Analytical and numerical modeling of apparent yield strength reduction induced by ultrasonic nanocrystalline surface modification in Inconel 718

초록

In this study, ultrasonic nanocrystalline surface modification (UNSM), one of the surface severe plastic deformation (SSPD) processes, was applied to an aging heat-treated Inconel 718 alloy. Contrary to the strengthening effects generally reported for SSPD processes, the 0.2% offset yield strength decreased by 9.6%, from 1070.0 MPa to 966.7 MPa. To explain and predict this behavior, a closed-form 2D plane-stress framework was developed for rounded (axisymmetric) tensile specimens that couples axial loading with near-surface circumferential compression and self-equilibrating core tension. The analysis predicts a net reduction in apparent yield strength (AYS) through area-fraction averaging of the stressed layers. In parallel, a numerical model was formulated by combining a modified Swift-Voce hardening law that captures UNSM-induced strengthening with a depth dependent residual stress field, enabling faithful reproduction of both the initial softening and the transient regime. By extending the analysis beyond sheet geometries and validating the proposed mechanism for rounded specimens, this study demonstrates that SSPD/UNSM can reduce the apparent yield strength while still enhancing surface durability and provides a practical framework for incorporating residual stress and hardening effects into constitutive models and FE simulations.

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