Combined FEM and DIC Techniques for the 2D Analysis of the Stress-Strain Fields and Hydrogen Diffusion Near a Blunt Crack Tip

A mathematical model of hydrogen diffusion in structural elements near stress concentrators has been developed and experimentally confirmed. A finite-element algorithm for solving elastic-plastic and diffusion problems in the case of arbitrary geometry of the structure has been created. The study was performed on flat hydrogenated samples with a central crack. The calculations were performed for three materials: 20, 40Kh, and 65G steels. Experimental true stress–strain diagrams were used in the calculations of the hydrostatic stress. Distributions of hydrostatic stress and hydrogen concentration on the extension of the crack line for the above three steels under tension at the moment of ultimate equilibrium and after unloading were calculated. The results of hydrostatic stress distribution for steel 20, which were obtained experimentally by digital image correlation technique at the ultimate force, are also presented. For steel 20, a comparison of the calculated hydrogen concentration with the results of experimental studies obtained at the Paton Institute of Electric Welding of the National Academy of Sciences of Ukraine by mass spectrometry after complete unloading has been made. In this case, the results correlate well with each other. The correspondence between the distribution of hydrogen concentration and hydrostatic stress near the crack tip has been established. Similar to hydrostatic stress, the maximum value of the hydrogen concentration is reached at a certain distance from the crack tip. As the hydrostatic stress increases, the amount of hydrogen in the pre-failure zone increases. The concentration of hydrogen near the crack tip in the loaded sample is 2–3 times higher than the initial one; in the unloaded sample, it is 1.5–3 times lower than in the limit-equilibrium state. Plots of hydrogen concentration near the crack tip versus on the mechanical characteristics of the material were constructed, which can be used to estimate the hydrogen concentration near sharp stress concentrators in a hydrogenated metal with a given yield stress The empirical dependence of hydrogen concentration on the mechanical characteristics of the material was established, which can be used to assess the load-bearing capacity of structural elements. An engineering method for estimating the maximum hydrogen concentration near a notch is proposed.