THE INVESTIGATION OF MECHANICAL BEHAVIOR OF LIGHWEIGHT ALUMINIUM FOAM SANDWICH (AFS)

ii

THE INVESTIGATION OF MECHANICAL BEHAVIOR OF LIGHWEIGHT ALUMINIUM FOAM SANDWICH (AFS)

BY

NUR ASMAWIYAH BINTI IBRAHIM

A thesis submitted in fulfilment of the requirement for the Master of Science (Manufacturing Engineering)

Kulliyyah of Engineering

International Islamic University Malaysia

AUGUST 2018

iii

ABSTRACT

Demand and interest for the use of porous materials in various applications are rapidly growing by years. Several types of porous materials had been introduced in market todays and one of the well-known types is metal foam. Yet, metal foam itself is weak and to overcome the limitation, sandwich structure had been introduced which is aluminum foam sandwich (AFS). It has many advantages including excellent stiffness to weight ratio is, high energy absorption and most importantly lightweight. There is the need for lightweight material in structural parts for reducing used of energy and eventually reduce fuel consumption. The applications of AFS are automotive, aerospace, shipbuilding and architectural design industries. There were many researchers who had done an investigation on mechanical behavior of AFS. However, a few numbers did a research on open-cell aluminum foam and none of them identify the effect of skin to core thickness ratio, if any. Therefore, this research was conducted to identify the effect of skin to core thickness ratio on mechanical behavior of AFS when loaded under tension and three point bending experimentally with validation of simulation study. AFS specimens were made of open-cell aluminum foam as a core and attached with 6061-0 aluminum skin sheets using epoxy and hardener. Full factorial design of experiment (DOE) was used and repeated three times for each test. Three levels of skin thickness and three levels of core thickness had been used for tensile test. While for three-point bending test, DOE was developed using three levels of skin thickness and two levels of core thickness. Experimental results showed that by increasing skin to core thickness ratio, strength, force and deflection of AFS also increase for both tension and bending. Besides, results show that core thickness play an important role in effecting behavior of open-cell aluminum foam sandwich because of the percentage of porosity of the foam. Increasing foam thickness, will increase percentage of pore which will weaken the sandwich panels. Simulation study was conducted using LS-DYNA software and showed an agreement with experimental result of sandwich panel’s deformation and force-displacement curve. Statistical analysis details show that both models of tensile and three-point test were significant and reliable with

‘Prob > F’ less than 0.05. The optimum skin to core ratio for tensile and three-point bending test were 0.1 and 0.12 respectively. Stiffness to weight ratio of AFS was increasing with higher core thickness. Lastly, stiffness of proposed porous material (open-cell foam) had better stiffness compared to other porous material with more than 40% higher stiffness.

iv

ثحبلا ةصلاخ

أ تراث داوملا ةيِماسَملا هابتنلاا

يف تاونسلا ةيضاملا

تدازو ةبسن

اهبلط ربع نينسلا امم ىدأ إ ىل راكتبا

عاونأ ةفلتخم هنم اهضرعو يف

،قوسلا دحأ

رهشأ هذه عاونلأا يه

ةوغرلا ةيندعملا

ريدج . ركذلاب نأ

ةوغرلا ةيندعملا

يف اهتاذ

،ةفيعض زايتجلاو

زجَعلا اذه مت لاخدا لكيهلا ىّمسُملا شتودنسب

وغر ة

موينموللأا aluminum foam sandwich)

عتمتي .) اذه لكيهلا كلاب ريث نم تازيمملا هتبلاصك

ةبسانملا ةبسنل

،هنزو و ةعرُس هصاصتما

،ةقاطلل مهلأاو

نم كلذ هنزو فيفخلا دوجو .

اذه رصنعلا

مهُم ليلقتلل نم ةبسن ةقاطلا ةمدختسُملا دحلاو

نم كلاهتسا طفنلا

نكمي . قيبطت شتودنس ةوغر

موينموللأا

ىلع ميماصتلا ةيرامعملا

ةيضرلأا ةيوجلاو

ةيرحبلاو كانه .

ديدعلا نم ثاحبلأا يتلا

تيرجأ ىلع كولسلا

يكيناكيملا شتودنسل

ةوغر موينموللأا (AFS)

َّ نكلو ثوحبلا ةصاخلا

ايلاخلاب ةحوتفملا

ةوغرل

موينموللأا دعُت

ةردان

،اًدج ليلقلاو اهنم طقف ن يَب نم

ريثأت ةبسن ةرشقلا ةاونلل نإ - دِجُو كلذل .

، نإف هذه

ةساردلا دق

تيرجُأ ك ي نّيبُت ريثأت ةبسن ةرشقلا ةاونلل ىلع كولسلا يكيناكيملا

شتودنسل ر

ةوغ موينموللأا

دنع هض ُّرعت طغضلل

ينثلاو يثلاُث

،طاقنلا امك مت دكأتلا نم ةحص جئاتنلا ةساردب ةاكاحُملا مت .

ذخأ تانّيع

نم ةاون ةيلخل ةحوتفم ةوغرل

موينموللأا ةقلعُم

6061-0 ب حئارش

روشقل ينيموللأا مو

مادختساب

يسكوبيلإا مت .

مادختسا ميمصت

براجتلا (DOE)

لماك لماوعلا ةداعإو

ةبرجتلا ثلاث

تارم لكل رابتخا .

كلذك مت مادختسا ثلاث

تايوتسم ةكامسل

ةرشقلا ثلاثو تايوتسم ةكامسل

ةاونلا رابتخلا شلا

. د امأ ةبسنلاب

رابتخلا ينثلا

يثلاُث طاقنلا دقف مت مادختسا ثلاث

تايوتسم ةكامسل

ةرشقلا نْيَيوتسمو

ل ةكامس

ةاونلا ترهظأ . جئاتنلا

هنأ املك تداز ةبسن ةرشقلا

،ةاونلل املك داز داهجإ ةوقو ةحازإو شتودنس

وغر ة

موينموللأا AFS)

َّ لكل ) نم طغضلا ينثلاو

بناجب .

،كلذ ترهظأ جئاتنلا

نأ كمُس ةاونلا اًرود بعلي

اًمه َُّم

يف كولس ةاون ةيلخلا ةحوتفملا غرل

ةو موينموللأا ببسب

ةبسنلا ةيوئملا ةيماسمل

،ةوغرلا ذل

ا نإف ةدايز

كمُس

،ةوغرلا ديزيس

نم ةبسنلا ةيوئملا تاماسملل يذلاو

يدؤيس ىلإ

فاعضإ تاحول

شتودنسلا مت .

دامتعلاا ىلع

جمانرب . لا

- سا يد ياو نا ( هيإ LS-DYNA ءارجلإ )

جذومن

،ةاكاحُملا يذلاو

تَمعد

هجئاتن ةبرجتلا معلا

ةّيل اميف صخي رييغتلا يلكشلا ةحولل

شتودناسلا ىنحنُمو

ةوق ةحازلإا أ .

ترهظ

جئاتنلا ةيئاصحلإا نأ

ًَّّلاُك يَرابتخا نم دشلا

و يثلاُث طاقنلا قوثوم ربتعُمو اًذخأ يف رابتعلاا نأ

ةيلامتحلاا

ربكأ نم ( فآ Prob > F’

ةبسنب ) لقت 0.05 نع

امك . نأ ىوتسملا لثملأا

َّ لُكل نْيَرابتخلاا نم

0.1 وه

،دشلل 0.12 و

يثلاثل طاقُنلا و املك تداز ةبلاص ةاونلا املك تداز ةبسن ةبلاصلا نزولل

ةصاخلا

شتودناسب ةوغر

موينموللأا AFS)

)

،اًريخأو . نإف

ةبلاص داوملا ةيِماسَملا ةحرتقُملا

ةوغر ( تاذ ةيلخ

ةحوتفم ناك )

اهيدل ةبلاص لضفأ نم داوملا ةيِماسَملا لآا

ىرخ يتلا كلتمت ةبسن ىلعأ 40 ب

نم %

ةبلاصلا

.

v

APPROVAL PAGE

I certify that I have supervised and read this study and that in my opinion, it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a thesis for the degree of Master of Science (Manufacturing Engineering)

………..

Muataz Hazza Faizi Al Hazza Supervisor

………..

Erry Yulian Triblas Adesta Co-Supervisor

I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a thesis for the degree of Master of Science (Manufacturing Engineering).

………..

Ahsan Ali Khan Internal Examiner

………..

Mohd Amri Lajis External Examiner

This thesis was submitted to the Department of Manufacturing and Materials Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Master of Science (Manufacturing Engineering).

………..

Mohamad Abd. Rahman

Head, Department of Manufacturing and Materials Engineering

This thesis was submitted to the Kulliyyah of Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science (Manufacturing Engineering).

………..

Erry Yulian Triblas Adesta Dean, Kulliyyah of Engineering

vi

DECLARATION

I hereby declare that this thesis is the result of my own investigations, except where otherwise stated. I also declare that it has not been previously or concurrently submitted as a whole for any other degrees at IIUM or other institutions.

Nur Asmawiyah binti Ibrahim

Signature... Date...

vii

COPYRIGHT PAGE

INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA

DECLARATION OF COPYRIGHT AND AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH

THE INVESTIGATION OF MECHANICAL BEHAVIOR OF LIGHTWEIGHT

ALUMINIUM FOAM SANDWICH (AFS)

I declare that the copyright holders of this thesis are jointly owned by the student and IIUM.

Copyright © 2018 Nur Asmawiyah binti Ibrahim and International Islamic University Malaysia. All rights reserved.

No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder except as provided below

1. Any material contained in or derived from this unpublished research may be used by others in their writing with due acknowledgement.

2. IIUM or its library will have the right to make and transmit copies (print or electronic) for institutional and academic purposes.

3. The IIUM library will have the right to make, store in a retrieved system and supply copies of this unpublished research if requested by other universities and research libraries.

By signing this form, I acknowledged that I have read and understand the IIUM Intellectual Property Right and Commercialization policy.

Affirmed by Nur Asmawiyah binti Ibrahim

……..……….. ………..

Signature Date

viii

ACKNOWLEDGEMENTS

‘In the name of Allah the Most Gracious, and the Most Merciful’

This research has been carried out in International Islamic University Malaysia (IIUM) since 2016 under supervision of Assoc. Prof. Dr. Muataz Hazza Faizi AL Hazza. I would like to express my deepest appreciation for his patient and continuous guidance with endless support and motivation. His comments and advices are very valuable to me. Thank you for spending your time guiding me and all the knowledge you had given to me. My sincere thanks also to Prof. Dr. Erry Yulian Triblas Adesta for his valuable comments and supports.

My unbounded gratitude goes to my mother, Hajah Mek Som for all her support and dua’, my brothers (Amir, Juhari, Burhan and Ahmad Feroz), my other family members and friends for their encouragement and support throughout the accomplishment of this study.

My sincere gratitude also to Br. Ameen Topa for his endless support and help on teaching me simulation study. Thank you so much for your time and knowledges. Besides, I also want to extend my gratitude to all staffs in Department of Manufacturing and Materials Engineering for their guidance and assistance throughout to finish this study.

This research was supported by International Islamic University Malaysia through Fundamental Research Grant Scheme (FRGS15-247-0488) by Ministry of Higher Education, Malaysia.

Last but not least, I dedicate this writing to my late father, Allahyarham Haji.

Ibrahim Mamat. May Allah S.W.T grant him happiness and Jannah. Amin.

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TABLE OF CONTENT

Page

Abstract ... iii

Arabic Abstract ... iv

Approval Page ... v

Declaration ... vi

Copyright Page ... vii

Acknowledgements ... viii

Table of Content ... ix

List of Figures ... xii

List of Tables ... xvi

List of Abbreviations ... xviii

List of Symbols ... xix

CHAPTER ONE: INTRODUCTION ... 1

1.1 Background ... 1

1.2 Problem Statement and Its Significance ... 4

1.3 Research Objecives ... 6

1.4 Research Methodology ... 6

1.5 Scope of the Project ... 7

1.6 Dissertation Outline ... 8

CHAPTER TWO: LITERATURE REVIEW ... 9

2.1 Overview ... 9

2.2 Introduction ... 9

2.3 Foam Structure ... 10

2.3.1 Theory and It’s Fabrication ... 10

2.3.2 Mechanical Characterization and Failure Modes of Foam Structure ... 11

2.4 Sandwich Structure ... 12

2.4.1 Foam and Skin Thickness Parameters ... 14

2.4.2 Specimens Cutting Method ... 17

2.4.3 Fabrication of Aluminium Foam Sandwich (AFS) ... 18

2.4.4 Mechanical Testing Technique ... 20

2.4.4.1 Flexural Test (Three Point Bending) ... 21

2.4.4.2 Tensile Test ... 24

2.4.4.3 Edgewise Compression Test ... 25

2.4.5 Failure Mode of Sandwich Structure ... 27

x

2.5 Material Modelling ... 30

2.5.1 Constitutive Models... 31

2.6 Statistical Analysis ... 34

2.6.1 Design of Experiment ... 35

2.6.2 Optimization method ... 36

2.7 Stiffness to Weight Ratio ... 36

2.8 Application of Aluminium Foam Sandwich ... 38

2.9 Summary ... 40

CHAPTER THREE: METHODOLOGY ... 41

3.1 Introduction ... 41

3.2 Material Preparation and Fabrication ... 42

3.2.1 Materials ... 42

3.3 Equipment and Testing Machine ... 48

3.4 Simulation Method and Material Modelling ... 50

3.5 Design of Experiment (DOE) ... 55

3.6 Data Analysis ... 57

CHAPTER FOUR: RESULTS AND DISCUSSION ... 59

4.1 Overview ... 59

4.2 Tensile Test... 60

4.2.1 Experimental Study ... 60

4.2.1.1 Experimental Result and data Compilation ... 60

4.2.1.2 Failure Modes Analysis ... 64

4.2.2 Simulation Study ... 67

4.2.2.1 Specimen Setup and Boundary Condition ... 67

4.2.2.2 Simulation Results and Comparison ... 68

4.2.3 Statistical Analysis ... 76

4.2.3.1 Tensile Stress Analysis ... 77

4.2.3.2 Force Analysis ... 78

4.2.3.3 Displacement Analysis ... 81

4.2.3.4 Scatter Diagram Analysis ... 82

4.2.3.5 Desirability Function ... 85

4.3 Three Point Bending (3PB) ... 86

4.3.1 Experimental Study ... 86

4.3.1.1 Experimental Results and Data Compilation ... 86

4.3.1.2 Failure Modes Analysis ... 90

4.3.2 Simulation Study ... 92

4.3.2.1 Specimen Setup and Boundary Condition ... 92

4.3.2.2 Simulation Result and Comparison ... 93

4.3.3 Statistical Analysis ... 98

4.3.3.1 Bending Stress Analysis ... 99

4.3.3.2 Force Analysis ... 101

4.3.3.3 Displacement Analysis ... 103

4.3.3.4 Scatter Diagram ... 105

4.3.3.5 Desirability Function ... 107

4.4 Stiffness to Weight Ratio ... 108

xi

CHAPTER FIVE: CONCLUSION ... 113

5.1 Conclusion ... 113

5.2 Future Recommendation... 116

REFERENCES ... 117

PUBLICATIONS ... 124

CONFERENCES ... 125

APPENDIX A: LIST OF EQUATIONS ... 126

APPENDIX B: LIST OF SUMMARIZATION TABLES ... 128

xii

LIST OF FIGURES

Figure 1.1 a) open-cell foam, b) closed-cell foam (Veale, 2010) 2 Figure 1.2 Bumper with crash box (Belingardi, Beyene, Koricho & Martorana, 2015) 3 Figure 2.1 Steps in manufacture the aluminium foam sandwich 18 Figure 2.2 Types of failure modes in three point bending (D’Urso & Maccarini, 2012) 27 Figure 2.3 Types of failure modes in edgewise compression test (Hou et. al., 2014) 29 Figure 3.1 Flowchart of the research 42

Figure 3. 2 Araldite epoxy resin and hardener 43

Figure 3. 3 Open-cell aluminium foam 43

Figure 3. 4 Aluminium sheet 44

Figure 3. 5 Electric discharge machining (wire cut) 45

Figure 3. 6 Scroll saw 45

Figure 3. 7 Dog-bone shape of AFS specimens 46

Figure 3. 8 Rectangular shape of AFS specimens 47

Figure 3. 9 Ultrasonic cleaner (Branson 2510) 47

Figure 3. 10 a) Dog-bone specimens for tensile test, b) Rectangular specimens for three

point bending 48

Figure 3.11 Flow-chart of specimens preparation 48

Figure 3.12 Universal testing machine (INSTRON) for tensile test 49

Figure 3. 13 Dog-bone specimen set up 49

Figure 3.14: Universal testing machine (INSTRON) for 3-point bending test 50

Figure 3. 15 Specimen set up for three point bending 50

Figure 3.16 Stress/strain curve of aluminium skin 52

xiii

Figure 4.1 Microscopic view of open-cell aluminium foam. 59 Figure 4.2 Force-displacement curve of tensile test of sandwich structure. 63 Figure 4.3 Force-displacement curves of AFS when loaded under tension. 64 Figure 4.4 Breakage of dog-bone specimen in gauge length after loaded under tension. 65 Figure 4.5 Breakage of dog-bone specimen outside of gauge length 65 Figure 4.6 Comparison between experimental and simulation result 66

Figure 4.7 Core shear failure of AFS. 67

Figure 4.8 a) Tensile test setup for experimental, b) Tensile test setup in simulation. 68 Figure 4.9 Force-displacement curve for core thickness of 3.2 mm and skin thickness of

0.4 mm of both experimental and simulation study 71

Figure 4.10 Force-displacement curve for core thickness of 3.2 mm and skin thickness of

0.6 mm of both experimental and simulation studies 71

Figure 4.11 Force-displacement curve for core thickness of 3.2 mm and skin thickness of

0.8 mm of both experimental and simulation studies 72

Figure 4.12 Force-displacement curve for core thickness of 6.35 mm and skin thickness of

0.6 mm of both experimental and simulation studies 72

Figure 4.13 Force-displacement curve for core thickness of 6.35 mm and skin thickness of

0.8 mm of both experimental and simulation studies 73

Figure 4.14 Force-displacement curve for core thickness 10 mm and skin thickness 0.4 mm

of both experimental and simulation studies 74

Figure 4.15 Force-displacement curve for core thickness 10 mm and skin thickness 0.6 mm

of both experimental and simulation studies 74

Figure 4.16 Force-displacement curve for core thickness 10 mm and skin thickness 0.8 mm

of both experimental and simulation studies 75

Figure 4.17 3D plot surface profiler of interaction between input parameters and tensile

stress 78

xiv

Figure 4.18 3D plot surface profiler of interaction between input parameters and bending

force 80

Figure 4.19 3D plot surface profiler of interaction between input parameters and

displacement of AFS 82

Figure 4.20 Scatter diagram of skin thickness factor on output responses 83 Figure 4.21 Scatter plot of core thickness factor on output responses 84

Figure 4.22 Prediction profiler of tensile test 85

Figure 4.23 Force-displacement curve for three point bending test 89

Figure 4.24 Failure modes of AFS under three point bending 91

Figure 4.25 a) 3PB setup for experimental, b) 3PB setup for simulation model 93 Figure 4.26 Force-displacement curve of three point bending on AFS 94 Figure 4. 27 Force-displacement curve for core thickness of 3.2 mm and skin thickness of

0.2 mm of both experimental and simulation studies 95

Figure 4.28 Force-displacement curve for core thickness of 3.2 mm and skin thickness of

0.4 mm of both experimental and simulation studies 96

Figure 4.29 Force-displacement curve for core thickness of 3.2 mm and skin thickness of

0.6 mm of both experimental and simulation studies 97

Figure 4.30 Force-displacement curve for core thickness of 6.35 mm and skin thickness of

0.6 mm of both experimental and simulation studies 97

Figure 4.31 3D plot surface profiler of input parameters against bending stress 101 Figure 4.32 3D plot surface profiler of interaction between input parameters on bending

force 103

Figure 4.33 3D plot surface profiler of input parameters on displacement 104 Figure 4.34 Scatter diagram of skin thickness with responses 105 Figure 4.35 Scatter diagram of core thickness with responses 106 Figure 4.36 Prediction profiler of three point bending test 108

xv

Figure 4.37 STW ratio for AFS with different core and skin thickness 111 Figure 5. 1 Summarization of tensile test analysis 114 Figure 5. 2 Summarization of 3 point bending test analysis 114

xvi

LIST OF TABLES

Table 2.1 Summary of skins and cores thickness used by previous researchers 16 Table 2.2 List of simulation software used by previous researchers 31 Table 3.1 Details of open-cell aluminium foam 44

Table 3.2 Details of aluminium skin sheets 44

Table 3.3 Physical and mechanical properties of aluminium skin 52 Table 3.4 Material parameters for the analyses with statistical variation 53

Table 3.5 Material parameters of aluminium foam core 54

Table 3.6 Material properties of steel 54

Table 3.7 Range of input parameters 56

Table 3.8 Design of experiment for tensile test 56

Table 3.9 Design of experiment for three point bending 57

Table 4.1 Average value of ultimate tensile stress at maximum load 61

Table 4.2 Force value at maximum stress 61

Table 4.3 Displacement of AFS at break 62

Table 4.4 Deformation behavior of AFS for experimentation and simulation. 69

Table 4.5 Summary of responses of tensile test 76

Table 4.6 Analysis of variance of stress 77

Table 4.7 Summary of fit model of tensile stress 77

Table 4.8 Analysis of variance of force 79

Table 4.9 Summary of fit model of force 79

Table 4.10 Analysis of variance of displacement response 81

Table 4.11 Summary of fit model of displacement response 81

xvii

Table 4.12 Bending stress of AFS at maximum force 87

Table 4.13 Bending force of AFS 88

Table 4.14 Bending displacement of AFS at break 88

Table 4.15 AFS deformation when loaded under bending for experimentation and

simulation 93

Table 4.16 Summary of responses of 3PB test 99

Table 4.17 ANOVA for bending stress 100

Table 4.18 Summary of fit model for bending stress 100

Table 4.19 ANOVA for force response 101

Table 4.20 Summary of fit model of force 102

Table 4.21 ANOVA for displacement response 103

Table 4.22 Summary of fit model for displacement 104

Table 4.23 Details of three point bending test on AFS for calculating STW ratio 110

Table 4.24 Summary of STW ratio results 110

Table 4.25 Flexural stiffness of different porosity types 112

Table 5. 1 Flexural stiffness of different types of porous material 115

Table 5. 2 Summary of optimization results 116

xviii

LIST OF ABBREVIATIONS

3D Three Dimensional

3PB Three Point Bending

AFS Aluminium Foam Sandwich

ANOVA Analysis of Variance

ASTM American Society for Testing and Materials

EDM Electric Discharge Machining

EDS Energy Dispersive X-ray Spectroscopy

EXD Energy Dispersive

FEA Finite Element Analysis

JMP JUMP Statistical Analysis Software

Kfile Text files and can be open in any text editor LS-DYNA Livermore Software Finite Element Program

LS-Prepost Livermore Software for Preprocessing and Post processing

OM Optical Microscope

SEM Scanning Electron Microscope

STW Stiffness to weight ratio

xix

LIST OF SYMBOLS

l Length of sandwich beam (mm)

b Width of sandwich beam (mm)

s Support length (mm)

t Skin thickness (mm)

c Core thickness (mm)

F Force (N)

d Displacement (mm)

ρ Density (kg/m³)

W Weight (kgf)

m Mass (kg)

g Gravity (mm/s²)

𝜌_𝑓 Foam density (kg/m³)

𝜌_𝑓𝑜 Foam base material density (kg/m³)

mm Millimeter

mm/min Millimeter per minutes

N Newton

MPa Mega pascal

E Young’s modulus

𝐸_𝑓𝑜 Young’s modulus of foam base material

PR Poisson’s ratio

α, β Dimensionless constants 𝛾 Kinematic hardening parameter 𝜀_𝐷 Densification strain

𝐶₁, 𝐶₂, n Material constants 𝜎_𝑝 Density stress

CHAPTER ONE INTRODUCTION

1.1 BACKGROUND

Demand and interest for the use of porous materials in various applications are rapidly growing by years. Porous material is a material containing pores and struts. Several types of porous materials had been introduced in market today such as foams, honeycomb and balsa wood. At the beginning of production of foam material, polymer- based foam had been introduced. However, polymer had limitation in heat resistant (Styles, Compston & Kalyanasundaram, 2005). Besides, there is an increasing in the amount of waste as increasing the used of polymer foam in shipbuilding industries (Crupi, Epasto, & Guglielmino, 2013). Thus, metal foam was developed and one of the famous metals foams with many industries applications is aluminum foam. Metal foam is also easier to recycle compared to polymer foam (Banhart, Schmoll & Neumann, 1998).

There are two categories of aluminum foam which is open-cell foam and closed- cell foam. Figure 1.1 below shows the different structure and porosity of both foam cell types.

2

Figure 1.1 a) open-cell foam, b) closed-cell foam (Veale, 2010)

Open-cell metal foam had many advantages compared to closed-cell such as high interconnectivity, high moisture absorption and chemical leached. Closed-cell metal foam had disadvantage in term of closed-cell may contain undesired chemical.

Aluminum foam can be different based material such as aluminum 6061 which produce by Banhart, Schmoll and Neumann (1998). They used powder metallurgical technology by using metal powder and foaming agent of TiH2. Yet aluminum foam itself is weak, thus aluminum foam sandwich (AFS) were produced which consist of foam as core and thin solid material as upper and lower skin (Banhart, Schmoll & Neumann, 1998).

There were many advantages of sandwich panels with metallic foam cores.

Crupi and Montanini (2007) stated that the properties of AFS was high energy dissipation, low specific weight, high damping, thermal insulation, and high strength impact. According to Banhart, Schmoll and Neumann (1998), the porous materials had grown in various application because of its excellent physical properties. This also supported by Styles, Compston & Kalyanasundaram (2005) who reported that metal foam has high impact energy absorption, good strength and stiffness to weight, electromagnetic wave absorption, good sound damping, non-combustibility and thermal insulation.

AFS geometries and physical properties can be varied according to each purpose such as core thickness, foam density, cellular morphology and face thickness (Crupi &

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Montanini, 2007). Sandwich panels can be failing with different failure or collapse mode depending on their geometries, physical and mechanical properties. Li, Zheng, Yu, Qian and Lu (2014) mentioned that the possible failure modes of sandwich beams are core yielding and shear, face wrinkling and yielding and indentation. This is also supported by Crupi and Montanini, (2007) which stated that failure mode for bending can be face wrinkling, face yielding, indentation and core shear.

Nowadays, production in automotive industries are rapidly increased year by year. It is important for the industries to minimize the production cost but have high quality of product. Besides that, it is also important to have a product with longer life and durability. One of the essential parts of automotive industries that need to be enhanced and investigated in terms of weight, stiffness and energy absorption are the crash box. Crash box is the structural parts that placed behind the bumper of the vehicles. It is important for the crash box to have high energy absorption to withstand the impact during crash and lightweight to reduce the weight of the vehicle. Figure 1.2 below shows the sample of crash box.

Figure 1.2 Bumper with crash box (Belingardi, Beyene, Koricho & Martorana, 2015) It is also stated by Banhart, Schmoll and Neumann (1998), the conventional steel used to make seat wall in car was replaced with sandwich panels and the results show that seat wall became lighter and ten times stiffer than conventional part. They also

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mentioned that, it is important to reduce the weight of parts for reducing the consumption of fuel and increasing the safety of passenger. The other well-known applications of sandwich panels were in shipbuilding, aerospace and architectural design.

Previously, there are several methods and techniques have been done by the previous researchers to identify mechanical behavior of the aluminum foam sandwich such as drop test (He et. al, 2016), quasi test (Jung, Pullen, & Proud, 2016), four-point bending test (An et. al, 2015) and impact test (Ismail et. al, 2015). All testing methods must be done by following the standard of ASTM and ISO standard. Although there were several reports on identifying mechanical behavior of sandwich structure, research of effect of skin to core ratio on sandwich structure has not been reported yet especially for sandwich panel with open-cell core, if any. In this study, a series of mechanical testing is proposed to investigate the effect of skin to core ratio on AFS using two approach of study which is experimental and simulation.

1.2 PROBLEM STATEMENT AND ITS SIGNIFICANCE

The requirement of producing complex parts with advanced material properties and lighter weight are generally needed nowadays especially in structural parts in aerospace, automotive and marine industries. The parts were usually made by heavy and expensive material. To overcome the problem, researchers had generated a new technology of lightweight material which is aluminum foam sandwich. Shunmugasamy and Mansoor (2018) stated that AFS have been used as energy absorbers, acoustic dampers and weight saving members in automotive and aerospace structures. Besides, Crupi and Montanini (2007) mentioned that the aluminum foam sandwich has a

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lightweight structure with good dissipation of energy under impact and high mechanical strength. By reducing the weight of structural parts, energy consumption also will reduce which will lead to lessen oil consumption.

Although, many researches had been done by experimentally to investigate the mechanical behavior of aluminum foam sandwich but there is still a gap need to be fill, solve, and complete. This can be supported by An et. al (2018) who stated that many scholars have devoted time and energy to improve the mechanical properties of aluminum foams for a preferable application. Lehmhus, Busse, Chen, Bomas and Zoch (2008) also mentioned that there are still lacking on reliable data of mechanical performance of AFS. Besides, Nesic et al., (2012) stated data of material behavior from shear, bending and tensile were needed for contributing better understanding and general description on various types of foams. The data develop also can be used as metal foam design guidelines for future use. Other than that, there also a gap needs to be fill in term of parameters variation, and method of testing to develop a new reliable data as mentioned by Luna, Barari, Woolley and Goodall, (2014).

There are two types of foam available in the market which are open-cell and closed-cell foam. However, most of past researchers were focusing on determine behavior of closed-cell foam while least of them discuss on open-cell foam. None of them investigate skin to core ratio effect on mechanical behavior of open-cell aluminum foam sandwich.

Therefore, this research was conducted to compile new reliable data of mechanical behavior of open-cell aluminum foam sandwich in term of ultimate stress, force-displacement curve when loaded under tension and bending. Effect of skin to core ratio on mechanical behavior of AFS were determined experimentally and validate

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using simulation study. Furthermore, the research was continued by calculating the stiffness to weight ratio and strength to weight ratio for proving the lightweight of sandwich structure with foam as a core.

1.3 RESEARCH OBJECIVES The objectives of the research are:

1. To investigate the effect of skin to core ratio on mechanical behavior and failure modes of aluminium foam sandwich using bending and tensile test.

2. To analyze lightweight properties of open-cell aluminium foam sandwich using stiffness to weight ratio through bending test.

3. To determine optimum values of core to skin ratio that maximize stress and force using desirability function methods.

1.4 RESEARCH METHODOLOGY

The proposed steps for completing this research will be divided into four categories which is fabrication method, experimental work, simulation modelling, data analysis and optimization. The fabrication of the sample was using adhesive bonding which consist of mixing epoxy resin and hardener. The samples were cut into desired shape following ASTM standard using electric discharge machine (wire cut). There are two types of mechanical testing conducted on samples which are tensile and three-point bending test. To validate the experimental results, simulation was run and material model were developed according to experimental work and previous research. The results were analyze based on stiffness to weight ratio, force-displacement curve and failure modes of sandwich beams. Lastly, ANOVA, scatter diagram, 3D plot surface