CONDITION MONITORING ON VIBRATION ('IIARACTERISTICS OF COMPOSITE STRUCTURES

WONG GING HUO

ti1ALA ) ýf ý9

ýý . ý "Y ý ý r ýý. i,

Universiti Malaysia Sarawak 1999

TA 418.9 C6 N872

BORANG PENYERAHAN TESIS

Judul: Condition Monitoring on Vibration Characteristics of Composite Structures

SESI PENGAJ IAN: 1996/97

Saya WONG GING HUO (HURUF BESAR)

i I

I

Mengaku membenarkan tests ini dtstmpan di Pusat Khidmat Makhrmat Akademik, Universiti Malaysia Sarawak dengan syarat-sýarat kcgunaan scpcati berikvt.

Hakmilik kcrtas projek adalah di bawah nama penulis melainkan penulisan sebagai projek bersama dan dibtavat oleh UNIMAS, hakmilikn) a adalah kcpunyaan UNIMAS. Naskhah salinan di dalam bentuk kcrtas atau mikro hanya boleh dibuat dengan kebenaran bcttttlis daripada penulis. Pusat Khidmat Maklumat Akademik, UNIMAS dibcnarkan membuat salinan untuk pengajian mere". Kcrtas projek hatn, a bolch ditcrbitkan dcngan kcbcnaran penulis ßavaran royalti adalah mengikut kadar

yang dipcxsctujur kclak " SavaTnembenmimt/ttdak mcmbcnari: an Pcrpustakaan membuat salinan kettas projdt ini scbagai bahan pcrtukaran di antara imtrtust pcngajian tinggi. '" Sila tandakan (J)

ý SULIT (Mengandungi maklumat yang berdarjah kesdamatan atau kopentingan Malaysia scperti yang termaktub di dalain AKTA RAHSIA RASMI 1972).

= TERHAD (Mcngandungi maUumat TERHAD yang telah ditcntukan oleh organisasi/ badan di mane penselidikan dijalankan)

[Tý] TIDAK Ti? RNAD

Disahkan oleh

.-

(TANDATANGAN PENULIS) (TANDATANGAN PENYELIA)

Alamat tetap No. 38C,

Tong Sang Road,

Sibu, Sarawak

I Tunlh 26 Hß FEBRUARY, 1999

L CAT AT AN

)ATANG P

DR HA HOW UNG

Nama Penvelia

Tankh 26 HB. FEBRUARY, 1999

Polorag yang tidak berkenam Jika Kertas Prejek ini SULIT atau TERHAD, sila laespfrkat+ srarst daripada pYrak berlcuasa/ orgwtaisasi berksYraaan derrpao roeeyertakan sekall tempre6 kerW projek. hd periu dlksiaakaa sebaXai SULiT atau TERHAD.

Approval Sheet

This project report entitled "CONDITION MONITORING ON VIBRATION

CHARACTERISTICS OF COMPOSITE STRUCTURES" was prepared by

Wong Ging Huo as a partial fulfillment for the Bachelor of Engineering with honors

(Mechanical) is hereby read and approved by:

ýý, / -'. -/ 91 Dr. Ha How Ung Date

(Supervisor)

'ucat Kh+dmat Maklumat Akadem"'

UNIVERSITI MALAYSIA SARAWAa P. KHIDMAT MAKLUMAT

UNIMAS

IliuIllIlull CONDITION MONITORING ON VIBRATION CHARACTERISTICS OF

COMPOSITE STRUCTURES

WONG GING HUO

This project report is submitted in partial fulfillment of the requirement for the

degree of Bachelor of Engineering (Hons. ) Mechanical Engineering from the

Faculty of Engineering

Universiti Malaysia Sarawak

1999

/

Dedicated to beloved family

Acknowledgements

This project could not have emerged in its present form without the helpful

suggestions made by many people during its preparation. First and foremost, I would

like to express my deep gratitude and appreciation to my supervisor, Dr. Ha for his

insightful suggestions, valuable advice and untiring help in the preparation of this

project.

My sincerest and warmest thanks also go to the lecturers and laboratory assistants

from Mechanical Program for their valuable insight and illustrative materials in

assisting me in this project. Special thanks also extended to my friends for their

helpful support and advice through the project implementation period

Last, but certainly not least, l sincerely appreciate the support and understanding of

my family towards the success of this project

III

Abstract

Composite structures have been widely adopted for the support of structural members

mainly in bridges and buildings. Since the damage of the composite structures such as

cracks is usually harmfil, a non-destructive evaluation method based on vibration

analysis to monitor the vibration behavior of composite structures was proposed.

Therefore, for the case of this project, testings were carried out by using the

equipment namely Erudite MKII in which effects of cracks in different types of

rectangular composite structures were systematically examined. Through vibration

testing method, it is expected to find the relationship between the presence of the

cracks together with their resonant frequencies which in turn would enable the

following up of preventive maintenance that would prevent any catastrophic failures

from occurring.

iv

Abstrak

Struktur komposit telah digunakan secara meluas dalam pembinaan struktur anggota

terutamanya pada jambatan dan bangunan. Memandangkan kemudarstan path

struktur komposit seperti rekahan adalab merbahaya, satu kaedah penilaian bukan

pembinasaan telab diusulkan. Kaedah ini merupakan asas kepada analisis getatan dan

boleh digunakan untuk mengawasi ciri-ciri getaran bagi struktur komposit yang diuji.

Oleh yang demikian, dalam projek ini, ujian getaran dijalankan dengan radas Erudite

MKII di mana peralatan ini dapat menguji kesan rekahan dalam pelbagai jenis

struktur komposit dengan sistematik. Melalui kaedah ujian getaran tersebut, hubungan

yang wujud di antara kehadiran rekahan dengan frekuensi resonans bagi setiap jenis

struktur komposit dapat diperolchi. Dengan itu, langkah penyelenggaran dapat

diambil untuk mengelakkan sebarang bahaya seperti rekahan daripada berlaku.

TABLE OF CONTENTS

Letter of Approval

Sheet of Approval

Project Title

Dedication

Acknowledgement

Abstract

Abstrak

Table of Contents

List of Tables

List of Figures

List of Graphs

CHAPTER

1. INTRODUCTION

1.1 Background

1.2 Non-destructive Testing

1.3 Vibration Techniques

2. AIMS AND OBJECTIVES

i'

ü

lll

iv

V

vi

ix

I

zfi

Page

1

3

4

6

vi

3. LITERATURE REVIEW

3.1 Vibration Concepts 8

3.2 Free Vibration 9

3.2.1 Undamped Free Vibration 9

3.2.2 Damped Free Vibration 13

3.3 Forced Vibration 17

3.3.1 Undamped Forced Vibration 17

3.3.2 Damped Forced Vibration 23

3.4 Vibration Techniques 27

3.4.1 The Global Methods 27

3.4.2 Vibrothermography 30

3.4.3 The Acoustic Spectral Method 31

3.5 Non-vibrating Techniques 32

3.5.1 Modal Analysis of Cracked Composite Beam 32

3.5.2 Investigation of the Eigen-nature of Cracked Beams 33

4. METHODOLOGY

4.1 Introduction

4.2 Equipment

4.3 Test Specimens

4.4 Experimental Procedures

5. RESULTS

M Introduction

S. 2 Blank Specimen

34

34

38

41

46

46

vii

5.3 Drilled Specimen

5.4 Welded Specimen

5.5 Welded-drilled Specimen

5.6 Bolted Specimen

5.7 Bolted-drilled Specimen

6. ANALYSIS AND DISCUSSION

6.1 Introduction

6.2 Blank Specimen

6.3 Welded Specimen

6.4 Bolted Specimen

6.5 Problem Encountered

7. CONCLUSIONS

BIBLIOGRAPHY

47

50

so

54

S4

58

59

61

63

65

67

69

viii

List of Tables

Table

5.1 Blank specimen without crack

5.2 Drilled specimen without crack

5.3 Drilled specimen with one crack

5.4 Drilled specimen with two cracks

5.5 Drilled specimen with three cracks

5.6 Drilled specimen with four cracks

5.7 Welded specimen without crack

5.8 Welded-drilled specimen without crack

5.9 Welded-drilled specimen with one crack

5.10 Welded-drilled specimen with two cracks

5.11 Welded-drilled specimen with three cracks

5.12 Welded-drilled specimen with four cracks

5.13 Bolted specimen without crack

5.14 Bolted-drilled specimen without crack

5.15 Bolted-drilled specimen with one crack

5.16 Bolted-drilled specimen with two cracks

5.17 Bolted-drilled specimen with three cracks

5.18 Bolted-drilled specimen with four cracks

Page

46

47

47

48

48

48

50

50

51

51

51

52

54

55

55

55

56

56

ix

List of Figures

Figure Page

3.1 Spring-mass schematic 9

3.2 A plot of x as a function of pt 11

3.3 Schematic of spring-mass-damper system 13

3.4 Spring-mass schematic 17

3.5 A plot of x, as a function oft 20

3.6 A plot of xp as a function of t 20

3.7 Aplotofxasafunctionoft 21

3.8 A plot of M. F as a function of the driving-frequency ratio aý'p 21

3.9 Schematic of spring-mass-damper system 23

3.10 A plot of M. F as a function of the driving-frequency ratio ap 26

3.11 Schematic diagram of apparatus used to check natural

frequencies of filament- wound CFRP tubes 28

3.12 Location chart of trapezoidal 0°, 60°, 30°, 90°, 90°, 30°, 60°, 0°

CFRP plate with impact damage 29

4.1 Erudite MKII front panel layout 36

4.2 Erudite MKII rear panel layout 36

4.3 A test bench 37

4.4 Blank specimen 38

4.5 Drilled specimen 39

4.6 Welded specimen 39

4.7 Welded-drilled specimen 40

4.8 Bolted specimen 40

X

4.9 Bolted-drilled specimen 41

xi

List of Graphs

Graph

5.1 Blank Specimen

5.2 Welded Specimen

5.3 Bolted Specimen

6.1 Blank Specimen

6.2 Welded Specimen

6.3 Bolted Specimen

49

53

57

60

62

64

xii

CHAPTER 1

INTRODUCTION

1.1 Background

A steel designated as a mild steel has a carbon content in the range approximately

from 0.10 % to 0.15 %. Sometimes, mild steels are also referred to as low carbon

steels. They are characterized by low strength and high ductility, and are

nonhardenable by heat treatment except by surface-hardening processes. Because of

their good ductility, mild steels are readily formed into intricate shapes. These steels

are also readily welded without danger of hardening and embrittlement in the weld

zone. Although mild steels cannot be through-hardened, they are frequently surface-

hardened by various methods (carburizing, carbonitriding, and cyaniding, for

example) which diffuse carbon into the surface. Upon quenching, a hard, wear-

resistant surface is obtained. [7]

The use of composite structures in various construction elements has increased

substantially over the past few years. These materials especially mild steels are

particularly widely used in situations where a large strength-to-weight ratio is

required for structural members mainly in bridges and buildings and for parts to be

carburized

I

For example, the development of mild steels in the 1960's made it possible to

overcome the major weakness of concrete in bridge construction that is low tensile

strength. Of these steel bridges, the steel girder or beam bridge is the most common.

type of bridge for highways or railroads since it is one of the most simple and

economical structures to build.

However, as a result of introduction of larger vehicle such as trucks with heavier

loads and after undergoing for a long period of loading, the steel bridge with the

supporting structural member may undergo a series of serious vibratory motion about

its equilibrium position. Therefore, damages such as cracks are likely to occur. If any

of these possible damages, faults or even cracks are present and sustained by these

steel bridges, these may affect the physical geometry of the component, and hence the

resonant frequency.

Therefore, for this project, in order to analysis and monitor the vibration

characteristics of mild steel components, faults will be introduced intentionally

throughout the experimental test. The faults onto the specimen are obtained by

manually machined i. e. by using machine tool like hammer and drill. The faults that

will be machined are, cracks on the specimen surface and voids or cavities within the

specimen.

Since such surface discontinuities as presented on the specimen surface can act as

stress raisers, that is, they can reduce mechanical strength, especially bend strength

and fatigue strength, and they can act as initiating points for fatigue failure. Therefore,

a good steel member, must resist a complex combination of tension, compression,

2

bending, shear, and torsion forces. For this reason, it is essential to have an effective

and appropriate monitoring practice in order to determine the design capacity and

ability of steel members in carrying the load

1.2 Non-Destructive Testing

Non-destructive testing (NDT) forms an integral part of quality control, a term used to

describe the procedures which contribute to total quality assurance. A formal

definition of the subject, agreed by the International Committee for Non-destructive

Testing (ICNDT) and accepted later by the International Standards Organization

(ISO) states: Non-destructive testing (NDT) is a procedure which covers the

inspection and/or testing of any material, component or assembly by means that do

not affect its ultimate serviceability.

Non-destructive testing (NDT) has three major functions for research, development

and applications testing in composite structures. They are:

" Initial inspection of test specimens and confirmation of the structural integrity of

new components;

" Monitoring sample tests in progress, or components subjected to service loads;

" Analyzing test results after failure, or proof loading of components during their

service life.

This method may be divided into two general groups. The first group consists of tests

used to locate defects. In it are various simple methods of examination such as visual

inspection of the surface as well as the interior by the use of drilled holes, tests

3

'usat Khidmat Makfumat Akadem, P

UNiVERSITI MALAYSIA SARAWAa

involving the application of penetrants to locate surface cracks, the examination of

welded joints by use of a stethoscope to detect changes in sounds caused by hidden

flaws, and highly technical methods involving radiographic, magnetic, electrical, and

ultrasonic techniques.

The second group of nondestructive tests consists of those used for drmining

dimensional, physical, or mechanical characteristics of a material or part. In this

group are tests for the thickness of paint or nickel coatings on metallic bases, the

thickness of materials from only one surface, the determination of moisture COMM of

wood by electrical means, certain hardness tests, proof tests of various kinds, surface-

roughness tests, and methods employing forced mechanical vibrations to determine

the changes in natural frequency of the system due to changes in the properties of the

material. One type of vibration test uses the sonic analyzer for determining the natural

frequency, from which the modulus of elasticity can be computed.

1.3 Vibration Techniques

For this project, Vibration Technique is used as an approach to determine the

vibration behavior of composite structures and the effects due to damage incurred in

some of the members of that structure. One of the definition of the Vibration Testing

given by Kenneth G. McConnell states: Vibration Testing is the art and science of

measuring and understanding a structure's response while exposed to a specific

dynamic environment; and if necessary, simulating this environment in a satisfact0ry

manner to ensure that the structure will either survive or function properly when

exposed to this dynamic environment under field conditions.

4

It is a light-weight, low cost, simple to use vibro-diagnostic device, which enables to

locate fatigue cracks, defects in multi-layer structures, failure of elements and other

structural problems. In addition, it can also be used to detect the oscillation of a

structure up to 25 kHz and is thereby very suitable to analyze the condition

monitoring of most of the structures.

Generally, this techniques requires several hardware components. They are consists of

a vibration exciter for providing a controlled input to the structure, a force transducer

to convert the mechanical motion of the structure into an electrical signal, a signal

conditioning amplifier to match the characteristics of the transducer to the input

electronics of the digital data acquisition system, and a frequency analyzer, in which

signal processing and modal analysis programs reside.

5

CHAPTER 2

AIMS AND OBJECTIVES

The main aim of this study is to examine the vibration behavior and characteristics of

composite structures which are made of mild steel as apply in the construction of

bridges and buildings. Such evaluation is important to enable any demotion of

vibration analyses of composite structures to be predicted in a systematic manner.

This can be achieved by investigating the possibility of applying suitable monitoring

technology to the monitoring of composite structures. Therefore, for the case of this

project, vibration testing method is used as an approach to monitor the vibration

behavior of these composite structures.

The analyses of this vibration test is established based on the relationship between the

fundamental natural frequencies of the composite structures and the dimension of the

composite structures. With this relationship, we can predict the acceptable load that

can be withstand by mild steel systems in the condition of vibratory motion.

To further enhance the analysis, this study also aims to investigate ways of

determining the effects due to damage on the performance and safety of the composite

structures and to control its effects using vibration analyses. In the case of a

composite structure with crack, this fundamental frequency will change and the

experiments will be repeated to establish again the relationship between the extent of

6

the cracks with the difference of resonance. In return, it is hope that by the end of this

analyses, maintenance schedule can be followed up from time to time to prevent any

failures from occurring.

7

CHAPTER 3

LITERATURE REVIEW

3.1 Vibration Concepts

A vibration is the periodic motion of a body or system of connected bodies displaced

from a position of equilibrium. In general, there are two types of vibration, free and

forced. Free vibration occurs when the motion is maintained by gravitational or elastic

restoring forces, such as the vibration of an elastic rod. Forced vibration is caused by

an external periodic or intermittent force applied to the system. Both of these types of

vibration may be either damped or undamped. Undamped vibrations can continue

indefinitely because frictional effects are neglected in the analysis. Since in reality

both internal and external frictional forces are present, the motion of all vibrating

bodies is actually damped.

Therefore, this chapter is mainly concerned with describing the fundamental

principles of vibration behavior of the above types of vibration motion and those

concepts that are most often involved in vibration testing. These vibration concepts

are important and will be used in developing a general vibration testing model.

8

3.2 Free Vibration

Simple harmonic motion or oscillation, is showed by stuctures that have elastic

restoring forces. Such system can be modelled by a spring mass schematic where it is

the most basic vibration model of a structure and can be used to describe a number of

devices, machines and structures. When a spring-mounted body is disturbed from its

equilibrium position, its ensuring motion in the absence of any imposed external

forces is termed free vibration. In every actual case of free vibration, there exists some

damping force due to mechanical and fluid friction which tends to diminish the

motion. In 3.2.1, we consider the ideal case where the damping forces are small

enough to be neglected. In 3.2.2, we treat the case where the damping is appreciable

and must be accounted for.

3.2.1 Undamped Free Vibration

Lets begin by considering the simplest type of vibrating motion, i. e. the horizontal

vibration of the simple frictionless spring-mass system of Figure 3.1.

cxxx m

Xi(t)

Figure 3.1: Spring-mass schematic

9