General Mechanical

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Struggling to Implement Friction in Needle Penetration Model

• dwidi
Subscriber

Hello Forum,

I am hoping someone can provide some suggestions as to how to implement friction effectively for a model I am working on as part of my research - I am having issues with the model convergence. Below is a quick description of the problem, and further details can be found further down my post as well.

Model: Create a 2D simulation of a needle propagating a crack through a pre-defined crack tip and crack path in a layer of skin. The whole skin section is 4 mm long and 1 mm high, with a prescribed thickness of 100 um (plane stress behavior is enabled). The needle is modeled as structural steel, and the skin is modeled as a visco-hyperelastic material whose properties were obtained through experimentation and past literature data. The left and right skin interfaces have CZM applied for contact debonding based on fracture energies. The needle moves 100 um/s up to 600 um. The model setup is shown below.

• Ashish Khemka
Ansys Employee
nnFor large sliding between contact and target can you try Normal Lagrange Formulation?.Regards,nAshish Khemkan
• dwidi
Subscriber
nnThank you for your suggestion! If I understood it correctly, you suggested to change the Formulation for the frictional contact (between the microneedle and the skin layers) to Normal Lagrange.nUnfortunately, I run into the same issue - the model fails to converge when the microneedle displaces ~173 microns. I've tried other Formulations too out of curiosity (eg: Augmented Lagrange, Pure Penalty), but the issue remains the same. The distorted elements persist for the elements located right beneath the pre-crack tip. n
• peteroznewman
Subscriber
nTry All Triangles instead of Quad element shape on the skin. Try Linear Element Order.nThis may be more tolerant of element distortion and immune from hourglass modes.nMaybe you only need this in the layer that makes contact with the needle. Further away, you can use Quad element shape.n
• peteroznewman
Subscriber
n
• dwidi
Subscriber
nThank you so much for responding! Regarding the suggestions you've posted above:n1) I will definitely try triangles instead of quad - that's something that's never crossed my mind before.n2) Linear Element Order - this is a setting I've been using consistently with quads, so I'll maintain this setting.nI will let you know if using all triangles and some clever meshing helps avoid these distortion errors. Thanks again!n
• dwidi
Subscriber
Hi, I implemented Triangle formulation in the middle of the skin as well as the microneedle, and the model was able to move along a little farther than with Quads but still fails at around 270 um of microneedle displacement. This only occurs with Frictional Coefficient = 0.1; the triangle formulation works very well with frictionless contact, so I don't think it's the formulation itself that's causing the issue. Here is a screenshot of the element violations I see with the Triangle Formulation + Frictional Contact:nSome troubleshooting I did include:n1) Varying load steps and substeps, but convergence issues occur pretty consistently at displacement = 260 umn2) Increased mesh density. The figure above shows elements at around 3-4 microns in size, and I re-did the model with the highest mesh density element size at around 1-2 microns. Still no luck nI haven't tried using different meshing patterns to see if maybe the placement or aspect ratio of the triangles can help avoid this issue, but I don't have much experience in how to control for the triangle shapes in meshing outside of the built-in face meshing and face sizing settings. Do you have any tips or suggestions on the best way to implement this, if it's worth pursuing?