Consider renaming target_strain(_rate)

[mikesmic]

target_strain becomes deformation gradient in the simulation definition which is confusing at first use e.g. a target_strain of 0.02 leads to a compression simulation rather than 0.02 tension

[aplowman]

How about target_def_grad and target_def_grad_rate, with the implication that these are only the relevant components, given the loading method (e.g. uniaxial, biaxial, etc)?

[mikesmic]

Yes I think that would be clearer.

Or you could keep strain and convert that to a def grad to define the simulation loading.