Nonlinear rheology of stress-controlled rheometers: Large amplitude oscillatory shear

Although stress-controlled rheometers are more popular than strain-controlled rheometers, the latter is preferred to the former. This is because stress-controlled rheometers are apt to suffer from inertia effects. This work demonstrates that stress-controlled rheometers can also be used in the same manner as strain-controlled rheometers to study large amplitude oscillatory shear (LAOS). We demonstrate that the inertia effect can be detoured by employing a sufficiently long conditioning time and that the stress decomposition theory remains valid for stress-controlled LAOS data even when strain is not sinusoidal. However, the decomposed stresses are hard to say elastic and viscous because non-sinusoidal strain and its time derivative do not satisfy γ2(t)+ω−2(dγ/dt)2=γo2, where γo is a positive constant. We are certain that cost-effective stress-controlled rheometers can be used if the purpose of LAOS utilization is to identify the fingerprint of rheologically complex materials.