FATIGUE BEHAVIOR AND STRUCTURAL STRESS ANALYSIS OF

COACH-PEEL AND LAP-SHEAR FRICTION STIR WELDED JOINTS OF

AZ31 MAGNESIUM ALLOY

by

MOHAMMED HAROON SHEIKH

J.B. JORDON, COMMITTEE CHAIR

M. BARKEY

Y. GUO

A THESIS

Submitted in partial fulfillment of the requirements for the degree of

Master of Science in the Department of Mechanical Engineering

in the

Graduate School of

The University of Alabama

TUSCALOOSA, ALABAMA

2014

All rights reserved

INFORMATION TO ALL USERSThe quality of this reproduction is dependent upon the quality of the copy submitted.

In the unlikely event that the author did not send a complete manuscriptand there are missing pages, these will be noted. Also, if material had to be removed,

a note will indicate the deletion.

Microform Edition ProQuest LLC.All rights reserved. This work is protected against

unauthorized copying under Title 17, United States Code

ProQuest LLC.789 East Eisenhower Parkway

P.O. Box 1346Ann Arbor, MI 48106 - 1346

UMI 1566243Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author.

UMI Number: 1566243

Copyright Mohammed Haroon Sheikh 2014

ALL RIGHTS RESERVED

ii

ABSTRACT

In this work the fatigue behavior of coach-peel and lap-shear friction stir linear welded joints

of AZ31 magnesium alloy sheet were evaluated under different loads and the results were

compared using structural stress analysis. Lap-shear coupons of 30 mm in width were obtained

from a welded overlap configuration. However, since coach-peel configuration coupons were

not available, aluminum L-shaped brackets were adhesed weld to create the coach-peel

configured specimen. The experimental fatigue life results showed an inverse relationship

between applied load and the number of cycles to failure for both types of coupons. However,

due to differences in the configuration of the joint leading to higher applied stress, coach-peel

coupons failed at comparatively lower loads than lap-shear coupons. In order to correlate the

fatigue data with stresses developed at the weld, finite element analysis, using shell/plate

elements, was employed to model the joints under cyclic loading. The modeling effort focused

on several post-process techniques including equivalent stress and structural stress methods.

While the structural stress approach has merits over conventional nominal equivalent stress

approach for welded joints including accuracy of the results and mesh insensitivity, it could

not correlate the lap-shear and coach-peel fatigue results into a master design curve. As such,

a fatigue damage parameter (FDP) was introduced to correlate the fatigue results of both the

joints, thus establishing a basic relation to account for differences in stress at the weld for lap-

shear and coach-peel welded configurations.

iii

DEDICATION

This thesis is dedicated to my awesome family.

iv

LIST OF ABBREVIATIONS AND SYMBOLS

FSW Friction Stir Welding

FSSW Friction Stir Spot Welding

DOF Degree of Freedom

F,f Force

M Moment

Stress

Shear Stress

E Young s Modulus of Elasticity

G Shear Modulus

Rho Density

Nu Poisson s Ratio

SS Structural Stress

R Load Ratio

t Sheet Thickness

CP Coach Peel

LS Lap Shear

TIG Tungsten Inert Gas

MIG Metal Inert Gas

Mg Magnesium

Al Aluminum

Zn Zinc

Mn Manganese

v

ACKNOWLEDGMENTS

I would like to express my gracious gratitude to Dr. J. Brian Jordon, the chairman of the thesis

committee for selecting me to work on this project and for his continued guidance, support and

encouragement which played a phenomenal role in success of this project.

Additional appreciation is given to my committee which consists of Dr. Mark Barkey and Dr.

Yuebin Guo for guidance and for sharing their lab facility. Also I would like to thank all of the

graduate students working with Dr. Jordon: Harish, Bobby, Rogie and Joao for their help and

co-operation.

Also I would like to take this opportunity to thank all of my friends, UA students and faculty

for their extended support and necessary encouragement. Also Johnny Goodwin and Rob

Holler at UA central analytical facility deserve special thanks for their kind co-operation.

The greatest thanks of all goes to my family for their unconditional love, continuous support

and un-wearying faith in me from a distance of thousands of miles.

vi

CONTENTS

ABSTRACT ..............................................................................................................................ii

DEDICATION .........................................................................................................................iii

LIST OF ABBREVIATIONS AND SYMBOLS ....................................................................iv

ACKNOWLEDGMENTS..........................................................................................................v

LIST OF TABLES .................................................................................................................viii

LIST OF FIGURES ..................................................................................................................ix

1. INTRODUCTION

1.1 Need for Lightweight ................................1

.................1

1.3 Merits of Magnesium Alloy..................................................................................................3

1.4 Fatigue Behavior of Friction Stir Welding.. ........ .......4

2. MATERIALS AND EXPERIMENTAL APPROACH

2.1 Materials and Specimen Preparation.....................................................................................7

2.2 .............................................9

3. FINITE ELEMENT ANALYSIS

3.1 Finite Element Analysis .....................................10

3.2 Structural Stress ..................................................11

3.3 Finite Element Setup ...............................................15

3.4 Fatigue Data Correlation Using Equivalent Stress Approach .............................................17

4. RESULTS AND DISCUSSIONS

4.1 Microstructure of FSW .......................19

4.2 Fatigue Test Results. .......................................21

4.3 Fracture Behavior .............................................................................................22

vii

4.4 .........................................................................25

4.5 ................................................................................................29

. .31

. 36

...37

viii

LIST OF TABLES

Table 2.1: Chemical Composition of AZ

Table 3.1: Material Properties 17

ix

LIST OF FIGURES

.....2

Figure 1.2 Comparison of specific Y ....4

Figure 2.1 Preparation Steps: Coach P ..

Figure 2.2 Prepared: Coach . ...

Figure 2.3 Coach Peel Specimen befor ..

..

Figure 3.1 Structural Stress Definit ... .. ...12

Figure 3.2 Structural Stress Procedure for Shell/plate FE .. .

Figure 3.3 Finite Element ...

Figure 4.1 Macro and Micro- structure of FSW AZ31 Lap Joint...............................................20

Figure 4.2 Cycles to Failure variation with load app ... 22

Figure.4.3 Failed lap shear specimens loaded on the advancing side........................................23

Figure.4.4 Coach Peel Failed specimens loaded on the advancing side.....................................24

Figure.4.5 Cycles to Failure variation with VonMises Stress....................................................25

Figure.4.6 Cycles to Failure variation with Equivalent Stress...................................................26

Figure.4.7 Bending Component variation along the weld line ..................................................27

Figrue.4.8 Membrane Component variation along the weld line..............................................27

Figrue.4.9 Maximum Shear Stress variation along the weld line..............................................28

Figure.4.10 FDP versus Cycles to Failure for Coach Peel.........................................................28