A Fracture Mechanics Approach to Study Hydrogen Embrittlement in High Strength Martensitic Steels
Abstract: High strength steels that are subjected to hydrogen experience embrittlement where the mechanical properties are reduced, and premature failure of components may occur. Although the phenomenon has been recognized for over 150 years, it is not clear what drives embrittlement. The goal of this thesis has been to investigate hydrogen embrittlement in high strength martensitic steels by fracture toughness testing. Since a well-recognized standard to test a materials susceptibility to hydrogen embrittlement is missing, the first step has been to develop an experimental-numerical method that produces reproducible results that can be transferred from laboratory to in-service components, which is presented in Paper I. Here it is seen that the environmentally driven ductile-to-brittle transition region in elastic-plastic fracture toughness depends on the hydrogen exposure time. The presented numerical evaluation approach removes the need to perform unloadings, and the results correlates well with standards. The proposed method is then applied to two different application areas presented in Paper II and Paper IV. In Paper II the proposed experimental method is utilized to develop a framework that can be used to study hydrogen kinetics ahead of a crack frontduring in-situ conditions for delayed hydrogen cracking using neutron imaging. In Paper IV, the experimental method is applied to specimens with different crack tip constraints to mitigate the gap between laboratory experiments and in-service components. It is seen that the environmentally driven ductile-to-brittle transition region is obtained for specimens with different constraints, and that both the plastic strains as well as the hydrostatic stress play a critical role in hydrogen embrittlement. The results from Paper I are used as the basis for the numerical framework presented in Paper III. Here, a conceptual modeling approach is adopted that incorporates two separate failure mechanisms observed in the experiments performed in Paper I. It is seen that both a ductile and brittle failure mechanism must be employed to capture the full range of crack extension resistance curves. Furthermore, to capture the slope of the degraded J-R curves, it is necessary to employ a degradation of fracture energy, the cohesive strength as well as the strain driven nucleation.
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