Theory Manual Version 3.4
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Subsubsection 5.6.5.2: Simo Damage Criterion Up Subsection 5.6.5: Constitutive Models Subsubsection 5.6.5.4: Von Mises Stress

5.6.5.3 Specific Strain Energy

It may be assumed that damage occurs when the specific strain energy 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 we have used the result of Eq.(5.6.5.1-2).
Subsubsection 5.6.5.2: Simo Damage Criterion Up Subsection 5.6.5: Constitutive Models Subsubsection 5.6.5.4: Von Mises Stress