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 elastic deformation, whereas above the yield stress the material response is dominated by viscous flow. Often, the yield stress is identified as the applied stress below which a material appears to not flow (where the shear rate ≈ zero). This observation can be dependent on the time scale of the measurement, the scan rate used to locate the change in the behavior, and the direction from which the transition is approached (i.e., sample history). Most measurement techniques identify the yield stress by locating the onset of viscous flow; however, penetrometry looks for the yield stress where the semisolid stops yielding. Although the yield stress for various materials is expected to correlate when measured with different techniques, the values should not be considered to be independent of the method. Therefore, it is recommended that measurements of yield stress be reported as apparent yield stress in order to emphasize that the quantitative result is dependent on the measurement method used. Yield stress is reported with units of shear stress [Pascals (Pa)].