No abstract

Two types of creep testing machine, one to apply bi-axial tension to cruciform specimens and the other, torsion to diseshaped specimens, have been constructed in the course of an investigation of the behaviour of copper subject to multi-axial stress system Problems of bending in biaxial tension and buckling in torsion were encountered subsequent to construction of the machines. In the former case the deficiency was accepted and eccentric loading used to control the bending within acceptable limits. In the latter case microscopic examination was found necessary in order to define creep failure.

Theories of failure of elastohydrodynamic lubrication are briefly reviewed, but none that relate to scuffing per se and no general criterion that accounts for the sensitivity of scuffing to rolling as well as sliding speed are found. A theoretical investigation of micro-EHL by Baglin, for surface finishes with a lay parallel to the sliding direction, predicts boundaries in the operating condition domain to a regime of mixed lubrication in which little elastic deformation of asperities by micro-EHL is expected. A new thermal model incorporating salient features of scuffing in mixed lubrication conditions is described. It is shown to give the form of a boundary in the sliding/rolling speed domain above which localized temperatures close to melting may be expected and below which lower temperatures suggest running-in without scuffing may be expected. Results of scuffing tests on circumferentially ground discs, at sliding and rolling speeds in the range 3-10 m/s, are reported and shown for surfaces with a distinguishable mainscale wavelength in their topography, (a) to provide further support for the location of the boundaries to the mixed lubrication regime in the operating domain predicted by Baglin and (b) to match the form of the thermal model in the speed domain. Implications for engineering practice are briefly discussed.

Twin-disc tests in microelastohydrodynamic lubrication (micro-EHL) confirm that prior running-in at lower sliding speed enhances load and temperature at scuffing. Examination of surface oxidation and smoothing gives a new insight into the running-in process and suggests that the latter is the dominant effect. Restatement of the operating conditions in terms of non-dimensional groups that take into account the current roughness of the surfaces shows there to be little difference at scuffing.

Results of tensile tests at constant strain rate on an AISI 316 stainless steel at 550°C are presented and shown to imply that mathematical modelling of the creep life of ductile materials subject to variable load must take into account their short term behaviour and a practical failure criterion.