Pure Material Instability and the Concept of Yield Stress

Abstract

Abstract The paper is concerned with the rheological study of gels, complex materials which are characterised by an internal network structure developed in viscous liquids. This category of materials exhibits in viscometric motion a yield state at a critical value of the applied shear stress. The yield shear stress defines the plateau behaviour in the steady flow curve. The creeping experiments and the dynamics of the Oldroyd’s 3 constants model put in evidence the connection between the loss of stability of the network structure and the coexisting strain rates at a constant shear stress. The correlation between theory, numerical simulations and experiments are established. All the results are qualitatively consistent with the statement that the concept of yield stress is ’natural defined‘ in the context of the pure material (structural) instability (i.e. instability at zero Reynolds number) of constitutive relations with non-monotone flow curve.