Author

Grohs

Date of Award

12-2019

Degree Name

Master of Science in Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Dr. Jinseok Kim

Second Advisor

Dr. Muralidhar Ghantasala

Third Advisor

Dr. Daniel Kujawski

Keywords

3D printing, fatigue, strain life, roughness, finite element

Access Setting

Masters Thesis-Open Access

Abstract

In this study, a method is developed to estimate the effects of surface roughness on the fatigue life of additively manufactured titanium Ti6Al4V, aluminum 7075–T6, and steel 4340 alloys through modified strain life parameters using finite element analysis (FEA). This method is highly beneficial to the fatigue analysis of as-built additively manufactured metal components, which possess rough surfaces that reduce fatigue life significantly but are challenging to analyze directly using finite element simulation because of complex geometries, i.e., modeling an exact surface profile is arduous.

An effective stress concentration factor, incorporating roughness data, is defined to quantify their effects on effective stress and fatigue life. Notched finite element models with effective stress concentration factor values ranging from 3.058 – 10.75, representing rough surfaces from as-built additively manufactured components, are analyzed, and the S–N curves obtained. A parametric study is done to reproduce these S–N curves by using a smooth finite element model with modified strain life parameters. A mathematical model describing the variation of stress concentration in fatigue is incorporated into these lines of constant stress concentration factors to produce a more realistic fatigue model. Finally, there is a relationship established between effective stress concentration factor and modified strain life parameters such that the fatigue life of as-built additively manufactured parts can be easily calculated using finite element simulation by measuring surface roughness data and calculating the new effective strain life parameters, without using an actual rough surface in FEA.

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