Modeling fatigue life and hydrogen embrittlement of bcc steel with unified mechanics theory

Abstract

The fatigue life estimation of metals operating in hydrogen-rich environments such as hydrogen pipelines, hydrogen-burning internal combustion engines, etc. is important. Studies in the past 40 years have shown that the diffusion of hydrogen into steel and other metals causes various chemical reactions, hydrogen-material interactions, and microstructural changes. That leads to hydrogen embrittlement (HE) and other types of hydrogen damage mechanisms including hydrogen environmentally assisted cracking (HEAC). Hydrogen embrittlement mechanisms, such as hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) can have synergetic effects in steel depending on the hydrogen concentration level. At concentrations above and below the critical hydrogen concentration, HEDE and HELP dominate the embrittlement process, respectively. Different HE mechanisms result in distinctly different fracture modes, both ductile and fully brittle. The ultrasonic vibration fatigue life of bcc steel with a ferrite-pearlite microstructure pre-charged with hydrogen at different concentrations is studied. Modeling is based on the unified mechanics theory (UMT), which does not need any empirical dissipation/degradation potential function or an empirical void evolution function. However, the UMT does require analytical derivation of the thermodynamic fundamental equation of the material, which is used to calculate the thermodynamic state index (TSI) of the material. The UMT is ab-initio unification of the second law of thermodynamics and the universal laws of motion of Newton [1]. Dissipation/degradation evolution is governed by Boltzmann's second law of thermodynamics entropy formulation. The original contribution of this paper is the derivation of the thermodynamic fundamental equation of pre-hydrogen embrittled bcc steel subjected to ultrasonic very high cycle fatigue and the numerical simulations of fatigue life estimation using the proposed novel model. The synergetic interaction of hydrogen embrittlement mechanisms in steel and other metallic materials, i.e., HELP and HEDE at different hydrogen concentrations (HELP + HEDE model) is also studied, reviewed, and applied. The synergetic effects between ultrasonic vibration fatigue life and synergistically active hydrogen embrittlement mechanisms in low carbon bcc steel (S355J2+N, equivalent to ASTM A656), according to the HELP + HEDE model for HE, is modeled for the first time using UMT and also thoroughly discussed.