The Rheology of Lubricants at High Shear Rates

A modified torsional Kolsky bar is used to shear a thin annular layer of lubricant held between concentric cylinders. The shear rates obtained range from 4 × 10³ to 4 × 10⁴ s⁻¹; the duration of the shear pulse is about 400 microseconds. The shear stress history and the corresponding shear-rate history are deduced using standard torsional Kolsky bar techniques. This experimental technique provides the capability of measuring the shear stresses sustained by fluids at very high shear rates, while the short test duration ensures a minimal rise in temperature due to the associated shear heating. Results are presented for a synthetic lubricant (5P4E) and a mineral oil (HVI650). The lubricants are both observed to be rate-sensitive over the entire range of shear rates examined; the rate-dependence of the shear response is strongly non-Newtonian at high shear rates. Softening during the test is observed for both materials; this softening appears to be largely driven by a thermal mechanism.