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

iii

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.

iv

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%.

v

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%.

vi

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

vii

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

viii

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

ix

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

x

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

xi

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

xii

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

xiii

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

xiv

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

xv

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

xvi

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

2

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.

3

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:

5

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.

6

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.

7

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.

8

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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