Over a critical stress level, ie. when the second invariant of the
stress tensor js
j exceeds the yield stress sY , viscoplastic materials
start flowing. In the general case, there may be solid-like areas
(unyielded regions) and fluid like areas (yielded regions). In some
cases, a viscoplastic material can flow and deform throughout its
whole domain, if the stress is everywhere above the yield stress,
or it may not flow at all, if the stress is below this value everywhere