Modelling of fatigue crack initiation in hydrogen-metal system

Rakesh Kumar, Indian Institute of Technology Ropar, Rupnagar, India
Dhiraj K. Mahajan, Indian Institute of Technology Ropar, Rupnagar, India

Hydrogen has emerged as a potential clean energy source providing new possibilities for replacing the polluting hydrocarbon-based fuels. This has led to focused efforts by several researchers to understand the behavior of hydrogen-metal system often characterized by degradation of fractomechanical properties and decrease in fatigue life of metals under hydrogen environment. To this end, we have developed a simulation framework based on crystal plasticity model coupled with hydrogen transport model to understand the crack initiation mechanism in hydrogen-charged metal samples. This work is motivated by the results of small-scale fatigue experiments performed under SEM in our group, revealing the importance of grain boundary engineering to control the crack initiation in hydrogen-charged nickel polycrystals. A representative volume element (RVE) with special grain boundary network is used for this study.

The nonlocal crystal plasticity model calculates statistically stored (SSD) and geometrically necessary dislocations (GNDs) which provide hardening to the material as well as back stress due to pileup of GNDs. The hydrogen transport model calculates the hydrogen concentration at normal interstitial lattice sites and traps sites affected by the concentration gradient, pressure gradient and strain rates. Within this modeling framework, fatigue indicator parameter (FIP) is established for hydrogen-metal system based on accumulated plastic strain, grain boundary normal stress and hydrogen concentration showing good match with the experimental results.

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