THE FLEXURAL BEHAVIOR OF COMPOSITE SANDWICH STRUCTURE
MOHD HAKIMI BIN OMAR
A report submitted in partial fulfillment of the requirements for the award of the degree of
Bachelor of Mechanical Engineering
Faculty of Mechanical Engineering
UNIVERSITI MALAYSIA PAHANG
OCTOBER 2008
SUPERVISOR’S DECLARATION
We hereby declare that we have checked this project and in our opinion this project is
satisfactory in terms of scope and quality for the award of the degree of Bachelor of
Mechanical Engineering.
Signature
Name of Supervisor :
Position :
Date :
Signature
Name of Panel :
Position :
Date :
STUDENT’S DECLARATION
I hereby declare that the work in this thesis is my own except for quotations and
summaries which have been duly acknowledged. The thesis has not been accepted for
any degree and is not concurrently submitted for award of other degree.
Signature
Name : Mohd Hakimi Bin Omar
ID Number : MA05037
Date :
DEDICATION
I humbly dedicate this thesis to
my lovely mom and father, Che Jamilah Binti Hasan and Omar Bin Muhamad
my dearest sister, Nur Hanani Binti Omar, Qistina Binti Omar, Hafizah Binti Omar,
Izzati Binti Omar and Nur Bazla Binti Omar
my dearest cousin, Nik Norma Binti Nik Idris
and to all my friends.
who always trust me, love me and had been a great source of support and motivation.
ACKNOWLEDGEMENTS
First of all, I want to thank The Almighty Allah SWT for the beautiful life that
has been given to me in the past 22 years and the present. I am very thankful to be given
the time and chances to finally complete this research.
I would like to express my sincere gratitude to my supervisor Mr. Mohd Ruzaimi
Bin Mat Rejab for his invaluable guidance, continuous encouragement and constant
support in making this research possible. He has always support me in times when I
faced difficulties during completing this research and constantly giving the best advice
to help me. I am always impressed with his effort in putting up with my attitude and still
treated me well as his student after giving him such a difficult time. I apologize for the
hard times. I also want to thank Dr. Ahmad Syahrizan Bin Sulaiman who have given his
best in making me understand how this research has to be done and have guided me
throughout the research.
My sincere thanks go to all my lab mates and members of the staff of the
Mechanical Engineering Department, UMP, who helped me in many ways whenever I
needed. Thanks for always putting up the best effort in helping me learn and familiarized
myself with the equipments in the lab so that I can finish this research.
The best thanks goes to my family especially to my parent. I am very thankful to
have them as my father and mother because they never gave up on me and constantly
support me morally and financially which are things that I needed the most in order to
complete this research. But most of all, thanks for the love and attention that they gave
to me which I will cherish until the end of time. Thanks for never stop believing in me
although I have let them down so many times. Thanks for always pray for my success
and happiness in the past, present and the future. Thanks for everything.
ABSTRACT
Sandwich construction is widely used because of its ability to provide high bending
stiffness coupled with light weight. The main objective of this study is to investigate the
effects of core under flexural test and to investigate performance of sandwich panels
from the energy absorption point of view. The fibre used is woven roving glass fibre
where the matrix used is based on saturated polyester resin. The composite skins of the
sandwich panels were prepared by hand lay-up technique. Three point bending test was
carried out to obtain flexural strength and flexural strain of the composite sandwich
structures. Two different cores of sandwich panels were tested in this project. Sandwich
panels with polyurethane core was found had higher flexural strength and higher energy
absorption compared with sandwich panels with polyethylene core. From the
observation, increasing the debonding strength of the core-face interface, the failure
mode changed from debonding of the core-face interface to the failure of the face. The
experimental results show that mechanical properties were improved when span
decreased and also depends on core materials. Thus, the influence of core materials and
span length of the sandwich panels has been evaluated.
ABSTRAK
Pembangunan struktur berlapis (sandwich structure) telah digunakan secara meluas
kerana kemampuan untuk menyediakan kekuatan lenturan yang tinggi dan struktur yang
lebih ringan. Objectif utama dalam kajian ini adalah untuk menyelidik kesan terhadap
perbezaan teras di bawah ujian lenturan dan mengkaji kemampuan struktur berlapis dari
sudut tenaga serapan. Bahan gentian yang digunakan adalah jenis gentian woven roving
dan polyester tak tepu sebagai bahan matrik. Permukaan komposit disediakan dengan
menggunakan teknik bengkalai tangan (Hand lay-up). Ujian 3 titik lenturan dijalankan
untuk mendapatkan kekuatan lenturan dan ketegangan lenturan struktur berlapis.
Struktur berlapis dengan menggunakan dua teras yang berbeza telah diujikaji dalan
projek ini. Struktur berlapis dengan teras polyurethane didapati menunjukkan kekuatan
lenturan dan penyerapan yang lebih tinggi dibandingkan dengan struktur berlapis yang
menggunakan polyethylene sebagai teras. Daripada pemerhatian, kekuatan antara muka
di antara teras dan permukaan meningkat, ragam kegagalan bertukar kepada kegagalan
berlaku di permukaan struktur. Keputusan ujikaji menunjukkan bahawa sifat mekanikal
dapat dipertingkatkan dengan mengurangkan panjang span dan juga bergantung kepada
bahan teras yang digunakan. Dengan demikian, pengaruh bahan teras dan panjang span
struktur berlapis telah dibuktikan.
TABLE OF CONTENTS
Page
SUPERVISOR’S DECLARATION ii
STUDENT’S DECLARATION iii
DEDICATION iv
ACKNOWLEDGEMENTS v
ABSTRACT vi
ABSTRAK vii
TABLE OF CONTENTS viii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF SYMBOLS xiv
LIST OF ABBREVIATIONS xv
CHAPTER 1 INTRODUCTION
1.1 Project Background 1
1.2 Problem Statement 2
1.3 Project Objective 3
1.4 Scope of Study 3
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction 4
2.2 History 4
2.3 Structure In Sandwich Panels 5
2.3.1 Faces/Skin 5 2.3.2 Principle of Skin 6
2.3.3 Fiberglass2.3.4 Properties of Fiberglass2.3.5 Core2.3.6 Principle of Core
788
11
2.4 Mechanical Testing 12
CHAPTER 3 METHODOLOGY
3.1 Introduction 16
3.2 Overview of Methodology 17
3.3 Define Problem 18
3.4 Literature Review 18
3.5 Sandwich Beam Geometry 18
3.6 Sandwich Panels Fabrication 19
3.6.1 Skin 193.6.2 Preparation of Mould 203.6.3 Preparation of Matrix Material 203.6.4 Preparation of Laminate 213.6.5 Preparation of Skin Specimens 213.6.6 Core 223.6.7 Bonding The Sandwich Panel 22
3.7 Mechanical Test 22
3.7.1 Three-point Bending Test 23
CHAPTER 4 RESULTS AND DISCUSSION
4.1 Introduction 26
4.2 Test Specimen 26
4.3 Load-Displacement Behavior 28
4.3.1 Polyethylene Core 284.3.2 Polyurethane Core 29
4.4 3-point Bending Result 31
4.4.1 Result for Polyethylene Core 314.4.2 Result for Polyurethane Core 324.4.3 Energy Absorption Result for Polyethylene Core 344.4.4 Energy Absorption Result for Polyurethane Core 35
4.5 Comparison Between Polyethylene and Polyurethane Core 38
4.6 Failure Mode of Sandwich Panels 40
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS
5.1 Conclusions 42
5.2 Recommendations 44
REFERENCES45
APPENDICESA Sample Calculation 48
B1 Stress-Strain Curve for Polyethylene Core 50
B2 Stress-Strain Curve for Polyurethane Core 54
C Comparison of Stress-Strain Curve Among Specimens 58
LIST OF TABLES
Table No. Page
4.1 Density of core material 27
4.2 Dimensions of Test Specimens 27
4.3 Experimental data of sandwich panels 29
4.4 Experimental data of sandwich panels 30
4.5 Experimental data of sandwich panels 32
4.6 Experimental data of sandwich panels 33
4.7 Energy absorption for polyethylene core 35
4.8 Energy absorption for polyethylene core 37
LIST OF FIGURES
Figure No. Page
2.1 The skin of the sandwich panels 5
2.2 How the stress act on the skin 6
2.3 Fiberglass 7
2.4 Types of core 9
2.5 Shear stiffness of typical core materials as a function of density 10
2.6 Sandwich construction with laminated-reinforced foam core 11
2.7 Core weak in shear 11
2.8 Core strong in shear 12
2.9 3-point and 4-point flexural tests 12
2.10 3-point bending test 13
2.11 3-point bending test apparatus 14
3.1 Flow chart of the project 17
3.2 Sandwich beam dimension used for three-point bend test 19
3.3 Woven roving glass fibre 20
3.4 The process of cutting the specimens using vertical T-jaw machine
21
3.5 Schematics of three point bend test fixture setup 23
3.6 Universal Testing Machine 3639 (INSTRON) 24
4.1 Load-displacement curve plots for polyethylene core 284.2 Load-displacement curve plots for polyurethane core 30
4.3 Stress-strain curves for polyethylene core 31
4.4 Stress-strain curves for polyurethane core 33
4.5 Energy absorption for polyethylene core at deflection 15mm 34
4.6 Energy absorption for polyurethane core at deflection 15mm 36
4.7 Stress-strain curves for polyethylene and polyurethane core at span 50mm
38
4.8 Stress-strain curves for polyethylene and polyurethane core at span 250mm
39
4.9 Energy absorption for polyethylene and polyurethane core 39
4.10 Core-face interface debonding 40
4.11 Failure of face by fiber breakage 41
4.12 Failure of the core 41
LIST OF SYMBOLS
Strain
Stress
f Flexural Strength
fE Modulus of Elasticity
P Load
L Support Span
b Width of Test Beam
d Depth of Test Beam
D Deflection
m Slope
Density
v Volume
m Mass
LIST OF ABBREVIATIONS
FRP Fiber reinforced plastic
GRP Glass reinforced plastic
PUR Polyurethane
PIR Polyisocyanurate
PF Phenolic Foam
EPS Expanded Polystyrene
XPS Extruded Polystyrene
ASTM American Society for Testing and Material
MEKP Methyl ethyl ketone peroxide
CHAPTER 1
INTRODUCTION
1.1 PROJECT BACKGROUND
A sandwich structure composite is a special class of composite materials that is
fabricated by attaching two thin but stiff skins to lightweight but thick core. The core
material is normally low strength material but its higher thickness provides the sandwich
composite with high bending stiffness with overall low density. As a result sandwich
components achieve the same structural performance as conventional materials with less
weight. Sandwich structures are used in many applications ranging from buildings to
aerospace systems because of their high specific bending stiffness and strength and good
acoustical insulation. Sandwich structure also ideal for large ship hulls because of their
good shock resistance combined with reasonable affordability. This project is to study
the effect of core sandwich structure under flexural test that is to determine the flexural
behavior based on core material and to identify the span length of different core
material. The other is to investigate performance analysis of sandwich structure in term
of energy absorption using different core material.
1.2 PROBLEM STATEMENT
Sandwich composite is highly subject to damage and crack. This damage and
defect is very difficult to detect in composite materials. For example in wind turbine, the
damage can occur from events such as dropped tools during maintenance or debris
impacting during periods of high winds. This will cause delaminations, core disbonds,
and crack. The defect will reduce the stiffness of the structures and will damage the
fatigue life of the component. Usually, the damage cannot see with naked eye. Besides
that, defects can also arise during manufacture, particularly where pre-pregs are not
used. This defect can take the form of resin dry spots, skin-core disbonds and such.
While the cause and nature of defects are wide ranging the effect is similar. Composite
sandwich structure such as wind turbines and wings are subjected to large flexural
loadings. Consequently, the flexural behavior of these structures is critical to their use.
To overcome this problem, testing methodology that can simulate the flexural loading is
needed so that the suitability of composite sandwich constructions can evaluate to the
application. In addition, the ability to determine the effect of defects on stiffness and
fatigue performance will allow for better service life prediction.
1.3 PROJECT OBJECTIVE
1.3.1 To investigate the effects of core sandwich structure under flexural test.
1.3.2 To investigate the performance of sandwich structure from the energy absorption
point of view.
1.4 SCOPE OF STUDY
1.4.1 Determination of flexural behavior based on different core material.
1.4.2 Identification the span length of different core material.
1.4.3 Performance analysis of energy absorption using different core material.
CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
International research effort continuously looking for new, better, and efficient
construction materials. The main goal is to improve the structural efficiency,
performance, and durability of civil engineering and transportation applications. In the
past decades various sandwich panels have been utilized in the construction of
aerospace, marine, architectural, and transportation industry [1]. Lightweight, excellent
corrosion characteristics and rapid installation capabilities created tremendous
opportunities for these sandwich panels in the industry [1]. This structure provides an
efficient use of the materials and utilization to each component to its ultimate limit. The
structures offers also very high stiffness-to-weight ratio [10]. It enhances the flexural
rigidity of a structure without adding a substantial weight therefore it provides
significant advantages in comparison to the use of the material alone for structural
system [1]. Sandwich construction has superior fatigue strength and excellent acoustical
and thermal insulation [9]. Sandwich structures also ideal for large ship hull because of
their good shock resistance combine with reasonable affordability [9].
2.2 HISTORY
Historically, the principle of using two cooperating faces separated by a distance
in between was introduced in 1820 by Delau [1]. The first extensive use of sandwich
panel was during the World War II. In the “Mosquito” aircraft, sandwich structure was
used, mainly because of the shortage of other materials in England during the war [3].
The faces were made of veneer while the core consisted of balsa wood. One of the early
uses of sandwich structure in an aeroplane application was in 1937 where balsa wood
core and cedar plywood face sheets was used in the construction of De Havilland
albatross airplane [1]. During World War II the first theoretical analysis of sandwich
theory was published [1]. By the completion of World War II and in the late 1940’s,
some of the first theoretical works on sandwich constructions documented [1].
2.3 STRUCTURE IN SANDWICH PANELS
Generally, single layer sandwich structure consists of three main parts that is two
face sheets and a core. With an additional sheet, called internal sheet, inserted into the
core, a two-layer sandwich panel is then formed.
2.3.1 Faces/Skin
The faces adhesively bonded to the core to obtain a load transfer between the
components. This way the properties of each separate component is utilized to the
structural advantages of the whole assembly leading to a very high stiffness-to-weight
and high bending strength-to-weight ratio. Typically, the facing layer realized by
aluminum plates, high pressure laminates, and glass fiber reinforced plastics. Basically,
the skins are thin, stiff, and very strong. Figure 1 below show the example of skin part in
sandwich structures.
Figure 2.1 The skin of the sandwich panels [5]
facingadhesive
adhesiveTricel honeycom
2.3.2 Principle of Skin
Figure 2.2 How the stress acts on the skin [4]
From the Figure 2.2, it shows how the stress acts on the skin of the sandwich
structures when load is applied. If the panels bend, the core and the skin elongate and
shrink linearly from the neutral axis. The thick black line represents the new section of
the panel after bending. Because the skin are glue to the core, both of them will stretch
the same amount where they bond together but physical properties are completely
different and therefore will react differently to this elongation. Equation below will show
the relation between stress and strain.
L
(2.1)
Strain = amount of stretch (in)/original length (in)
E (2.2)
Stress (psi) = Modulus of elasticity (psi) x strain
Logically, the force acting on the skin is larger than the core because of the large
modulus of elasticity for the skin material. Equal strain at the boundary multiplied by
larger modulus will produce larger stress in the skin.
2.3.3 Fiberglass
Figure 2.3 Fiberglass [6]
Fiberglass or also called glass fiber (Figure 2.3) is a material made from
extremely fine fiber of glass. It is use as a reinforcing agent for many polymer products.
The basis of textile grade glass fiber is silica, 2SiO and in pure form it’s exists as a
polymer nSiO )( 2 . It has no true melting point but softens up to 2000 Co where it starts to
degrade. Glass fibers are useful because of their high ratio of surface area to weight. The
resulting composite material, properly known as fiber-reinforced polymer (FRP) or
glass-reinforced plastic (GRP), is called "fiberglass" in popular usage in the industries.
Glassmakers throughout history have experimented with glass fibers, but mass
manufacture of fiberglass was only made possible with the advent of finer machine-
tooling. In 1893, Edward Drummond Libbey exhibited a dress at the World's Columbian
Exposition incorporating glass fibers with the diameter and texture of silk fibers. What is
commonly known as "fiberglass" today, however, was invented in 1938 by Russell
Games Slayter of Owens-Corning as a material to be used as insulation. It is marketed
under the trade name Fiberglas, ® which has become a genericized trademark.
2.3.4 Properties of Fiberglass
Glass fibers are useful because of their high ratio of surface area to weight.
However, the increased surface area makes them much more susceptible to chemical
attack. By trapping air within them, blocks of glass fiber make good thermal insulation,
with a thermal conductivity of 0.05 W/m-K.
Glass strengths are usually tested and reported for "virgin" fibers: those which
have just been manufactured. The freshest, thinnest fibers are the strongest because the
thinner fibers are more ductile. The more the surface is scratched, the less the resulting
tenacity [13]. Because glass has an amorphous structure, its properties are the same
along the fiber and across the fiber [12]. Humidity is an important factor in the tensile
strength. Moisture is easily adsorbed, and can worsen microscopic cracks and surface
defects, and lessen tenacity.
In contrast to carbon fiber, glass can undergo more elongation before it breaks
[12]. The viscosity of the molten glass is very important for manufacturing success.
During drawing (pulling of the glass to reduce fiber circumference) the viscosity should
be relatively low. If it is too high the fiber will break during drawing, however if it is too
low the glass will form droplets rather than drawing out into fiber.
2.3.5 Core
The core is the structure that placed between two thin faces of the sandwich
structure. The core material is normally low strength material but its higher thickness
provides the sandwich composite with high bending stiffness with overall low density
[7]. The function of the core is to support the thin skin so that they don’t deform and stay
fixed relative to each other. The core of a sandwich structure can be almost any material
Energy Absorbtion at deflection 15mm
7.356962
4.516049
1.6800741.4848270.915761
0
1
2
3
4
5
6
7
8
Polyurethane
En
erg
y A
bso
rbti
on
(J)
50mm
100mm
150mm
200mm
250mm
Figure 4.6 Energy absorption for polyurethane core at deflection 15mm
From Figure 4.6, the maximum energy values were obtained at 50mm span
which is 7.356962J and then values were started to decreasing. The value of energy
absorption decreased at 100mm span and this decrement continues at span 150mm,
200mm and 250mm. The lowest energy absorption occurred at 250mm span which is
0.915761J. The others specimens started from 100mm to 200mm span, the energy
absorption are located in the range of 5.0J to 1.0J. The specific energy absorption for
polyurethane core are summarize in Table 4.8.