Date

2018

Document Type

Campus Only Thesis/Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Harlow, Gary

Abstract

The viability of modern engineering structures and materials depends on actively designing against fatigue, which accounts for the majority of mechanical failures. While additive manufacturing (AM) offers numerous advantages to conventional manufacturing techniques, the cyclic behavior of AM parts must be understood before they can be safely used in areas such as the aerospace and biomedical industries. In this research, the behavior of austenitic steel components produced via direct energy deposition (DED) wire and arc additive manufacturing (WAAM) under cyclic loading was investigated. Paris-Law fatigue crack growth parameters were determined from experimental testing in an effort to deduce location-dependent properties and to compare to published data for the conventional material. Statistical ANOVA analysis provided inconsequential evidence of the dependence of damage accumulation on location or position within the WAAM structure. Further scanning electron microscopy (SEM), light optical microscopy (LOM), x-ray diffraction (XRD), and electron backscatter detection (EBSD) investigation showed compressive residual stress, texture and large grain sizes resulting from high thermal gradients in the AM process. Primarily it was discovered that WAAM thermal gradients and resulting material behavior, led to a decrease in damage propagation as compared to the conventional material at a given stress intensity range within the Paris-Law regime in austenitic steel. Additionally, Safe Life analyses were undertaken for WAAM austenitic steel. Experimental results of the cyclic response matched closely with literature for wrought austenitic steel, indicating that materials produced by WAAAM are not adversely affected by processed induced imperfections (e.g. porosity) such as other metal AM processes. In subsequent fracture surface investigation, large shear lips and microvoid coalescence was discovered which is typical in conventionally processed austenitic steels. Inelastic strain accumulation (ratchetting) behavior was compared to predictions from finite element analyses using a combined Isotropic and Chaboche Non-Linear Kinematic Hardening material model which accurately captures the yield surface translation during cyclic loading. Experimental results displayed comparable ratcheting strains as predicted by the finite element model. Primarily it was discovered that continuum scale plasticity modeling can be used to predict ratcheting strain in AM austenitic steel; however discrepancies existed due to complex material behavior observed at the microstructural level using SEM and EBSD. Further investigations are therefore necessary for better prediction of the deformation response of additively manufactured metals, which will require multiscale efforts such as phase field modeling, crystal plasticity, and probabilistic simulation techniques.

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