Viscoelastic relaxation spectra of lubricating oils and their component fractions

Abstract
The early work of Barlow & Lamb (1959) on linear viscoelastic relaxation at high frequencies showed that the behaviour of mineral oils could be represented by a spectrum of relaxation frequencies. It also gave an interpretation of the spectrum in terms of the hydrocarbon type analysis of the oil. We have made the first test of this interpretation by making measurements on other samples of oil and on a suitably fractionated oil. It is concluded that the relaxation spectrum cannot be interpreted solely in terms of the hydrocarbon type analysis and that other unknown characteristics of the oil have at least as great an influence. A new mechanical model for the representation of viscoelasticity, proposed by Barlow. Erginsav & Lamb (1967), fits the experimental results for HVI 330 oil and its saturate and monoaromatic fractions over a wide range of the results. Results for these oils obtained at low temperatures and all results for MVI (N) 170 and LVI 260 oils fit less well. Evidence is presented which supports a conclusion drawn by Miles (1962) that the relaxation spectrum of an oil broadens as the temperature is lowered.