I am running a static contact simulation of a hip replacement prosthesis for an assignment and getting an error I don’t understand. The assembly consists of a stem, stem head (femoral head), acetabulum, and replacement hip socket. All parts were positioned and aligned on the assembly page. A tie constraint links the femoral head to the stem. The cup has a boundary condition, and a pressure load is applied to the bottom of the stem, pushing upward along the stem axis toward the head.

Set up:

All parts were positioned and aligned on the assembly page.
I have made a tie between the femur head and stem, with a boundary condition placed on the cup and a pressure force being applied to the bottom of the stem, pushing upwards towards the rest of the stem.

I have a surface contact between the stem head and the acetabulum. I've also put my contact properties as frictionless, and as a hard contact with default enforcement and separation is allowed after contact.

I have my step increments set to automatic (Initial 0.01, Minimum 1E-08, Maximum 0.1) and a maximum number of increments at 1000. 

Error:

The problem I'm having is that I am getting an error I don't understand (see attached for full): "The system matrix has three negative eigenvalues". It mentions unconnected regions and that 6 elements are distorted, and "displacement increment for contact is too big".

can anyone spot what could be wrong with the setup or have any ideas on how I can fix this?