Fatigue Crack Propagation Behaviour of Welded and Weld Repaired 5083 Aluminium Alloy Joints

155 Pages·2012·2.02 MB·English

by  Wu Weidong

School of Aerospace and Mechanical Engineering
University College
The University of New South Wales

Fatigue Crack Propagation Behaviour of Welded and Weld
Repaired 5083 Aluminium Alloy Joints

A thesis subm it for the degree of
Master of Me chanical Engineering

Weidong Wu

June 2002
DECLARATION

I hereby declare that this submission is my own work and to the best of my knowledge
it contains no materials previously published or written by another person, nor
materials which to a substantial extent has been accepted for the award of any other
degree or diploma at UNSW or any other educational institution, except where due
acknowledgment is made in the thesis. Any contribution made to the research by
colleagues, with whom I have worked at UNSW or elsewhere, during my candidature,
is fully acknowledged.

I also declare that the intellectual content of this thesis is the product of my own work,
except to the extent that assistance from others in the project(cid:146)s design and conception or
in style, presentation and linguistic expression is acknowledged.

Signature ___________________

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ABSTRACT

Welding, as one of the most effective joining methods for metals, has been extensively
applied in engineering usage for a long time. When cracks occur in the vicinity of
weldments, weld repairs are frequently considered for crack repair to extend service
life. In order to evaluate to what extent the weld repair has improved the fatigue life of
a cracked welded structure, it is necessary to be able to determine the residual life of
the cracked welded joint, as well as the life of the weld repaired joint. Both these
assessments require that the fatigue crack growth data be available. The determination
of crack propagation rates of welded and weld repaired structures is thus of paramount
importance to implement a damage tolerant approach to structural life extension.
However, since most studies on welded joints so far have concentrated on fatigue life
evaluation, at the present time only limited information is available on crack
propagation rates in welded joints, and virtually none on fatigue behaviour and crack
propagation in weld repaired joints. This thesis has focused on examination of fatigue
and crack propagation behaviour in as welded and weld repaired aluminium alloy 5083,
a weldable marine grade alloy extensively used in construction of high speed ferries
and aerospace structures.

Crack growth rates were measured during constant amplitude fatigue testing on
unwelded, as-welded and weld repaired specimens of 5083-H321 aluminium alloy. A
3-D finite element analysis was conducted to determine the stress intensity factors for
different lengths of crack taking into account the three-dimensional nature of the weld
profile. The effects of crack closure due to weld residual stresses were evaluated by
taking measurements of the crack opening displacements and utilised to determine the
effective stress intensity factors for each condition. Metallurgical examinations and
fractography of the fracture surface were conducted using an optical microscope and
SEM. It was found that crack growth rates in welded plates are of the same order of
magnitude as those of parent material when effective stress intensity factors were
applied. However weld repaired plates exhibit higher crack growth rates compared to
those of unwelded and once-only welded plates.

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ACKNOWLEDGEMENTS

I would like to acknowledge my supervisor, Dr. Krishna Shankar, for his tireless
supports, encouragement and inspiration through out this research project. I would also
like to thank Mr. Alan Fien, who was my co-supervisor, for his kind assistance and
valuable pieces of advice. It is also a pleasure to record my appreciation to the
following staff in the school of Aerospace and Mechanical Engineering who
contributed much to this research project.

Professor Joseph Lai and Professor John Baird for their endeavouring supports
on this project.

Mr. Tony Bennett for assistance with Hardness tests and Strain Gage
installation.

Mr. Bob Clark and Mr. Pat Nolan for their help in all static and fatigue
experiments.

Mr. Tony Carthy, Mr Keiran and other workshop staff for their assistance in
material preparation and specimen processing.

Mr. Andrew Roberts and Mr. Errol Brown for providing electronic support in
this research project.

My special thanks go to my family, and in particular, my wife Mei Li for encouraging
me to undertake this research project.

This research was carried out with financial support from the Overseas Postgraduate
Research Scholarship (OPRS) and UNSW University College Postgraduate Scholarship
(UCPRS).
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TABLE OF CONTENTS
ABSTRACT..................................................................................................................iii
ACKNOWLEDGEMENTS.........................................................................................iv
TABLE OF CONTENTS...............................................................................................v
LIST OF TABLES........................................................................................................ix
LIST OF FIGURES........................................................................................................x
LIST OF PUBLICATIONS.......................................................................................xvi
NOMENCLATURE..................................................................................................xvii
CHAPTER 1 INTRODUCTION..................................................................................1
CHAPTER 2 LITERATURE REVIEW.....................................................................5
2.1. FATIGUE CRACK PROPAGATION..............................................................................5
2.2. LEFM AND STRESS INTENSITY FACTOR (SIF) CALCULATION................................7
2.3. CRACK CLOSURE AND EFFECTIVE SIF IN WELD RESIDUAL STRESS (WRS)..........10
2.3.1 Crack Closure and Effective SIF....................................................................10
2.3.2 Evaluation of Weld Residual Stress (WRS) Effects Using Crack Closure....12
2.4. FATIGUE CRACK GROWTH AND EVALUATION FOR WELD JOINTS.........................14
2.4.1 Approaches for Evaluation of Fatigue Crack Growth in Welded Joints........14
2.4.2 Fatigue Crack Growth in Welded Joints.........................................................15
2.4.3 Factors Affecting Fatigue Crack Growth in Welded Joints............................19
2.4.3.1 Weld Geometry Factors............................................................................20
2.4.3.2 Weld Residual Stress (WRS) and Effects on Fatigue Crack Growth.......22
2.4.3.3 Weld Defects and Weld Metallurgy.........................................................22
2.4.3.4 Materials and Welding Techniques..........................................................23
2.5 FATIGUE IN WELD REPAIRED JOINTS .....................................................................24
2.5.1 Weld Repaired Applications...........................................................................24
v
2.5.2 Fatigue Crack Growth in Weld Repaired Joints.............................................25
2.6 THE 5000 SERIES ALUMINIUM ALLOY...................................................................26
CHAPTER 3 FINITE ELEMENT MODELLING FOR STRESS INTENSITY
FACTOR SOLUTIONS...............................................................................................29
3.1 INTRODUCTION.......................................................................................................29
3.2 ASSUMPTIONS FOR FINITE ELEMENT MODEL.........................................................30
3.3 STRESS FIELD AT THE CRACK TIP...........................................................................30
3.4. FINITE ELEMENT SOFTWARE.................................................................................34
3.5. CONSTRUCTION OF 3-D SINGULAR ELEMENTS AT THE CRACK TIP.......................34
3.6. VALIDATION OF SIF CALCULATION IN 3-D MODEL...............................................36
3.7 FINITE ELEMENT MODEL OF CRACKS IN WELDED SPECIMEN.................................39
3.8 EFFECT OF WELD GEOMETRY ON STRESS INTENSITY FACTOR SOLUTIONS............42
CHAPTER 4 SPECIMEN DESIGN AND MANUFACTURE................................45
4.1. TEST MATERIAL (cid:150) COMPOSITION AND PROPERTIES..............................................45
4.2 WELDING PROCESSES.............................................................................................46
4.2.1 Weld Parameters.............................................................................................46
4.2.2 Manufacture of Initial Butt Weld Joints.........................................................50
4.2.3 Weld Repair on Welded Joints with Cracks...................................................51
4.3. TEST SPECIMENS...................................................................................................54
4.3.1 Static Test Specimens.....................................................................................54
4.3.2 Fatigue Crack Growth Test Specimens..........................................................55
CHAPTER 5 EXPERIMENTAL PROCEDURES...................................................62
5.1 STATIC TESTS.........................................................................................................62
5.2 HARDNESS TESTING AND WELD METALLURGY EXAMINATION..............................63
5.3 FATIGUE CRACK GROWTH (da/dN) EXPERIMENTS.................................................65
5.3.1 Testing Machine.............................................................................................65
5.3.2 Test Procedures...............................................................................................66
5.3.3 Crack Growth Test Matrix..............................................................................67
5.4. CRACK LENGTH MEASUREMENT...........................................................................68
5.5 CRACK TIP OPENING DISPLACEMENT MEASUREMENT...........................................70
5.6 CRACK GROWTH DATA PROCESSING.....................................................................72
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5.7 CRACK CLOSURE CALCULATION............................................................................72
5.8 SCANNING ELECTRONIC MICROSCOPE (SEM) FATIGUE FRACTURE ANALYSIS......75
CHAPTER 6 STATIC AND HARDNESS TEST RESULTS..................................76
6.1 STATIC TENSION PROPERTIES.................................................................................76
6.2 HARDNESS TESTS...................................................................................................80
CHAPTER 7 DETERMINATION OF CRACK CLOSURE EFFECTS...............84
7.1 ESTIMATION OF CRACK OPENING STRESS AND EFFECTIVE SIF RANGE.................84
7.2 EFFECTIVE STRESS INTENSITY FACTOR RANGE FOR UNWELDED SPECIMENS.........85
7.3 EFFECTIVE STRESS INTENSITY FACTOR RANGE FOR AS-WELDED SPECIMENS.......86
7.4 EFFECTIVE STRESS INTENSITY FACTOR RANGE FOR WELD REPAIRED SPECIMENS 89
7.5 COMPARISON OF EFFECTIVE SIFS FOR UNWELDED, AS-WELDED AND WELD
REPAIRED SPECIMENS...................................................................................................91
CHAPTER 8 ANALYSIS OF CRACK GROWTH DATA.....................................93
8.1 INTRODUCTION.......................................................................................................93
8.2 CRACK LENGTH AS A FUNCTION OF NUMBER OF CYCLE........................................93
8.3 CRACK PROPAGATION IN PARENT MATERIAL........................................................95
8.4 CRACK PROPAGATION RATE OF AS-WELDED PLATES............................................97
8.4.1 Comparison of Crack Growth Rates (CGRs) in Pro and Anti Direction........97
8.4.2 Comparison of Crack Growth Rates (CGRs) of As-Welded with Unwelded
Applying Nominal Stress Intensity Factor (SIF).....................................................98
8.4.3 Comparison of Crack Growth Rates (CGRs) of Welded with Unwelded Plates
Applying Effective Stress Intensity Factor (SIF)....................................................99
8.5 CRACK PROPAGATION RATES IN WELD REPAIRED PLATES..................................101
8.5.1 Comparison of Crack Growth Rates (CGRs) between Single Sided Weld
Repair (SSWR) and Double Sided Weld Repair (DSWR) in (cid:1)K and (cid:1)K ......101
nom eff
8.5.2 Comparison of Crack Growth Rates (CGRs) of Double Sided Weld Repair
(DSWR) with Unwelded and As-Welded in (cid:1)K and (cid:1)K ...............................103
nom eff
8.5.3. Paris Relation for Unwelded, As-Welded and Weld Repaired Specimens.106
CHAPTER 9 METALLURGICAL EXAMINATION AND FRACTOGRAPHIC
ANALYSIS..................................................................................................................109
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9.1 MICROSTRUCTURE EXAMINATION.......................................................................109
9.2 FRACTOGRAPHIC ANALYSIS.................................................................................111
CHAPTER 10 CONCLUSIONS AND RECOMMENDATIONS..........................114
10.1 CONCLUSIONS....................................................................................................114
10.2 LIMITATIONS OF THE PRESENT WORK................................................................116
10.2 RECOMMENDATIONS AND FURTHER WORK(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)(cid:133)117

REFERENCES...........................................................................................................119
APPENDIX A (cid:150) TEST DATA...................................................................................128
A.1 STATIC TEST RESULTS.........................................................................................128
A.2 DIMENSIONS OF WELD REINFORCEMENTS IN WELDED SPECIMENS.....................130
APPENDIX B COMPUTER PROGRAM FOR CREATION OF 3-D MODELS
AND STRESS INTENSITY CALCULATION........................................................131
B.1 MACRO PROGRAM FOR STRESS INTENSITY CALCULATION .................................133

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LIST OF TABLES

Table 4.1 Typical chemical composition of 5083-H321 plate.......................................45
Table 4.2 Typical mechanical properties of 5083-H321 plate.......................................46
Table 4.3 Welding parameters of weld-only and weld repair process...........................49
Table 5.1 Typical technical parameters of applied strain gauges for static tests...........62
Table 5.2 Test matrix for static tests..............................................................................63
Table 5.3 Chemical composition of etchant for weld metallography.............................65
Table 5.4 Fatigue test parameters...................................................................................67
Table 5.5 Fatigue crack growth test plan........................................................................68
Table 6.1 Average static mechanical properties of unwelded, as-welded and weld
repaired specimens.........................................................................................................79
Table 6.2 Half range of HAZ for as-welded and weld repaired plates...........................83
Table 7.1 Empirically determined values for the constants C and C in equation 7.1 for
1 2
all the different types of specimens................................................................................91
Table 8.1 Values of Paris coefficient and exponent using nominal SIF......................106
Table 8.2 Values of Paris coefficient and exponent using effective SIF......................107
Table A 1. Static properties for unwelded, as-welded and weld repaired plates..........128
Table A2. Dimensions of weld reinforcements in welded specimens..........................130

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LIST OF FIGURES

Figure 2.1 Typical fatigue crack growth rate curve..........................................................5
Figure 2.2 D. Bowness 3-D FE model for stress intensity factor calculation in T-butt
welded joints...................................................................................................................10
Figure 2.3 Weld toe magnification factor change for semi-elliptical crack...................10
Figure 2.4 Definition of effective stress intensity range................................................11
Figure 2.5 Typical longitudinal WRS distribution at butt weld (a) mild steel (b)
aluminium alloy (c) high alloy structural steel and (d) filler metal................................12
Figure 2.6 Crack opening load and crack opening displacement curve.........................15
Figure 2.7 Flow chart for predicting crack growth rate in WRS....................................17
Figure 2.8 Weld geometry parameters at butt weld joints..............................................20
Figure 3.1 Three types of loads that can be applied to a crack......................................30
Figure 3.2 Stress field at crack tip..................................................................................31
Figure 3.3 Stress normal to crack plane in mode I loading............................................33
Figure 3.4 20 nodes brick element collapse into wage elastic singular element............35
Figure 3.5 Elastic singularity element of 3-D FE model for welded plate.....................35
Figure 3.6 Mesh near crack tip on 2-D FE model of flat plate.......................................37
Figure 3.7 Mesh in the vicinity of crack tip on 2-D FE model of flat plate...................37
Figure 3.8 Stress (X-direction) distribution in the vicinity of crack tip for flat plate (2-D
FE model).......................................................................................................................38
Figure 3.9 3-D FE model (mesh) for flat plate...............................................................38
x