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Theory Manual Version 3.6
 Subsection 5.6.5: Constitutive Models for Damage and Yield Criteria Up Subsection 5.6.5: Constitutive Models for Damage and Yield Criteria Subsubsection 5.6.5.2: Simo Damage Criterion 

5.6.5.1 Strain Energy Density

It may be assumed that damage occurs when the strain energy density achieves a certain threshold . In that case, the constitutive model for the damage measure is This damage measure is valid for isotropic or anisotropic materials, since the strain energy density must satisfy the frame-invariance of eq.1↓ by construction. The resulting damage surface normal is where is the second Piola-Kirchhoff stress associated with the material.
 Subsection 5.6.5: Constitutive Models for Damage and Yield Criteria Up Subsection 5.6.5: Constitutive Models for Damage and Yield Criteria Subsubsection 5.6.5.2: Simo Damage Criterion