Harmonic analysis in grease rheology

Yield stress, and small‐ and large‐ amplitude oscillatory shear measurements were performed on greases in a cone‐plate fixture. Below the yield point, the grease behavior mimics that of a lightly crosslinked polymer. In large‐amplitude oscillations that exceed the yield stress, harmonic distortion appears in the stress waveform. The elastic component of the stress is continuous across the transition, while the viscous, or out‐of‐phase component, is a transient stress that occurs in the high shear rate portion of each oscillation cycle. Our newly proposed viscous slip model accurately reproduces the measured stress waveform for greases, as well as for other yield stress materials. From measured harmonic amplitudes, this model provides the strain amplitude dependence of the elastic modulus and the equivalent of a dynamic yield stress. These properties of channeling grease complement the conventional measurements of yield stress and worked and unworked penetration. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 334–343, 2003