Implement LinearElasticDamage for PLANE_STRESS

[TTitscher]

The class performs a deviatoric/volumetric split of the stresses and strains for its calculations. This was not (super) trivial for PLANE_STRESS conditions since the Kronecker delta (named D or mD) for this situation differs. For PLANE_STRAIN you can write

eps_volumetric = 1/3 D.transpose() * strain
eps_deviatoric = strain - D * eps_volumetric
with
D.transpose() = [ 1  1  0 ] (in 2D PLANE_STRAIN)

which is, with trivial modifications, also valid for 1D and 3D.

For PLANE_STRAIN, we have to introduce a second Kronecker D2, namely:

eps_volumetric = 1/3 D2.transpose() * strain
eps_deviatoric = strain - D * eps_volumetric
with
D.transpose() = [ 1  1  0 ] (in 2D, always)
D2 = (1 + nu/(nu - 1)) * D (only for 2D PLANE_STRESS)

I am not :100: % sure if this is correct. But the tests

• LinearElasticDamage::Stress(strain, omega) == (1 - omega) * LinearElastic::Stress(strain)
  • (the same for all tangents, PLANE_STRAIN and PLANE_STRESS)

run smoothly.

[codecov[bot]]

Codecov Report

Merging #237 into PDE_reviewed will increase coverage by 0.01%. The diff coverage is 100%.

@@               Coverage Diff                @@
##           PDE_reviewed     #237      +/-   ##
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+ Coverage         84.43%   84.45%   +0.01%     
================================================
  Files               296      296              
  Lines             10749    10754       +5     
================================================
+ Hits               9076     9082       +6     
+ Misses             1673     1672       -1

Flag	Coverage Δ
#integrationtests	62.34% <95.45%> (ø)	:arrow_up:
#unittests	87.91% <100%> (+0.01%)	:arrow_up:

Impacted Files	Coverage Δ
nuto/mechanics/tools/AdaptiveSolve.h	100% <ø> (ø)	:arrow_up:
...hanics/constitutive/damageLaws/SofteningMaterial.h	100% <ø> (ø)	:arrow_up:
nuto/mechanics/tools/AdaptiveSolve.cpp	92.3% <100%> (+0.41%)	:arrow_up:
...est/mechanics/constitutive/LinearElasticDamage.cpp	100% <100%> (ø)	:arrow_up:
nuto/mechanics/constitutive/LinearElasticDamage.h	100% <100%> (+2.56%)	:arrow_up:
test/mechanics/integrands/GradientDamage.cpp	100% <0%> (ø)	:arrow_up:

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