New displacement-based finite element analysis method for predicting the surface residual stress generated by ultrasonic nanocrystal surface modification

Peening techniques can be applied to relieve high tensile residual stress, which is one of the main causes of primary water stress corrosion cracking (PWSCC). Ultrasonic nanocrystal surface modification (UNSM) is a peening technique that generates a high compressive residual stress on the surface of a structure. Finite element (FE) analysis can be utilized for quantitative surface stress prediction according to the UNSM in various materials and loading conditions. In this paper, a new method to predict the surface residual stress generated by UNSM process through FE analysis is proposed. In order to accurately simulate the UNSM process through FE analysis, it is necessary to define an equivalent displacement calculation method that can replace the complex load states of UNSM. For this, we proposed the new equivalent displacement calculation method based on the contact mechanics theory and the energy conservation law. The new equivalent displacement calculation method was validated by comparing the indentation depth obtained by single-path UNSM FE analyses and experiments under various loading conditions. The actual UNSM process is carried out in multiple paths over a large area of the structure. By applying the new equivalent displacement calculation method, the compressive residual stresses were calculated through multi-path UNSM FE analyses and compared with the results obtained from the multi-path UNSM experiments. The surface residual stress results of the experiments and analyses showed good agreement, and it was confirmed that the UNSM process can be simulated well if the new equivalent displacement calculation method is used for the FE analysis.