Date of Award

6-2022

Degree Name

Doctor of Philosophy

Department

Mechanical and Aeronautical Engineering

First Advisor

Daniel Kujawski, Ph.D.

Second Advisor

Judah Ari-Gur, D.Sc.

Third Advisor

Muralidhar Ghantasala, Ph.D.

Fourth Advisor

Lee Wells, Ph.D.

Keywords

Cyclic indentation fatigue, fatigue, fatigue incremental step test, fatigue surface damage, low alloy steels, microhardness

Abstract

Fatigue is the most common type of failure in many structures and components subjected to cyclic loading. Surface effects in fatigued materials are of particular importance, since in most cases the surface is the preferred site of fatigue crack initiation. Microcracks originate at the metal surface due to stress concentration introduced by macro notches and micro defects in the surface layers. The microcrack initiation and propagation are controlled by the localized plastic deformation in the surface layers especially in the initial stage of fatigue failure process. Therefore, the knowledge regarding the cyclically induced changes in deformation resistance at the metal surface layers is of practical importance in fatigue analysis of engineering structure and components subjected to cyclic loading.

On the other hand, fatigue evaluation using conventional methods may not be always feasible to conduct considering the size, with well-defined geometry and quantity of the test samples. Also, for some particular applications (e.g components of turbine engine) significant savings can be realized by conducting limited small-scale fatigue testing instead of conducting conventional fatigue on a large number of processed fatigue specimens. Therefore, some other semi/non-destructive methods need to be employed to assess their cyclic response with a good reliability.

The objective of this research study is to investigate a technique to monitor the fatigue damage accumulation throughout the fatigue life using the non-destructive quantifiable changes in the material surface properties. The present contribution also is investigating a non-destructive method to evaluate the fatigue properties of the material.

For this purpose, two kind of ferritic-pearlitic low alloy steels, 8620 cold drawn round bar and 4340 steel cold drawn round bar in annealed condition, have been studied. At first, a series of experimental procedures are defined to investigate a correlation between parameters results from the cyclic indentation to the fatigue properties results from the traditional fatigue tests. A potential correlation was obtained from the results. However, further studies on more metals and alloys need to examine the general existence of a correlation between conventional material fatigue property and the parameter determined through the cyclic indentation test.

In order to pursue the second major aim of present work, development of the surface damage parameters, surface hardness and roughness, were monitoring throughout the fatigue life. For this purpose, the uniaxial cyclic total strain control tests under two different strain amplitudes were applied to the fatigue specimen of the materials. The cyclic test was interrupted in random different life intervals to measure the surface hardness and roughness and monitor their variation throughout the cyclic test. The results show that at least for these two steels, the surface hardness development can only predict the cyclic softening/hardening. Surface roughness monitoring show an increasing trend throughout the fatigue life but in three different stages. Because the rate of increasing the surface roughness values is increasing by progressing the applied cycles. It looks monitoring the surface roughness development throughout the fatigue life is a potential method for indicating the macrocrack nucleation.

Access Setting

Dissertation-Open Access

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