Ha-rok Bae (Committee Member), Ramana Grandhi (Advisor), Kristina Langer (Committee Member), Rajiv Shivpuri (Committee Member), Raghavan Srinivasan (Committee Member)
Doctor of Philosophy (PhD)
Generating compressive residual stresses at the critical stress locations can prevent or delay structural component failure. Laser peening is a well-known method for inducing compressive residual stresses. The plastic deformation created by the planar shock waves near the surface regions is the major cause for the generation of compressive residual stresses. Apart from the planar waves, release waves at the border of impact are generated that act against the plastic deformation created by planar waves. This decrease of plastic deformation reduces the compressive residual stress generated near the surface regions of peened components. Laser peening of curved geometries creates compressive residual stresses - which is dissimilar to flat geometries - because of the influence of release waves on different curvatures. This research investigates the effects of reduction in the amount of plasticity in convex, concave, and flat geometries using the plastic dissipation energy as the measure of plastic deformation imparted on the component. Finite element models are created in Abaqus to predict the effects of "reduced plasticity" in residual stresses generated on curved geometries of laser peened components. An analytical formulation is derived based on the plasticity incurred inside the material and the results are compared with the prediction by numerical simulation. The consistency in the analytical formulation with the simulation model indicates the behavior of laser peening for curved geometries. However, the compressive residual stresses can relax due to the loading conditions and thus reduce the laser peening effectiveness. Under such a condition, re-peening or re-laser peening a component already in service can further increase its component life. This research develops a method to predict the optimal re-peening time for maximum fatigue life under realistic loading conditions. An optimization problem is set up to illustrate the application of this method to an aircraft lug problem. A novel surrogate modeling technique called the Sorted k-fold Approach (SKA) is developed to perform the optimization. Results from the investigation indicate that re-peening the component ~50-55% of its expected fatigue life maximizes the component's fatigue life. The proposed approach, proven to be able to obtain optimal process parameters for improving the fatigue resistance of the component, can significantly reduce the costs for experimental testing.
Department or Program
Ph.D. in Engineering
Year Degree Awarded
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