack growth
energy release rate" with "tendency". Now it reads "[...] the tendency
towards the interface increases for a crack in a stiffer material and
decreases for a crack in a weaker material." Now, looking at Fig. 13 in
[1], we exactly observe what is stated! This is also in line with your
first considerations, where the crack is further away from the
interface than (any) of the introduced length scales. Therefore, an
influence of the heterogeneity of the fracture toughness can be
excluded. This is good news, because the phase-field method does, what
LEFM predicts :-)

Investigating heterogeneities and their manifold influence on crack
propagation, crack delay or increase of overall cracking resistance is
indeed a topic of great interest and subject to ongoing research of us
[4,5] but also of many other research groups such as [6-10]. The list
is not exhaustive and further work of these and other research groups
can be found in the references of the cited literature.

Warm regards,

Arne Hansen-Dörr and Franz Dammaß

[1]: Hansen-Dörr, Dammaß, de Borst and Kästner, Phase-field modeling of
crack branching and deflection in heterogeneous media, Eng Fract Mech
232: no. 107004, 2020.

[2]: Zak and Williams, Crack point stress singularities at a bi-
material interface, GALCIT SM 62-1, 1962.

[3]: He and Hutchinson, Crack deflection at an interface between
dissimilar elastic materials, Int J Solids Struct 25(9): pp. 1053-1067, 
1989.

[4]: Hansen-Dörr, Hennig, de Borst and Kästner, Phase-field modelling
of interface failure in brittle materials, Comput Method Appl M 346:
pp. 25-42, 2019.

[5]: Hansen-Dörr, Brummund and Kästner, Phase-field modeling of
fracture in heterogeneous materials: jump conditions, convergence and
crack propagation, Arch Appl M, 2020.

[6]: Hossain, Jsueh, Bourdin and Bhattacharya, Effective toughness of
heterogeneous media, J Mech Phys Solids 71: pp. 15-32, 2014.

[7]: Kuhn and Müller, Phase field modeling of interface effects on
cracks in heterogeneous materials, PAMM 19(1), 2019.

[8]: Nguyen, Yvonnet, Zhu, Bornert and Chateau, A phase field method to
simulate crack nucleation and propagation in strongly heterogeneous
materials from direct imaging of their microstructure, Eng Fract Mech
139: pp. 18-39, 2015.

[9]: Schneider, Schoof, Huang, Selzer and Nestler, Phase-field modeling
of crack propagation in multiphase systems, Comput Method Appl M 312:
pp. 186-195, 2016.

[10]: Lebihain, Leblond and Ponson, Effective toughness of periodic
heterogeneous materials: the effect of out-of-plane excursions of
cracks, J Mech Phys Solids 137: no. 103876, 2020

-----------

Posted by ESIS on behalf of Drs. Hansen-Dörr and Dammaß / PS

The call has been closed.

Sun, 2020-12-06 18:17

Dear Frederick,

Wed, 2020-11-25 13:44

In reply to Great initiative!

Dear Frederick,

Thank you for your interest in the solid mechanics symposium and look forward to meeting with you at conference in PEI or virtually depending on the pandemic situation.

Hamid

This position has been filled

Tue, 2020-11-24 12:07

In reply to Postdoc position at University of Glasgow

This position has been filled

Great initiative!

Mon, 2020-11-23 08:36

In reply to CSME Congress 2021 - Symposium on Solid Mechanics

Dear Hamid,
Great initiative! I'm looking forward to this. Hopefully, we will be allowed to travel to PEI.

Thanks for sharing

Fri, 2020-11-20 11:56

In reply to D. Rittel - DYMAT webinar

I read some of your work.

Re: Cloaking in plates

Tue, 2020-11-17 18:14

In reply to Transformation Cloaking in Elastic Plates

Dear Michele:

Thank you for your message. I was not aware of your PML paper and just read it. In Eq. (17) of your PML paper the transformed elastic constants are correct. You cited your older papers but I wonder why the discrepancy between the two transformed elastic constants was not discussed? The continuity conditions on the PML boundary (your Eq. (30)) are correct. However, this will not make your cloaking formulation in your previous papers work. There are still some constraints (coming from matching the different terms in the governing equations of the virtual and physical plates) that will ultimately force the cloaking map (or your "transformation" map) to be the identity. Exact cloaking is not possible. I am not claiming that approximate cloaking is not possible. However, it should be formulated properly as an optimization problem. 

Regards,

Arash

Cloaking in plates

Sun, 2020-11-15 13:54

In reply to Transformation Cloaking in Elastic Plates

Dear Arash Yavari,

thank you for your interesting work.
Concerning your criticisms on our work concerning your eqs. (1.1) and (1.2), they were already solved in the paper I published in (2018), i.e. https://www.sciencedirect.com/science/article/pii/S0020768317305140?via%3Dihub

Transformed equations and interface/boundary conditions have been reported in Section 4.

In Section 5, we propose the eigenfrequency analysis as a tool in order to check the transformation.

There, higher order polynomials were implemented in order to respect interface conditions between untransformed and transformed domains.

 

Anyway, concerning the formulation reported in 
1) D. J. Colquitt, M. Brun, M. Gei, A. B. Movchan, N. V. Movchan, and I. S. Jones. Transformation elastodynamics and cloaking for flexural waves. Journal of the Mechanics and Physics of Solids, 72:131–143, 2014. 

2) M. Brun, D. Colquitt, I. Jones, A. Movchan, and N. Movchan. Transformation cloaking and radial approximations for flexural waves in elastic plates. New Journal of Physics, 16(9):093020, 2014.

and 

3) I. Jones, M. Brun, N. Movchan, and A. Movchan. Singular perturbations and cloaking illusions for elastic waves in membranes and kirchhoff plates. International Journal of Solids and Structures, 69:498–506, 2015.

the comparison between untransformed and transformed domain show an excellent agreement apart from the neighbourhood of the interface between untransformed and transformed domains. The difference between solutions in the reference and transformed domains was considered quantitatively in (1) through the scattering measure.
Anyway, the difference is small.

May you guess why?

 

Best,

Michele Brun 

Great event

Mon, 2020-11-09 17:04

In reply to Event of ASME Technical Committee on Mechanics of Soft Materials 2020

Thank you for sharing. Great event with great speakers!

pull-through test

Mon, 2020-11-09 13:53

In reply to VUMAT viscoelasticity (Maxwell model)

Hello Surot

just give it a try and see if the result matches your data.

How is this pull-through achieved ? By suction induced by vacuum ? You might check the literature on thermoforming, a process where a disk is heated and then makes contact with a mold.

I advise you to subscribe to and seek assistance from the ABAQUS mailing list
https://groups.yahoo.com/neo/groups/Abaqus/info
Search the archive of the list before posting in it.
The list does not accept attachments.

If you have a UMAT for generalized Maxwell implementation, you might also look at this interface:
https://soilmodels.com/download/vumat_umatinterface-zip/
No comment from me on that; I never worked with it.

If that Kelvin-Voigt VUMAT somewhere available on the Internet ? Just curious, I was also searching for that a few years ago.

Good luck,

Frank

Mineral processing

Mon, 2020-11-09 10:19

In reply to Faculty positions at Northeastern University in China

Mineral processing

Tribology, Mechanics,

Fri, 2020-11-06 06:49

In reply to Postdoc in Mechanics/Tribology and/or Triboelectric Nanogenerators

Tribology, Mechanics, Triboelectric

Thanks, Emilio, we are

Fri, 2020-11-06 02:55

In reply to Great opportunity in a great

Thanks, Emilio, we are flattered!

Great opportunity in a great

Fri, 2020-11-06 01:11

In reply to Two postdoc positions in computational mechanics at Czech Technical University in Prague

Great opportunity in a great environment. I couldn't recommend more!

Thank you Cheikh for the

Thu, 2020-11-05 09:28

In reply to Phase field modelling of fracture and fatigue in Shape Memory Alloys

Thank you Cheikh for the clarification. In my view, the phase field method is a mathematical model, not pertaining to any scale. It is of course widely used in microstrutural evolution, as you say. But it is also very widely used in other interfacial problems at many differente scales. Fracture is one of them, where the phase field order parameter is used to differenciate between the cracked and solid parts of the material. Hope that this clarifies things.

Thank you Emilio for the

Thu, 2020-11-05 08:46

In reply to Phase field modelling of fracture and fatigue in Shape Memory Alloys

Thank you Emilio for the answer. When I saw phase-field modeling which is known as a microscale method, I expected more details about the microstructure evolution such as the competition between the different martensite variants instead an averaged martensite at the macroscale.

Regards,

Cheikh 

Unfortunately, the #EURAXESS

Thu, 2020-11-05 05:56

In reply to Two postdoc positions in computational mechanics at Czech Technical University in Prague

Unfortunately, the #EURAXESS link was inactive, but it is working again. Apologies for any inconvenience caused.

Comment

Tue, 2020-11-03 12:39

In reply to PhD opportunity within the ITN NreFrac Computational Fracture Mechanics

Please feel free to ask any doubt about the offer

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