Hi All,

I am working on a project that involves a multilayered hyperelastic tubing that I would ultimately like to be able to place displacement compressions, twist it, bend it, etc. involving nonlinear frictional contact. I am currently treating these as Isotropic, but do have uniaxial, biaxial, and shear test data available for some Mooney-Rivlin parameter fittings... but figured I'd figure out Isotropic FIRST.

**Geometry/Materials:**

- Inner tube: Silicone Rubber; Young's Modulus: 250 psi (0.49 Poisson's Ratio)
- Second layer: Polyethylene terephthalate (PET) monofilament (treated as a 3D surface wrapped around in contact with the silicone); Young's Modulus: 71,000 psi (0.49 Poisson's Ratio)
- Third Layer: Polyurethane 80A; Young's Modulus: 500 psi (0.49 Poisson's Ratio)
- Fourth outer-most layer: Polyurethane 55D; Young's Modulus: 2,500 psi (0.49 Poisson's Ratio)

The tubing below is 0.500" long.

**Contact:**

- Silicone Rubber->PET: Frictional Contact with a coefficient of 0.1
- PET->PU80A: Frictional Contact with a coefficient of 0.1
- PU80A->PU55D: Bonded Contact

**Mesh:**

- I am currently utilizing hex dominant method. I'm not sure if Contact Match or Node Merge could come in handy here. Open for opinions regarding this kind of surface contact.

**BCs:**

- One end is fixed, and the other end faces have constraints in the x-z plane with a displacement of 0.1" in the negative y direction to compress towards the fixed end in hopes to induce a buckling shape.
- There is also a pressure (fluid moving through the tubing) of 1 psi inside the silicone tubing.

**Issues:**

- As with most people, convergence. "Contact Status has Experienced an abrupt change". Highly distorted elements.
- I've even run my Eigenvalue Buckling to get my critical load factor that would induce my first buckling shape:

I took the Load Multiplier and multiplied that by my load applied to this tubing (1 lbf) to determine the critical buckling load, and then added ~20% increase to this value along with a perturbation in Nonlinear Structural to try to obtain some post-buckling behavior but had no success with this.

I'm a little lost with this, let alone how to even start including non-linear material modeling, so any help or thoughts is GREATLY appreciated!

Regards,

Zach