Large Amplitude Oscillatory Shear of Pseudoplastic and Elastoviscoplastic Materials
by Ewoldt, R.H., Winter, P., Maxey, J. and McKinley, G.H.

We explore the utility of strain-controlled large amplitude oscillatory shear (LAOS)
deformation for identifying and characterizing apparent yield stress responses in
elastoviscoplastic materials. Our approach emphasizes the visual representation of the LAOS
stress response within the framework of Lissajous curves with strain, strain-rate, and stress as the
coordinate axes, in conjunction with quantitative analysis of the corresponding limit cycle
behavior. This approach enables us to explore how the material properties characterizing the
yielding response depend on both strain amplitude and frequency of deformation. Canonical
constitutive models (including the purely viscous Carreau model and the elastic Bingham model)
are used to illustrate the characteristic features of pseudoplastic and elastoplastic material
responses under large amplitude oscillatory shear. A new parameter, the perfect plastic
dissipation ratio, is introduced for uniquely identifying plastic behavior. Experimental results are
presented for two complex fluids, a pseudoplastic shear-thinning xanthan gum solution and an
elastoviscoplastic invert-emulsion drilling fluid. The LAOS test protocols and the associated
material measures provide a rheological fingerprint of the yielding behavior of a complex fluid
that can be compactly represented within the domain of a Pipkin diagram defined by the
amplitude and timescale of deformation.