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Updated: 9 hours 31 min ago

Thanks nskatim :)  

Thu, 2015-08-27 12:35

In reply to You may read the input file

Thanks nskatim :)

 

 

Wed, 2015-08-26 19:46

In reply to Physics and Computational Physics Links...

 

Possible seo jung for mechanics?

Re: moisture content

Wed, 2015-08-26 17:00

In reply to moisture content

Hi Steve,

 

Thanks for your comment. You’ve touched on two important issues to which we’ve given a lot of thought and that we do plan on incorporating into the NGF modeling framework. To your first point, we’ve only considered dry granular materials so far, and the situation changes when the material is partially saturated with a pore fluid, leading to cohesion due to the liquid bridges between particles. Mathematically, the NGF model revolves around the granular fluidity − a scalar state parameter which relates the stress quantity driving flow to the consequent strain rate. For cohesionless, dry granular materials, this stress quantity is μ=τ/P, a stress invariant. (Note that one would expect the same approach to work for the fully saturated case, since it is cohesionless as well. Hence, the path forward for fully saturated granular materials is more straightforward in my opinion.) Clearly, a modified stress quantity, which accounts for cohesion, must be identified for the partially saturated case. As a first cut, one might incorporate the capillary pressure arising due to the liquid bridges into the definition of μ. The simplest definition is μ=τ/(P + P_cap), where τ and P are the stress invariants as before and P_cap is the capillary pressure due to liquid bridges, which is a function of the degree of saturation. (One may think of P_cap as the maximum hydrostatic tension that a partially saturated granular material may sustain before coming apart. Hence it should be zero for both the unsaturated and fully saturated cases but greater than zero in between.) The idea is that the normal force felt at granular contacts is a combination of both the confining loads which give rise to the pressure P and the capillary effects which give rise to P_cap. Of course this is just a first hypothesis and will need to be tested and refined by experiments.

 

In your second point, you ask if there are models that are aimed at the transition between solid-like and fluid-like behavior. I’d like to think that the NGF model provides such a unified framework. In fact, I would argue that the phenomenologies discussed in my post are both more solid-like than fluid-like, involving a yield condition and rate-independent behavior (as demonstrated in the lower left figure of Fig.4). The NGF model unifies this rate-independent, solid-like regime of deformation with the fluid-like regime (in which the μ(I) rheology provides an excellent description) and does so in a manner which is quantitatively consistent with experiments.

 

However, with all of this said, the solid-like aspect of the NGF model is not as detailed as most granular plasticity models like critical state theory. In particular, we have ignored the effects of loading and preparation history as well as shear-induced dilatancy thus far, instead focusing only on the “critical state” so as to quantitatively test the nonlocal aspect of the model. One strategy for remedying this would be to incorporate the effect of loading history into the local response, i.e., through the function g_loc. In the current model, g_loc only depends upon stress, but in a history-dependent version, it could depend on strain as well (as in the paper you cited) or some other system of internal variables (for example, as in Anand and Gu). The resulting model would be capable of describing (i) nonlocal effects, (ii) transient shear weakening/strengthening and the effect of the initial state, and (iii) the smooth transition to the rate-dependent regime as strain-rate is increased.

 

Finally, since you mentioned modeling the process of repeated yielding, I'll raise one additional complicating wrinkle: Yield in granular materials is hysteretic. For example, in the inclined plane geometry discussed in my post, one will robustly measure an Hstart curved based upon the initiation of flow, which is distinct from the Hstop curve based upon flow cessation (see MiDi for more). Hysteretic yield has been attributed to the local μ(I) rheology actually being non-monotonic, i.e., not invertible (see Dijksman et al.). Incorporating hysteretic yield into a continuum model is a significant challenge going forward.

 

-David

moisture content

Tue, 2015-08-25 07:57

In reply to Journal Club Theme of August 2015: Nonlocality and yield in granular materials

David, thanks for the fantastic reviews on the nonlocaility and yielding in granular flow. I am actually wondering if you and Ken are planning to incorporate the effect of moisture content and liquid bridges in your model? What is the potential challenge to do that? 

Another question is related to the transition from granular flow to granular solid. In geotechnical engineering and geomechanics research, there is a significant body of work on modeling granular solid under confining pressure in which case the stress depends on loading history and strain instead of strain rate and we considered a material in critical state under a certain combination of shear stress, volumetric strain and mean pressure. However, for granular flow, strain rate and normal pressure seem to be dominating factors for the shear stress. Is there any work aimed to provide a unified framework to allow a smooth transition from fluid-like state to solid-like state and vice versa? What is the major difficulty to model such transitions? What are the major difference in those two transitions? The (re-)activiation of the fault guoge layer mentioned by Ahmed seems to be more related to a transition from solid-like status to the fluid-like states, and a few works are attempted to model that for dense assemblies, e.g. [1]. However, I am interested to see whether it is possible to model such transition back and forward. Any guidance is greatly appreciated. 

[1] Andrade, J. E., Chen, Q., Le, P. H., Avila, C. F., & Evans, T. M. (2012). On the rheology of dilative granular media: Bridging solid-and fluid-like behavior. Journal of the Mechanics and Physics of Solids, 60(6), 1122-1136.

 

 

I appreciate your help. 

Sun, 2015-08-23 04:08

In reply to Re: EMU Peridynamics code

I appreciate your help. 

Very interesting

Sat, 2015-08-22 23:14

In reply to Continuous Assessment

The development on this is very good. Good ideas all around Mr Tan.

 

 

 

 

 

 

 

 

 

 

 

kizi

perfect plasticity

Sat, 2015-08-22 15:50

In reply to Stresses go beyond yield limit in perfecly-plastic model...

modify the command to

*PLASTIC

4.85 E8, 0.0

4.85001E8, <a strain value that approximate "infinity">

 

Open Postdoctoral Position in Computational Science Engineering

Fri, 2015-08-21 14:51

In reply to Open Postdoctoral Position in Computational Science and Engineering

Postdoctoral Associate

 

            The University of Notre Dame, Center for Shock Wave-processing of Advanced Reactive Materials (C-SWARM), is seeking a highly qualified candidate for the postdoctoral associate position in the area of computational mechanics/physics with emphasis on parallel numerical methods. C-SWARM is center of excellence established by National Nuclear Security Administration (NNSA) whose primary focus is on the emerging field of predictive science. The main mission of C-SWARM is to predict shock conditions under which new materials can be synthesized using predictive computational models that are verified and validated with quantified uncertainty on future high-performance Exascale computer platforms.

            The successful candidates will be a key part of a team that is developing and implementing adaptive, multiscale, high-performance (parallel) computational algorithms for numerical solutions of chemo-thermo-mechanical PDE’s with emphasis on complex heterogeneous materials, such as heterogeneous reactive composites, etc.

 

Qualifications:

·   Ph.D. in Mechanical Engineering, Theoretical and Applied Mechanics, Applied Mathematics, Physics or related engineering or science discipline

·   Knowledge of numerical methods such as collocation schemes, meshless methods, wavelet methods for solution of PDEs, etc.

·   Knowledge of computational nonlinear solid/fluid mechanics

·   Knowledge of C/C++ and UNIX operating system is required

·   Experience in parallel programming

 

Close Date:

Review of applications will begin immediately and continue until the position is filled.

 

Salary:

Salary will commensurate with qualifications and experience.

 

Contact:

Interested applicants should send a CV with a cover letter, names of at least three references, and a summary of recent work. All applications should be submitted electronically (paperless process) as a single PDF document to:

 

Gail Small

C-SWARM Administrative Coordinator

Department of Aerospace and Mechanical Engineering

University of Notre Dame

117M Cushing Hall

Notre Dame, IN 46556

574-631-3957

cswarm@nd.edu

www.cswarm.nd.edu

 

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            The University of Notre Dame is an Affirmative Action, Equal Opportunity Employer. Women and minorities are encouraged to apply.

Mechanics of cell-nanomaterial interfaces

Fri, 2015-08-21 10:19

In reply to Guiding principles on nanoparticle-based therapeutic targeting

A review focusing on the fundamentals: why is cellular uptake of nanoparticles size selective, shape and stiffneess sensitive, and microenvironment regulative ?

thanks

Fri, 2015-08-21 05:46

In reply to FGM

thanks for your reply.

You may read the input file

Thu, 2015-08-20 22:35

In reply to add a python script to an input file

You may read the input file as a regular text file and add/insert commands wherever necessary. OR

You may use the scripting functionality keywordBlock at the end of your python script to add stuff to the input file just created by the script.

The code will run, but

Thu, 2015-08-20 05:15

In reply to the limits

The code will run, but probably slowly. What you can do is define a set with only those nodes that need to be included in the periodic boundary conditions. The code will loop over all nodes in the set to determine if it has another node in that set that is located periodically, so the less nodes the faster.

thanks for your reply

Wed, 2015-08-19 17:44

In reply to There is no thing like adding

thanks for your reply

how can I read the input file via python? do you mean to change the input file to python language? how?

thanks

There is no thing like adding

Wed, 2015-08-19 15:58

In reply to add a python script to an input file

There is no thing like adding a (Python) script to an input file. But for your purpose, what you may need is to read the input file via Python script and perform the required changes to the same input file.

Please let us know what exactly you need to do if the above doesn't answer your query.

 

Regards

Re: sphere modelling

Wed, 2015-08-19 14:48

In reply to FGM

Dear Rakhshi, 

To model a full sphere it is useful in Ansys batch mode programming to use: CS command in order to define your selected nodes or keypoints after that choose a 3D curved element.

thanks

Wed, 2015-08-19 06:24

In reply to FGM

your guidance was very useful.thanks alot.

VUMAT subroutine file:A multiscale model required

Wed, 2015-08-19 02:40

In reply to Sharing ABAQUS UMAT and VUMAT subroutines

Hi Everyone,

I want to implement the multi-scale model for ceramics under ballistic impact. My reference paper: Dynamic fragmentation of brittle solids: a multi-scale model,Christophe Denoual, François Hild. I have studied the Dassault systems 'Writing user subroutines in ABAQUS'. I have read the examples also. But the model I have has a new parameters which are not similar to the example models. I want to do Abaqus/Explicit analysis.If any one can help me regarding this it would be great. Please read the paper and give me some idea of how to implement in Abaqus using Subroutine VUMAT.If you have the subroutine it would be great help if you please send the subroutine to my email Id:kosuribs99@gmail.com

Dear Mina Rakhshi

Wed, 2015-08-19 02:39

In reply to FGM

Dear Mina Rakhshi

It is very well explained in the paper and references therein (in fact, one of the papers cited uses ANSYS).

You can assign spatially varying properties at integration points by defining properties as a function of temperature and providing the model with an initial temperature distribution that matches the elastic modulus variation desired. The assignment of a zero thermal expansion coefficient then eliminates unwanted thermal strains.

I hope that this is useful for you.

Emilio Martínez Pañeda

SIMUMECAMAT Research Group
www.simumecamat.com

 

 

Problem in implementing a new multi-scale model for ceramics

Tue, 2015-08-18 21:54

In reply to Sharing ABAQUS UMAT and VUMAT subroutines

Hi,

I want to implement the multi-scale model for ceramics under ballistic impact. My reference paper: Dynamic fragmentation of brittle solids: a multi-scale model,Christophe Denoual, François Hild. I have studied the Dassault systems 'Writing user subroutines in ABAQUS'. I have read the examples also. But the model I have has a new parameters which are not similar to the example models. I want to do Abaqus/Explicit analysis.If any one can help me regarding this it would be great. Please read the paper and give me some idea of how to implement in Abaqus using Subroutine VUMAT.Please send the subroutine to my email Id:kosuribs99@gmail.com

a

temperature

Tue, 2015-08-18 12:30

In reply to FGM

thanks, I will study your paper, But I don't discover, I how can use the temperature to have graded finite element in ANSYS. please more explain.

 

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