Viscoplastic Materials

Abstract

In contrast to fluids (chapter 9) viscoplastic materials can sustain a shear stress even at rest. They begin to flow with viscous stresses after a yield condition has been satisfied. Thus, viscoplastic materials are considered as solids. The first viscoplastic model was proposed by Bingham (1922). In the introduction to the Proceedings of the “First Plasticity Symposium” in Lafayette College (1924) Bingham writes: Our discussion of plasticity therefore concerns itself with the ’flow of solids’. The Greek philosopher Heraklitus was literally correct when he said that ‘everything flows’ (Panta rhei). It is therefore necessary to limit our discussion by excluding the flow of those things which we are accustomed to refer to as fluids, i.e., the pure liquids and gases. But the circle of our lives is not concerned principally with the fluids, even air and water, but with plastic materials. Our very bodies, the food we eat, and the materials which we fashion in our industries are largely plastic solids. Investigation leads us to the belief that plasticity is made up of two fundamental properties which have been made ‘yield value’ and ‘mobility’, the former being dependent upon the shearing stress required to start the deformation and the mobility being proportional to the rate of deformation after the yield value has been exceeded.