Mechanisms and modeling of near-threshold fatigue crack propagation

First, this paper proposes a comprehensive framework for the modeling of the intrinsic FCP (i.e. after elimination of any environmental and closure effects). On the basis of numerous experimental data on Al alloys, steels,and Ti alloys, three intrinsic crack growth regimes have been identified: i) stage I regime, observed in single crystals or in the early growth phase of short cracks; ii) stage II regime, commonly observed when the crack advance proceeds along a plane normal to the load axis and results from the activation of symmetrical slip systems; iii) crystallographic stage I-like regime which prevails near the threshold. Second, this contribution is dedicated to the description of environmentally assisted propagation and specially focused on the understanding of the role of water vapor and the complex interactions existing between environment and microstructure. The effective FCP behavior is described by superimposing two:distinct stage II regimes: i) a propagation assisted by water vapor adsorption which can be operative under vary low partial pressure or at very low frequencies; ii) hydrogen-assisted propagation which is operative when some critical conditions are encountered. Constitutive laws are proposed for both intrinsic propagation and water-vapor assisted propagation.