Molecular Dynamics Simulations on Crack Growth Behavior of BCC Fe under Variable Pressure Fluctuations

• Author / Creator

  Xing, Xiao

• Hydrogen embrittlement is an industrial problem involving in environment, loading mode, materials. Clarifying the mechanisms of hydrogen embrittlement not only makes economic sense but also academic significance. Although laboratory tests have gained significant understanding in the research of stress corrosion cracking (SCC), there are still limitations to study the effects of diffusible-atomic hydrogen on cracking. To get insights into nano-scale problem, such as the initiation of the micro-cracks, molecular dynamics (MD) simulation could be a good choice. There are some obstacles in applying MD simulations to field operations. Firstly, the current MD model is in nano-scale, however, the classical cracking mechanism is related to micro or centimeter scale plastic zone; secondly, former theoretical models only consider hydrogen movement during loading but ignore its movement during unloading. To bridge the nano-scale simulations with field operations, we are aiming to build a new model, i.e., the hydrogen atoms will diffuse out or accumulate into the plastic zone based on the stress intensity and hydrogen concentration gradient. In particular, an annulus region outside the plastic zone in bcc structure offers or depletes the hydrogen atoms to the plastic zone. Based on this hypothesis and the simulated results, the model could be used to predict the crack growth rate in different loading spectra. To study the crack growth mechanism, we simplify our model to a nano-bcc structure plate, where the crack is located at one side of the plate or at the center of the loading direction. The static single loading is applied on the models with different concentration of hydrogen. By tracing the hydrogen concentration in a nano region ahead of the crack tip, we found that the atomic ratio of H/Fe would reach 0.4 as ductile to brittle (DTB) transition happens. The total number of hydrogen atoms to saturate the plastic zone can be assessed with this critical hydrogen concentration. In addition, the asymmetric hydrogen diffusion in minor cycles could be quantified in our new model. In addition, cyclic loading was applied to those models. The purpose of simulations is to detect the discontinuous crack propagation process. In MD simulations, hydrogen atoms do not diffuse out during unloading and consequently, form hydrogen-rich region ahead of crack tip. The hydrogen rich region blocks the dislocation emission and enhances the brittle cleavage of crack. The crack will propagate during both loading and unloading. However, as the crack pass through the hydrogen-rich region, the crack will become dormant. Several cycles of loading will then accumulate hydrogen atoms form new hydrogen rich region and brittle the crack, again. Moreover, we found the number of hydrogen atoms accumulated at the crack tip is also linearly related to the number of minor cycles. In addition, the linear accumulation mechanism could be applied to field operation and be used to predict the hydrogen accumulation acceleration effects. This hydrogen-accumulation model had been used to predict critical loading frequency and could be further modified to predict the crack growth rate. The new model has offer a combined factor, which consider both stress intensity change and loading frequency change. The pH and temperature effects have been included in an environmental factor. The new model has been used to predict crack growth rate in different loading spectra and steels and show good convergence. It is compared with previous combined factor and shows a power law relationship, which ensure its predictive capability.

• Subjects / Keywords

  • Predictive model of crack growth in pipeline
  • Molecular dynamics simulations
  • Hydrogen embrittlement

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3WP9TH8R

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.