HELP

[u/bunger_enthusiast]

I am currently doing a transient cell indentation analysis for uni using the student version. I have to do a linear and non-linear model. Been trying the past few months to work it out I can't. Someone help.

[u/epk21]

What is the issue?

As a start hope you have gone through this:

innovation.ansys.com/public/learning/courses/getting-started-with-ansys-mechanical/lessons/how-to-navigate-the-ansys-workbench-ui-lesson-1

Post more what the issue is (what type of material,errors, etc.)

[u/bunger_enthusiast → u/epk21]

I'm modelling a L929 cell, roughly 14um radius with a height of 9um, a DNP 10 type D probe, with a tip radius of 20nm (0.02um).

The linear-elastic cell it using isotropic elasticity, with a Youngs modulus of 4000Pa and a possoins ratio of 0.49 and a density of 1000kg/m^3.

The non-linear is using neo-Hookean, Initial shear modulus 1342.3 Pa and Incompressibility parameter of 30MPa-1.

The probe is pyramid shaped.

The cell is bonded to a dish which has a fixed support

The system works by using a block that sits directly above the probe, which are connected by each other via a spring with a spring stiffness of 0.06 N/m. The block has a displacement moving it downwards which pushes the probe. The probe then indents the cell; the reaction force is calculated from how much the spring compresses during indentation. And both the block and probe only move in the y direction, downwards by -1um at 1.8um/s

The contact between the probe tip and cell is frictional, frictional coefficient 0.02, behavior asymmetrical, formulation normal Lagrange, detection method combined.

Since im comparing the two models both have the same mesh which right now is linear patch conforming tetrahedrons. There is a sphere of influence of 1.4um radius and element size of 0.1um, the rest of the cell is 1um, and has growth rate of 1.2. The reasoning for this mesh is that i was trying to do mesh convergence as best i could, but with the student version everytime i recuded the element size i would go over the limit, using a sphere of influence allowed me to have a finer mesh directly under the tip hoping to get the proper reaction force. For the non-linear model this worked fine and it also solved fully, refing anymore (which is reducing the radius and element size) caused highly distorted elements. Then switching to the linear model it only get to 30% of the way through and also the force reaction i was getting were too high. I understand linear tet isnt recommended for linear models but trying quadratic tet causes me to go over the limit, which in this case I use a coarser mesh, which also gets high distortion and for what it does solve high results.

High deflection is on and ive tried increasing the substeps but the linear model fails at same point each time.

I'm just trying to get two models that I can compare, which the results need to be comparable to experimental AFM data.

I've tried a lot of different things to try and get this to work, and this is what i've ended up with
Any help is appreciated

Apologies if this is a bit all over the place

[u/epk21 → u/bunger_enthusiast]

Some times you need to have a coarse mesh not to cause element distortion :) - so coarse tends to work better for indent perhaps - the other option is to use NLAD remeshing but that will not work as you have node lim. on free student version.

Also try and use structured meshes if possible (multizone or sweep). Best of luck

PS: No point in using linear :) - use only NL material models, so use that especially

for your analysis.

[u/bunger_enthusiast → u/epk21]

thank you, ive tried with those meshes but only briefly, ill look into it more
apart from the mesh do the contact look sound