  Experiments (Mullin and Kreswell, 2005) show that transition to turbulence
can start at Reynolds numbers lower than it is predicted by the linear
stability analysis - the subcritical transition to turbulence. To explain these
observations qualitatively we suggest that the onset of subcritical instability
is related to decline of viscosity of the fluid: friction between fluid layers
fails with the increase of the velocity gradient. To describe the declinie of
friction theoretically we relax the the assumption of the stability of the
fluid material and introduce a constant of fluid strength. Particularly, we
enhance the Navier-Stokes model with a failure description by introducing the
fluid strength in the constitutive equation for the viscous stress. The
clasical model is obtained from the enhanced one when strength goes to
infinity. We use the modified Navier-Stokes model to analyze the Couette flow
between two parallel plates and find that the lateral perturbations can
destabilize the flow and the critical Reynolds number is proportional to the
fluid strength. The latter means that the classical Navier-Stokes model of a
stable material with infinite strength does not capture the subcritical
transition to turbulence while the mofified model does.
