Flow curves and stress-strain curves generated by a nonlinear model for shear flow of thixotropic viscoelastic media accounting for structure evolution

A systematic analytical study of the mathematical properties of the nonlinear shear flow model of thixotropic viscoelastic-plastic media is continued. It takes into account the mutual influence of а deformation process and structure evolution (the kinetics of the formation and destruction of intermolecular bonds and associates of macromolecules). The model is reduced to the system of two nonlinear differential equations for the dimensionless stress and the degree of structuredness (i.e. cross-links density and so on). Assuming six material parameters and an (increasing) material function governing the model are arbitrary, the phase portrait of the system is analytically studied in the vicinity of its single equilibrium point. Basic properties of flow curves and stress-strain curves with constant shear rate generated by the model are examined. Thus, the analysis of the model ability to describe the behavior of both liquid-like media and solid-like (thickening, hardening, solidifying) viscoelastic-plastic media has been started: the effects of strain-rate and strain hardening, relaxation, creep, recovery, etc. The stress-strain curves dependence on the shear deformation (monotonicity, convexity, instantaneous modulus, tangent modulus evolution), on the shear rate and initial structuredness and on the material parameters and function of the model (in particular, the parameters that control the effect of structuredness on viscosity and shear modulus and the influence stress on the rate of destruction of the structure) has been studied. It is proved that stress-strain curves can be both increasing and have sections of decrease, resembling a yield-drop, and damped oscillations; that all stress-strain curves have horizontal asymptotes (steady flow stress), monotonically dependent on shear rate, and the flow stress strictly increases with increasing shear rate; that their instantaneous shear modulus, on the contrary, depends on the initial structuredness, but does not depend on shear rate. Under certain restrictions on the material parameters, the model is also capable to provide a bilinear form of stress-strain curves, which is intrinsic for an ideal elastoplastic model, but with strain rate sensitivity. It has been established that the family of stress-strain curves does not have to be increasing function of initial structuredness or shear rate: in a certain range of shear rates, in which the equilibrium point is a “mature” focus and pronounced oscillations of stress-strain curves are observed, it is possible that stress-strain curves with different shear rates may interweave with each other. It is studied how structuredness changes in the process of deformation depending on shear rate, stress, material parameters and material function of the model. The initial structuredness affects only the initial arc of stress-strain curves, but does not affect their asymptotes and the steady value of the structuredness, which monotonically decreases with increasing shear rate. A variety of scenarios of structuredness behavior over time (in particular, the observed sharp collapse of the structuredness when critical stress values are reached) generates a number of unusual effects (unusual properties) in comparison with typical properties stress-strain curves of structurally stable materials.