**1. n. [Geology]**

The elastic limit, or the point at which a material can no longer deform elastically. When the elastic limit is exceeded by an applied stress, permanent deformation occurs.

**Synonyms:**
elastic limit

**See:**
elastic deformation,
plastic

**2. n. [Drilling Fluids]**

A parameter of the Bingham plastic model. YP is the yield stress extrapolated to a shear rate of zero. (Plastic viscosity, PV, is the other parameter of the Bingham-plastic model.) A Bingham plastic fluid plots as a straight line on a shear rate (x-axis) versus shear stress (y-axis) plot, in which YP is the zero-shear-rate intercept. (PV is the slope of the line.) YP is calculated from 300- and 600-rpm viscometer dial readings by subtracting PV from the 300-rpm dial reading. YP is used to evaluate the ability of a mud to lift cuttings out of the annulus. A high YP implies a non-Newtonian fluid, one that carries cuttings better than a fluid of similar density but lower YP. YP is lowered by adding deflocculant to a clay-based mud and increased by adding freshly dispersed clay or a flocculant, such as lime.

**Alternate Form:**
yield point

**See:**
aggregation,
apparent viscosity,
clay-water interaction,
deflocculated mud,
direct-indicating viscometer,
drill solids,
gelled-up mud,
mixed-metal hydroxide