Pde gradient damage calibration

[TTitscher]

In local damage models, the fracture energy parameter of the damage laws can be directly related to the global fracture energy. Damage localizes in one (layer of) elements and you can analytically integrate that. Due to the nonlocality of the gradient damage model, damage does not localize in a single element, but in many, to different degrees. This makes an analytical integration hard/impossible. With additional features like a damage dependent nonlocal parameter, it gets even harder.

These commits adds an example that provides this calibration. Using the secant methods (we cannot calculate derivatives here), the local gf parameter is calibrated to match a given global Gf parameter in a uniaxial tensile test.

Edit: ... and the global Gf is what you measure in experiments.

[codecov[bot]]

Codecov Report

Merging #234 into PDE_reviewed will decrease coverage by <.01%. The diff coverage is 86.66%.

@@               Coverage Diff                @@
##           PDE_reviewed     #234      +/-   ##
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- Coverage         84.33%   84.32%   -0.01%     
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  Files               295      295              
  Lines             10669    10676       +7     
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+ Hits               8998     9003       +5     
- Misses             1671     1673       +2

Flag	Coverage Δ
#integrationtests	62.4% <86.66%> (ø)	:arrow_up:
#unittests	87.74% <ø> (ø)	:arrow_up:

Impacted Files	Coverage Δ
nuto/mechanics/tools/AdaptiveSolve.h	100% <ø> (ø)	:arrow_up:
nuto/mechanics/tools/AdaptiveSolve.cpp	91.89% <86.66%> (-4.78%)	:arrow_down:

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