Date of Award

3-2020

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Aeronautics and Astronautics

First Advisor

Anthony N. Palazotto, PhD

Abstract

Laser shock peening (LSP) is a form of work hardening by means of laser induced pressure impulse. LSP imparts compressive residual stresses which can improve fatigue life of metallic alloys for structural use. The finite element modeling (FEM) of LSP is typically done by applying an assumed pressure impulse, as useful experimental measurement of this pressure impulse has not been adequately accomplished. This shortfall in the field is a current limitation to the accuracy of FE modeling, and was addressed in the current work. A novel method was tested to determine the pressure impulse shape in time and space by optimization driven data-matching. FE model development and material model verification was completed in Abaqus. A 2D and 3D model type study was conducted. A proof of concept data-matching optimization tool was developed and verified. This data-matching optimization tool, using the Hooke-Jeeves optimization algorithm, was then applied to match experimentally collected residual stress measurements from single LSP treated spots in 2024-T351 aluminum specimens. Validation of this “best-fit” pressure impulse was attempted in a 6Al-4V titanium material model for the same LSP treatment process. A combination Johnson-Cook viscoplasticity and Mie-Grüneisen equation of state (EOS) material model was shown to be amply sufficient for modeling the highly dynamic LSP event. A 2D axisymmetric FE model was shown to adequately represent a square LSP treatment process, in terms of residual stress field results with the use of a linear adjustment factor. The Hooke-Jeeves optimization algorithm proved highly successful at working through a FE model “black box” to match a target residual stress outcome. Further, this method was successful in matching the residual stress field of experimentally collected data. The validation of the best-fit pressure impulse in titanium was not a perfect match, but exhibited enough accuracy to be useful to design engineers in certain cases, and further shows potential for improvement and implementation toward this impulse matching goal.

AFIT Designator

AFIT-ENY-DS-20-M-260

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