Fatigue fracture and microstructural analysis of Friction Stir Welded butt joints of aerospace

163 Pages·2017·5.13 MB·English

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LSU Master's Theses Graduate School
2009
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Vinay Raghuram
Louisiana State University and Agricultural and Mechanical College
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Raghuram, Vinay, "Fatigue fracture and microstructural analysis of Friction Stir Welded butt joints of
aerospace aluminum alloys" (2009). LSU Master's Theses. 1827.
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FATIGUE FRACTURE AND MICROSTRUCTURAL ANALYSIS
OF FRICTION STIR WELDED BUTT JOINTS OF AEROSPACE
ALUMINUM ALLOYS

A Thesis

Submitted to the Graduate Faculty of the
Louisiana State University and
Agricultural and Mechanical College
in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical Engineering

in

The Department of Mechanical Engineering

by
Vinay Raghuram
B.S., R.V. College of Engineering, Visweswaraiah Technological University, 2003
December 2009
ACKNOWLEDGEMENTS
I would like to express my deepest , and most sincere gratitude to my supervisor Dr. M.
A. Wahab, Associate Professor, Department of Mechanical Engineering, LSU for his continuous
guidance, encouragement, , and sharing valuable time throughout the work. It is my pleasure to
acknowledge his untiring help by supplying supporting valuable references, information, and
financial support, without which this work could not have been completed.
I would like to thank Dr. Guoquiang Li, and Dr. Shengmin Guo for being my thesis
committee members, and for their valuable comments and suggestions, which has certainly
improved the quality of this work. I would like to extend special thanks to Laspace for
sponsoring the research, Lockheed Martin for providing the Al-2195 T8, Al-2219-T8 material ,
and NASA/NCAM-New Orleans authorities for providing the facilities for the FSW. My special
thanks to ME staff Mr. Charlie Smith and Mr. Mark Brennen for their help with the Universal
Testing Machine (MTS) in all my experimental tests. I also wish to thank my colleagues and
staff of the Department of Mechanical Engineering for their sincere co-operation during this
research work.
I am grateful to my parents Mr. Raghuram Rao and Mrs. Sheela Raghuram for being
there, always for me, and for their continued encouragement. I would like to thank my beloved
wife, Bharathi for being a great support throughout. I am thankful to all my friends; Harsha
Chatra, Shrinidhi Shetty, Prasad Kalghatgi, Sunada Chakravarthy, Ajay Kardak, Ashok
Badigannavar, and many more; here in Baton Rouge for their help in keeping my high spirits ,
and making my stay so enjoyable.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS........................................................................................................ ii
NOMENCLATURE .................................................................................................................. vi
ABSTRACT ............................................................................................................................. ix

1 INTRODUCTION ...............................................................................................................1
1.1 Introduction to Fatigue Crack Growth ...........................................................................1
1.2 Fatigue Crack Growth Behavior of Engineering Metals.................................................2
1.3 Damage Tolerant Design ...............................................................................................3
1.4 Motivation and Objectives ............................................................................................5
2 LITERATURE REVIEW ....................................................................................................7
2.1 Welding Al-Li Alloys ...................................................................................................7
2.2 Problems with Welding Aluminum Alloys ....................................................................7
2.3 Friction-Stir-Welding (FSW) ...................................................................................... 11
2.4 Microstructural Features ............................................................................................. 15
2.5 Theory and Mechanisms of Fatigue Failure ................................................................. 17
2.6 Intrinsic and Extrinsic Mechanisms ............................................................................. 18
2.7 Environment and Corrosion Fatigue ............................................................................ 20
2.8 Variables Affecting Corrosion Fatigue ........................................................................ 23
2.8.1 Effect of Fatigue Frequency ................................................................................. 23
2.8.2 Effect of Environment .......................................................................................... 23
2.8.3 Effect of Waveform ............................................................................................. 25
2.8.4 Effect of Temperature .......................................................................................... 26
2.8.5 Metallurgical Variables ........................................................................................ 26
2.9 Effect of Corrosion-Prevention-Compounds (CPC) on Fatigue Crack Growth ............. 27
2.10 Materials for Aerospace Structures .......................................................................... 28
2.10.1 Aluminum Alloy 2219 ......................................................................................... 28
2.10.2 Aluminum-Lithium Alloy (Al-2195) .................................................................... 29
2.11 Crack-Growth-Rates ................................................................................................ 31
2.12 Crack Closure Mechanisms ..................................................................................... 32
2.13 Plate-Orientation Effects.......................................................................................... 34
2.14 Fatigue Crack Growth Under Overload and Variable Amplitude Conditions ............ 34
2.15 Crack Growth Retardation Due to Overloads (OL) .................................................. 37
2.16 Numerical Analysis ................................................................................................. 44
2.17 Finite Element Simulation of Fatigue Crack Growth ................................................ 46
2.18 Weld Residual Stress (WRS) and Its Effect on Fatigue Life ..................................... 51
2.19 Weld Defects and Weld Metallurgy ......................................................................... 53
2.20 Conclusions from Literature Review........................................................................ 53

3 EXPERIMENTAL program .............................................................................................. 55
3.1 Introduction ................................................................................................................ 55
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3.2 Test Specimen ............................................................................................................. 55
3.3 Recommended Specimen Configuration & Size .......................................................... 57
3.4 Materials and Welding Procedure ................................................................................ 58
3.5 Self-Reacting FSW (SRFSW) ..................................................................................... 59
3.6 Experimental Setup ..................................................................................................... 61
3.7 Effect of Periodic Overloads on Fatigue Life ............................................................... 62
3.8 Corrosion Fatigue Crack Growth Test Method ............................................................ 64
3.9 Fracture Mechanics Approach to Analyzing Fatigue Data ........................................... 65
3.10 Corrosion Chamber ................................................................................................. 66
3.11 Corrosive Environment ............................................................................................ 66
3.12 Analysis of Fracture Surface .................................................................................... 68
3.13 Characteristics of LPS-3 Heavy-Duty Inhibitor (CPC) ............................................. 69

4 EXPERIMENTAL RESULTS AND DISCUSSIONS ........................................................ 71
4.1 Fatigue Test Objectives ............................................................................................... 71
4.2 Fatigue Life Results and Analysis ............................................................................... 72
4.3 Effect of Overloads on Fatigue Life ............................................................................ 75
4.4 Fatigue Crack Growth Characteristics ......................................................................... 81
4.5 Micrographs and Analysis ........................................................................................... 83
4.6 Conclusions ................................................................................................................ 98

5 Numerical Analysis ......................................................................................................... 100
5.1 Introduction .............................................................................................................. 100
5.2 Finite Element Programming..................................................................................... 102
5.2.1 Element Type ..................................................................................................... 102
5.2.2 Equation Solver ................................................................................................. 102
5.2.3 Symmetrical Analysis ........................................................................................ 103
5.2.4 Material Properties ............................................................................................. 103
5.2.5 Meshing ............................................................................................................. 103
5.2.6 Crack-tip Propagation ........................................................................................ 105
5.2.7 Load Step Increment .......................................................................................... 105
5.3 Traditional FEA Results ............................................................................................ 107
5.4 Overload Analysis ..................................................................................................... 110
5.5 Interface Element Technique ..................................................................................... 112
5.6 Traditional and New Approach ................................................................................. 116
5.7 Theoretical Formulation for The Interface Element Method ...................................... 117
5.8 Finite Element Method for Interface Element Technique ........................................... 121
5.9 Modeling Crack-tip Singularity in FEA ..................................................................... 123
5.10 Extraction of Stress Intensity Factor SIF ................................................................ 128
5.11 Overall Methodology Followed for Fatigue Life Calculation ................................. 129
5.12 Results and Discussions ......................................................................................... 133
5.13 Convergence.......................................................................................................... 138
5.14 Validation .............................................................................................................. 139
5.15 Conclusions ........................................................................................................... 139

6 Conclusions and Recommendations ................................................................................. 141
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6.1 Conclusions .............................................................................................................. 141
6.2 Recommendations for Future Work ........................................................................... 143

REFERENCES ......................................................................................................................... 145
VITA….. .......... ........................................................................................................................ 153

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NOMENCLATURE
Symbol Definition
a Half crack length for a central crack, constant
a Final crack length
f
a Initial crack length
o
B Breadth, width of plate
c A constant
C Material dependent constant
C Specific heat
p
da /dN Crack growth rate (crack length per cycle)
E Modulus of elasticity
f Load vector, restraining force
h Tangent modulus
t
DXZ Dynamically Stirred action zone.
HAZ Heat-affected zone.
TMAZ Thermo-mechanical affected zone.
K Stress intensity factor (MPa√m) (SIF)
K Fracture toughness
c
K Critical stress intensity factor
crit
K Maximum stress intensity factor
max
K Minimum stress intensity factor
min
K Crack opening stress intensity factor
open
vi
K Spring stiffness
s
K Theoretical stress concentration factor
th
ΔK Range of stress intensity factor
ΔK Range of threshold stress intensity factor
th
L Length, latent heat of fusion
m Material dependent constant
m (x, a) Weight function
N Number of cycles
n Shape parameter
N Shape function
i
N Fatigue crack propagation life
p
r Distance from crack-tip
r Overload radius
ol
R Stress ratio
r Scale parameter
o
r Plastic zone radius
p
S Nominal stress
S Yield stress (same as ζ)
ys y
t Thickness of plate
ΔS Range of stress
Π Total energy
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α Weld toe angle
δ Crack opening displacement
ε Strain
φ Lennard-Jones surface potential
γ Surface energy per unit area
η Natural (local) coordinate, efficiency of welding
κ A coefficient, constant
μ Shear modulus
ν Poisson‟s ratio
θ Angle in radian
π Density
ζ Nominal stress (local)
ζ Critical bonding strength
cr
ζ Yield stress (same as S )
y ys
ζ Crack opening stress
οpen
ξ Natural (local) coordinate

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ABSTRACT
Friction-Stir-Welding (FSW) has been adopted as a major process for welding Aluminum
aerospace structures. Al-2195, which is one of the new-generation Aluminum alloys that has
been used on the external tank of the new super lightweight external tank of the space shuttle.
The Lockheed Martin Space Systems (LMSS), Michoud Operations in New Orleans is
continuously pursuing Friction-Stir-Welding technologies in its efforts to advance fabrication of
the external tanks of the space shuttle. The future launch vehicles which will have to be reusable,
m, an dates the structure to have good fatigue properties, which prompts an investigation into the
fatigue behavior of the friction-stir-welded aerospace structures. The butt joint specimens of Al-
2195 and Al-2219 are fatigue tested according to ASTM-E647. The effects of: (i) Stress ratios,
(ii) Corrosion Preventive Compound (CPC), and (iii) Periodic Overloading on fatigue life are
investigated. Scanning electron microscopy (SEM) is used to examine the failure surface, and
examine the different modes of crack propagation i.e. tensile, shear, and brittle modes. It is found
that fatigue life increases with increase in stress ratio; the fatigue life increases from 30-38%
with the use of CPC, the fatigue life increases 8-12 times with periodic overloading, , and crack
closure phenomenon predominates the fatigue facture.
Numerical Analysis in FEA has been used to model a fatigue life prediction scheme for
these structures, the interface element technique with critical bonding strength criterion for
formation of new surface has been used to model crack propagation. The Linear Elastic Fracture
Mechanics (LEFM) stress intensity factor is calculated using FEA, and the fatigue life
predictions made using this method are within acceptable 10-20% of the experimental fatigue
life obtained. This method overcomes the limitation of the traditional node release scheme, and
closely matches the physics of crack propagation.
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