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Updated: 2 hours 21 min ago

Hi

Sun, 2018-04-15 06:37

In reply to Check this position

Thanks for this valuable information. 

Hello! How do I apply?

Sat, 2018-04-14 09:30

In reply to Finite element software developer

Hello! How do I apply?

Hi

Fri, 2018-04-13 23:53

In reply to You can try to contact Prof.

Hello Chinguyen,

Thankyou,

The FEM job requisition is re-openned

Fri, 2018-04-13 19:59

In reply to FEM Job Opportunity at Apple

The job requisition has been reoppend recently to recruit more candidate. Please use the new link to submit your resume. 

https://jobs.apple.com/us/search?#&ss=52347662&t=0&so=&lo=0*USA&pN=0&ope...

Or directly send it to me at jhzhao at apple.com if you prefer.

 

 

You can try to contact Prof.

Thu, 2018-04-12 01:54

In reply to looking for Postdoc position in the fields of Impact Mechanics, Ballistic load analysis, FRP composite structure, Fracture Mechanics, Numerical modelling and Experimentation

You can try to contact Prof. Tuấn Ngô in University of Melbourne. He is one of the leading personal on this topics with numerous on-going projects.

http://www.ie.unimelb.edu.au/people/staff.php?person_ID=6717

dtngo@unimelb.edu.au

Release of VEMLab v2.0

Mon, 2018-04-09 22:59

In reply to VEMLab: a MATLAB library for the virtual element method

VEMLab: a MATLAB library for the virtual element method                     
Release of VEMLab v2.0
  

From VEMLab v1.0 to VEMLab v2.0: the following features have been added 

  • Two-dimensional Poisson problem.
  • Setup of plot and output options in function "plot_and_output_options" located in folder "config".
  • Additional plotting options (stresses, strains, fluxes and gradients) to MATLAB figures, text files and GiD files.
  • Option to plot solutions to VTK files.

Browse and get the code: http://camlab.cl/research/software/vemlab/

friction and enengy dissipation

Mon, 2018-04-09 21:53

In reply to energetics of friction

Dear Mike,

Thanks for the clarification. I got your point/concern now. I think the confusion lies in the different meaning of "friction". I think your "friction" means specifically the dissipative force. While my "friction" actually means the lateral force (not necessarily dissipative). For example, in the traditional Prandtl-Tomlinson model, when one drives a particle with a linear spring along an energy landscape, if the energy corrugation is relatively small or the spring constant is relatively high, the particle will slide smoothly along the landscape without any exhibiting any stick-slip instability. In this case, sliding forward and sliding backward will be essentially reversible and no energy will be dissipated. According to your definition, friction will be zero. However, one still needs a lateral force (sometimes positive and sometimes negative) to move the particle forward. What I meant previously was that this lateral force could be predicted by the energetics. I fully agree with you that the friction force (defined from the energy dissipation) cannot be predicted soley from energetics.

At the continuum level, friction defined from energy dissipation seems to be more relevent. At the atomic scale (or nanotribology you mentioned), people are interested in both lateral force and mean friction (this will have the same meaning as the dissipative force). However, in many cases, when people think about the sliding process in nanotribology, they often focus more on variation of the lateral force while assuming that the system is always overdamped. Maybe, such types of work made you feel that they were too idealized and not physically meaningful :)   I agree with you that once a particle overcomes an energy barrier and starts to slip, its energy is not necessarily fully damped when it reaches the next energy barrier. How energy dissipates during this slip process is a fundamental question, which essentially determines the level of dissipative friction. Nowadays, it is believed that the energy is dissipated primarily through lattice vibrations (phonon and eventually heat) and sometimes electronic excitations[45-48].  Of courses, these two mechanisms are for elastic and wearless sliding. For macroscale sliding process, more dissispative mechanisms can be involved, e.g., plastic deformation, crack or new surfaces, capillary ruptures....  All these make friction at the larger scales very complicated.

 

45. Witte, G. et al. Damping of molecular motion on a solid substrate: Evidence for electron-hole pair creation. Phys. Rev. Lett. 80, 121-124, doi:DOI 10.1103/PhysRevLett.80.121 (1998).
46. Park, J. Y., Ogletree, D. F., Thiel, P. A. & Salmeron, M. Electronic control of friction in silicon pn junctions. Science 313, 186-186, doi:10.1126/science.1125017 (2006).
47. Qi, Y., Park, J. Y., Hendriksen, B. L. M., Ogletree, D. F. & Salmeron, M. Electronic contribution to friction on GaAs: An atomic force microscope study. Phys. Rev. B 77, 184105 (2008).
48. Kisiel, M. et al. Suppression of electronic friction on Nb films in the superconducting state. Nat Mater 10, 119-122 (2011).

Interlayer sliding is another interesting aspect of 2d materials

Mon, 2018-04-09 12:30

In reply to Shear deformation of graphene-based interfaces

Dear Zhaohe,

Thanks for sharing the interesting and nice work.

Interlayer sliding between 2d materials and their contacting surfaces is another hot topic in tribology. Depending on the atomic stacking and the nature of the inter-molecular forces, the frictional behavior can be very intriguing. For example, friction tuning via modifying the chemistry of the substrate surface as shown in your papers, or suppressing friction to acheive so-called superlubricity (nearly a frictionless state) via adjusting interface stacking as  partially shown in your PRL paper.

 

Qunyang

Please participate in SES 2018, Madrid, October 10-12

Mon, 2018-04-09 00:23

In reply to Principal Stresses explained with an experiment

Dear colleagues,

 The Society of Engineering Science 2018 (SES 2018) meeting will be help in Madrid, Spain, October 10-12, 2018:

http://ses2018.org/

The abstract submission deadline for the 55th Annual Meeting of the Society of Engineering Science has extended until April 15th, 2018 and thus you still have an opportunity to participate in this most exciting event. To submit your abstract, just go to the following url:

http://ses2018.org/?s=abstract-submission.

 The comprehensive conference program can be found at:

http://ses2018.org/?s=track-symposia.

There are 40 carefully selected symposia divided in 5 tracks, which cover very important areas in Engineering Science as well as the interface between Engineering and other disciplines!

Looking forward to seeing you in Madrid in October!

Glaucio H. Paulino

SES President, 2018

energetics of friction

Sat, 2018-04-07 03:27

In reply to energetic approach

Qunyang

 there is some confusion about how to apply energetic approaches to friction.  It is not so obvious how friction, which dissipates energy, could be "predicted" in continuum models.  JKR approach predicts adhesion, because we postulate that energy is needed to form new surfaces. But what about a sliding body?  These atomistic models of friction of course get some numbers out of some assumptions of potentials, but they are limited to very idealized assumptions and I am not sure if they work outside "nanotribology"....

Shear deformation of graphene-based interfaces

Thu, 2018-04-05 15:14

In reply to Journal club for April 2018: Friction of atomically-thin sheets

Dear Qunyang,

Thanks for sharing this very interesting review. These tip-based friction characterizations are very attractive. Many important properties (e.g. electronic performance) of 2D material-based devices can be tuned by simply deforming/sliding/twisting the interface formed by the 2D material to its substrate and by 2D materials themselves.

I'd like to add our recent works on the "sliding" of graphene on a polymer substrate (https://pubs.acs.org/doi/abs/10.1021/acsami.6b03069, https://www.sciencedirect.com/science/article/pii/S0266353816303098), graphene on silicon oxide, and graphene on graphene (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.036101). By "sliding" here, I mean the shear deformation of the graphene-based interface, which may be small deformation and not large enough to be termed friction. One interesting point is that the shear response of these graphene-based interfaces was modified by chemical treatments, such as hydrogen bonding and boron doping.(https://pubs.acs.org/doi/abs/10.1021/acsami.6b03069https://www.sciencedirect.com/science/article/pii/S0266353816303098,https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b05771)

Thanks

Zhaohe

energetic approach

Wed, 2018-04-04 13:06

In reply to thanks. For energetic approach, I meant continuum ones

Congratualtions on your new JMPS paper. Your approach of handling the adhesive contact of rough surfaces is very elegant.

I think the idea of configurational force should be fine if one considers all the energy associated the system. I did not get your point of violating the approach....

 

Qunyang

The unique deformation revealed in the added ref

Wed, 2018-04-04 12:57

In reply to fantastic review

Dear Shuze,

Thanks for sharing the very relevant paper. The simualtion results presented in your work nicely showed that the interlayer deformation between the top-layer graphene and bottom-layer graphene made a significant contribution to the frictional resistance, or the configurational force as I mentioned. So these atomically-thin lamellar materials indeed behave distinctively from bulk materials upon adhesive contact.

 

Qunyang

Position now closed

Wed, 2018-04-04 03:52

In reply to Post Doctoral Researcher in Smart Machining (Testing and Simulation)

This position is now closed. 

Dear Ahmed,

Tue, 2018-04-03 21:34

In reply to Very interesting review

Dear Ahmed,

Regarding your first question:

These assembled structures show some kind of periodicity. Has there been any studies on their dynamic response and their wave mitigation properties? Periodic structures may exhibit frequency band gaps and may have extreme properties (e.g. auxetic response, negative refraction,..etc).

Yes, there have been. Materials with these microstructure might present bandgaps for shear modes, since dilatational waves would have continuous translational symmetry.

I would like to suggest our previous work related to the dynamic behavior of shear stress waves in the Stomatopod's dactyl club [1]. We found that the extraordinary absorbing energy capabilities existing in the dactyl club of Stomatopods can be partially attributed to the bouligand like structure present in its material microstructure.

[1] Nicolás Guarín-Zapata, Juan Gomez, Nick Yaraghi, David Kisailus, Pablo D. Zavattieri, Shear Wave Filtering in Naturally-Occurring Bouligand Structures, Acta Biomaterialia, Vol 23, N 1 (2015), doi: 10.1016/j.actbio.2015.04.039. Preprint available at <http://arxiv.org/abs/1505.04203>.

thanks. For energetic approach, I meant continuum ones

Tue, 2018-04-03 07:58

In reply to Dear Mike,

...not the discrete atomistic ones. For example, JKR theory is a balance of strain energy, and interface energy (with potential mechanical energy if we have load control), which can be generalized. We can obtain adhesive solutions from adhesionless solutions, provided the contact area has the same shape.  However, it may be that these "configurational forces" case violates this approach.

Ciavarella, M. (2018). An approximate JKR solution for a general contact, including rough contacts. Journal of the Mechanics and Physics of Solids.

fantastic review

Tue, 2018-04-03 03:08

In reply to Journal club for April 2018: Friction of atomically-thin sheets

Dear Qunyang, 

Thanks for putting together such a fantastic review! Indeed, the deformation of the interface matters to the abnormal load dependence. Here I am adding a reference to the abnormal load dependence of friction in lamellar materials, in which we studied the effects of surface compliance and relaxation on the changing of contact area upon indenting and retracting the tip. 

http://pubs.rsc.org/en/content/articlelanding/2014/ra/c4ra03810j/unauth#...

Re: some additional works

Mon, 2018-04-02 23:12

In reply to some additional works

Dear Yanfei,

Thanks for kind words and sharing the two inspiring papers. Structural lubricity and thermal lubricity are two interesting and important phenomena in friction, which might lead to revolutionary applications if properly controlled.

I fully agree with you that the collective behavior exhibited by the flexibile graphene interface shares the same physical origin as the structural lubricity you mentioned. Interface commensurability is an important variable of the contact state. Recently, we have experimentally demonstrated that it indeeds can be actively tuned by straining the system. We will keep posted for this.

Regarding the thermal effect, most availabe theories adopt the approach of thermal activation. This usually leads to a logarithmic dependence on sliding velocity. However, our recent experiments show an unsual power law dependence for friction on moire superlattice structure. Do you happen to have any thoughts on this abnormal behavior? 

 

Qunyang

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