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 Corufia (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 of K-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 growth v = v (K) are discussed: the role of the far stressstrain field (i.e., the stress-strain field which is not K-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 the v (K)-curve. As far as the influence of the history is concerned, we discovered that the processes of hydrogenation and crack growth are coupled, each of them affects the other, and, hence, the kinetic diagram of crack growth v = 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 a K-dominated process and the corresponding plot of the steady-state rate v as a function of K 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,

Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking

“…[1][2][3][4][5] Hudak and Wei [2] conducted crack growth rate experiments on 4340 steel in distilled water to better understand the earlier static load results of Landis and Wei, [1] which showed that K th is not unique and that stage I crack velocity is not a unique function of K when K increases with crack growth. Both were found to be dependent on the initial value of K. By testing specimens for which K remains constant during crack growth under constant load, Hudak and Wei concluded that the behavior observed by Landis and Wei is ''due, at least in part, to non-steady state crack growth.''…”

“…[2], [5], [13], and [15] given values for each of the parameters in these equations. However, use of Eq.…”

“…Assuming p = 0.5, Eq. [5] can be inverted to find the minimum, or critical, grain boundary occupancy,ĥ gb , required to obtain intergranular fracture by substituting the applied stress intensity factor K for K th :ĥ…”

Optimization of the simulation of stress-assisted hydrogen diffusion for studies of hydrogen embrittlement of notched bars

“…As regards hydrogen diffusion, phenomenological consideration attending the stress effect on hydrogen behavior in metals yields the equation of stress-assisted diffusion under isothermal conditions as follows [5,8,16,17,25]:…”

“…Taking into account that the diffusion driving force and flux are nil at equilibrium, it can be easily obtained [5,8,25] …”