Please can someone give me guidance on SimulationXpress.

I would just use F=Ft/A for this, where Ft is the ultimate strength of the material

The first basic thing:

SimulationXPress is a static, linear FEA solver.

That linear part is key here:  the results are only "accurate" for linear loading scenarios. Once you reach your yield strength (where you start seeing plastic deformation in the part), the solver isn't taking into account changes in the body geometry and the results are not accurate.

Ergo... At best, you're getting a "ballpark" figure for the breaking point.  Even then, it can vary wildly depending on what the plastic deformation portion of your material's stress-strain curve looks like.  A better way to say it would be: this isn't the tool to find what you're looking for.

And that's before we get into the possibility of singularities around the edges of your tethers, the lack of being able to test for convergence in SimulationXpress, etc.

Since you are discussing failure of a material you are already using the wrong tool, you need a non-linear solution at the minimum.  Worse yet, you are looking at the failure of a plastic material (non-isotropic). 

To mis-quote Obi-Wan, "This is not the tool you're looking for."