ROSLINDA BINTI IDRIS
Transcript of ROSLINDA BINTI IDRIS
MODELING AND EXPERIMENTAL VERIFICATION OF MULTIPHASE
STEEL FOR COMPONENTS SUBJECTED TO FATIGUE LOADING
ROSLINDA BINTI IDRIS
UNIVERSITI TEKNOLOGI MALAYSIA
MODELING AND EXPERIMENTAL VERIFICATION OF MULTIPHASE
STEEL FOR COMPONENTS SUBJECTED TO FATIGUE LOADING
ROSLINDA BINTI IDRIS
A thesis submitted in fulfillment of the
requirements for the award of the degree of
Master of Mechanical Engineering
Faculty of Mechanical Engineering
Universiti Teknologi Malaysia
DECEMBER 2012
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ACKNOWLEDGEMENT
I would like to thank and express my utmost sincere gratitude and appreciation to my
supervisor Assoc. Prof. Dr. Yunan Prawoto for his invaluable guidance, significant
comments and essential suggestions throughout the project. Not to forget my co-
supervisors, Dr. Nazri Kamsah and Prof. Dr. Mohd Nasir Tamin for their generosity
in providing me continuous support, advice, and encouragements had been a great
motivation to me in completing the project.
Many kind of friends have contributed to my study in the subject attempted here.
Among them are Mohammad Azfar Che Azmi, Computational and Solid Mechanics
(CSM) - lab members and FKM technicians who were always generous enough to
offer their guidance and assistance whenever I need them.
Last but not least, I would like to express a heartfelt “thank you” to my family for
their overwhelming love, patience, and support throughout the project. To all my
friends and course-mates, I am so grateful and sincerely appreciate them for being so
willingly to share their precious knowledge and experience with me, supporting me
always and being so helpful to me in times of need during the entire journey of
completing this thesis.
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ABSTRACT
This research is done to acquire microstructure combinations that resist fatigue
loading more than typical conventional microstructure (tempered martensite) and to
study the effects of microstructure on fatigue properties of multiphase steels. A
multiphase (polygonal ferrite and martensite) microstructure is developed. 2D and 3D
square models with variation of ferrite fraction are used as local models and placed in
front of the CT specimen in a global model. The ferrite shapes are from actual
microstructure of multiphase material. It is found that the plastic zone size changes, as
the ferrite fraction varies, and saturated at approximately 65% and 60% for 2D and 3D
modelling, respectively. The influence of a ferrite areal fraction within a martensite
matrix on fatigue crack propagation is studied. The variation of the areal fraction is
achieved by means of intercritical thermal treatment, which specifically aims at
optimizing the resistance to fatigue loading. The steels are annealed at different
temperatures followed by water quenching and tempering process. Within the
intercritical annealing temperature range, the areal fraction of ferrite increases with
decreasing soaking temperature. Fatigue crack propagation tests are conducted
according to ASTM E647-00 to obtain fatigue crack growth, FCG behaviour. It is found
that the highest fatigue strength is achieved when the ferrite areal fraction is
approximately 65%, which in this particular test, corresponds to 748 0C annealing
temperature. It is concluded and is verified by computational modelling that appropriate
thermal treatment can contribute to a significant improvement of fatigue properties and
strength. The optimum ferrite fraction found from both computation and experiment is
approximately 60% –65%.
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ABSTRAK
Kajian ini dijalankan untuk memperoleh kombinasi mikrostruktur yang
menentang kelesuan lebih daripada mikrostruktur tipikal konvensional (martensit
terbaja) dan untuk mengkaji kesan mikrostruktur pada sifat lesu keluli berbilang. Oleh
itu, mikrostruktur berbilang (poligon ferit dan martensit) telah dibangunkan. Model
segiempat 2D dan 3D dengan variasi pecahan ferit telah digunakan sebagai model
setempat dan diletakkan di hadapan spesimen CT dalam model global. Dalam hal ini,
bentuk ferit terbentuk daripada mikrostruktur sebenar bahan berbilang. Didapati bahawa
perubahan saiz zon plastik sebagai pecahan ferit berbeza, dan tepu pada anggaran 65%
dan 60% masing-masing bagi model 2D dan 3D. Seterusnya pengaruh keluasan pecahan
ferit dalam matriks martensit pada perambatan retak lesu telah juga dikaji. Perubahan
keluasan pecahan dicapai dengan cara rawatan pengkhususan haba, yang bertujuan
untuk mengoptimumkan rintangan beban kelesuan. Keluli disepuh lindap pada suhu
yang berbeza diikuti oleh pelindapkejutan air dan proses pembajaan. Dalam julat suhu
penyepuhlindapan kritikal, keluasan pecahan ferit meningkat dengan pengurangan suhu
rendaman. Ujian kelesuan perambatan retak telah dijalankan mengikut ASTM E647-00
untuk mendapatkan sifat pertumbuhan retak lesu, FCG. Didapati bahawa kekuatan
kelesuan tertinggi tercapai apabila keluasan pecahan ferit adalah dalam anggaran 65%,
di mana dalam ujian tertentu, ia sepadan dengan suhu 748 0C. Dirumuskan dan disahkan
oleh model pengiraan bahawa rawatan haba yang sesuai telah menyumbang kepada
peningkatan ketara kepada sifat kelesuan dan kekuatan. Pecahan ferit yang optimum
didapati dengan cara pengiraan dan eksperimen dianggarkan berada pada julat 60% -
65%.
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TABLE OF CONTENTS
CHAPTERS TITLES PAGES
DECLARATION ii
ACKNOWLEDGEMENTS iii
ABSTRACT iv
ABSTRAK v
TABLE OF CONTENTS vi
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF ABBREVIATIONS xiv
LIST OF SYMBOLS xv
LIST OF PUBLICATIONS xvi
1 INTRODUCTION
1.1 Introduction 1
1.2 Objective of the Research 4
1.3 Scope of the Research 4
1.4 Methodology of the Research 5
1.5 Significant of the Research 6
1.6 Thesis Structure 7
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1.7 Summary 8
2 LITERATURE REVIEW
2.1 Introduction 9
2.2 Mechanisms of Fracture 9
2.2.1 Cleavage Fracture 10
2.2.2 Ductile Fracture 11
2.2.3 Fracture Surface Measurements 13
2.3 Recent Developments of Research in Dual Phase Steel 14
2.3.1 Through observation of chemical metallurgy 15
2.3.2 Through observation of dislocation 15
2.3.3 Through thermal treatment 15
2.3.4 Through microstructure research with a focus on
morphology 16
2.4 The Relationship between Fatigue Crack Growth Rate
and Striation Spacing 16
2.5 Fatigue Cracking 17
2.5.1 Fatigue Crack Initiation 18
2.5.2 Fatigue Crack Propagation 19
2.6 The Effect of the Plastic Zone 20
2.7 The Shape of Plastic Zone 21
2.8 Stress Intensity Factor 22
2.9 Paris Law Theory 24
2.10 Finite Element Analysis 25
2.11 Plain Carbon Steel 27
2.11.1 Low Carbon Steels 27
2.11.2 Medium-carbon Steels 29
2.11.3 High-carbon Steels 30
2.12 Dual Phase Steel 31
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2.13 Intercritical Heat Treatment 33
2.14 Summary 34
3 RESEARCH METHODOLOGY
3.1 Introduction 35
3.2 Computational Method 35
3.2.1 Global Model 36
3.2.2 Local Model 38
3.2.3 Two-dimensional, 2D Method 39
3.2.3.1 Developing FEA Model 40
3.2.3.2 Estimation of the Plastic Zone Area 41
3.2.3.3 FEA of Homogeneous and Multiphase
Material 42
3.2.4 Three-dimensional, 3D Method 43
3.2.4.1 Developing FEA Model 45
3.2.4.2 Estimation of the Plastic Zone Volume 45
3.2.4.3 FEA of Homogeneous and Multiphase
Material 46
3.3 Experimental Procedure 50
3.3.1 Material Selection 51
3.3.2 Determination of Chemical Composition 52
3.3.3 Preliminary Heat Treatment 52
3.3.4 Sample Preparation 53
3.3.5 Heat Treatment Process 55
3.3.6 Metallographic studies 56
3.3.7 Fatigue Crack Propagation Test 58
3.3.8 Fractography 60
3.4 Summary 62
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4 RESULTS AND DISCUSSION
4.1 Introduction 63
4.2 Analysis Results for 2D Modeling 63
4.2.1 Strain Components at Integration Points of Global
Model 64
4.2.2 Plastic Zone Sizes and Shape 65
4.2.3 FEA of Multiphase material 65
4.3 Analysis Results for 3D Modeling 76
4.3.1 Strain Components at Integration Points of Global
Model 77
4.3.2 Plastic Zone Sizes and Shape 79
4.3.3 FEA of Multiphase material 80
4.4 Composition Test Analysis 85
4.5 Intercritical Annealing Temperature Determination 85
4.6 Microstructure Study 86
4.7 Fatigue Crack Propagation Test Results 90
4.8 Fractography 95
4.9 Comparison Between Experimental and Computation 102
4.10 Summary 102
5 CONCLUSIONS
5.1 Introduction 103
5.2 Conclusions 104
5.3 Suggestions for Future Work 106
REFERENCES 107
APPENDICES
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LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Classification of Plain Carbon Steels 27
3.1 Material Property of Elastic-Plastic 37
3.2 Material Property of Microstructure for 2D and 3D model 39
3.3 The element pictures with various percentage of ferrite for
2D model 42
3.4 Various shape of ferrite 48
4.1 Results of the plain strain and plastic zone of different
percentage of ferrite 66
4.2 The results of the von Mises stress and strain components 72
4.3 Chemical composition of the steel, wt% 85
4.4 Ferrite areal fraction 85
4.5 Conditions of all samples at all stages and their ranks 93
4.6 Variation of threshold stress intensity factor range, ∆KTH
and Paris coefficient, m 94
4.7 Variation of fatigue striation spacing constant 100
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LIST OF FIGURES
FIGURE NO. TITLE PAGE
1.1 Example of car parts needs to improve the fatigue life 2
1.2 Schematic plot of the area grouping and their mechanisms 3
2.1 Brittle-ductile transition of steel 11
2.2 The steps in ductile fracture (in pure tension) 12
2.3 Wood’s model for fatigue crack initiation 18
2.4 Possible model for fatigue crack growth 19
2.5 Plastic zone shapes according to Von Mises criteria 21
2.6 The Paris law for fatigue crack growth rates 24
2.7 Microstructure of low carbon steel 28
2.8 Microstructure of medium-carbon steel 29
2.9 Microstructure of high-carbon steel 30
2.10 Dual phase microstructure 31
2.11 Formation of austenite, ferrite and martensite during heating
and cooling 33
3.1 Global model with load and boundary condition :
(a) 2D model (b) 3D model 37
3.2 Concept of the modeling used to compute and compare the
morphology and the plastic zone size in front of the crack tip 40
3.3 Finite element modeling of the ASTM 647 sample. The
multiphase microstructure (local model) is embedded to
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the sample (global model) 41
3.4 The modeling concept used to compute and compare the
morphology and the plastic zone size in front of the crack tip 44
3.5 Basic creation of the polygonal ferrite 47
3.6 Examples of the multiphase material model for different level
of ferrite. The ferrite colonies are embedded inside the
martensite matrix 49
3.7 Summary of the experiment procedure 50
3.8 Iron carbon phase diagram (the red circle shows the area of
interested) 51
3.9 Compact tension, CT specimen (dimensions in mm) 54
3.10 Schematic heat treatment diagrams 55
3.11 Performing area counts 57
3.12 Fatigue test machine (INSTRON 8801) 58
3.13 Example of fractured specimen 59
3.14 Areal of observation for striation 61
4.1 Strain Components at Integration Points of Global Model for
2D model 64
4.2 Plastic zone size and shape of global model for 2D model 65
4.3 Relation between ferrite fraction and plastic zone sizes 76
4.4 Strain Components at Integration Points of Global Model for
3D model 78
4.5 Plastic zone size and shape of global model for 3D model 79
4.6 Example of Von-Mises stress distribution of the local model
at two different ferrite fractions 81
4.7 Summary of the plastic zone size computation results. It is
comparable with that of 2D and closer to the actual 82
4.8 Simplified explanation of dispersed and independently
distributed ferrite results in larger PZ than ferrite that are
touching each other and form colony 84
4.9 Microstructure of as-received materials, low carbon steel 86
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4.10 Optical micrograph of multi-phase material consists of
polygonal ferrite (white region) and martensite (dark region)
at different temperatures and consists of different ferrite
fraction 87
4.11 Crack size versus elapsed cycles data (a versus N) 91
4.12 Fatigue crack growth behavior of MPM samples 92
4.13 Variation of threshold stress intensity factor with areal
percentage of ferrite 95
4.14 Representative fractographs, on which the striations are
evaluate 96
4.15 Fatigue striation spacing constant, ζ versus ferrite fraction 101
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LIST OF ABBREVIATIONS
2D - Two-dimensional
3D - Three-dimensional
ASTM - American Society for Testing and Materials
CAE - Computer Aided Engineering
CT - Compact tension
DPS - Dual phase steel
FCG - Fatigue crack growth
FCP - Fatigue crack propagation
FEA - Finite element analysis
GDS - Glow discharge spectrometer
HSLA - High strength low alloy
IHT - Intercritical heat treatment
MPM - Multiphase material
MVC - Microvoid coalescence
MVF - Martensite volume fraction
PZ - Plastic zone
SEM - Scanning electron microscopy
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LIST OF SYMBOLS
a - Crack length
B - Specimen thickness
da/dN - Fatigue crack growth rate
E - Young modulus
Fe-C - Iron carbon
K - Stress intensity factor
KTH - Threshold stress intensity factor
m - Paris coefficient
N - Number of load cycles
P - Load
R - Load ratio
rp - radius of the plastic zone
W - Specimen width
α - Ferrite
γ - Austenite
σ - Stress
σys - Yield stress
ϴ - Angle
ζ - Fatigue striation spacing constant
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LIST OF PUBLICATIONS
JOURNAL NO. JOURNAL
1 Y. Prawoto, R. Idris, N. Kamsah, N. Tamin. Two-dimensional modeling
to compute plastic zone in front of compact tension sample of a
multiphase material. Computational Materials Science. (2009). 47: 482 -
490.
2 Y. Prawoto, R. Idris, N. Kamsah, N. Tamin. Three-dimensional modeling
to compute plastic zone in front of crack in compact tension sample of
multiphase material. Computational Materials Science. (2011). 50: 1499-
1503.
3 Roslinda Idris, Yunan Prawoto. Influence of ferrite fraction within
martensite matrix on fatigue crack propagation : An experimental
verification with dual phase steel. Material Science and Engineering A.
(2012). 552: 547-554.
CHAPTER 1
INTRODUCTION
1.1 Introduction
The microstructure of conventional steels often makes it impossible to obtain
concurrently good ductility and high strength. However, some applications, especially in
the transportation industries, require economical high strength steel with good
formability [1]. It is generally understood that among other forms of microstructure
available in industrial steel, martensite has the best tensile properties [2]. However, at
the same time it also has poor toughness due to its brittleness. It has been known and
practiced extensively that tempering increases the toughness while sacrificing the
material’s tensile properties. Tempered martensite, also formerly known as sorbite, is the
microstructure resulting from quenching followed by tempering.
To achieve weight reductions and fuel saving in the vehicles and to fulfill the
energy and resource requirements, a new kind of steel named as multi-phase steel was
developed. This type of steel exhibited a microstructure which was constituted basically
on ferrite and martensite. Multi-phase is of interest because of better ductility and could
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provide a high mechanical resistance than other steels. They are very important for the
automotive industry since they reduce weight and cost and also improve the fatigue life
of products of many car parts such as wheels, radiator support, doors, spring support, etc
(see Figure 1.1) [2]. The service life of many components of this type is dependent on
materials fatigue life which is an important consideration in material selection.
Figure 1.1 Example of car parts needs to improve the fatigue life [2]
Multi-phase steels can be obtained from the presence of martensitic and austenite
islands dispersed in a ferrite matrix. These steels are produced by transforming the
pearlite areas of ferrite-pearlite microstructure in low carbon steel to austenite by heating
into the ferrite-austenite (α-γ) region and followed by quenching process. There are
several factors that significantly affect the microstructure of multi-phase steel, namely
the chemical composition of steel, temperature of intercritical annealing and the rate of
cooling after annealing. The strength of such steels is determined mainly by the volume
fraction of martensite, whereas the ductility is determined by the volume fraction of
ferrite.
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There is no doubt that controlling and determining the FCP rate is the most
essential part of the fracture mechanics design approach. The best way to acquire it is by
knowing the metallurgical mechanisms as well as the continuum mechanics of fatigue
crack propagation. Nonetheless, it is difficult to combine these approaches since in the
area where the continuum mechanism approach can be used, the metallurgical influence
is small and vise versa, see Figure 1.2 [3]. Furthermore, research on microstructure often
lacks the continuum mechanics approach; the same way research on continuum
mechanics lacks microstructural observation.
Figure 1.2 Schematic plot of the area grouping and their mechanisms [3]
In this research, the conceptual design, as well as the computation results and
parts of the static and impact experiments are discussed lightly. In contrast, the
discussion is focused on the fatigue crack propagation experimental procedure and the
verification of the results with computational results. This research was done to obtain
Stage I
Non-continuum
mechanism.
Large influence
of:
-Microstructure
-Mean stress
-Environment
Stage II
Continuum
mechanism.
Little influence of:
-Microstructure
-Mean stress
-Environment
-Thickness
Stage III
Static-
mechanism.
Large influence
of:
-Microstructure
-Mean stress
-Thickness
Little influence
of:
-Environment
Consideration for
this research
Log
da
/dN
Log ∆K
4
an optimized sample that grains its fatigue performance from its high toughness yet
possesses high tensile properties due to its tailored microstructure. The objective
ultimately is to achieve a microstructure that resists fatigue more than the
microstructures conventionally available, and in an economical manner.
1.2 Objective of the Research
The main objective of the present research is to investigate and to provide a
better understanding of the effects of heat treatment on the mechanical properties of
multi-phase steels. The primarily objective of the research are:
1. To obtain microstructure combination that can resists to fatigue loading more
than typical conventional microstructure (tempered martensite) and the effect of
microstructure on fatigue properties of multi-phase steels.
2. To acquire the optimize phase composition of the microstructure which
presumably the combination between polygonal ferrite and martensite by means
of computational approaches.
3. To formulate heat treatment method for producing the optimum microstructure.
1.3 Scope of the Research
The scopes of this research work are:
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1. Model creation and computational approach by using finite element method
(Abaqus CAE) for two-dimensional, 2D modeling.
2. Model creation and computational approach by using finite element method
(Abaqus CAE) for three-dimensional, 3D modeling.
3. Sample making and heat treatment to produce multi-phase material.
4. Fatigue crack propagation test to get the optimized sample which have the best
characteristics resist to fatigue loading.
5. Fractography analysis by using Scanning Electron Microscope (SEM).
1.4 Methodology of the Research
To achieve the objective of this study, several methods have been used in the
implementation of this research. These methods can be briefly explained as follows.
a) Computational method
Two-dimensional, 2D and three-dimensional, 3D of plane strain models are developed
to estimate the plastic zone size in front of a crack. 2D and 3D square models with
variation of ferrite fraction are used as local models and placed in front of the CT
specimen in a global model. The model is developed and meshed in ABAQUS/CAE
(ABAQUS pre-processor).
b) Experimental procedure
i. Sample making and heat treatment
Compact tension samples appropriate for fatigue crack propagation and
static test are made prior to heat treatment.
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ii. Metallographic analysis
The microstructures of the samples are examined from the point of view
of microstructure and chemical composition in term of the specific
changes due to heat treatment. The quantitative approach is considered in
Chapter 3, using image analysis coupled with scanning electron
microscopy.
iii. Fatigue crack propagation testing
The heat treated samples are investigated with fatigue crack propagation
testing and are carefully examined in terms of microstructure and
mechanical properties, to achieve the optimized characteristics that resist
to fatigue loading.
iv. Fractography analysis
The fatigue striations spacing formed on the fracture surface of the
samples are analyzed using scanning electron microscopy (SEM). The
quantitative relationships between microstructure and mechanical
properties are investigated.
1.5 Significant of the Research
It is concluded that appropriate thermal treatment can contribute to a significant
improvement of fatigue properties and strength, which is verified by computational
modeling. The variations of the areal fractions are achieved by means of intercritical
thermal treatment, which specifically aims at optimizing the resistance to fatigue
loading.
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1.6 Thesis Structure
The thesis comprises of five chapters. A brief explanation on every chapter is
described as follows.
Chapter one, the introduction overviews the application and importance of multi-
phase steel and highlights it as an important element of the automobile industries. It
discusses the research objectives, scope, methodology, significant of the research and
thesis structure.
Chapter two highlighted the theory of fracture mechanics and the formation of
multi-phase steel that resist to fatigue loading. It covers the literature review on
metallurgical mechanism of fatigue cracking and methodology for assessment of the
influence of ferrite fraction within martensite matrix on fatigue crack propagation, the
literature on the formation of multi-phase steels that resist to fatigue loading and also
other researchers works on determination of the quantitative relationship of the multi-
phase steel for specific application.
Chapter three narrates in details the research methodology of this research. It
proposed improvement for life prediction methodology by discussing a research frame
work employed in this research and presenting step by step process in accomplishing the
results in the form of research methodology flow chart. This chapter gives adequate
details regarding computational approach and detailed experimental procedures carried
out throughout this research work.
Chapter four narrates the final discussion of this research. The results of the
computational method and the experimental data are discussed comprehensively and are
validated for model predictions.
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Finally, the conclusions for the whole research and future recommendations are
presented in chapter five.
1.7 Summary
In this chapter presented the definition to the problem, the objective and the
scope of the project. An explanation of the methodology and significant of the project
and also the report structure are enhanced in this chapter.
107
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