The lubrication of porous elastic solids with reference to the functioning of animal joints

Abstract

This investigation is concerned with the elasto-hydrodynamic squeeze-films generated by the normal approach of two surfaces. It is inspired by the excellent functioning of healthy animal joints under the adverse conditions of high load and low, or even zero, sliding speeds. The effects on the film of four features are examined from both the theoretical and experimental viewpoints. These four features are the elasticity of the materials bounding the film, the permeability of these materials, an extreme interpretation of the concept of "weeping" lubrication and the presence of a lubricant enrichment in the film. This latter feature is considered only in its theoretical aspects. It is shown experimentally that when a thin layer of rubber is used as the soft boundary of the film, entrapment occurs, due to the high poisson's ratio involved. The agreement between theoretical predictions and experimental results for the effects of permeability and of a "weeping" mechanism is good. The models, as examined, are found to be dominated by viscous forces rather than the inertia of the moving surfaces. It is found that high permeability of the surfaces tends to decrease film life and that high flexibility tends to increase it. However, very low permeability material appears to promote slight "weeping" tendencies and hence prolong film lives. The effect of an additive confined to motion in the fluid flow is found to be small unless present at extremely high concentrations and in joints it seems likely to be effective only at very thin films. When the additive is confined not only to the film but also prevented from flowing in the film, squeeze times are found to be increased by several times. The increase is not considered to be enough to make the mechanism totally convincing as a mode of operation of an animal joint.