the exponential growth of the yield-stress term of the classical Bingham model in regions subjected to very small strain-rates, may be pointed. Thus, he proposed an exponential regularization of eq., by introducing a parameter m, which controls the exponential growth of stress, and which has dimensions of time. The proposed model (usually called Bingham-Papanastasiou model) has the form: and is valid for all regions, both yielded and unyielded. Thus it avoids solving explicitly for the location of the yield surface, as was done by Beris et al. Papanastasiou's modification, when applied to the Bingham model, becomes in simple shear flow (1-D flow): Bingham-Papanastasiou model: where η is the apparent viscosity. References Plasticity (physics)