TF conduit shear stress correction - 2

A fudge factor was put in to adjust the maximum shear stress (used in the Tresca yield criterion) for the TF coil, based on collaboration with CEA. The adjustment factor was taken from equation 7 of Tools Used at CEA for Designing the DEMO
Toroidal Field Coils Winding Pack:

where

and

In other words, the vertical force is distributed only over the winding pack, with none taken by the case. This may be approximately valid if there is zero friction between the winding pack and case.

The "mechanical safety coefficient λ" acts to reduce the allowable stress (or increase the actual stress) to agree with the results from stress linearisation (a more sophisticated procedure) in
[11] A. Panin et al., “Analysis approaches to resolve structural issues of the European DEMO toroidal field coil system at an early design stage,” presented at the 24th Int. Conf. Magnet Technology, Seoul, Korea, 2015.
[12] A. Panin et al., “Analysis approaches to resolve structural issues of the European DEMO toroidal field coil system at an early design stage,” IEEE Appl. Supercond.

Note that this differs from the original PROCESS formulation in three ways:

1. PROCESS assumes the winding pack (WP) is bonded to the case,
2. PROCESS also includes the tangential stress, and
3. Because of the λ formula above.

PROCESS offers the use of the λ formula, but even if this is selected, the axial stress before adjustment $\sigma_{zmax}$ is still calculated assuming the WP is bonded to the case.

Proposed solution

I am not too comfortable about a correction like this that's based on a single design. I am happy to allow for the zero friction case as an option.