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A material may accumulate damage over a single cycle or multiple cycles of loading which alters its properties. In the classical framework of damage mechanics this attenuation of the material properties is described by a single scalar damage variable $D$ when the material is isotropic ($0\leqslant D\leqslant1)$ . For anisotropic materials however, classical frameworks require that we introduce a function of the fourth-order damage tensor $\mathcal{D}$ to account for anisotropic damage. In FEBio, we use a reactive damage mechanics framework where the elastic response is proportional to the total number of intact bonds in the material and where, at any given time in the loading history, $D$ represents the mass fraction of bonds that have broken. In this reactive framework, it is possible to also model damage in anisotropic materials by assuming that multiple bond types exist in the material, each of which may get damaged under different circumstances. Each bond type $b$ may be described by a distinct solid constituent within a solid mixture (see Sections 4.1.2.14↑ and 4.1.3.22↑), each having its own scalar damage variable $D^{b}$ .