Discussion Forum

Posted: 1 decade ago Feb 1, 2012, 3:08 p.m. EST

Hi what about telling us slightly more, i.e. size of sphere compared to your 0.5um indentation, thickness of layer pinched, what kind of change to nu ? It's difficult to give you any clear int so long one cannot better understand de details here ;) -- Good luck Ivar

Posted: 1 decade ago Feb 1, 2012, 4:39 p.m. EST

Hi Dear Ivar, Thanks for your reply. In fact I am using axisymmetry geometry and I have a very stiff shell (Something like 1/8 of a half sphere) pressing against a block of material. The material height is 100 micron and in R direction, I have used perfectly match layer mesh elements. Apart from that pressing force, I have a boundry force that acts between the shell and polymer. That is about 11.98 MPa and pulls the surface up. That is a relatively large force that acts on some nm lenght scale. something like figure 3 in this page http://rsta.royalsocietypublishing.org/content/366/1870/1627.full The difference is that my particle is really pressing the surface and this pressure is deforming the soft surface upward. Well I start from nu of 0.33 for 3 GPa polymer and come down to 0.4999 for 20 MPa. I wish to go evenmore down. The softer the film gets the more it deviates from Hertz model. Right now I consider nonlinear geometry for polymer film but not for the stiff shell . If I do not change the nu for my film, comsol cannot solve my problem Thanks again, Marjan [QUOTE] Hi what about telling us slightly more, i.e. size of sphere compared to your 0.5um indentation, thickness of layer pinched, what kind of change to nu ? It's difficult to give you any clear int so long one cannot better understand de details here ;) -- Good luck Ivar [/QUOTE

Posted: 1 decade ago Feb 21, 2013, 11:07 a.m. EST

Hi Marjan, Are you doing a time-dependent study or stationary study? Thanks a lot.

Posted: 1 decade ago Feb 21, 2013, 12:06 p.m. EST

....Both. -- Marjan

Posted: 1 decade ago Feb 22, 2013, 8:47 a.m. EST

Hi Marjan, Thanks for your reply, I saw other post from you.http://www.uk.comsol.com/community/forums/general/thread/34/ I'm simulating contact between tip and sample, but it is difficult to converge doing time dependent study. I use small time step and the discretization of displacement is larger than contact. As there is no inertial contact, the inertial pressure should be zero right? More over, have you met the 'warning: New force constraints node detected' when tip indents the sample? I appreciate if you could give me some advice. Thanks a lot. Regards, Wenqi Deng

Posted: 1 decade ago Feb 22, 2013, 10:19 a.m. EST

Hi Wenqi, What are you using for your time dependent tip movement control? Parametric sweep or continuation. I would say that you would better use continuation. What is the material that you are indenting? How much is the elastic modulus? I would suggest that you first try to solve your problem and get convergence when your tip is in contact with your sample surface and then extend it to a case that the tip is not in contact. About the initial pressure: The initial pressure is the pressure that can be 2 orders of magnitude larger than the maximum pressure that occurs in contact area. It helps convergence and depending on your material elastic modulus you can estimate that. You may also need to change some parameters is solver but first please tell me what is your elastic module and if you are moving a boundary or your whole indenter. I have also faced the error you have faced but it can have different meanings. I can only give my comments if I know your boundary conditions and see your mesh. It can be due to both. I hope it can help you :). -- Marjan

Posted: 1 decade ago Feb 22, 2013, 11:28 a.m. EST

Hi Marjan, The tip movement is controlled by a sinusoidal force(which depends on time variable 't'), it should be the contiuation you mentioned? The sample elastic modulus is 3e9[Pa] (PPMA). Both the tip and sample are defined as linear elastic model. I define tip surface and sample surface as contact pair and use tetrahedral mesh(whose minimum element size is 1[nm]). And bottom of the sample is fixed. Thanks. Looking forward to more advice. Regards, Wenqi Deng

Posted: 1 decade ago Feb 22, 2013, 12:12 p.m. EST

Hi Wenqi, You can give values to " t" in continuation of parametric sweep. Use continuation. Please see the attached file. The second point is check that size of your mesh elements on the PMMA is 1.5 times smaller than the mesh size of your sphere. Check 2 times as well. For penalty factor you should not have problem if you set the initial pressure about 1000 Pa or even larger. So your simulation is 3d. Is it true that your system is only fixed at the bottom of your sample or support onto which the indenter indents and is everything else free? Then you should reconsider your model. Comsol faces problem to solve it. What is the theory you want to compare your simulations with? I used Hertz model. As it is a model for half infinite surface, I used infinite element domains in definition part of comsol (see the attached file please) and decreased the number of mesh elements a lot. But my support is divided to several domains . The model became also very exact. It is as if you had a half space surface. I did the reverse of what you did. I controlled displacement and with week constraints, I calculated reaction forces. Do you want to see the indentation? It is also interesting :). Regards from Germany, Marjan

Attachments:

• 1.PNG

Posted: 1 decade ago Feb 22, 2013, 12:27 p.m. EST

..... See the attached new file for the continuation time dependent solver -- Marjan

Attachments:

• 2.PNG

Posted: 1 decade ago Feb 25, 2013, 9:37 a.m. EST

Hi Marjan, Thanks for your reply. I've been thinking about my model. I apply a dynamic load on the tip and fix the bottom of my sample, it is not a complicated model, I cannnot figure out why comsol faces problem to solve it? Did you come across this problem or have you found out the source of this problem? Regards, Wenqi

Posted: 1 decade ago Feb 25, 2013, 12:22 p.m. EST

Dear Wenqi .... you do not have enough boundary conditions. It is too much free everywhere. It is difficult for comsol to solve it. you should think of putting somehow more boundary condition. try to solve such a problem for a hard rigid surface both analytically and with FEM. Then you can understand which assumptions you want to use and you immediately understand that for solving the problem on hard surfaces you need some boundary conditions. Can you share only your geometry with me? I can give you my comments. -- Marjan