Y

The point at which material loading behaviour changes from elastic to inelastic.

The stress at which yielding takes place in soils and other materials; the stress at which the swelling-recompression line joins the normal compression line.

(Also stiffness modulus). The ratio of the change in applied uniaxial stress to the induced direct axial strain, when the lateral (radial) stress remains constant.
EŽ is referred to effective stresses
E_u is referred to total stresses (see undrained modulus)