Some considerations on the modelling of oxide-induced fatigue crack closure using solutions for a rigid wedge inside a linear elastic crack

The formation of insoluble corrosion deposits within slowly growing cracks exposed to active environments can produce a mechanical wedging action which can either promote sustained-load cracking or retard cracking under cyclic loads. In the latter case, this wedging action has been shown to be particularly significant for fatigue cracks propagating at near-threshold levels. In such instances oxide deposits can accumulate near the crack tip to thicknesses some 20 times the naturally-occurring oxide thickness in ambient temperature moist air atmospheres due to a mechanism of ''fretting oxidation'' from the combined action of plasticity-induced crack closure and Mode II crack tip displacements characteristic of near-threshold crack extension. The result of such oxide deposits under cyclic loading is to promote crack tip blunting and more importantly to enhance crack closure through a mechanism which has become known as oxide-induced crack closure. This effect, however, is most relevant to fatigue cracks growing at near-threshold levels, at low load ratios, in lower strength materials, and of course in environments which promote the formation of corrosion deposits. Recently, attempts have been made to model the role of oxide-induced closure in suppressing rates of environmentally-influenced fatigue crack growth using stress intensity solutions for a rigid wedge inside a linear elastic crack. The purpose of this note is to examine the significance and limitations of this approach in the light of recently available experimental information on near-threshold corrosion fatigue behavior