Finite strain uniaxial stress

[KnutAM]

When using UniaxialStress stress state for finite strain, we have 8 free components, but these involve rotation, and in some cases a singular matrix has been observed. Therefore, it could make sense to reduce the unknowns further, preventing rotation around the x-axis. Alternatively augment the stiffness to prevent rotation as a modified Newton.

[p-antonioni]

Singular matrix problem when solving the linear system at:

MaterialModelsBase.jl/src/stressiterations.jl

Line 295 in 2defce2

dσᶜdϵᶜ = ∂σᶜ∂ϵᶜ - ∂σᶜ∂ϵᶠ * (∂σᶠ∂ϵᶠ \ ∂σᶠ∂ϵᶜ)