DUCTILE FAILURE ANALYSIS OF API STEEL PIPE USING STRAIN ...

DUCTILE FAILURE ANALYSIS OF API STEEL PIPE

USING STRAIN BASED FAILURE CRITERIA

SITI FIRDAUS BINTI MD SALLEH

Report submitted in partial fulfillment of

the requirement for the award of the degree of

Bachelor of Mechanical Engineering

BACHELOR OF MECHANICAL ENGINEERING

UNIVERSITI MALAYSIA PAHANG

JUNE 2013

vii

ABSTRACT

This project was performed to propose ductile failure criteria as a function of the

stress triaxiality for the API X42 steel pipes. The objective of this project is to determine

the burst pressure of modeled pipe using strain based failure criteria. In this project,

uniaxial tension test was performed using three types of specimen. The specimen extracted

from API X42 5L steel pipe. The steel pipe was machined to desired dimension and obeys

the international standard of ASTM-E8 specimen. Three type of specimen which is smooth,

notch radius 1.5 mm, 3 mm and 6 mm prepared and undergoes uniaxial tension test. The

engineering stress-stress data retrieved from the test converted to the true stress-strain

curve. The true stress-strain data become as an input data to the simulation analysis. Initial

and final diameter of specimen was taken to calculate the strain fracture of the pipe. Stress

modified critical strain criteria were proposed by using the strain fracture. The findings of

the main parameter which is burst pressure predictions precede by using FEA software.

Finite Element analysis was performed by using MSC Patran/Marc 2008r1 software. In

MSC Patran, the API X 42 steel pipes was modeled. Burst pressure predicted compared to

the available industrial pipe design assessment in order to validate the obtained results.

viii

ABSTRAK

Projek ini telah dilaksanakan untuk mencadangkan kriteria kegagalan mulur sebagai

tekanan fungsi triaxiality untuk paip keluli API X42. Objektif projek ini adalah untuk

menentukan tekanan pecah paip dimodelkan menggunakan kriteria kegagalan berasaskan

ketegangan. Dalam projek ini, ujian ketegangan ekapaksi dilakukan dengan menggunakan

tiga jenis viiipecimen. Spesimen dikeluarkan dari API 5L X42 paip keluli. Ia telah dimesin

untuk menjadi contoh dan menurut standard antarabangsa ASTM-E8. Tiga jenis

viiipecimen yang licin, jejari bertakuk 1.5 mm, 3 mm dan 6 mm disediakan dan menjalani

ujian ketegangan ekapaksi. Data tegasan-terikan kejuruteraan dari ujian, ditukar kepada

graf tegasan-terikan benar. Data telah ditukar dijadikan sebagai input data untuk analisis

simulasi. Diameter awal dan akhir viiipecimen telah diambil untuk mengira patah tekanan

paip. Terikan kriteria tegasan kritikal yang diubahsuai telah dicadangkan dengan

menggunakan tekanan patah. Hasil parameter utama iaitu tekanan ramalan pecah telah

diteruskan dengan menggunakan perisian FEA. Analisis Unsur Terhingga dilakukan

dengan menggunakan MSC Patran / Marc perisian 2008r1. Dalam MSC Patran, API X 42

paip keluli telah dimodelkan. Bagi mengesahkan keputusan yang diperolehi, tekanan letus

yang diramalkan, dibandingkan dengan penilaian reka bentuk paip industry.

ix

TABLE OF CONTENT

Page

EXAMINER’S DECLARATION ii

SUPERVISOR’S DECLARATION iii

STUDENT’S DECLARATION iv

DEDICATION v

ACKNOWLEDGE MENT vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENT ix

LIST OF TABLE xii

LIST OF FIGURES xiii

LIST OF SYMBOLS xvi

LIST OF ABBREVIATIONS xvii

CHAPTER 1 INTRODUCTION

1.1 Research Background 1

1.2 Problem Statements 2

1.3 Objectives 2

1.4 Scopes of Study 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.2 Fundamental of API Pipe 4

2.3 Pipelines 5

2.3.1 Defects On Pipes 6

x

2.4 Engineering Stress-Strain Curves 9

2.5 True Stress-Strain curve 11

2.6 Finite Element Analysis 15

2.7 Failure Criteria 17

2.7.1 Stress Modified Failure Strain Criteria 17

CHAPTER 3 METHODOLOGY

3.1 Introduction 20

3.2 Research Flow Chart 20

3.3 Specimen Preparation 23

3.4 Machining Process 24

3.5 Uniaxial Tension Test 27

3.6 Simulation 31

3.6.1 Development of failure criteria equation 33

CHAPTER 4 RESULTS AND DISSCUSSION

4.1 Introduction 36

4.2 Uniaxial Tension Test Result 36

4.2.1 Results from Uniaxial Tension Test 36

4.2.2 Engineering Stress-Strain curve 42

4.2.3 Conversion of Engineering stress-strain curve 44

4.3 Materials 45

4.4 Finite Element Analysis 46

4.4.1 Determination of Stress Modified Critical Strain Criteria 47

4.4.2 FE Analysis Results 51

4.5 Effect of Corrosion Defect on Burst Pressure 56

4.5.1 Burst Pressure on Different Depth Defect 57

4.5.2 Burst Pressure on The Different Length Defect 58

4.5.3 Burst Pressure On The Different Width Defect 58

xi

4.6 Comparison of FEA to the Available Code 59

4.6.1Variation of Burst Pressure on the Different Length Defect 60

4.6.2Variation of Burst Pressure on the Different Depth Defect 61

4.6.3Variation of burst pressure on the Different Width Defect 63

CHAPTER 5 CONCLUSION

5.1 Conclusion 65

5.2 Recommendation 66

REFERENCES 67

APPENDICES

Appendix A 68

Appendix B 69

Appendix C 70

Appendix D 71

Appendix E 72

Appendix F 73

xii

LIST OF TABLES

Table No. Page

2.1 Chemical Composition of API X42 steel pipe 5

2.2 Mechanical Properties of API X42 steel pipe 5

3.1 The standard requirement in ASTM E8 procedures for smooth

specimens

23

3.2 The chemical composition of the material selected 29

3.3 Design of pipe defect for FEA 31

4.1 Final area of the specimen 41

4.2 Chemical Composition 45

4.3 Mechanical properties 46

4.4 Failure results CS1 52









4.13 Burst pressure result for each case 56

4.14 Variation of burst pressure on selected design code 59

xiii

LIST OF FIGURES

Figure No. Pages

2.1 External corrosion defect on pipe 6

2.2 Corrosion on the pipeline 8

2.3 The irregular length, width and depth of a typical corrosion

defect

9

2.4 Stress-strain curve 10

2.5 True stress-strain curve 14

2.6 FE meshes for notched tensile bars, (a) notch = 0.2R

(b) notch = 1.5R (c) notch 3R

15

2.7 Comparison of experimental engineering stress-strain data for

smooth tensile bars with FE results

16

2.8 Equivalent strain and stress triaxiality distribution of smooth

tensile bars

18

2.9 Stress triaxiality distribution 19

3.1 The process flow chart 22

3.2 The standard dimension for notch specimen 23

3.3 API X42 steel pipe 24

3.4 Band saw machine 24

3.5 Raw material of API X42 25

3.6 Conventional Milling machine for facing purpose 25

3.7 Standard specimen 26

3.8 Lathe machine 26

3.9 Cutting tool for notch shaping 27

3.10 Ready specimens 27

3.11 Profile projector machine 28

3.12 Spectrometer machine 28

3.13 Uniaxial tension test machine, INSTRON 29

3.14 Testing of the smooth specimen 30

xiv

3.15 Testing on the notch specimen 30

3.16 Tested specimen 30

3.17 Selecting the analysis type and start to build the pipe

coordinate

32

3.18 (a)The process of curving the coordinate (b) checking the

element of surface to ensure the next solid extrude will have

no element inside out

32

3.19 Meshed one-four pipe with the simulated corrosion defect 33

3.20 Pipe with simulated corrosion defect 34

4.1 Smooth specimen with d = 5.77mm 37

4.2 Smooth specimen with d = 5.82mm 37

4.3 Notched R1.5 with d = 6.41mm 38

4.4 Notched R1.5 with d = 6.09mm 38

4.5 Notched R3 with d = 6.30mm 39




4.9 Notched R6 d = 6.20mm 41

4.10 Engineering stress-strain graph 42

4.11 (a)Engineering stress-strain graph for notched specimen R1.5

(b) R3 (c) R6

43

4.12 Engineering stress-strain graph for notched R1.5, R3,R6 and

smooth specimen

44

4.13 True plastic stress-strain curve for API X42 45

4.14 Comparison of uniaxial tension test results to FE analysis 46

4.15 (a)Fracture strain vs. stress triaxiality for CS1 (b)CS2 (c)CS3

(d)CS4 (e)CS5 (f)CS6 (g)CS7 (h)CS8 (i)CS9

50

4.16 (a) strain deformation of CS1 pipe (b) Stress deformation 52



xv







4.25 Burst pressure vs. depth of defect 57

4.26 Burst pressure vs. length of defect 58

4.27 Burst pressure vs. width of defect 58

4.28 (a)Burst pressure vs. length of defect d/t = 0.2 (b) d/t = 0.4 (c)

d/t = 0.6

61

4.29 (a)Burst pressure vs. defect depth l = 50mm (b) l = 100mm

(c) l =150mm

63

4.30 Burst pressure vs. defect width for d/t = 0.2 (b) d/t 0.4

(c) d/t =0.6

65

xvi

LIST OF SYMBOLS

σ stress

σy Yield Strength,

σs Tensile stress

σE Engineering stress

σT True strain

σe Equivalent stress

σm normal (hydrostatic) stress

Stress triaxiality

ɛe Engineering strain

ɛT True strain

Specimen fracture strain

A0 Original cross-sectional area

Af Final cross-sectional area

L0 Original length

Final length

deflection

F Force

F0 Original force

Ff Final force

L Length of pipe

t Thickness of pipe

d/t Ratio of defect depth to the pipe thickness

d Defect depth

l Defect length

c Defect width

xvii

LIST OF ABBREVIATIONS

ASTM E8 American Society for Testing and Material E8

API X42 American Pipe Institute X42

ANSI American National Standard Institute

HAZ heat affected zone

FEA Finite Element Analysis

FE Finite Element

SMCS Stress Modified Critical Strain

CS Case Study

ASME American Society of Mechanical Engineer

UTM Universal Testing Machine

CHAPTER 1

INTRODUCTION

1.1 RESEARCH BACKGROUND

The long-distance pipeline for transportation of the natural gas, one of the green energy

resources, is now under construction in the world wide range, due mainly to the expensive

requirement of the energy resources. One of the major difficulties is that, usually, the

production area is far from the usage area, resulting in higher transportation cost of the

natural gas, although this has been known for many years.

In additions, underground gas pipelines are often subject to damages due to

surroundings environment such as corrosion, and the third party accidents such as dents and

gouges. The negligence by the human factor will cause those defect. The high costs of

maintenance have to be provided to ensure the smooth transportation of the gas.

The installation cost for the thus steel gas pipe higher than the cost of the pipe itself. To

minimize those excessive cost, lots of analysis have been proposed and running over a

decade. Lots of efforts give to ensure the last longer of the gas transportation’s

performance.

The study was focusing on API X42 steel pipe material. Three type of sample specimen

were machine according to the Tensile testing standard requirement. Tensile test has been

performed on the selected material to determine the engineering stress – strain curve. Then,

detailed elastic–plastic, finite element analyses perform to simulate tensile tests specimens

and thus to determine variations of the triaxial stress and strain.

2

1.2 PROBLEM STATEMENT

Nowadays, the fitness-for-service analyses of underground gas pipelines, engineering

assessment methods against possible effects need to be developed, it will prevent such as

corrosion, gouges and dents defects type. Thus possible causes may not distress in a day,

but it will bounce on results in few years which consist of extraordinary maintenance cost.

Nothing that, typical gas pipelines are made of sufficiently ductile materials, the net-

section limit load approach can be used, where a damaged pipe is assumed to fail at the

load when the net section is in the fully plastic state. Meanwhile, design of pipelines is one

of the important processes in developing the engineering structure. During design stage,

few processes were involved such as analysis of deflections, stress analysis, cost reliability

and others.

Stress analysis appears as a crucial process for many engineering structure fail due to

lack of consideration on the analysis. One of the important parameters involve in stress

analysis is uniaxial fracture strain. The failure on engineering structure normally predicted

based on maximum stress that withstand the structure. Unfortunately, the fracture strain

becomes critical to be determined.

1.3 PROJECT OBJECTIVES

The research proposes ductile failure criteria as a function of the stress triaxiality for

the API X42 steel pipes. Smooth and notched tensile rods with three notch radii are tested

to determine parameter of ductile failure criteria. The strain based failure criteria will be

applied for failure prediction of the defective API steel pipes. For this purpose, the strain-

based criterion will be developing in advance. Otherwise, the burst pressure will be

determined for different defect dimension.

3

1.4 SCOPES OF STUDY

The scopes of research are as follows:

i. Preparation of both smooth and notched specimens

ii. Uniaxial Tension test- at room temperature obey the ASTM E8

iii. Finite Element Analysis

MSC PATRAN/MARC

Non-linear

Homogeneous material

iv. Validations - Compare result between equation and available industrial codes

for pipelines defect assessment.

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

This chapter will provide the detail descriptions literature review done accordingly

to tittle of ductile failure analysis of API steel pipe on strain-based failure criteria.

Literature regarding any development or experiment about fracture strain and state of stress

is useful in this project. This chapter will explain about the fundamentals of API X42 steel

pipes, pipelines, engineering stress-strain curve, true stress-strain curve, finite element

analysis, and failure criteria that available in this project.

2.2 FUNDAMENTAL OF X 42 API STEEL PIPES

The material used in this study was API X42 steel. Specimens for uniaxial tensile

test were extracted in longitudinal direction from pipe. Table 2.1 and 2.2 shows the

chemical compositions and mechanical properties of API X42

5

Table 2.1: Chemical composition of API X42 steel pipe

Sources: N.A.Alang (2009)

C P Mn S Fe

Experimental

API SPEC 5L

0.03

0.28(max)

0.01

0.08(max)

0.98

1.3(max)

0.003

0.03(max)

98.6

Balance

Table 2.2: Mechanical Properties of API X42 steel pipe in room temperature.

Young Modulus,

E (GPa)

Poisson Ratio,

V

Yield

Strength,

σy (Mpa)

Tensile stress,

σs (MPa)

Experimental 207 0.3 284.7 464.4

2.3 PIPELINES

API pipeline tubes belong to ANSI (American National Standard Institute)

Petroleum standards. The function of line pipe is to pump the oil, gas, water from field to

the refinery. Pipeline tubes include seamless tube and welded tube. The development of

pipeline steel plate technology and welding technique widen the application scope of

welded pipe. Each of pipe type have their own mechanical properties which as vary it from

each other.

Pipelines have been employed as one of the most practical and low price method for

large oil and gas transport since 1950. The pipe line installations for oil and gas

transmission are drastically increased in last three decades. Consequently, the pipeline

failure problems have been increasingly occurred. The economic and environmental and

eventually in human life considerations involve the current issue as structural integrity and

safety affair.

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2.3.1 Defects on Pipes

The explosive characteristics of gas provide high wakefulness about the structural

integrity. Therefore, the reliable structural integrity and safety of oil and gas pipelines

under various service conditions including presence of defects should be warily evaluated.

The external defects, corrosion defects, gouge, foreign object scratches, and pipeline

erection activities are major failure reasons of gas pipelines.

Figure 2.1: External corrosion defect on pipe

Source: M.Hadj, 2010

A typical external of a corrosion defect is given in Figure 3.1. Several types of

pipes failures can be distinguished as longitudinal, circumferential or helicoidally failures

(M.Hajd,Y.G Matvienko, G. Pluvinage, 2010 ) . These types depend mainly on pipe

diameter. For small diameter pipes, where bending stresses are the major, circumferential

failure occurs. For large diameters, hoop stresses are more important than bending stresses

and longitudinal failure appears. When bending and hoop stresses are of the same

importance, fracture path becomes spiraled. Pipe steels have yield stress up to 700 MPa for

the most recent quality in order to ensure enough ductility and weld ability.

Defects occurring during the fabrication of a pipeline are usually assessed against

recognized and proven quality control limits. However, a pipeline will regularly contain

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larger defects at some stage during its life and these will require a fitness- for-purpose

assessment to determine whether or not to repair the pipeline.

Line pipe steels is generally tough and ductile. Initiation and propagation of a part-

wall flaw through the wall occurs under a ductile fracture mechanism, involving some

combination of plastic flow and crack initiation and ductile tearing, involving a process of

void nucleation, growth and coalescence. The relative importance of plastic flow and crack

initiation and tearing depends on the toughness of the material and the geometry of the

defect.

As the toughness decreases the burst strength of a defect will decrease. As the

toughness increases the burst strength of a defect will increase, but tending towards an

upper limit corresponding to the plastic collapse limit state, where failure occurs due to

plastic flow .Therefore, if the toughness is greater than some minimum value then the

failure of a defect will be controlled by plastic collapse and only knowledge of the tensile

properties of the material is required to predict the burst strength.

Corrosion is an electrochemical process. It is a time dependent mechanism and

depends on the local environment within or adjacent to the pipeline. Corrosion usual

appears as either general corrosion or localized (pitting) corrosion. There are many different

types of corrosion, including galvanic corrosion, microbiologically induced corrosion, AC

corrosion, differential soils, differential aeration and cracking. Corrosion causes metal loss.

It can occur on the internal or external surfaces of the pipe, in the base material, the

seam weld, the girth weld, and or the associated heat affected zone (HAZ). Internal and

external corrosion are together one of the major causes of pipeline failures. Figure 2.2

shows example of worse corrosion effect on the pipeline. It will cause higher maintenance

cost and wasting time.

8

Figure 2.2: Corrosion on the pipeline

Sources: M.Hajd, 2010

Corrosion in a pipeline may be difficult to characterize. Typically, it will have an

irregular depth profile and extend in irregular pattern in both longitudinal and

circumferential directions it may occur as a single defect or as a cluster of adjacent defects

separated by full thickness (uncorroded) material.

There are no clear definitions of different types of corrosion defects. The simplest

and perhaps most widely recognized definitions are as pitting corrosion. It defined as

corrosion with a length and width less than or equal to three times the uncorroded wall

thickness. While, general corrosion, defined as corrosion with a length and width greater

than three times the uncorroded wall thickness.

9

Figure 2.3: The irregular length, width and depth of a typical corrosion defect.

Source: A.Cosham, 2007

2.4 ENGINEERING STRESS-STRAIN CURVE

Stress-strain curves are an extremely important graphical measure of an API steel

pipe material’s mechanical properties. Perhaps the most important test of a material’s

mechanical response is the tensile test. The engineering measures of stress and strain,

denoted in this module as σe and ɛe respectively, are determined from the measured the load

and deflection using the original specimen cross-sectional area A0 and length L0 as

σe =

(2.1)

10

ɛe =

(2.2)

When the stress σe is plotted against the strain ɛe, a typical engineering stress-strain curve

such as that shown in figure 2.4 is obtained.

Figure 2.4: Stress-strain curve

Sources: David, 2001

As strain is increased, many materials eventually deviate from this linear

proportionality, the point of departure being termed the proportional limit. This nonlinearity

is usually associated with stress-induced “plastic” flow in the specimen. Here the material is

undergoing a rearrangement of its internal molecular or microscopic structure, in which

atoms are being moved to new equilibrium positions. This plasticity requires a mechanism

for molecular mobility, which in crystalline materials can arise from dislocation motion.

Materials lacking this mobility, for instance by having internal microstructures that block

11

dislocation motion, are usually brittle rather than ductile. The stress-strain curves for brittle

materials are typically linear over their full range of strain, eventually terminating in

fracture without appreciable plastic flow. The stress needed to increase the strain beyond

the proportional limit in a ductile material continues to rise beyond the proportional limit;

the material requires an ever-increasing stress to continue straining, a mechanism termed

strain hardening.

These microstructural rearrangements associated with plastic flow are usually not

reversed when the load is removed, so the proportional limit is often the same as or at least

close to the material’s elastic limit. Elasticity is the property of complete and immediate

recovery from an imposed displacement on release of the load, and the elastic limit is the

value of stress at which the material experiences a permanent residual strain that is not lost

on unloading. The residual strain induced by a given stress can be determined by drawing

an unloading line from the highest point reached on the curve at that stress back to the strain

axis.

Until the neck forms, the deformation is essentially uniform throughout the

specimen, but after necking all subsequent deformation takes place in the neck. The neck

becomes smaller and smaller, local true stress increasing all the time, until the specimen

fails. This will be the failure mode for most ductile metals. As the neck shrinks, the

nonuniform geometry there alters the uniaxial stress state to a complex one involving shear

components as well as normal stresses.

2.5 TRUE STRESS STRAIN CURVE

While stress testing, the stress-strain curve is a graphical representation of the

relationship between stress, obtained from measuring the load applied on the sample.

Meanwhile, strain derived from measuring the deformation of the sample. The nature of the

curve varies depends on the type of materials. Strain describes quantitatively the degree of

deformation of a body. It is measured most commonly with extensometers and strain

gauges. For uniaxial deformation strain can be expressed as

12

ɛe =

(2.3)

Where

ɛe = engineering strain

= original length of the undeformed specimen

= final length of the deformed specimen

Based on this definition, if a sample were stretched such that Lf = 2L0, the tensile

engineering strain would be 100%. On the other hand, if a sample were compressed to the

limit such that Lf = 0, the compressive engineering strain would again be 100%. These

extreme examples show that for large strain the definition of equation (2.3) is not

meaningful.

For purely elastic deformation stresses are uniquely defined by the final

configuration of a material, regardless of how this final state is reached. Because of the

presence of irreversible elements in the deformation a plastic analysis has to follow the path

along which the final configuration is reached. So that, the total deformation is generally

divide into small increments.

Considering the uniaxial case, let dL be the incremental change in gauge length and L the

gauge length at the beginning of that increment. Then, the corresponding strain increment

becomes

dɛ =

(2.4)

and the total strain for a change of the gauge length from L0 to Lf

ɛ = ∫

= ∫

= ln

(2.5)

DUCTILE FAILURE ANALYSIS OF API STEEL PIPE USING STRAIN ...

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