Most of the gas turbine equipment use nickel-based superalloy, Inconel 718, and their service life is influenced by the alloy's response to fatigue, wear, oxidation and corrosion, when exposed to thermo mechanical loads. Majority of these mechanisms originate from the surface and are very sensitive to the microstructure. Shot peening is one of the widely used surface treatments, to enhance microstructural properties of the surface. One of the key outcomes of shot peening is Compressive Residual Stress (CRS), that helps in improving fatigue life. But CRS is observed to relax and re-distribute under various thermo mechanical loads. The amount of relaxation depends on the increased surface hardness caused due to grain refinement/fragmentation during shot peening process, owing to higher dislocation density near the surface. Finite Element Modeling (FEM) in combination with Discrete Element Method (DEM) has been extensively used for decades to simulate shot peening process, but very limited studies focused on microstructural changes during shot peening. This warrants a need for modeling shot peening process at a microstructural level, capturing the effects of grain size and dislocation density, for accurate estimation of relaxation of CRS.
The objective of this work is to study and model the micromechanics during shot peening process and subsequent cyclic loading. First, a dislocation density-based crystal plasticity model is developed, to model the target surface. All the complex cyclic deformation characteristics of IN718, (1) Bauschinger effect, (2) Mean stress relaxation and (3) Cyclic softening are included in the rate dependent material model. The model is also capable of capturing the grain size effects and reduction in precipitate size due to dislocations shearing through them. The model predictions are shown to be in good agreement with experimental data for both monotonic and cyclic loads. Next, the constitutive model is used to simulate the shot peening process. A two-step numerical framework is proposed, that uses 3D FEM, to simulate shot peening process in first step and subsequent relaxation in second step.