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