Periodic bcs with non-zero offset

[jasondhales]

I need an enhancement to the periodic bcs. For a pair of nodes on opposite faces, the current constraint is

u,,i,, - u,,j,, = 0.

I need to be able to define the constraint this way:

u,,i,, - u,,j,, - c = 0

where c is user-defined. This allows an average gradient across the domain and enables us to impose a net strain, e.g.

[friedmud]

If you just do this in one direction (like x), it should be possible to do it with a DirichletBC and the NearestNodeLocator...

[jasondhales]

It will be for (x,y,z) degrees of freedom for all three pairs of faces. I need to specify three axial (xx, yy, zz) and three shear (xy, yz, zx) strains.

Is it hard to add the non-zero offset to the periodic bcs? I suppose I could wire sets of NearestNodeLocators with specialized DirichletBCs, beautified with Actions. But by then I would have essentially replaced the periodic bcs.

If possible, I'd much prefer to avoid duplicating effort. Thanks.