The theoretical mechanics of viscoplastic soft solids.

Many intermediates and final products behave as a viscous suspension, but may also have sufficient structure to display plastic yield-like behaviours. Some theoretical attention has been paid, in recent years, to the flow of visco-plastic fluids. This has been limited to the confines of simple geometries, parallel platens, capillary flows, etc. A definitive understanding of the mechanics of these systems has not yet been established. This situation has arisen since there is controversy in the literature about the possibility of unyielded ''plugs'' or domains existing within a flowing, yielded region. Specifically, there has been a perceived difficulty in incorporating a three dimensional yield criterion into a fluid mechanics analysis. Previous analyses of the compression of viscoplastic materials have employed the lubrication approximation developed for fluid flows with the result that a kinematic inconsistency in the velocity fields is predicted.