BACKGROUND

Semisolids are viscoelastic materials that exhibit a yield stress. The yield stress for a raw material or dosage form is the applied stress at which a change in the viscoelastic properties of a semisolid is observed: below the yield stress the material response is dominated by completely reversible elastic deformation, whereas above the yield stress the material response is changing to irreversible elastic behavior (as the material remains a viscoelastic semisolid) or is dominated by viscous flow behavior. Often, the yield stress is identified as the applied stress below which a material appears to  not show any motion or flow (i.e., where the shear rate is near-zero).