Two tribology problems are studied analytically from the viewpoint of fracture mechanics. One is an inclined semicircular surface crack under Hertzian contact loading and the other is an elliptical crack embedded parallel to the surface under Hertzian contact loading.
The behavior of crack opening/closure for a surface crack formed on lubricated rolling/sliding line-contact surface is analyzed theoretically on the basis of fracture mechanics. The crack opening displacement is controlled mainly by surface traction, contact pressure, and oil hydraulic pressure. Both the direction and the magnitude of the surface traction govern the oil seepage into the crack. The oil hydraulic pressure on crack faces is induced by two types of mechanism depending on the movement of contact pressure: the contact pressure transmitted directly by the oil to the crack faces and the oil blocking phenomenon caused by closure of mouth of crack. It was concluded by the analytical results that the oil seepage into a surface crack is the crucial factor which causes the pitting phenomena in rolling/sliding contact fatigue. From this point of view, the reason why pitting phenomena are more frequently observed on the follower surface than on the driver surface can be clearly explained.
The variations of three-dimensional, mixed-mode stress-intensity factors for an embedded elliptical crack under a Hertzian contact pressure moving over the surface are analyzed. The crack opening occurs near the leading crack tip as well as near the trailing crack tip during one pass of contact load. This crack opening behavior was confirmed by the experimental measurements. The directions of crack extension are also discussed.