Title

Large Strain Plastic Deformation of Traditionally Processed and Additively Manufactured Aerospace Metals

Date of Award

6-1-2021

Degree Name

M.S. in Mechanical and Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Robert Lowe

Abstract

To numerically simulate and predict the plastic deformation of aerospace metal alloys during extreme impact events (e.g., turbine engine blade-out and rotor-burst events, and foreign object damage), accurate experimental knowledge of the metaĺ⁰₉s hardening behavior at large strains is requisite. Tensile tests on round cylindrical specimens are frequently used for this purpose, with the metaĺ⁰₉s large-strain plasticity ultimately captured by an equivalent true stress vs. equivalent true plastic strain curve. It is now well known that if axial strain is measured using an extensometer, the equivalent true stress-strain curve calculated from this measurement is valid only up to the onset of diffuse necking. That is, once the strain field heterogeneously localizes in the specimen gage (onset of necking), extensometers, which average the strain field over the gage section, are unable to capture the local strain at the site of fracture initiation.Thus, a number of approaches have been proposed and employed to correct the post-necking hardening response. One commonly-used technique is an iterative approach commonly referred to as finite-element model updating (FEMU). This approach involves inputting a suite of candidate post-necking equivalent true stress-strain curves into finite-element software. The true stress-strain curve that produces the best agreement between simulation and experiment is ultimately adopted. In this document, a novel variation of this iterative approach is presented, aimed at decreasing computational expense and iterative effort with a better first guess that bounds this fan of prospective true stress-strain curves. In particular, we use local surface true (Hencky) strain data at the fracture location in an approximate analytical formula to generate a first guess curve and upper bound on the candidate true stress-strain fan of curves.To assess its performance and robustness, the proposed approach is verified using experimental data for a menu of aerospace relevant metal alloys (In-625, In-718, Al-6061, 17-4 PH stainless steel, and Ti-6Al-4V) that span various crystallographic structures and exhibit different plastic (hardening) behaviors. For each of these metals, our approaches substantially decreases the number of candidate curves and meaningfully reduces iterative effort, a trend that holds true across a broad range of crystal structures and corresponding hardening behaviors.Next, using the above improved iterative post-necking hardening correction, a plastic deformation model was generated for AM Ti-6Al-4V. A series of tensile experiments were completed across varying strain rates and multiple additive build orientations. The true stress-strain data from these mechanical tests help to build a database of material behavior. Parallel finite element simulations (in LS-DYNA) of the tensile experiments were completed and corrected with the novel, iterative post-necking correction method. A tabulated material card was populated with the corrected true stress-strain data from the various tensile tests, which can be widely employed to help constrain a ductile fracture model, or to qualify in-use parts and assemblies through the prediction of deformation and damage accumulation felt under a specific loading condition or impact environment.

Keywords

Aerospace Engineering, Engineering, Mechanical Engineering, Mechanics, Aerospace Materials, Post-Necking Correction, Plastic Deformation Modeling, Laser Powder Bed Fusion Ti-6Al-4V, Material Characterization, LS-DYNA Parallel Numerical Simulation

Rights Statement

Copyright 2021, author

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