INVESTIGATE THE EFECTIVENESS OF STOP DRILL HOLE IN DELAYING

CRACK FROM CRACK INITIATION

SYAFIQ BIN HASHIM

Thesis submitted in fulfilment of the requirements for the award of the degree of

Bachelor of Mechanical Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

JUNE 2012

v

ABSTRACT

Many failures in engineering applications or machine components have been caused

by a crack initiated from points at which stress was concentrated. As the stress

concentration level is higher than a critical value, continuous crack-growth results

in failure in the machine components. Crack initiation behavior has been evaluated

to prevent an early failure of machine equipment. Delaying the crack initiation is

important for the maintenance of machine elements as well as for the detection of

crack initiation. Hence, the main objective of this study is to investigate the delay of

crack using stop drilled holes. The investigation was based on the diameter of the

stop drilled holes, the location of stop drilled hole and the ancillary stop hole

drilled. The experiment and selected experiment on finite element analysis were

done in order to validate the result. The experiment was carried out by using tensile

test to obtain maximum applied load to the specimen while the finite element

analysis was analyzed by using ALGOR software to obtain the stress of the

specimen.The experimental results and the finite element analysis results show that

the crack initiation life of the specimen was longer when the diameter of the stop

drilled holes increased. The further the location of the stop drilled holes with the

slits location, the longer crack initiation life of the specimen. The influence of extra

stop-hole drill increase more time for delaying the crack on the specimen. From the

experiment the longest time on delaying crack propagation is 92.32% increase

compare before install stop-hole drill gained from Experiment 3. Thus, the

influence of the stop drilled holes on the crack initiation life has been proved.

vi

ABSTRAK

Kebanyakan kegagalan dalam bidang kejuruteraan dan komponen mesin adalah

disebabkan permulaan retak yang bermula dari titik pada tekanan yang tertentu.

Apabila paras penumpuan tegasan adalah lebih tinggi daripada nilai kritikal bahan,

kesinambungan keretakan akan menyebabkan kegagalan dalam komponen mesin.

Permulaan retak telah dinilai bagi mencegah kegagalan awal dalam komponen

mesin. Melambatkan permulaan retak adalah penting bagi tujuaan penyeliaan

elemen-elemen dalam mesin. Maka, objektif utama kajian ini adalah untuk

mengkaji kelambatan permulaan retak dengan menggunakan penahan lubang yang

ditebuk ke atas besi lembut. Kajian adalah berdasarkan diameter, susunan penahan

lubang dan penambahan penahn lubang. Eksperimen dan juga analisis finite element

yang telah dipilih dijalankan untuk mengesahkan keputusan ujikaji. Eksperimen

telah dijalankan dengan munggunakan mesin tegangan untuk mendapatkan daya

maksimum yang dikenakan ke atas spesimen manakala analisis finite element

dianalisis menggunakan perisian ALGOR bagi mendapatkan nilai tekanan yang

dikenakan ke atas spesimen. Keputusan eksperimen dan analisis finite element

menunjukkan hayat permulaan retak lebih adalah lebih lama apabila saiz diameter

penahan lubang dititngkatkan. Semakin jauh lokasi penahan lubang dari lokasi

belahan, semakin lama hayat permulaan retak. Pengaruh daripada kehadiran ekstra

penahan lubang juga meningkatkan lagi masa untuk menahan retak pada spesimen.

Daripada eksperimen , masa terpanjang untuk melambatkan perambatan retak

adalah 92.32% peningkatan berbanding sebelum menebuk penahan lubang yang

diperolehi daripada eksperimen 3. Oleh itu, pengaruh penahan lubang pada

permulaan retak telah dibuktikan.

vii

TABLE OF CONTENTS

Page

SUPERVISOR’S DECLARATION ii

STUDENT’S DECLARATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xii

LIST OF SYMBOLS xv

LIST OF ABBREVIATIONS xvi

CHAPTER 1 INTRODUCTION

1.1 Project Background 1

1.2 Problem Statement 2

1.3 Scope Of Study 2

1.4 Objectives of the Project 3

1.5 Summary 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.2 Crack 4

2.2.1 Concept of Fracture Mechanic 5

2.2.1.1 Crack modes and stress intensity factor 5

2.2.1.2 Plastic zone at the crack tip 8

2.2.2 Factors influencing crack initiation behaviour 9

2.2.2.1 Stress Concentration factor 10

2.2.2.2 Fatigue notch factor 11

2.3 STOP-HOLE DRILL 12

2.3.1 Stop-hole Parameters 13

2.3.1.1 Diameter of stop-hole drill 13

viii

2.3.1.2 Location of stopping holes 21

2.3.1.3 Additional of stop-holes 26

CHAPTER 3 METHODOLOGY

3.1 Flow Chart 37

3.2 Materials and Specimens 38

3.3 Stop Drilling Procedure 41

3.3.1 Specimen for Experiment 1 41

3.3.2 Specimen for Experiment 2 42

3.3.3 Specimen for Experiment 3 47

3.4 Tensile test 48

3.5 Finite element analysis 50

3.6 Model Validation 51

CHAPTER 4 RESULT AND DISCUSSION

4.1 Introduction 52

4.2 Tensile Machine Results 52

4.2.1 Result of Experiment 1 52

4.2.2 Result of Experiment Two 55

4.2.2.1Result of Category A 55

4.2.2.2 Result of Category B 57

4.2.3 Experiment 3 60

4.3 Result of Numerical Analysis 62

4.3.1 FEA Result Crack Plate 62

4.3.2 FEA Result Experiment 1 66

4.3.2.1 FEA Result Experiment 2 Category A 69

4.4 Result Validation 71

CHAPTER 5 CONCLUSION

5.1 Conclusion 74

5.2 Recommendation 75

ix

REFERENCES 76

APPENDICES

79

A1 Gantt chart for FYP1 79

A2 Gantt chart for FYP 2 80

B1 Crack behaviour on crack plate 81

B2 Crack behaviour on Experiment 1, diameter ∅ 2.8mm 82

B3 Specimen preparation for Experiment 2, category A at 11.2mm

from the edge specimen

83

B4 Specimen preparation for Experiment 2, category B (x=3.6mm,

y=3.6mm)

84

B5 Crack behaviour on Experiment 2, category B (x=3.6mm,

y=3.6mm)

85

B6 Specimen Preparation for Experiment 3 86

B7 Crack behaviour for Experiment 3 87

C1 Data for Crack test 88

C2 Data for Experiment (Figure 4.1) 89

C3 Data for Experiment 2, category A (Figure 4.2) 90

C4 Data for Experiment 2 Category B, y-axis constant 3.6mm

(Figure 4.3)

91

C5 Data for Experiment 2 Category B, y-axis constant 5.2mm

(Figure 4.4)

92

C6 Data for Experiment 3 (Figure 4.5) 93

x

LIST OF TABLES

Table No. Page

2.1 Crack initiation and total fatigue lives in 6061-T651 and AISI 304

specimens with different stop-holes

15

2.2 Factors influencing 6061-T651 and AISI 304 crack initiation lives 19

2.3 Mechanical properties of the Aluminium alloy 22

2.4 Experimental result of test 24

2.5 Percentage of decreasing of K_c versus (a⁄W) for different locations

and diameters of hole

25

2.6 Mechanical properties of2024-T3 33

3.1 Mechanical properties of aluminium alloy 1050 39

3.2 Dimensions of the Aluminium Alloy 1050 specimen with the initial

crack.

40

3.3 Distance of each location for stop hole for category A 44

3.4 Distance of stop-hole drill from crack tip at constant 3.6mm y-axis and

various x-axis to centre of stop-hole drilled

46

3.5 Distance of stop-hole drill from crack tip at constant 5.2mm y-axis and

various x-axis to centre of stop-hole drilled

46

3.6 Location of first hole and additional hole 48

4.1 Result of Experiment 1 54

4.2 Stress Concentration factor for Experiment 1 55

4.3 Result from Experiment 2 Category A 57

4.4 Result of Experiment 2 Category B, y-axis constant 3.6mm 58

4.5 Result of Experiment 2 Category B, y-axis constant 3.6mm 60

xi

4.6 Comparison of distance of y-axis for constant 3.6mm and 5.2mm 60

4.7 Result of Experiment 3 62

4.8 Distribution of stress along the cut of crack plate 65

4.9 Von Misses Stress along x-axis (denote as triangle, Figure 4.11),

Experiment 1

67

4.10 Von Misses Stress along x-axis (denote as triangle, Figure 4.11)

Experiment 2, category A

70

4.11 Result comparison experiment 1 72

4.12 Result comparison experiment 2, category A 73

xii

LIST OF FIGURES

Figure No. Page

2.1 Three modes of loading that can be applied to a crack 5

2.2 Centre cracked plate 7

2.3 Double edge notched plate 8

2.4 Three bearing, elasto-plastic and elastic zone in the crack tip 9

2.5 elliptical hole in a plate 10

2.6 Location of stop-hole 13

2.7 Effect of stophole diameter on crack initiation 14

2.8 Stress gradient along edges of 6061-T651 stop-holes with different

diameters.

16

2.9 Stress gradient along the edges of AISI 304 stop-holes with different

diameter

17

2.10 Post-initiation crack growth retardation in AISI 304 specimens 18

2.11 N_i versus ∆J⁄〖ρα〗_c relationship 20

2.12 Three locations of stopping hole 21

2.13 Schematics of compact tension (CT) specimen 22

2.14 Curve of P_cr versus displacement for the specimen without a hole and

three locations A, B, and C of hole diameter 6mm

23

2.15 Boundary conditions and loading applied to the compact tension (CT)

specimen

24

2.16 curve of K_c versus (a⁄W) in three locations A, B, and C with hole

diameter of 2mm.

26

2.17 Analysis model 27

2.18 Definitions of a and B 28

xiii

2.19 Stress concentration factor, K_t at point A in various angles of Ɵ with

δ=1.6mm ;(a) d_1=2mm (b)d_1=3.2mm

29

2.20 Stress concentration factor, K_t at point A in various angles of Ɵ with

δ=0.8mm ;(a) d_1=2mm (b)d_1=3.2mm

30

2.21 Relationship between stress concentration factor, K_t at point A and

ratio of various hole diameters R_d with θ=0°, where R_d=d_2⁄d_1 ; (a)

d_1=2mm

32

2.22 Geometry of specimen 33

2.23 Schematic representation of the stop-hole drill method of the specimen

experiments of base type (a), and (b) and (c) the case where adding an

hole

34

2.24 Fatigue lives of specimens where δ=0.8mm and θ=0° under applied

stress σ_a=62MPa

35

3.1 Project flow chart 38

3.2 Schematics of Aluminium Alloy 1050 40

3.3 Actual specimen of Aluminium Alloy 1050 with initial Crack 41

3.4 Specimen for Experiment 1 42

3.5 Specimen for Experiment 2 for category A 43

3.6 Specimen for Experiment 2 for category B 45

3.7 Specimen for Experiment 3 47

3.8 Universal Tensile Machine DTU-900 MH Series 49

3.9 Clamping position of the plate 50

3.10 Meshed Specimen using ALGOR software 51

4.1 Graph of Experiment 1 53

4.2 Graph of Experiment 2 Category A 56

4.3 Graph of Experiment 2 Category B, y-axis constant 3.6mm 58

xiv

4.4 Graph of Experiment 2 Category B, y-axis constant 5.2mm 59

4.5 Graph of Experiment 3 61

4.6 Model of Crack Plate 63

4.7 Model of Crack plate half cut 63

4.8 Von Misses Stress of Crack Plate 64

4.9 View at Max Von misses Stress of Crack Plate 65

4.10 Crack Von Misses Stress along the cut of x-axis (Figure 4.8, denote by

triangle shape)

66

4.11 Model specimen for experiment 1 67

4.12 Experiment 1 Von Misses Stress along the cut of x-axis (Figure 4.11,

denote by triangle shape)

68

4.13 simulation of Maximum Von Misses Stress for Experiment 1 69

4.14 Experiment 2, Category A Von Misses Stress along the cut of x-axis

(Figure 4.11, denote by triangle shape)

70

4.15 ALGOR simulation of Maximum Von Misses Stress for Experiment 2,

category A

71

xv

LIST OF SYMBOLS

a Crack length, m

Material constant

Distance of plastic zone, m

K factor, MPa

p Radius of curvature, m

q Notch sensitivities

r Radius of plastic zone, m

Stress, MPa

Subscript

Stress at the tip of A axis

Critical stress intensity factor

Stress Tensor

f Fatigue notch factor

Stress Concentration factor

y Yield strength

Stress gradient

xvi

LIST OF ABBREVIATIONS

AISI American Iron and Steel Institute

ASTM American Society for Testing Materials

FE Finite Element

FEA Finite Element Analysis

FEM Finite Element Model

1

CHAPTER 1

INTRODUCTION

1.1 Project Background

Many failures in engineering applications or machine components have been

caused by a fatigue crack. Any plates that undergoing vibration or the defect on

manufacturing of the plates can cause an initiation crack. The fatigue crack initiated

from points at which the stress concentrated. Where the stress concentration level is

higher than a critical value the crack-growth will grow larger and longer until the

plate failed. Thus, introducing repairing method could be useful to arrest the crack-

growth before the failure.

A number of investigations have been performed to prolong the fatigue life

of the cracked components. The crack repair lies in retarding or arresting the crack

propagation, which can be achieved by reducing crack-tip intensity, introducing

residual compressive stresses, and reducing crack-tip stress concentrations (Broek,

1986). The reduction of crack-tip stress intensity occurred by crack filling (Song,

1998; Shin, 2000), composite patches (Schubbe, 1999; Baker, 1993) and welding

repair (Linnert, 1967). The introduction of residual compressive stresses was

through overloading (Novotny, 1986; Vardar, 1988), indentation (Miyagawa, 1985;

Song, 2002) cold expansion (Buxbaum, 1987; Ball, 1998) and defence hole drilling

(Miyagawa, 1985, Goto, 1996). The reduction of crack-tip stress concentrations was

by stop-hole drilling (Ghfiri, 2000).

One of the most popular techniques of arresting fatigue crack propagation

was stop-hole method. The stop-hole method is an emergency repair technique that

2

has been employed for a long time to extend the fatigue life of cracked structural

components that cannot be replaced as soon as the crack discovered (Broek, 1986;

Shubbe, 1999). This classical resource is used in many maintenance crews all over

the world since it is relatively inexpensive, simple and fast to apply. In its simplest

form, this method consists of boring a hole in the vicinity of, or centred at the crack

tip, to transform the crack into notch, increasing in this way the residual fatigue life

of the cracked structure in comparison to the life it would have if not repaired.

However, the appropriate modelling of stop-holes drill is not simple. Several

parameters can influence the fatigue life increment caused by stop-hole. Among

them, at least the radius, the position and the surface finish of hole; the type and the

size of the crack; the geometry and the mechanical properties of component; the

magnitude of the load; and the residual stresses around the stop-hole border can all

influence the effectiveness of the repair.

1.2 Problem Statement

Stop-hole drill needs an appropriate way to be model so that can delay

propagation of the crack. But there are several parameters that need to be followed

to make sure stop-drilled hole effectively delay the crack plate.

1.3 Scope Of Study

In this research, the fatigue crack initiation lives prediction of specimen

aluminium alloy improving with stop-hole method will be investigate. The initiation

cracks will be delayed by diverting the cracks to detour to less important section.

This investigations concern to the stop-hole drill method with various techniques

applied. The techniques that will be studies are the improvement of fatigue life of

mild steel with different type of diameters of stop-hole. Besides that, the location of

stop-hole drill will also determined as one of the factor influence in improvement of

fatigue crack of specimen. Also the study focuses on the effectiveness of providing

additional holes or ancillary holes following the drilling at the crack tips. The

3

effectiveness of thus techniques will be developed by doing an experiments using

tensile machine.

1.4 Objectives Of the Project

The objectives of the research are to investigate the effectiveness with

appropriate techniques of stop-hole to delay the crack line from crack initiation by;

1. Suitable diameter of the stop-hole drill.

2. Suitable location for the stop-hole drill.

3. Application of additional hole following the stop-hole drill at the crack

tip.

1.5 Summary

Chapter 1 has been discussed briefly about project background,

problem statement, objective and scope of the project on the effectiveness of

the stop-hole drill in enhancing the fatigue crack life of material. This chapter is as a

fundamental for the project and act as a guidelines for project research completion.

4

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The purpose of this chapter is to provide a review of past research efforts

related to crack behaviour, factor influencing crack initiation, stop-hole drilled and

also finite element analysis. A review of other relevant research studies is also

provided. Substantial literature has been studied on stress history computation,

techniques of arresting crack using stop-hole drill, and fatigue life prediction after

introducing stop-hole drill. However, little information can be found on integrated

durability evaluation methods. The review is organized chronologically to offer

insight to how past research efforts have laid the groundwork for subsequent

studies, including the present research effort. The review is detailed so that the

present research effort can be properly tailored to add to the present body of

literature as well as to justly the scope and direction of present research.

2.2 Crack

A fracture happen when an object is separate into two or more pieces under

applied of stress. A crack say to accompany fracture which the crack propagates

through the material that end up separate the material into two pieces. Crack

properties are different for ductile material and brittle material. In ductile material,

cracks slowly propagate through material and it is accompanied with large plastic

deformation. The crack will stop unless extra stress is applied. For brittle material,

crack is spreading fast with no plastic deformation. The crack that propagates in

brittle material will continue to grow once they are initiated (Anderson, 2005).

5

2.2.1 Concept of Fracture Mechanic

2.2.1.1 Crack modes and stress intensity factor

There are three types of loading that a crack can experience, as shown in

Figure 2.1.

Figure 2.1: Three modes of loading that can be applied to a crack

Source: Anderson T.L 2005

A tensile stress field gives rise to mode 1, the opening crack propagation mode.

Mode 2 is the sliding mode, which due to in-plane shear and mode 3 is tearing

mode, which arises from out-of-plane shear. In this experiment we will only

consider the mode 1, since it is common in practice and crack behaviour almost

related to mode 1.

In study of crack, stress intensity factor plays a big role that take account the

stresses in the vicinity of crack tip. If the quantity reaches its critical values, crack

will initiates (Khoshravan, 2007).Irwin (1957) (Irwin, 1957) research found a

6

method of calculating the stress field in any linear elastic cracked body if polar

coordinate axis with the origin at the crack tip was define. The equation is given by

(2.1)

where is stress tensor, is the distance from the crack tip, is the angle with

respect to the plane of the crack, and fij are functions that are independent of the

crack geometry and loading conditions. Irwin called constant K as stress intensity

factor. Since fij is dimensionless units , stress intensity factor, has units of

stress. .( .). Since experiment is conducted only stress on tensile

field, calculation taken only on mode 1 crack, the opening crack propagation mode

is taken. The equation is given as

(2.2)

Equation 2.2 can written as

(2.3)

Broek (Broek, 1972) then derive relationship between stress intensity factor and

crack length:

(2.4)

F is the correction factor of , which is given in the different forms by researchers.

Since our specimen is initially crack at the edge of the plate, the correction factor of

specimen must be considered. The edge of the crack can be obtained by slicing the

plate in Figure 2.2 through the middle of the plate.

7

Figure 2.2: Centre cracked plate

Source: Michael Janssen

The stress intensity factor for the edge of the crack is given by

(2.5)

which increase by 12% increase in for the edge of the crack that cause from

different boundary conditions at the free edge. Further study by Tada (Michael) is

related to our test specimen shown the calculation of double edge notched plate

given as

(2.6)

8

which a and W can be measure as shown from Figure 2.3

Figure 2.3: Double edge notched plate

Source: Michael Janssen

2.2.1.2 Plastic zone at the crack tip

Plastic zone exist in the vicinity of crack tip for an elastic plastic material

with yielding stress ( which follows the maximum yield stress criteria. M.R

Kshorsravan (Khoshravan, 2007) state that inside the plastic zone, there is a bearing

zone which appear at the time the crack initiation. There is also an elastic zone

outside the plastic zone which has greater area than the plastic zone. The plastic

zones are depending on the thickness of the specimen, the crack length and strain

rate. Figure 2.4 shows the location of the bearing, electro-plastic and elastic zones at

the crack tip.

9

Figure 2.4: Three bearing, elasto-plastic and elastic zone in the crack tip

Source: Li.H, 2003

The size of the crack-tip-yielding zone can be estimated using Irwin approach. To

estimate the extent of the plastic deformation, Irwin equated yield strength to the y-

direction stress along the x-axis and solved the radius. For the plane stress state, the

radius of the plastic zone is:

(2.7)

In the plastic zone strain state, yielding is suppresses by the tri-axial stress state, and

the Irwin plastic zone correction is smaller by factor of 3:

(2.8)

2.2.2 Factors influencing crack initiation behaviour

Crack initiate when the material come to its limit of yield stress. When the

applied stress is more than material yield stress, crack will initiate. There are two

10

main factors that influence the crack initiation behaviour that are stress

concentration factor, and fatigue notch factor.

2.2.2.1 Stress Concentration factor

Stress concentration is a location in an object where stress concentrated. An

object is strongest when force is evenly distributed over its area, so a reduction in

area for example caused by a crack, results in a localized in stress. Material will fail

when the crack propagated cause from exceeding stress concentration of the

materials theoretical cohesive strength. Fatigue cracks always start at stress

concentration, so removing such defects increases the fatigue strength. Inglis

(Inglis, 1913) have provided a quantitative evidence for the stress concentration

effect of flaws, analyzed elliptical holes in flat plates. His analyses included an

elliptical hole 2a long by 2b wide with an applied stress perpendicular to the major

axis of the ellipse (Figure 2.5).

Figure 2.5: Elliptical hole in a plate

Source: Michael Janssen

11

The stress at the tip of the major axis (which is point A) is given by

(2.9)

The ratio

is defined as the stress concentration factor, . Then Inglis found

more convenient to express equation 2.9 in terms of radius of curvature :

(2.10)

Where

(2.11)

For delaying crack, we must design the geometry to minimize stress concentrations.

The geometry can be achieve by drill hole with relatively large diameter causes a

smaller stress concentration then a sharp end of crack.

2.2.2.2 Fatigue notch factor

The effect of notches on the fatigue life is accounted for by the fatigue notch

factor, , which depends on not only notch geometry and loading modes but also

on material types, and its follows the Peterson expression (Peterson, 1959).

(2.12)