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18.104.22.168 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.