Fracture mechanics approach to hydrogen assisted cracking: Analysis of theK-dominance condition

Abstract

We summarize the achievements of a major research line at the Department of Materials Science of the University of La Coruna (Spain) in the field of environmentally-assisted cracking in general and hydrogen degradation in particular. We analyze the meaning and significance of the fracture-mechanics approach to hydrogen-assisted cracking and study the problem ofK-dominance not only over the mechanical aspects of the phenomenon but also over the environmental (physicochemical) factors affecting the entire coupled process of hydrogenation and failure. Two key factors capable of violating the unambiguous behavior of the kinetic diagram of crack growthv=v (K) are discussed: the role of the far stress-strain field (i.e., the stress-strain field which is notK-dominated) and the effect of the history of hydrogenation and crack growth. For this purpose, we consider the stress-strain-assisted diffusion of hydrogen regarded as a rate-controlling factor of hydrogen-assisted cracking under sustained or quasistatic loads. It is shown that the far field produces a minor effect on the near-tip diffusion of hydrogen. This can only increase the spread in crack growth rates in the near-threshold part of thev (K)-curve. As far as the influence of the history is concerned, we discovered that the processes of hydrogenation and crack growth are coupled, eachof them affects the other, and, hence, the kinetic diagram of crack growthv=v (K) is not one-to-one as should be for an intrinsic property of the material. However, there exists a special mode of steady-state crack growth when hydrogen-assisted cracking becomes aK-dominated process and the corresponding plot of the steady-state ratev as a function ofK becomes unambiguous (as a characteristic curve of the material) and, thus, can be used in the engineering practice for a more conservative evaluation of the crack-growth resistance of materials and structural integrity.