Study on failure of surface and interface of the laminated composite structures of a small ship

공학박사 학위논문





복합적층구조 소형선박의 표면 및 층간 손상에 관한 연구

Study on the failure of surface and interface of the
laminated composite structures of a small ship









울산대학교 대학원
기계공학과
Pham Thanh Nhut




Doctor of Philosophy

Study on the failure of surface and interface of the
laminated composite structures of a small ship











The Graduate School
of the University of Ulsan
Department of Mechanical Engineering
Pham Thanh Nhut




Study on the failure of surface and interface of the
laminated composite structures of a small ship

이 논문을 공학박사 학위 논문으로 제출함

2013년 12월






울산대학교 대학원
기계공학과
Pham Thanh Nhut




Study on the failure of surface and interface of the
laminated composite structures of a small ship

Supervisor: Prof. Young Jin Yum

A Dissertation

Submitted to
the Graduate School of the University of Ulsan
In partial Fulfillment of the Requirements
for the Degree of

Doctor of Philosophy

by

Pham Thanh Nhut




Department of Mechanical Engineering
University of Ulsan, Republic of Korea
Dec., 2013





Pham Thanh Nhut 의 공학박사 학위 논문을 인준함




심 사 위원장
주석재
인

심 사 위 원
정일섭
인
심 사 위 원
천두만
인
심 사 위 원
최승태
인
심 사 위 원
염영진
인






울산대학교 대학원
2013






Study on the failure of surface and interface of the
laminated composite structures of a small ship

This certifies that the dissertation

of Pham Thanh Nhut is approved.




Committee Chairman

Prof. Seok Jae Chu

Committee Member

Prof. Il Sup Chung

Committee Member

Prof. Doo Man Chun

Committee Member

Prof. Seung Tae Choi

Committee Member
Prof. Young Jin Yum





Department of Mechanical Engineering
University of Ulsan, Republic of Korea

Dec., 2013
ACKNOWLEDGEMENTS



I would like to express my heartfelt gratitude to my advisor, Prof. Young Jin Yum, who
not only gives me priceless guidance in scientific work but also supports me the financial
help for three years studying in the University of Ulsan.
I would like to thank the professors in the committee for their suggestions and
comments throughout the study. Sincerely thank to University of Ulsan that grants me the
scholarship from the 2011 Research Fund. I also express my gratitude to professors and
staff members of the School of Mechanical Engineering for their kind help. Special thank
to Korea for living and studying opportunity and Korean people for their civilization and
friendship.
I would like to thank my family members, who encourage and suffer a separation during
the period I live away from home. Specially thank to all members of the Structure and
Composite Materials laboratory who support me a lot in my research works.
I would like to thank the Union of Vietnamese Students at University of Ulsan for
spiritual help and consulting abroad living and studying experience, and close friends in
Vietnam who always look forward to my returning our motherland.
Ulsan, Republic of Korea. Dec., 2013
PHAM THANH NHUT





i

CONTENTS

Abstract iv
List of figures vi
List of tables ix
Nomenclature x
Abbreviations xii
Chapter 1 : Introduction ………………………………………………………………… 1
1.1 Overview ……………………………………………………………………… 1
1.1.1 Thermoset composite materials …………………………………………… 1
1.1.2 Properties and application of composite materials in shipbuilding …………. 3
1.2 Surface characteristics of lamina composite …………………………………… 4
1.3 Problem statement, necessity, objectives and thesis layout ……………………… 6
Chapter 2: Analysis of typical surface structures and design of test specimens …… 9
2.1 Joining structures ………………………………………………………………… 9
2.1.1 Analysis of structures ……………………………………………………… 9
2.1.2 Design of test specimens ………………………………………………… 11
2.2 Propeller structures ……………………………………………………………… 14
2.2.1 Analysis of structures ……………………………………………………… 14
2.2.2 Design of test specimens ………………………………………………… 16
2.3 Coating structures .……………………………………………………………… 19
2.3.1 Analysis of structures ……………………………………………………… 19
2.3.2 Design of test specimens ………………………………………………… 19
Chapter 3: Failure of lap-joints with different surface properties ………………… 21
3.1 Introduction ……………………………………………………………………… 21
3.2 Theory and experiment ………………………………………………………… 23
3.2.1 Method …………………………………………………………………… 23
3.2.2 Theory …………………………………………………………………… 23
3.2.3 Materials and making of specimens ……………………………………… 25
3.2.4 Mechanical tests ………………………………………………………… 25
ii

3.3 Results and discussions ……………………………………………………… 28
3.3.1 Result of tensile and in-plane test ………………………………………… 28
3.3.2 Shear strength in y-z plane ………………………………………………… 32
3.3.3 Shear strength in x-z plane ………………………………………………… 35
3.3.4 Shear strength in x-y plane ………………………………………………… 37
3.3.5 Combination of shear stress for maximum and average values …………… 39
3.4 Conclusions ………………………………………………………………………. 41
Chapter 4: Failure of propeller blades surface by cavitation erosion phenomenon 42
4.1 Introduction ……………………………………………………………………… 42
4.2 Theory and experiment ………………………………………………………… 44
4.2.1 Method …………………………………………………………………… 44
4.2.2 Materials and samples preparation ………………………………………… 44
4.2.2.1 Manufacturing process of mold …………………………………… 45
4.2.2.2 Evaluation of mold ………………………………………………… 46
4.2.2.3 Manufacturing process of propeller …………………………………. 56
4.2.2.4 Preparation of circulating water channel (CWC) ……………………… 57
4.2.3 Test conditions …………………………………………………………… 58
4.3 Results and discussions ………………………………………………………… 59
4.3.1 Weight loss of blades ……………………………………………………… 59
4.3.2 Position and damage on blade surface …………………………………… 62
4.3.3 Area and propagation of cavitation ……………………………………… 65
4.4 Conclusions ………………………………………………………………………. 68
Chapter 5: Failure of gelcoat surface by print-through phenomenon ……………… 70
5.1 Introduction ……………………………………………………………………… 70
5.2 Theory and experiment ………………………………………………………… 71
5.2.1 Method …………………………………………………………………… 71
5.2.2 Theory …………………………………………………………………… 72
5.2.3 Materials and specimens preparation ……………………………………… 73
5.2.4 Experimental organization ………………………………………………… 75
iii

5.3 Results and discussions ………………………………………………………… 76
5.3.1 The values of surface roughness ………………………………………… 76
5.3.2 The microphotograph of surface ………………………………………… 80
5.3.3 Prediction of surface roughness …………………………………………… 86
5.4 Conclusions ………………………………………………………………………. 91
Chapter 6: Conclusions and future research …………………………………………. 93
6.1 Conclusions ………………………………………………………………………. 93
6.2 Future researches 94
6.2.1 Single lap-joints problem 94
6.2.2 Composite propeller problem 97
6.2.3 Gelcoat surface problem 97
References ………………………………………………………………………………. 99
Bibliography …………………………………………………………………………… 103
List of publications …………………………………………………………………… 106
Appendix ………………………………………………………………………………. 108

iv

ABSTRACT
Lamina composite is used in most structures of composite ship like hull, bulkhead,
frame or propeller. Surface and interface properties of this material should be considered
thoroughly because they can affect the strength, hardness or aestheticism of products. The
failures of surface and interface were studied and evaluated by experimental investigation
through three typical structures of a composite boat.
For failure of single lap-joints, unsaturated polyester/glass fiber (woven and mat) was
used for composite panel samples manufactured by hand lay-up and vacuum method at
room temperature. The polyester resin was used as an adhesive. Joining process of the
samples was carried out on six types of surface with two different
methods: terminal-joint and secondary-joint. Geometry properties of all bondings were the
same. Mechanical properties of adhesive, composite adherends, terminal-joint and
secondary-joint specimens were determined by experiments. Combinations between the
experiment results and bonding theory were used in this study. The maximum and average
shear stresses were calculated by maximum tensile and in-plane force and geometry
parameters of specimens. The results of maximum and average shear stresses were
compared and evaluated in six joints together. The results showed that grinding,
grinding/acetone terminal-joint and mat-mat, mat-woven secondary-joint had the high
strength. Conversely, no treatment and woven-woven bonding had very low strength. In
addition, the failure occurred strongly at two ends and then moved toward the middle area
of overlap length.
For failure of composite propeller blades surface, the cavitation erosion phenomenon
was tested directly. A small three blades propeller was made from fiberglass combined
with epoxy, polyester and gelcoat resin. This propeller was tested in the circulating water
channel whose revolution and depth were determined by conditions of cavitation number.
Cavitation erosion was observed and evaluated by images, weight loss and cavitation area.
From the results, it was observed that the cavitation erosion occurred around 0.4R and
0.7R (on both pressure and suction surface and both leading and trailing edge). Most

v

damage regions propagated along the circumferential direction of propeller. Ge-GF blade
had the best property, Po-GF blade had the worst property and the quality of Ep-GF blade
was medium.
For failure of gelcoat surface, print-through phenomenon (PTP) was investigated on the
specimens. The samples were fabricated with gelcoat resin material only. Three input
factors (variables) were studied including thickness of layer, hardener fraction and
temperature of making. The specimens were tested to determine the roughness and
microphotograph of PTP. Then, the multiple regression analysis was used to obtain the
experimental regression equations. Results indicated that the effect of three input factors
on PTP was different: hardener fraction was the strongest and thickness was the weakest.
The PTP increases quickly with upper 25
o
C and 2% hardener. The amount, area and depth
of points and cracks also increased when the input factors value increased. Besides, the
values of roughness or input factors can be predicted by experimental regression equations.
Finally, future works were given and performance method was suggested.

Key words: surface, terminal-joint, secondary-joint, composite propeller, cavitation
erosion, gelcoat, PTP.








vi

LIST OF FIGURES
Fig. 1-1 Influence of reinforcement type and quantity on composite properties………… 2
Fig. 1-2 Manufacturing of E-glass mat and woven roving………………………………… 4
Fig. 1-3 Surfaces of lamina composite structure…………………………………………. 5
Fig. 2-1 Co-curing bonding……………………………………………………………… 9
Fig. 2-2 Making of secondary layer……………………………………………………… 10
Fig. 2-3 Making of terminal-joint………………………………………………………… 11
Fig. 2-4 Three surface pairs of secondary-joint………………………………………… 11
Fig. 2-5 Surface of terminal-joint: (a) Three pairs of surface, (b) Morphology of surface 12
Fig. 2-6 Geometry of tensile test specimen………………………………………………. 12
Fig. 2-7 Geometry of in-plane test specimen…………………………………………… 13
Fig. 2-8 Position of propeller of small boat………………………………………………. 14
Fig. 2-9 Types of small propeller………………………………………………………… 14
Fig. 2-10 Structure of small propellers…………………………………………………. 15
Fig. 2-11 Linking of blade and hub………………………………………………………. 15
Fig. 2-12 Aluminium alloy mold of blade……………………………………………… 17
Fig. 2-13 Circulating water channel standard……………………………………………. 17
Fig. 2-14 Vibratory apparatus for cavitation erosion test………………………………… 18
Fig. 2-15 Composite circulating water channel………………………………………… 18
Fig. 2-16 Application of gelcoat resin……………………………………………………. 19
Fig. 2-17 Print-through phenomenon…………………………………………………… 20
Fig. 2-18 Structure of mold and manufacturing process…………………………………. 20
Fig. 3-1 Results of tensile loading test: (a) No treatment terminal-joint, (b) Grinding
terminal-joint, (c) Grinding/acetone terminal-joint, (d) Mat/mat secondary-joint, (e)
Mat/woven secondary-joint, (f) Woven/woven secondary-joint ………… 27
Fig. 3-2 Results of tensile loading test: (a) No treatment terminal-joint, (b) Grinding
terminal-joint, (c) Grinding/acetone terminal-joint, (d) Mat/mat secondary-joint, (e)
Mat/woven secondary-joint, (f) Woven/woven secondary-joint .……………………… 30

vii

Fig. 3-3 Results of in-plane loading test…………………………………………………. 32
Fig. 3-4 Shear stress in y-z plane along y………………………………………………… 33
Fig. 3-5 Ratio of (
yz
)/(
yz
)
ave
vs. y/c in tensile test case………………………………… 34
Fig. 3-6 Shear stress in x-z plane along y………………………………………………… 34
Fig. 3-7 Ratio of (
xz
)/(
xz
)
ave
vs. y/c in in-plane test…………………………………… 37
Fig. 3-8 Shear stress in x-y plane along y……………………………………………… 38
Fig. 3-9 Ratio of (
xy
)/(
xy
)
ave
vs. y/c in in-plane test…………………………………… 39
Fig. 3-10 Average shear stress for all cases……………………………………………… 40
Fig. 3-11 Maximum shear stress for all cases……………………………………………. 40
Fig. 4-1 Aluminium propeller model…………………………………………………… 45

Fig. 4-2 Parts of composite mold………………………………………………………… 45
Fig. 4-3 Making of mold from propeller model………………………………………… 46
Fig. 4-4 Hardness specimens and testing………………………………………………… 46
Fig. 4-5 Bending and compression testing……………………………………………… 48
Fig. 4-6 Load-displacement of mold only……………………………………………… 48
Fig. 4-7 Load-displacement of mold & mixture inside……………… …………………. 49
Fig. 4-8 Results of deflection along radius: a) Load-displacement at 0.7R , b) Load-
displacement at 0.8R , c) Load-displacement at 0.9R ……………………………… 51
Fig. 4-9 Comparison of deflection……………………………………………………… 51
Fig. 4-10 Position and method of measurement………………………………………… 52
Fig. 4-11 Error of Al propeller/GFPG mold pair………………………………………… 54
Fig. 4-12 Error of Al propeller/GRP propeller pair………………………………………. 55
Fig. 4-13 Manufacturing process…………………………………………………………. 56
Fig. 4-14 Composite propeller……………………………………………………………. 56
Fig. 4-15 Composite CWC……………………………………………………………… 57
Fig. 4-16 Experiment of composite propeller……………………………………………. 59
Fig. 4-17 Weight loss of three blades…………………………………………………… 61
Fig. 4-18 Weight loss rate of three blades……………………………………………… 61
Fig. 4-19 Weight loss ratio of three blades………………………………………………. 61
viii

Fig. 4-20 Position and area of cavitation for each time………………………………… 64
Fig. 4-21 Microphotograph of collapse of surfaces……………………………………… 65
Fig. 4-22 Propagation of cavitation………………………………………………………. 66
Fig. 4-23 Development of cavitation area……………………………………………… 67
Fig. 4-24 Rate of cavitation area every hour…………………………………………… 67
Fig. 5-1 Gelcoat and hardener 73
Fig. 5-2 Manufacturing process of specimens: a) Nine specimens at 15
o
C, b) Nine

specimens at 25
o
C, a) Nine specimens at 35
o
C………………………………………… 75
Fig. 5-3 Measurement of roughness 76
Fig. 5-4 Effect of thickness on roughness: a) Roughness R
a
, b) Roughness R
z
………… 77
Fig. 5-5 Effect of hardener fraction on roughness: a) Roughness R
a
, b) Roughness R
z
…78
Fig. 5-6 Effect of temperature on roughness: a) Roughness R
a
, b) Roughness R
z
……… 79
Fig. 5-7 Forms of PTP on gelcoat surface: a) At 0.5mm thickness, b) At 1.0mm thickness,
c) At 1.5mm thickness 83
Fig. 5-8 Change of surface state………………………………………………………… 85
Fig. 6-1 Change of overlap length 95
Fig. 6-2 Reinforcement at two ends 95
Fig. 6-3 In-plane test specimens: b) T shape b) Y shape 96
Fig. 6-4 PIRANHA propellers are 17% stronger than aluminum propeller 97
Fig. 6-5 Change of underlying lamina structure………………………………………… 98

ix

LIST OF TABLES
Table 3-1 Geometric characteristics of joint samples……………………………………. 26
Table 3-2 Material properties of polyester resin and composite.………………………. 27
Table 3-3 Results of tensile loading test and the calculated shear stresses from theory…. 33
Table 3-4 Results of in-plane loading test and calculated shear stress 
xz
……………… 36
Table 3-5 Results of in-plane loading test and calculated shear stress 
xy
……………… 38
Table 4-1 Characteristics of propeller model……………………………………………. 44
Table 4-2 Hardness of materials…………………………………………………… 47
Table 4-3 Roughness of materials……………………………………………………. 47
Table 4-4 Coordinate of pressure surface………………………………………………… 53
Table 4-5 Thickness of blade………………………………………………………… 53
Table 4-6 Coordinate error of mold and thickness error of blade compared to Al
propeller………………………………………………………………………………… 54
Table 4-6 Weight of blade after 500 hours of experiment……………………………… 60
Table 4-7 Rate of weight loss…………………………………………………………… 60
Table 4-8 Ratio of weight loss and weight of blade……………………………………… 60
Table 4-9 Area of cavitation erosion…………………………………………………… 66
Table 5-1 Code of specimens…………………………………………………………… 73

Table 5-2 Properties of gelcoat resin…………………………………………………… 74
Table 5-3 Input data for SPSS software………………………………………………… 86
Table 5-4 Simple regression analysis: a) For dependent variable R
a
, b) For dependent
variable R
z
……………………………………………………………………………… 87
Table 5-5 Multiple regression analysis for group 1: a) For dependent variable R
a
, b) For
dependent variable R
z
……………………………………………………………………. 88
Table 5-6 Multiple regression analysis for group 2: a) For dependent variable R
a
, b) For
dependent variable R
z
……………………………………………………………………. 88
Table 5-7 Result of roughness error: a) Error of R
a
, b) Error of R
z
89
Table 5-8 R
a
& R
z
of hull surface from practical conditions…………………………… 91

Table 5-9 Hardener fraction at the same Ra (minimum)……….………………………… 91
Table 5-10 Hardener fraction for different surfaces at 20
o
C, 1mm…………………… 91
x

NOMENCLATURE
A/A
0
: Area ratio
b : Bonding width (mm)
2c

: Overlap length, mm
D(D
p
) : Diameter of propeller (m)
e : Random error
i
y
E

: Young’s modulus of inner adherend (MPa)
o
y
E

: Young’s modulus of outer adherend (MPa)
g : Gravitational acceleration (m/s
2

)
G
a

: Shear modulus of adhesive (MPa)
h : Depth of immersion of propeller axis (m)
n : Rotational speed (rpm)
N
xy

: In-plane loading (N/mm)
N
y

: Tensile loading (N)
P : Pitch of propeller, significance level
p
A
: Atmospheric pressure (KN/m
2
)
p
v
: Saturation vapour pressure (KN/m
2
)
R : Radius of propeller (m), coefficient of correlation
R
2
: Coefficient of multiple determination

R
a
: Arithmetic mean value of roughness (m)
R
z
: Mean height of profile irregularities of roughness (m)
t
a

: Thicknesses of adhesive, mm
t
i

: Thicknesses of inner adherend, mm
t
o

: Thicknesses of outer adherend, mm
V
A
: Advanced speed (m/s)
x
i
: independent variable
x
1
: Thickness of specimen (mm)
x
2
: Hardener fraction (%)

x
3
: Temperature (
o
C)
y : Length of joint (mm), dependent variable
z : Number of blade propeller

xi

Greek symbols

i
: Coefficient determining the contribution of the independent variable
 : Density of sea water (T/m
3
)


: Cavitation number, standard deviation
a
xz


: Shear stress of adhesive in x-z plane (MPa)
a
yz


: Shear stress of adhesive in y-z plane (MPa)

o
xy


: Shear stress of outer adherend in y-z plane (MPa)
τ
ult
: Ultimate shear stress



















xii

ABBREVIATIONS

Al : Aluminium
ASTM : American Society for Testing and Materials
ave : Average
Coef. : Coefficient
COP : Cavitation occurrence probability
CWC : Circulating water channel
deg. : Degree
Ep-GF : Mat and woven glass fiber and epoxy resin
exp : Experiment
FEM : Finite element method
Fig. : Figure
FRP : Fiberglass reinforced plastic
Ge : Gelcoat
Ge-GF : Mat and woven glass fiber and gelcoat resin
GFPG : Glassfiber/Polyester/Gelcoat
GFRP : Glass fiber reinforced plastics
Hard. : Hardener
hrs : Hours
ITTC : International Towing Tank Conference
lim : Limit
max : Maximum
MEKP : Methyl ethyl ketone peroxide
min : Minimum
No. : Number
Po-GF : Mat and woven glass fiber and polyester resin
Pos. : Position
pre. : Pressure
pro. : Propeller
xiii

PVC : Polyvinyl chloride
rpm : Revolution per minute
RTM : Resin transfer molding
SJ : Secondary-joint
SJ1 : Mat-mat secondary-joint
SJ2 : Mat-woven secondary-joint
SJ3 : Woven-woven secondary-joint
spec. : Specimen
SPSS : Statistical Package for the Social Sciences
suc. : Suction
Temp. : Temperature
Thick. : Thickness
TJ : Terminal-joint
TJ1 : No treatment terminal-joint
TJ2 : Grinding terminal-joint
TJ3 : Grinding/acetone terminal-joint
vs. : Versus
wt. : Weight

1



Chapter 1
Introduction




1.1 Overview
1.1.1 Thermoset composite materials
A composite material can be defined as a combination of two or more materials that
results in better properties than those of the individual components used alone. In contrast
to metallic alloys, each material retains its separate chemical, physical, and mechanical
properties. The two constituents are a reinforcement and a matrix. The main advantages of
composite materials are their high strength and stiffness, combined with low density, when
compared with bulk materials, allowing for a weight reduction in the finished part. The
reinforcing phase provides the strength and stiffness. In most cases, the reinforcement is
harder, stronger, and stiffer than the matrix. The reinforcement is usually a fiber or a
particulate [1]. Popular fibers include glass, carbon and aramid, which may be continuous
or discontinuous. Continuous-fiber composites normally have a preferred orientation,
while discontinuous fibers generally have a random orientation. Examples of continuous
reinforcements include unidirectional, woven cloth, and helical winding, while examples
of discontinuous reinforcements are chopped fibers and random mat. Continuous-fiber
composites are often made into laminates by stacking single sheets of continuous fibers in
different orientations to obtain the desired strength and stiffness properties with fiber
volumes as high as 60 to 70 percent. The type and quantity of the reinforcement determine
the final properties (Fig. 1-1).
The continuous phase is the matrix, which is a polymer, metal, or ceramic. Polymers

have low strength and stiffness, metals have intermediate strength and stiffness but high
ductility, and ceramics have high strength and stiffness but are brittle. The matrix
2

(continuous phase) performs several critical functions, including maintaining the fibers in
the proper orientation and spacing and protecting them from abrasion and the environment.
In polymer and metal matrix composites that form a strong bond between the fiber and the
matrix, the matrix transmits loads from the matrix to the fibers through shear loading at the
interface.

Fig. 1-1 Influence of reinforcement type and quantity on composite properties
The advantages of composites are many, including lighter weight, the ability to tailor
the layup for optimum strength and stiffness, improved fatigue life, corrosion resistance
and with good design practice, reduced assembly costs due to fewer detail parts and
fasteners. The specific strength (strength/density) and specific modulus (modulus/density)
of high strength fibers are higher than those of other comparable aerospace metallic alloys.
Applications include aerospace, transportation, construction, marine goods, sporting
goods, and more recently infrastructure, with construction and transportation being the
largest. In general, high-performance but more costly continuous-carbon-fiber composites
are used where high strength and stiffness along with light weight are required, and much
Fiber volume fractions (%)
3

lower-cost fiberglass composites are used in less demanding applications where weight is
not as critical. For the marine industry, corrosion is a major source of headache and
expense. Composites help minimize these problems, primarily because they do not corrode
like metals or wood.
1.1.2 Properties and application of composite materials in shipbuilding
During the second world war, just after polyester resins were first developed, glass
fibers became available following the accidental discovery of a production process using

blown air on a stream of molten glass. Soon, fiberglass reinforced plastic (FRP) came into
use and FRP boats started to become available in the early 1950’s. Since the 1950’s, resins
(polyester, epoxy and vinylester) have improved steadily and GRP has become without
doubt the most prevalent composite used in boatbuilding. Today, the reinforcement
materials are widely used as E-glass, S-glass and carbon fiber. Here, the E-glass fiber is
most commonly used with two types: fiberglass mat and fiberglass woven roving (Fig. 1-2).
Mat is a non-woven felt consisting of chopped glass fibers 1" to 2" in length, crisscrossed
and randomly interlocked and held together with a binder. Since it is not a woven cloth it
can be stretched to fit into difficult areas. Because it is highly absorbent it soaks up a great
deal of resin, and the resultant structure is stiffer than a layer of woven cloth, although not
as strong. Matting is often used between layers of woven cloth or roving, or as a
waterproof layer when there is no gel-coat. Woven rovings are composed of direct rovings
woven into a fabric. The input rovings are designed to give rapid wet-out and excellent
laminate properties. The construction gives bi-directional (0°/90°) reinforcement and the
strength of continuous filaments. Woven roving is a high strength, coarsely woven fabric
widely used in all phases of fiberglass molding. It has replaced all other lightweight grades
where speed of wet-out, durability and flatness of weave are important. Higher physical
properties are possible in laminates of this grade because of the reduced number of
interstices in the fabric.
Because of low cost, unsaturated polyester resin, fiberglass mat (density of 225, 300,
450g/m
2
, etc.) and fiberglass woven roving (density of 150, 360, 570g/m
2
, etc.) are used
for most structures of ship (hull, deck, bulkhead, etc.). These products can be manufactured
4

by hand lay-up or machine at room temperature. Epoxy, vinylester resin and carbon fiber
are used only for high strength or resistant chemical structures (propeller, engine-bed or

fuel tank). For the products making from glass or carbon fiber and polyester, epoxy or
vinylester resin, the mold and surface layer are very important. The mold can be made
from wood or metal and most surface layer is gelcoat resin. Gelcoats are designed to be
durable, providing resistance to ultraviolet degradation and hydrolysis. Specialized
gelcoats can be used to manufacture the molds which in turn are used to manufacture
components. Suitable resin chemistries for the manufacture of gelcoats vary, but the most
commonly encountered are unsaturated polyesters or epoxies.

Fig. 1-2 Manufacturing of E-glass mat and woven roving
(fs-yingzhe.en.made-in-china.com; rongbay.com)
1.2 Surface characteristics of lamina composite
The lamina composite products are formed from several layers. So the surface is first
considered including interface and outer surface (Fig. 1-3). Interface occurs in whole of
structures of composite boat and called joining (layer – layer, layer – plate, plate – plate,


Woven roving
Mat
5

etc.). There are three kinds of joints for glass fiber layers: co-curing, secondary-joint and
terminal-joint. Co-curing is the joining of layers together continuously during curing
process (lay-up process). When pre-curing upper and lower adherends (unperfected surface)
were bonded together, it is secondary-joint. If upper and lower adherends are perfect
surface, that bond is called terminal-joint. Co-curing is used usually for manufacture since
it is easier to carry out and has a better ability to link between layers, while surfaces of
secondary and terminal-joint must be well prepared to obtain necessary mechanical
properties [2]. Clearly, the quality of joints depends greatly on the preparation of surface of
the adherends.
Interface

Topcoat
Fiber/resin layer
Coating resin
Molding
Out-surface

Fig. 1-3 Surfaces of laminated composite structure
Besides, the outer surface (surfaces contact to mold) is quite important. When
fabrication of composite structures, a resin layer is applied first and after finish and
removing the mold, it becomes the coating. It relates to the strength, hardness and
aesthetics (smooth, color, etc.) of product. The outer surfaces can be gelcoat resin,
polyester or epoxy resin. For marine boat, the corrosion of surface occurs easily. The
gelcoat resin is used for this purpose with resistant corrosion property. However, for some
typical structures of ship such as propeller, the damage can cause by corrosion or
Outer surface