Reopen #23: Small strain plasticity

[KnutAM]

Previous fork got unsynced when this repo became public...., see #23 for previous discussion.

It should be more cleaned up now, and the main ideas are:
Modular elastic, isotropic hardening, and kinematic hardening laws. Via dispatch, it should specialize functions depending on the material definition.

Examples

A few example of how different materials could be defined (see also test/test_small_strain_plasticity.jl)

• The same material as in Plastic.jl (Linear isotropic elasticity, Voce isotropic hardening, and Armstrong-Frederick kinematic hardening):

m = Chaboche(elastic=LinearElastic(E=200.e3, ν=0.3),
             σ_y0=200.0,
             isotropic=(Voce(Hiso=25.0, κ∞=13.0),),
             kinematic=(ArmstrongFrederick(Hkin=25.0, β∞=13.0),))

• Another material with linear isotropic elasticity, 2 isotropic hardening contributions (one Voce and one Swift), and 2 kinematic hardening contributions (back-stresses), Armstrong-Frederick and Ohno-Wang:

m = Chaboche(elastic=LinearElastic(E=200.e3, ν=0.3),
             σ_y0=200.0,
             isotropic=(Voce(Hiso=25.0, κ∞=13.0),
                        Swift(K=100.0, λ0=1.0e-2, n=0.5)),
             kinematic=(ArmstrongFrederick(Hkin=25.0, β∞=13.0),
                        OhnoWang(Hkin=25.0, β∞=20.0, mexp=4.0)))

TODO

Remaining to-do's before merging:

• Reuse the existing elasticity definitions (see e.g. mk/elasticity-models) and make fallback for cases not given a specific routine for
• Come up with an appropriate name and limitations (e.g., we could restrict to vonMises-type yield criterion, and then J2Plastic could work): "VonMisesPlasticity"
• Move utility functions to a general utility routine
• Make a complete documentation of the material
• Add proper unit testing

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.66%. Comparing base (34ab8ef) to head (c8c0d31).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   94.24%   95.66%   +1.41%     
==========================================
  Files           7       11       +4     
  Lines         313      415     +102     
==========================================
+ Hits          295      397     +102     
  Misses         18       18              

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