CREEP CRACK GROWTH TESTING OF PLASTIC

Bajy @ Mohd Azlan Bin Doimin

Bachelor of Engineering with Honours TA (Mechanical Engineering and Manufacturing Systems) 455 2004 8165 2004

UNIVERSITI MALAYSIA SARA WAK

R13a

BORANG PENGESAHAN STATUS TESIS

Judul: CREEP CRACK GROWTH TESTING OF PLASTICS

SESI PENGAjIAN: 2003/2004

Saya BAjY @ MOHD AZLAN BIN DOIMIN (HURUF BESAR)

mengaku membenarkan tesis • ini disimpan di Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dcngan syarat-syarat kegunaan seperti berikut:

1. Tesis adalah hakmilik Universiti Malaysia Sarawak. 2. Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dibenarkan mcmbuat salinan untuk

tujuan pengajian sahaja. 3. Membuat pendigitan untuk membangunkan Pangkalan Data Kandungan Tempatan. 4. Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dibenarkan mcmbuat salinan tcsis ini

sebagai bahan pertukaran antara institusi pengajian tinggi. 5. Sila tandakan ( ., ) di kotak yang bcrkenaan U

(Mengandungi maklumat yang berdarjah kcselamatan atau kepentingan Malaysia scperti yang termaktub di dalam AKTA RAHSIA RASMI 1972).

D TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/ badan di mana penyelidikan dijalankan).

[ZJ TIDAK TERHAD

(T~~ENUUS) Alamat tetap: P.O BOX 603,

89307 RANAU, ASSOCDR.SININ HAMDAN

Nama Penyelia SABAH.

Tarikh: ) '2­ - (\ if ­ -:J-Q 0 4­ Tarikh:

CATATAN * Tesis dimaksudkan sebagai tesis bagi Jjazah Doktor Falsafah, Sarjana dan Sarjana Muda.

** Jika tesis ini SULIT atau TERHAD. sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT dan TERHAD.

Laporan Projek Tahun Akhir berikut:

Tajuk: CREEP CRACK GROWTH TESTING OF PLASTICS

Nama penulis: BAJY @ MOHD AZLAN BIN DOIMIN

Matrik: 5509

telah dibaca dan disahkan oleh:

Tarikh

Penyelia

O IO{;-1.~"bO

Pusat Khidmat Maklumat Akademl UNIVERSITl MALAYSIA SARAWAY,

94100 Kota Samarahan

CREEP CRACK GROWTH TESTING OF PLASTIC

P.KHIDMA TMAKLUMATAKADEMtK UIlIMAS

1111111111111111111 t11111 1000133644 .

BAjY @ MOHD AZLAN BIN DOIMIN

This project is submitted in partial fulfillment of the requirements for the degree of Bachelor of Engineering with Honours.

(Mechanical Engineering and Manufacturing System)

Faculty of Engineering UNIVERSITY MALAYSIA SARAWAK

2004

J

l

For my beloved father and mother

ACKNOWLEDGEMENTS

Thanks to Allah because I finally succeed to finish this Final Year Project.

A lot of thanks to my supervisor, Assoc.Dr.Sinin Hamdan for giving guidance and giving

comments on my project. With his guidance, I manage to finish this project.

This appreciation also goes to the all person who is involved in this project and my friends

who helped indirectly in my project progress.

To my family, especially my parent who give full support and motivation to do the best in my

studies.

Thanks to you all.

ABSTRACT

Almost everything that is around us is made from plastics. Many researches had been done to

improve the quality of this material. One of the research is concerning about its crack

behavior.

The objective of this project is to design and develop a test system to study the crack behavior

under constant load effect for a period of time.

For the weight range between 35 to 40 N, the plastic start to experience fast cracking and

finally rupture. The Ultimate Tensile Strength (UTS) for Polylex Acrylic plastic was 5136

MPa.

The experiment seems to be easy but less equipment and technical problem has made it

difficult for gathering the data. Recommendations for further works are taken based on the

problem facing and summary of data.

olylex Acrylic Plastic is a glassy and brittle material. It breaks suddenly without much

arning.

11

ABSTRAK

Hampir semua bend a yang berada di sekeliling kita diperbuat daripada plastik. Banyak kajian

mengenainya diJakukan untuk memperbaiki mutu bahan ini. Salah satu kajian tersebut adalah

mengenai sifat rekahannya.

Tujuan projek ini adalah untuk mereka dan membentuk satu system ujian bagi mengetahui

sifat rekahan di bawah pengaruh daya bebanan yang konsisten dalam jangka masa yang

tertentu.

Plastik dalam ujian ini mengalami rekahan yang pantas dan akhirnya pecah dalam julat 35

hingga 40 N. Kekuatan regangan tertinggi adalah 5136 MPa.

Ujian ini nampak seperti mudah tetapi kekurangan peralatan dan masalah teknikal

menyebabkan data susah untuk diperolehi. Cadangan untuk kerja-kerja pad a masa hadapan

diambil berdasarkan masalah yang timbul dan rumusan data.

Polylex Acrylic Plastik bersifat kaca dan bahan rapuh. Ia pecah tanpa memberi banyak

amaran.

III

1

I

TABLE OF CONTENTS

CONTENTS

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF FIGURE

LIST OF TABLE

CHAPTER ONE: INTRODUCTION

1.0 Introduction

2.0 Plastics

3.0 Objectives

CHAPTER TWO : LITERATURE REVIEW

2.0 Introduction

~.1. Linear Elastic Fracture Mechanics Background

~.2 Historical Overview

;Z.3 Linear Elastic Fracture Mechanics Assumptions

~.4 Loading Modes

~.5 C

IV

Pusat Khidmat Maklumat Akadend UNIVERSITI MALAYSIA SARAWAJ<

4)4100 KOla Samarahan

PAGE

ii

iii

iv-v

vi

vii

1-2

2-3

3

4

4

5-6

6-7

7

8

j

2.6 Failure and fracture

2.7 Crack Growth

2.8 Stress Intensity Factor

2.9 Elastic Modulus

CHAPTERTHREE:METHODOLOGY

3.0 Introduction

3.1 Test System Requirements

CHAPTER FOUR: RESULT AND DISCUSSION

4.0 Introduction

4.1 Compact Type Specimens

.2 Crack of Poly lex Acrylic plastic

.3 Stress Intensity Factor of Polylex Acrylic plastic

.4 Elastic Modulus for Poly lex Acrylic plastic

.5 Problem Facing

HAPTER FIVE: CONCLUSION AND RECOMMENDATION

1 Conclusion

Recommendations for Further Work

ENDIXS

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10-12

13-14

14-15

16

17-20

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21-22

23-27

28

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30-31

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34-35

36-43

,..

LIST OF FIGURE

I

Figure 2.1 Location of local stresses near a crack tip in 6

cylindrical coordinates.

Figure 2.2 Three loading modes 7

Figure 2.3 Schematic illustration of a creep curve 8

J'igure 2.4 Compact Type specimen with main dimensions 13

' i gure 2.S Compact Type specimen including details for compliance I'

Calculation 15

Ilgure 3.la Experiment set up 17

"gure 3.lb Basic test system requirement 18

igure 3.lc Compact type specimen 19

igure 3.ld Battery cell laser 20

~ure 3.le l00mm2 centronic photodiode 20

I !lure 4.1a Crack Opening Displacement against applied load graph 23

~re4.1b Crack rates graph 24

~re 4.1c Schematic illustration of polymer chains 25

Jure 4.1d Specimen before the experiment 25

ture 4.1e Specimen after the experiment 26

lure 4.2 Crack speed against stress intensity factor graph 28

~re 4.3 Elastic modulus against time graph 29

VI

-

LIST OF TABLE

~able 1 Data result for crack length, crack opening displacement

and time 22

Irable 2 Data result for stress intensity factor, elastic modulus and crack

Speed 27

L

VII

I

CHAPTER!

INTRODUCTION

1.0 Introduction

The use of plastics in structural engineering application has attracted many

interests in the area of creep especially the failure behavior of these materials. The failure

behavior of several plastics under long-term static loads has been analyzed by many

researchers and different fracture mechanic methods have been developed. Currently,

there are many test method standards and test equipments are commercially available.

l'he test equipments are usually designed for specific investigations and thus are limited

ill terms of variability of specimen type and geometry , test parameters and data

leneration.

Fracture mechanic testing techniques are typically utilized for evaluation of the

ffects of environmental variables where the specimen contains a sharp crack. One of the

~t common and relatively simple techniques for incorporation of fracture mechanic

~hniques is through the use of constant load or constant deflection specimens. In the

~ of constant load specimens, a load is applied to a fracture mechanic specimen using

lirectly applied dead weight or through a pulley or lever system to magnify the dead

_ght load. Dead weight-loaded specimens are often used to monitor time to crack

1

initiation and can be used to monitor crack growth rate vs. stress intensity K. This is

normally performed by measurement of crack opening displacement, which can be

related to crack length for a particular specimen geometry using compliance techniques.

Depending on the viscoelastic state and the loading conditions, failure under long­

term static loads may occur either by ductile failure or by a mechanically induced stable

crack growth process. Another important area where creep crack growth takes place is

under the simultaneous effect of stresses and certain liquid environments known as

environmental stress cracking (ESC).

1.1 Plastics

Plastics are defined as polymers (solid materials), which become mobile

on heating and thus can be cast into moulds. Plastics are synthetic polymers, which can

be deformed easily. They are formed by the addition or condensation polymerization.

here are two main types of plastics, Thermoplastics and Thermosetting Plastics.

Thermoplastics are the plastics that become soft and melt on heating and can be

ulded again. Repetitive heating of thermoplastics does not cause permanent change in

perties or composition. They are addition polymers. Examples for thermoplastics are

lyethylene, Nylon, PVC PVA and etc.

2

Thermosetting Plastics are the plastics that can be softened on heating but they

become permanently hardened on cooling. They cannot be remoulded again. They are

insoluble in any solvent whether organic or inorganic. They are condensation polymers.

Examples for thermosetting plastics are Bakelite, Urea Aldehyde, Silicones and etc.

1.2 Objectives

The objectives of this project is mainly to design and develop a test system

whereby fracture tests are performed under long term static loading conditions at

different environment. Different data acquisition for creep crack growth tests under static

loading conditions using concepts of linear elastic fracture mechanics (LEFM) will be

applied and comparison is then performed. It offers maximum flexibility as to the

fulfillment of a broad range of test condition while allowing simultaneously the study of

ack growth and specimen creep under static loads.

Other objective is to expose the student to use and perform the theory and

owledge being teaches in class to solve engineering problems. It also gives student the

nee to develop new ideas and be innovative.

3

,...

CHAPTER 2

LITERATURE REVIEW

2.0 Introduction

This chapter will describe different technique and data acquisition for creep crack

growth tests under static loading conditions using concepts of linear elastic fracture

mechanics.

2.1. Linear Elastic Fracture Mechanics Background

Linear elastic fracture mechanics (LEFM) principles are used to relate the stress

IIl8gnitude and distribution of stress near the crack tip with remote stresses applied to the

~acked component, the crack size and shape and the material properties of the cracked

~ponent.

4

I

2.2 Historical Overview

In the 1920s, Griffith formulated the concept that a crack in a component will

propagate if the total energy of the system is lowered with crack propagation. That is, if

the change in elastic strain energy due to crack extension is larger than the energy

required creating new crack surfaces, crack propagation will occur.

Griffith's theory was developed for brittle materials. In the 1940s, Irwin extended

I' the theory for ductile materials. In the mid-1950s, Irwin made another significant

ac:ontrriibbulllti· He showed that the local stresses near the crack tip are of the general form:

I

~t~l~ rand 9 are cylindrical coordinates of a point with respect to the crack tip as shown

In Figure2.1 and KI is the stress intensity factor.

5

Figure 2.1: Location of local stresses near a crack tip in cylindrical

coordinates. [7]

LiDear Elastic Fracture Mechanics Assumptions

Linear elastic fracture mechanics (LEFM) is based on the application of the

of elasticity to bodies containing cracks or defects. The assumptions used in

illlhlMtv are also inherent in the theory of Linear Elastic Fracture Mechanics (LEFM):

displacements and general linearity between stresses and strains.

A singularity exists such that as r, the distance from the crack tip, tends toward

the stresses will tend to be infinity. Since materials plastically deform as the yield

. exceeded, a plastic zone will form near the crack tip. The basis of linear elastic

6

cture mechanics (LEFM) remains valid, though, if this region of plasticity remains

in relation to the overall dimensions of the crack and cracked body.

There are generally three modes of loading, which involve different crack surface

.iSpllaa~mEmts as shown in Figure 2.2. The three modes are:

1: opening or tensile mode (the crack faces are pulled apart)

".Ode 2: sliding or in-plane shear (the crack surfaces slide over each other)

.,..,ooe 3: tearing or anti-plane shear (the crack surfaces move parallel to the leading edge

of the crack and relative to each other)

Figure 2.2: Three loading modes [7]

7

p

Creep is the pennanent elongation of a component under a static load maintained

a period of time. The creep te t typically consists of subjecting a specimen to a

.ant load and measuring the changes in length at various time increments. A creep

ually consists of primary, secondary and tertiary stages. The specimen eventually

by fracture called rupture. This can be shown in Figure 2.3

Ru lure

.5

Time

Figure 2.3 Schematic illustration of a creep curve [3]

8

Failure and fracture are the most important aspects of material behavior because it

influences the selection of a material for certain application, the methods of

IlUllWlCh.lFiJllg and the service life of the component. Fracture is classified into two

categories, ductile and fracture.

Ductile fracture is characterized by plastic deformation which precedes failure of

pan. It generally takes place along planes on which the shear stress is a maximum.

is initiated with the formation of tiny voids, usually around small inclusions or

~~ml1lg voids, which then grow and coalesce, developing into cracks which grow in

Brittle fracture occurs with little or no plastic deformation. In tension, fracture

place along the crystallographic plane (cleavage plane) on which the normal tensile

• a maximum. Fatigue fracture typically occurs in materials of a basically brittle

The external or internal crack develops at preexisting defects in material. The

the1I propagate and lead to total failure of the part.

9

Crack growth under cyclic loading is considerably affected by proximity or

peueiS to the grain boundary. The small crack growth cannot be predicted by using a

fracture mechanic parameter. It is mainly due to the effect of microstructural

When the cracked specimen is loaded under compliance condition, the stability of

Drnwth [OK / aa]IlT strongly depend on the compliance of the loading device. 12]

speed, da/dt, was determined from the monitored length, a, and the test

iD three different ways. This is discussed in details in subsequent subsections.

Sec.... (point-to-point) Technique

speed was simply computed by calculating the slope of the straight line

adjacent data points on the a versus t curve. Because the calculated value

• an a erage growth rate, the average crack length aassociated with this crack

U$ed to calculate the corresponding value for stress intensity factor,K.

10

= (a j +J _a j )

• (t i+J - tj ) (2)

(3)

·MII..... Polynomial Method

.UIIle all crack length versus time data with a single, third polynomial as follows:

(4)

calculated as the slope of the curve at corresponding test times.

(5)

parameters Co, C], C2 and C3 for the fit of the polynomial were

11

Secant and An Incremental Polynomial Method

l two values of crack length, da/dt was calculated with the secant method.

d--order polynomials were fi ts to sets of (2n+ 1) successive data points, where

2 or 3, depending on the number of available data points. To avoid numerical

fblJlil~ in determining the regression parameters bo, bI and b2 by the least squares

tb time values, TI and T2 are scaled as follows:

HI + tii?!) and T1 == ( t ii?! - t i-n )

2 2 (6)

of the equation for the local fit of the parabola is:

(7)

• the fitted value of crack length at the test time t~ The stress intensity factor

~.k:8late:d using this crack length at the corresponding time ti. The crack growth

• . alalled as the first derivative of the above parabola, which is given by following

(8)

12