THE MATHEMATICAL FRAMEWORK AND AN APPROXIMATE SOLUTION OF SURFACE CRACK-PROPAGATION UNDER HYDRAULIC PRESSURE LOADING

Material degradation and failure in rolling contact components are oft en caused by surface microcrack initiation and propagation. Experiment al evidence shows that surface crack growth rate is higher with the pr esence of lubricating fluid than without, possibly due to high fluid p ressure within the crack. The mathematical framework to analyze a surf ace crack under hydraulic pressure loading is established. A surface c rack filled with incompressible, Newtonian viscous fluid is considered . The solid is considered to be linear elastic. A pressure loading his tory is prescribed at the mouth of the crack. The governing equations are found to be two coupled non-linear integral equations of pressure distribution and crack opening displacement distribution. An approxima te solution is obtained by assuming a local pressure-opening displacem ent constitutive law, and by using the method of separation of variabl es. The results indicate that upon a sudden decrease of pressure loadi ng at the crack mouth, the crack-tip stress intensity decreases rapidl y at the beginning followed by a long tail of diminishing decreasing r ate; whereas upon a sudden increase of pressure loading, an incubation time exists before the pressure can be transmitted deep into the crac k. A very important parameter, the characteristic penetration time, is identified and can be used to determine whether hydraulic pressure ha s significant influence on surface crack propagation.