ORCID Identifier(s)

0000-0002-1863-1017

Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Haiying Huang

Abstract

Fatigue has long been studied as the most critical loading mode for man-made structures. Despite the advances in material behavior characterization and numerical calculation, many structures unexpectedly fail before their designed life. Most of the studies in crack initiation are solely for describing the material and failure behavior and are not capable of predicting the failure before it starts. In addition, most of the fatigue crack growth models are based on the Paris Model with some modifications, which their application is limited to long crack. This implies that finding new methods which can reliably predict material failure is very important. In this study, two main stages of the fatigue life, i.e. crack initiation and microstructural dependent crack growth which accounts for 90% of the components’ life under high cycle fatigue (HCF), are studied. Our objective in this study is to introduce a fatigue indicator parameter which may be used to predict the crack initiation and early growth behavior. For this purpose, due to the importance of the out-of-plane deformation, the surface topography analysis based on Scanning White Light Interferometry technique is used. By studying the evolution of the surface topography on the nickel sample subjected to cyclic loading, we observed that a narrow valley in front of the crack forms through which, the crack grows. Large surface topography changes near the tip of the arrested crack were also observed. An image processing technique was developed which enabled us to measure the surface topography changes with high precision. By defining the surface roughness parameters as the damage index, we found that the damage parameters in the areas where the crack eventually grew were higher than the other areas. Inspired by these observations, an algorithm was developed which could predict the future path of the microstructural dependent crack up to a few grains. In addition, a method was proposed for measuring the sizes of the monotonic and cyclic plastic zones. The observations of the first study were utilized to predict crack initiation behavior. Using the method similar to that developed in crack propagation study, the critical areas on the surface were determined, and the crack initiation location well before the crack initiated was predicted. Besides, investigating the evolution of the damage index with applied fatigue cycles showed a steady increase in the damage index parameter which was followed by a plateau. The time at which the damage index plateaued was attributed to the crack initiation time. In addition, by studying the surface height change of the fatigue processing zone, the crack initiation time using a second method was found. The comparison between the results of proposed methods and Scanning Electron Microscopy (SEM) imaging technique verified the effectiveness of proposed methods. Since the damage initiation in the material is a result of strain localization due to material microstructure, which may occur in in-plane or out-of-plane directions, in the last part of the study a new method was developed which calculates both in-plane and out-of-plane deformation and strain. This new method enabled us to determine the effect of material microstructure on damage formation. Utilizing image enhancement techniques and correlation points rather than subsets, the resolution of strain calculation in this method was found to be higher than the digital image correlation (DIC) method. The superiority of the developed technique is that it can calculate the 3D strain and deformation with high precision at the sub-grain level using a single camera. The findings and the methods developed in this study may shed more light into the physics of crack formation and growth, and they may provide valuable tools for failure prevention.

Keywords

Monotonic Loading, Fatigue loading, Surface topography, Damage index, Out-of-plane deformation, In-plane strain, Crack initiation, Crack propagation, Mis-orientation, SWLI, Plastic deformation

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

Download

Share

COinS