Learning Forum

[asarma33]

I am trying to simulate penetration of skin (Dermis) by a needle. Going through a lot of the posts in the forum, it seems like LS-Dyna would be solver to go for. However, I am trying to build on a previous model, which was built using Static Structural.

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. Plane strain behavior is enabled, since the skin is much thicker than the needle. The needle is modeled as structural steel, and the skin is modeled as a visco-hyperelastic material whose properties are obtained through this paper. The left and right skin interfaces have CZM applied for contact debonding based on fracture energies. The model setup is shown below.

In trying to recreate the model, I was able to achieve convergence for the above meshing as shown:

However, this is unrealistic. The contact between the needle and the skin (top surface and interface) is frictionless. For the interface, a bonded contact is given with "Pure Penalty" formulation.

In trying to conduct the same simulation with a finer mesh, the convergence fails:

close up image near the crack tip:

I have been through the following forum posts that were somewhat helpful:

https://forum.ansys.com/forums/topic/simulating-skin-penetration-viscoelasticity/

https://forum.ansys.com/forums/topic/how-to-create-needle-penetration-insertion-model/

https://forum.ansys.com/forums/topic/needle-tissue-interaction/

https://forum.ansys.com/forums/topic/cannot-run-model-with-hyperelasticity-and-czm-enabled/

Basically, I'm trying achieve the same goal as the following post (I am, in fact, continuing the work by this user):

https://forum.ansys.com/forums/topic/struggling-to-implement-friction-in-needle-penetration-model/

I implemented some of the suggestions above (except with frictionless contact, but will move on the frictional once I have the former working) but the major cause of the error remains: "high distortion of elements".

Any advice or suggestions?

[Armin Abedini]

Hi Asarma,

I'm not very familiar with CZM but wanted to point out that in case you decided to pursue with Ansys LS-DYNA (as you indicated), this type of application should be handled relatively easy using the MAT_ADD_EROSION card that is also available to use via Workbench LS-DYNA.
Just a suggestion.

[asarma33]

Dear Armin,

Thank you for your suggestion. I did see in several places that using LS-DYNA would be a better approach towards solving this problem. I am not familiar with this solver at all, however when trying to learn, I am unable to find good resources. LS-DYNA is a separate software altogether, and while ANSYS offers this, I am unable to find good starting points for the project. Would you happen to know of any good resources?

As for the static structural analysis, I am still interested to pursue this approach. It has had success according to the paper and I just want to atleast recreate what is given here, followed by implementing friction.

[Armin Abedini]

I see your point.

Regarding Workbench LS-DYNA training, I'll let you know of good training courses. Do you have access to Ansys Learning Hub (https://www.ansys.com/services/learning-hub)?

[asarma33]

Yes, I have access to the learning hub.