===== User Function USF_Damage =====

==== General Informations ====
  * **User Function Fortran File:** ''USF_Damage.f90''
  * **Function 1 Name:** ''EnhancementFactor''
  * **Function 2 Name:** ''SourceDamage''

==== General Description ====
{{:userfunctions:usf_damage.pdf|USF_Damage.pdf}}
 
The first function changes the rheology of the damage ice by impacting the enhancement factor.

The second function is used to compute the source term of the advection-reaction solver.

==== SIF contents ====
The required keywords in the SIF file for this User Function are:
<code>
Constants
  Dev Tensile Strength Modifier = Real 0.05 ! standard deviation for the stress threshold distribution
End

Body Force 1
  DGD Source = Variable Damage
    Real Procedure "ElmerIceUSF" "SourceDamage"
End

Material 1
  Glen Enhancement Factor = Variable Damage
    Real Procedure "ElmerIceUSF" "EnhancementFactor" 
    
  Damage Enhancement Factor = Real 2.00 ! damage enhancement factor
  Damage Parameter sigmath = Real 0.05 ! stress threshold for damage increase
End
</code>


Additionnally, for output visualisation, the damage criterion Chi is saved as a variable named **Chi**, which need to be exported in a solver, such as :
<code>
Solver 3
  Equation = Sij
  Procedure = "ElmerIceSolvers" "ComputeDevStress"
[...]
  Exported Variable 1 = Stress[Sxx:1 Syy:1 Szz:1 Sxy:1 Syz:1 Sxz:1]
  Exported Variable 1 DOFs = 6
  Exported Variable 2 = -dofs 1 "Chi"
[...]
End
</code>

==== Examples ====
A 3D example can be found in ''[ELMER_TRUNK]/elmerice/Tests/Damage''.

==== Reference ====
Krug, J., J. Weiss, O. Gagliardini and G. Durand, 2014. Combining damage and fracture mechanics to model calving, The Cryosphere, 8, 2101-2117, doi:10.5194/tc-8-2101-2014.