THE DETERMINATION OF SHEAR PROPERTIES OF BRITTLE MATERIALS USING ARCAN TEST METHOD FAHIS BIN TUMIN

Transcription

1 THE DETERMINATION OF SHEAR PROPERTIES OF BRITTLE MATERIALS USING ARCAN TEST METHOD FAHIS BIN TUMIN UNIVERSITI TEKNOLOGI MALAYSIA

2 PSZ 19:16 (Pind. 1/97) UNIVERSITI TEKNOLOGI MALAYSIA BORANG PENGESAHAN STATUS TESIS JUDUL: THE DETERMINATION OF SHEAR PROPERTIES OF BRITTLE MATERIALS USING ARCAN TEST METHOD SESI PENGAJIAN : 2005/2006 Saya FAHIS BIN TUMIN (HURUF BESAR) mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah)* ini disimpan di perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut: 1. Tesis adalah hakmilik Universiti Teknologi Malaysia. 2. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian sahaja. 3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institusi pengajian tinggi. 4. **Sila tandakan ( ) SULIT TERHAD (Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia sepertimana yang termaktub di dalam AKTA RAHSIA RASMI 1972) (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan) TIDAK TERHAD Disahkan oleh (TANDATANGAN PENULIS) (TANDATANGAN PENYELIA) Alamat Tetap: No 18, Jln Kinabalu 2, Taman Koperasi, 86000, Kluang, Johor. Dr. Yob Saed Ismail Tarikh: 16 Disember 2005 Tarikh: 16 Disember 2005 CATATAN: * Potong yang tidak berkenaan. ** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD. Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek Sarjana Muda (PSM).

3 Fakulti Kejuruteraan Mekanikal Universiti Teknologi Malaysia PENGESAHAN PENYEDIAAN SALINAN E-THESIS Judul tesis Ijazah Fakulti : THE DETERMINATION OF SHEAR PROPERTIES OF BRITTLE MATERIALS USING ARCAN TEST METHOD : Sarjana Muda Kejuruteraan Mekanikal : Fakulti Kejuruteraan Mekanikal Sesi Pengajian : 2005/2006 Saya FAHIS BIN TUMIN (HURUF BESAR) No Kad Pengenalan mengaku telah menyediakan salinan e-thesis sama seperti tesis asal yang telah diluluskan oleh panel pemeriksa dan mengikut panduan penyediaan Tesis dan Disertasi Elektronik (TDE), Sekolah Pengajian Siswazah; Universiti Teknologi Malaysia, November (Tandatangan pelajar) (Tandatangan penyelia sebagai saksi) Alamat tetap : 18, JLN KINABALU 2, Nama penyelia : Dr. YOB SAED BIN ISMAIL TAMAN KOPERASI, KLUANG, Fakulti : Fakulti Kejuruteraan JOHOR DARUL TA ZIM. Mekanikal Tarikh : 16/12/2005 Tarikh : 16/12/2005

4 I/We* hereby declare that I have read through this thesis and in my/our* opinion this thesis has fulfilled the requirements in term of the scope and the quality of the purpose of awarding the Bachelor of Mechanical Engineering Degree Signature : Supervisor I : Dr. Yob Saed Ismail Date : 16 December 2005 Signature : Supervisor II : Mr. Shukur Abu Hassan Date : 16 December 2005 *Potong yang tidak berkenaan

5 THE DETERMINATION OF SHEAR PROPERTIES OF BRITTLE MATERIALS USING ARCAN TEST METHOD FAHIS BIN TUMIN This thesis is submitted in partial fulfillment of the requirement for the Degree of Bachelor in Mechanical Engineering Faculty of Mechanical Engineering Universiti Teknologi Malaysia DECEMBER 2005

6 ii I declared that this thesis is my own work except the ideas and summaries which I have clarified their sources. Signature : Author s Name : FAHIS BIN TUMIN Date : 16 DECEMBER 2005

7 iii Dedicated to My ever loving parents, Tumin B. Hussein and Peredah Bt. Haris, my sisters and brother, Azlina, Hasnida, Anuar Zamani, Faezah, Fazilah, Noraini, family members and lastly for my sweetheart Amilia Abd. Rahim, I love you so much.

8 iv ACKNOWLEDGEMENT I would like to express my gratitude and appreciation to my respectful supervisors, Mr. Shukur Abu Hasan and Dr. Yob Saed Ismail for their guidance, advice, support, and patience throughout the completion of this project. I also wish to thank the staffs of Strength Lab, Mr. Rizal and Mr. Fadli, staffs of Material Lab, Mr. Ayub and Mr. Jefri for their advice and help. A special gratitude to my friends, Mr. Shamsul Izwari, Mr. Syed Muammar, Mr. Tahrail, Mr. Haidir, Mr. Kesavan and Mr. Munisvaran for their support and help. Last but not least, I am very grateful to my parents and family for their love, support and encouragement.

9 v ABSTRACT This thesis presented the test results of brittle epoxy material by using the Arcan test method. This method was used to determine the shear strength, τ, and shear modulus, G, of the test specimens by the relation of stress and strain. The butterfly shape specimen was prepared and tested with the Arcan rig, in order to produce pure shear by tensile load conditions. The Resifix-31 is one of the epoxy adhesives widely used in civil engineering for bonding CFRP plate onto concrete surface. Thus, it is important to test and determine the shear behaviour of the adhesive due to tropical exposure condition. There are four types of exposure conditions that represent tropical environment selected in this project ; namely laboratory (LB), outdoor (OD), plain water (PW) and salt water (SW). The data obtained from the test results had showed that the controlled specimen shear strength, τ, and shear modulus, G are 21.84% higher and 33.05% lower from the manufacturers quoted data. Apart from that, the Resifix-31 had showed the value of shear strength decrease when exposed to selected exposure conditions, which are about 25.4% in (LB), 18.43% in (OD), 32.01% in (PW), and finally 26.6% in (SW), as compared to the controlled specimen. The shear modulus value had also decreased when exposed to selected exposure conditions, which is about 12.8% in (LB), 9% in (OD), 11.1% in (PW) and 14.5% in (SW). On the other hand, the Arcan test method has verified that a state of pure shear was present during the testing as the stress strain curves linearly propagated on each ± 45 degree direction from the loading axis although the different recorded strain value between strains gauges +45 and -45º is about 7%, which may be caused by the effect of porosity (i.e. air bubbles) in the specimens.

10 vi ABSTRAK Tesis ini membentangkan hasil kajian terhadap kekuatan tegasan ricih, τ, dan modulus ketegaran, G, bahan perekat epoksi Resifix-31 dengan menggunakan kaedah pengujian Arcan. Pengujian kekuatan tegasan ricih ini dibuat memandangkan perekat epoksi ini digunakan secara meluas dalam bidang kejuruteraan awam. Kaedah pengujian Arcan ini dibina dengan menggunakan perkaitan di antara tegasan ricih dan terikan ricih. Spesimen berbentuk kupu-kupu telah disediakan dan diuji dengan menggunakan rig Arcan yang akan menyebabkan spesimen gagal secara ricihan terhadap kesan bebanan secara tegangan. Terdapat empat keadaan persekitaran yang diuji iaitu sampel makmal (LB), jemuran pada keadaan sekeliling (OD), rendaman di dalam air paip (PW) dan juga rendaman di dalam air masin (SW). Daripada ujian ricih yang telah dijalankan, didapati bahawa nilai kekuatan tegasan ricih dan modulus ketegaran sampel kawalan telah meningkat sebanyak 21.84% dan berkurang 33.05% daripada nilai yang disertakan oleh pengeluar. Selain daripada itu, bagi keempat-empat sampel ujian didapati bahawa nilai tegasan ricihnya telah berkurang sebanyak 25.4% bagi sampel (LB), 18.43% bagi (OD), 32.01% bagi (PW) dan 26.6% bagi (SW) berbanding spesimen kawalan (CO). Di samping itu juga, nilai modulus ketegaran bagi sampel yang dikaji juga menurun sebanyak 12.8% dalam (LB), 9% (OD), 11.1% (PW) dan 14.5% (SW). Berpandukan kepada graf tegasan ricih melawan terikan ricih yang diplot bagi arah bebanan ± 45 darjah, terbukti bahawa hubungan linear di antara nilai tegasan ricih dan terikan ricih dapat dicapai walaupun terdapat ketidakselanjaran sebanyak 7% pada nilai terikan bagi kedua-dua arah. Secara kesimpulannya, kaedah pengujian Arcan ini terbukti dapat digunakan untuk mencari nilai tegasan ricih tulen dan modulus ketegaran bagi bahan komposit.

11 vii TABLE OF CONTENTS CHAPTER SUBJECTS PAGE THESIS TITLE THESIS DECLARATION DEDICATION ACKNOWLEDGEMENT ABSTRACT ABSTRAK TABLE OF CONTENT LIST OF TABLE LIST OF FIGURE LIST OF SYMBOL LIST OF APPENDIX i ii iii iv v vi vii xi xiii xviii xix 1 INTRODUCTION Background of the Study Objective Scope of Study Methodology Planning and Scheduling 8

12 viii 2 BRITTLE MATERIAL Introduction to Brittle Materials Brittle Fracture The Definition of Stress and Strain of a Solid Material Stress Analysis of Plane Stresses Strain Principal Strains and Planes Stress-Strain Curve Hooke s Law for Shearing Stress and Strain Relationship Between Young s Modulus and 25 Shear Modulus of Inorganic Material 2.4 Adhesives Epoxy Adhesive Adhesive Compositions Conclusion 32 3 THE ANALYSIS STUDY ON ARCAN TEST METHOD Introduction The Evolution of the Arcan Test Fixture and Specimen Theoretical Analysis Background Shear Analysis On Specimen Recent Research and Development Using Arcan 48 Test Method 3.6 Conclusion 55

13 ix 4 SPECIMEN PREPARATION, EXPERIMENTATION AND 56 TESTING 4.1 Introduction Material Details Specimen Preparation Experimentation Environmental Exposures Strain Gauge Installation Arcan Fixture Setup Arcan Fixture Installation Instrumentation and Measurement Testing Procedure Microstructure Analysis Conclusion 81 5 RESULTS AND DISCUSSION Introduction Results Result Discussion Test Data Sample Calculation Experimental Results of Control Specimen Experimental Results of Laboratory Exposure Experimental Results of Outdoor Exposure Experimental Results of Plain Water Wet-Dry 97 Exposure Sample Experimental Results Salt Water Wet-Dry 99 Exposure Sample 5.3 Result Analysis Microstructure Analysis 108

14 x 5.5 Chemical Elements in Resifix-31 Epoxy Adhesive 110 Determined by FESEM 5.6 Conclusion CONCLUSION AND RECOMMENDATION Conclusion Recommendation 113 LIST OF REFERENCES 114 APPENDICES 117 APPENDIX A1-D1

15 xi LIST OF TABLE TABLE TITLE PAGE 1.1 Gantt chart for first semester Gantt chart for second semester Average shear modulus and shear strength test results of 49 various materials 4.1 Chemical formulation of Resifix-31 structural adhesives Product data for Resifix-31 epoxy adhesive Specimen average width, thickness and significant area Exposure conditions for resifix-31 epoxy specimens Water quality measurement tested in environmental 68 laboratory, Faculty of Civil Engineering, UTM 4.6 Plain water and Salt water experimentation data conditions Rossete type strain gauge specifications Raw experimental data for Resifix-31 ESST-C Testing results for ESST-CO sample Raw experimental data for Resifix-31 ESLT-LB Testing results for ESLT-LB sample Raw experimental data for Resifix-31 ESLT-OD Testing results for ESLT-OD sample Raw experimental data for Resifix-31 ESLT-PW Testing results for ESLT-PW sample 97

16 xii 5.9 Raw experimental data for Resifix-31 ESLT-SW Testing results for ESLT-SW sample Comparison of average shear strength,τ, and average shear modulus, G of Resifix-31 exposure samples Chemical element in Resifix-31 epoxy for experimentation 110 purposes

17 xiii LIST OF FIGURE FIGURE TITLE PAGE 1.0 Cylinder torsion test method [9] Arcan test method [12] Flow chart of the thesis methodology Sectioning of a body [5] The normal and shearing component of stress [5] The most general state of stress acting on an element [5] Successive steps in the analysis of a body for stress [5] Stress element showing two-dimensional state of stress [5] Mohr s circle illustrating equation 2.3 and 2.5 [19] Mohr s circle illustrating equation 2.6 and 2.7 [19] Strain of cubical element subjected to uniaxial tension [5] Cubical element subjected to plane shear strain [5] Mohr s circle representation of state of strain [5] Normal stress-strain curve for a ductile materials [5] Normal stress-strain curve for a brittle type materials [5] An element of a body in pure shear [5] Three representations of a state of pure plane shear stress [5] Three representations of a state of pure plane shear strain [5] Classifications of an adhesive [3] Epoxy chemical structure [6] 30

18 xiv FIGURE TITLE PAGE 3.1 The early concept of Arcan test method [2] Significant section of the Arcan s butterfly specimen [1] Butterfly specimen bonded to aluminum circular plane [1] Test fixture set-up and butterfly specimen modified by 36 Yen et al. [14] 3.5 Arcan fixture and butterfly specimen modified by Yen et al. [14] Butterfly specimen used by Yen et al. [14] Arcan fixture for shear testing with different loading 39 configurations [20] 3.8 Internal mean shear and normal stresses acting along the 39 significant section [20] 3.9 Prismatic element in state of equilibrium [19] Mohr s circle due to stress analysis [19] Element deformation due to shear [19] The strain results of a Graphite/PEEK specimen in shear [14] Shear failure of an aluminum specimen [14] Shear failure of a Graphite/PEEK specimen [14] Failure mode of a Plexiglas specimen [14] Effect of notch radius on shear stress profile along gage 52 section [14] 3.17 Effect of sharp notch on shear stress along the gage section [14] Shear stress strain response from Arcan shear test [14] Measured strain profiles at center of transverse butterfly 54 specimen during pure shear tests [14] 4.1 Two parts structural adhesives of Resifix-31 58

19 xv 4.2 The butterfly specimen geometry Adhesive system Part A/epoxy and Part B/hardener A mixing process using slow speed electric mixer Male and female parts of the butterfly mould A complete assembly of mould parts Surface cleaning process using soft cloth with Carnauba wax Flat plate attached to male part by screws Casting process of epoxy mixture Male part attached to the female part A 10 kg mass used to press the female part from top side De-moulding process of specimen from mould Surface grinding and polishing process of the specimen Specimens ready to be exposed Adding salt in plain water Mixing ocean tropical salt and plain (tap) water Specimens in plain water condition Specimens in salt water condition Specimens in control room condition Specimens exposed to outdoor condition Rosette type strain gauge installation axis at ± 45 0 and at 70 centre of AB line 4.22 Complete gauge installation onto the butterfly specimen The adhesive film bonded onto specimen area Butterfly specimen mounted to grip Arcan male grip attached onto female grip Complete assembly of butterfly specimen Screws tightening process Soldering lead wire to terminals 73

20 xvi 4.29 Arcan fixture set-up Assembly of Arcan fixture Attachment grip to holder by pin Complete Arcan fixture attachment Instrumentation set-up Arcan test rig ready to be test Strain reading initialization Test data collection during testing Relationship between shear stress, shear strain and shear modulus Field-emission Scanning Electron Microscope (FESEM) Specimen geometry for sample calculation Mohr s circle constructed based on principal strains of Resifix ESLT-LB01 at 1000N loading condition 5.3 Shear stress-strain curve for Resifix-31 ESST-C Specimen ESST-C01 after failed Brittle failure of ESST-C Shear stress-strain curve for Resifix-31 ESLT-LB Specimen ESLT-LB01 after failed Brittle failure of ESLT-LB01 specimen Shear stress-strain curve for Resifix-31 ESLT-OD Specimen ESLT-OD01 after failed Brittle failure of ESLT-OD01 specimen Shear stress-strain curve for Resifix-31 ESLT-PW Specimen ESLT-PW01 after failed Brittle failure of ESLT-PW Shear stress-strain curve for Resifix-31 ESLT-SW Specimen ESLT-SW01 after failed Brittle failure of ESLT-SW01 102

21 xvii 5.18 Shear strain versus shear stress for ESST-CO Shear strain versus shear stress for ESLT-LB Shear strain versus shear stress for ESLT-OD Shear strain versus shear stress for ESLT-PW Shear strain versus shear stress for ESLT-SW Shear strength versus exposure condition for all test samples Shear modulus, G, versus sample exposure conditions for all 107 test samples 5.25 Fracture surface of Resifix-31 epoxy for ESST-CO sample Porosities in ESLT-LB sample Porosities in ESLT-OD sample Porosities in ESLT-PW sample Porosities in ESLT-SW sample Porosities size in Resifix-31 epoxy ESST-CO sample 110

22 xviii LIST OF SYMBOL SYMBOLS SUBJECT P,F - Force τ - Shear stress σ - Normal stress A - Cross sectional t - Thickness of specimen ε - Normal strain γ - Shear strain α, ø, θ - Radius G - Shear modulus E - Elastic Modulus l - Length r - Notch radius h - Gauge length w - Width v - Poisson s ratio

23 xix LIST OF APPENDIX APPENDIX NO. TITLE PAGE A1 A2 B1 C1 C2 D1 Adhesive compositions Arcan shear test result for Resifix-31 Shear stress-strain curve for Resifix-31 Microstructure of fracture surface of Resifix-31 Elements spectrum of Resifix-31 Arcan fixture mould and rig drawing

24 1 CHAPTER 1 INTRODUCTION 1.1 Background of The Study A brittle material is one which exhibits relatively small extensions to fracture so that the partially plastic region of the tensile test graph is much reduced. In the overview of brittle materials, mechanical behavior is determined by stress and strain associated with material points throughout the material. Mechanical properties are measured in test of samples in which loads or boundary displacements are applied in such way that the relation between stress and strain at a typical point can be inferred. In brittle materials, the situation is further complicated by the occurrence of fracture. During deformation, the material structure is changed due to the initiation and propagation of cracks at different locations throughout the material [8]. Among the problem in conducting the test for brittle materials is to devise specimen and loading configuration in order to produce a state of uniform plane stress. The problem has assumed increased significance with the application of brittle composite materials and brittle adhesives system nowadays in mechanical and civil engineering structures applications. For example by referring to fibre reinforced

25 2 polymer composites (FRP) that form of laminates in which the laminae are stacked by the principal of bonding using resin-matrix system. In most engineering applications, the laminae are in states of plane stress. It is therefore necessary to test single laminae in plane stress in order to obtain mechanical properties such as shear and stress properties and also to determine the failure criteria. The shear properties of epoxy adhesive is one aspect that user tends to neglect as it is only a small part but crucially important in designing the externally bonded FRP-concrete. Selection of the adhesive must provide material integrity of the bonded systems in order to produce uniform stress throughout the bond area. This is important to avoid the premature failure of the systems such as adherends, cleavage and interfacial failure of adhesive. For the same reason, this project aim to determine the shear properties of two parts epoxy adhesives system, namely Resifix-31 under lab controlled and tropical exposure conditions. In general, the typical shear test method used to determine the shear properties of most materials is the cylinder-torsion test, as shown in Fig Unfortunately, this method has an advantage which is unable to produced significant section on the sample, the grips strongly influence the state of stress. Fig. 1.0 : Cylinder torsion test method [9]

26 3 In 1978, Arcan et al. [1] introduced a new method of testing material shear properties under uniform plane stress conditions by means of specially designed plane specimen, as shown in Fig Fig. 1.1 : Arcan test method [12] The fixture was used to determine shear properties for various materials such as polymer composite, sandwich materials system, human bones and solid polymers. Photoelastic analysis [1] had show that in the significant section of the specimen it is possible to produced uniform plane stress with high degree of accuracy. The compact nature of the Arcan fixture offers an advantage to obtain the shear properties in all in-plane directions in a relative simple manner [10]. The Arcan fixture can be used to apply both shear and axial forces to the test specimen and this special case of loading produces pure shear on the significant section and the experimental results are encouraging and acceptable with high degree of confident [1].

27 4 In this project the studies concentrated on the Arcan fixtures in order to obtain the shear properties of the brittle structural epoxy adhesive material, namely ; Resifix-31. The apparatus consist of specimen with butterfly shape and Arcan test fixture. The shear properties obtained due to various types of exposure conditions will be used for analyzing the bond stress characteristic of CFRP-epoxy-concrete under final load test (i.e. which also exposed to various tropical conditions). 1.2 Objective The main objective of this project is to study the in-plane shear properties of brittle structural epoxy material using Arcan Test Method. The study focus on the following properties ; i) An average shear stress and shear modulus properties, τ and G of structural epoxy adhesive Resifix-31 due to tropical exposure conditions. ii) Microstructure analysis due to exposure conditions. iii) The reliability of test data. 1.3 Scope of study The scope of this study covers the following topics ; i. Literature study on the epoxy adhesive material (i.e. Resifix-31, which is two parts epoxy system) supplied by Exchem, United Kingdom ii. Literature study on the Arcan test method fixture iii. Specimen preparation

28 5 iv. Specimen experimentation and final shear load test v. Results and discussion vi. Report writing 1.4 Methodology The detailed project methodology are describes as follows ; i. Literature Study The literature studies were carried out by sourcing the related information from journals, handbooks, books, previous thesis, and websites. Firstly, literature review was carried out to understand the mechanical characteristic of the adhesive system used in this study. Then, the studies were focused on the Arcan test method in order to investigate the theoretical background of Arcan test method which related to shear properties and to determine suitable butterfly specimen geometry for the study. Apart from that, a study on Arcan test rig development also been conducted in order to identify problems faced by previous researchers. ii. Problems identification and solving The problems encountered during the literature studies and experiment set-up was then discussed with project supervisor. iii. Specimen testing The two parts epoxy adhesive was cast in a closed mould to produce butterfly shape specimens. The specimens were exposed to four (4) tropical

29 6 conditions, namely ; Control Specimens (ESST), Laboratory Exposure (LB), Outdoor (OD), Plain Water (PW-wet/dry) and Salt Water (SW-wet/dry). These specimens were exposed to their respective conditions for duration of six months before prior final shear load test. iv. Data collection and analysis Data were gathered from the instrumented measurement that been established during testing. The specimens failure mechanism was observed during testing and microstructure analysis has been done in order to investigate in depth the source of failure mechanism. v. Report writing All the findings and results from the experiment will be discussed and make a comparison with previous research if necessary. The project methodology flow chart is shown in Fig. 1.2.

30 7 Literature Study Problems Identification Problem Solving Arcan Test Rig Study Test Samples Experimentation Testing Data Analysis Comparison Satisfy? Discussion Conclusion & Suggestion End Fig. 1.2 : Flow chart of the project methodology

31 8 1.5 Planning and Scheduling In order to make sure this thesis is on schedule, a Gantt chart was produced from the second week after the discussion with the supervisor, as shown in Table 1.1 and Table 1.2. Table 1.1 : Gantt chart for first semester Activities Project Briefing * Planning and Scheduling * * Week Literature Study * * * * * * * * * * * * * * Testing Set-up * * * * * * * * * Problems Identification and Solving Final Draft and Presentation I * * * * * * * * * * * Table 1.2 : Gantt chart for second semester Activities Project Briefing * * Week Planning and Scheduling for Sample Preparation * * Testing Set-up * * * * * * Result Analysis * * * * * Final Draft and Presentation II * * * *

32 9 CHAPTER 2 THE MECHANICAL PROPERTIES OF BRITTLE MATERIALS 2.1 Introduction to Brittle Materials Brittle material behaved elastically to fracture and shows low-fracture toughness characteristic. Brittle polymers exhibit some yielding, but it is negligible compared to ductile polymers such as thermoplastic and metals. Brittle materials, which have complicated crystal structures usually consist of two or three kinds of atoms, produced high resistance slip to system among them. Even if there is limited plasticity, electrical neutrality has also to be satisfied in covalent and ionic-bonded materials. For all these reason, we usually do not observe any macroscopic plasticity in these materials. By definition, the material those failure is control primarily by the presence of flaws or cracks, are termed brittle materials. In ductile materials, cracks may be present, but the resulting stress concentration is relieved by plastic deformation, which produced brittle fracture. Typical examples of brittle materials are cast iron, glass, and polymer concrete. Some strength characteristic of brittle materials are briefly described as follows ;

33 10 1. Scatter of Strength Properties Brittle materials exhibit a scatter of failure strength, unlike ductile materials, in which plastic deformations takes place. In ductile material, the variability of strengths is nominally identical specimen is generally not more than 4% to 8% of its mean strength. Hence, the mean strength of a ductile material can be used in designing, as a measure of strength. In a brittle material, the variation of strength in nominally identical specimens can be as high as 100% of the mean strength. Therefore, the mean strength is not an adequate measure of the strength of a brittle material. 2. Effect of Volume Fraction Experimental observations have revealed that the mean strength of a brittle material depends on the volume of the material, especially when subjected to tensile stresses. The result show that when volume is increased, mean strength decrease (under tension). 3. Effect of Loading Systems / Conditions The mean strength depends on the type of loading system. This can be interpreted as when the brittle material is subjected to different types of loading, they will behave differently. 2.2 Brittle Fracture When a solid material is subjected to high (near to failure load) increasing load, the solid may fracture. If such breakage comes about before the piece has thinned down to zero thickness, it is called fracture; and if the amount of permanent deformation preceding fracture is negligible, it is called brittle fracture. On the other hand, in a ductile fracture, plastic deformation occurs in the final fracture such as necking [4]. According to Griffith [4], brittle fracture is due to minute crack like defects. In many

34 11 cases, brittle fracture gives rise to fast growth of a crack in the body [4]. Although the ideal strength is very high for a glassy solid whose atoms are held together with primary bonds, the severe stress concentration at sharp cracks can reduce this strength to ordinarily observes low values. Brittle fracture results from submicroscopic cracks with atomically sharp roots, where the stress can become concentrated beyond the capacity of the body to resist it. Brittle fracture is the expected mode of failure of materials like cast iron, glass, concrete, porcelain, ceramic, are expected to show considerable distortion (and to absorb substantial energy) before fracture can occur. This fracture often occurs suddenly and without warning. They are associated with a release of a substantial amount of elastic energy which may cause a loud noise. The primary factors influencing the material brittle fracture are as follows ; 1. Low temperature Reduced temperatures increase the resistance of the material to slip but not to cleavage due to tensile loading. 2. Rapid loading Rapid rates of shear deformation require greater shear stresses, and this may be accompanied by normal stresses which exceed the cleavage strength of the material. 2.3 The Definition of Stress and Strain of a Solid Material In all engineering construction and application the elements of a structure must be assigned definite physical size. Such elements must be properly proportioned to resist the actual or probable forces that may be imposed among them. Thus, it is important to

35 12 understand the concepts of stress and strain, and their relation in order to summarize the properties of the brittle materials used in this project Stress In general, the internal force acting on infinitesimal areas of a cut may be of varying magnitude and directions, as shown diagrammatically in Fig These internal forces are vectorial in nature and maintain in equilibrium the externally applied forces in the section. In mechanic of materials it is partially significant to determine the intensity of these forces on the various portions of the cut, as resistance to deformation and the capacity of materials to resist forces depend on these intensities. These intensities of forces acting on infinitesimal areas of the cut vary from point to point, and they are inclined with respect to the plane of the cut. Fig. 2.1 : Sectioning of a body [5] In engineering practice it is customary to resolve this intensity of force perpendicular and parallel to the section investigated. Such resolution of the intensity of the force on an infinitesimal area is shown Fig. 2.2.

36 13 Fig. 2.2 : The normal and shearing component of stress [5] The intensity of the force perpendicular or normal to the section is called the normal stress at a point. In this thesis it will be designated by the Greek letter σ (sigma). As a particular stress generally holds true only at a point, it is defined mathematically as ; σ = lim { A 0 P A [2.1] where P is a force acting normal to the cut, while A is the corresponding area. It is customary to refer to the normal stresses that cause traction or tension on the surface of a cut as tensile stresses. On the other hand, those that are pushing against the cut are compressive stresses. An infinitesimal cube, as shown in Fig. 2.3, could be used as the basis for an exact formulation of the problem in mechanics of materials. However, the methods for the behavior of such a cube (which involve the writing of an equation for its equilibrium and making certain that such a cube, after deformations caused in it by the action of forces) are beyond the scope of this thesis. They are in the realm of the mathematical theory of elasticity.

37 14 Fig. 2.3 : The most general state of stress acting on an element [5] In many practical situations, if the direction of the imaginary plane cutting a member is judiciously selected, the stresses that act on the cut will be found both particularly significant and simple to determine. One such important case occurs in a straight axially loaded rod in tension, provided a plane is passed perpendicular to the axis of the rod. The tensile stress acting on such a cut is the maximum stress, as any other cut not perpendicular to the axis provides a larger surface for resisting the applied force. The maximum stress is the most significant one, as it tends to cause the failure of the material. To obtain an algebraic expression for this maximum stress, consider the case illustrated in Fig. 2.4 (a). If the rod is assumed weightless, two equal and opposites forces P are necessary, one at each end to maintain equilibrium. The whole body is in equilibrium, so any part of it is also in equilibrium. A part of the rod to either side of the cut x-x is in equilibrium. At the cut, where the cross sectional area of the rod is A, a force of equivalent to P as shown in Fig. 2.4 (b) and (c) must be developed. Where upon, from the definition of stress, the normal stress which acts perpendicular to the cut is ;

38 15 σ = A P or force crossectional area N m 2 [2.2] Fig. 2.4 : Successive steps in the analysis of a body for stress [5] Analysis of Plane Stresses Most stress analysis problems of engineering involve one surface which is free of stress. In this case, all stresses on a stress element act on only two pairs of faces, as shown in Fig The stress surfaces of the element is by the definition on the principal planes (since they are subjected to zero shear stress), and the normal of these surfaces is the principal axis. According to the shear-stress convention in previous section, both τ xy and τ yz would be positive, and the element as a whole would be regarded as being subject to positive shear. As the elements are viewed, positive shear stress tends to deform the element to the right and negative shear stress to the left.

39 16 Fig. 2.5 : Stress element showing two-dimensional state of stress [5] The derivation of the analytical expressions relating the normal and shear stress to the angle of cutting plane φ is given in elementary texts book on strength of materials and will not be repeated here. If the stresses shown in Fig. 2.5 are known, the principal stress with the principal directions, and the maximum shear stress may be found from the following equations ; σ 1, σ 2 = σ x + σ y 2 ± 2 2 σ x σ y 2 τ + xy [2.3] 0 The principal shear stresses act on planes whose normal make angles of 45 to σ 1 and σ 2. Principal shear stresses occur when the normal stresses are equal. The maximum principal shear stress, τ max, (for the case of a two-dimensional stress systems) is ; τ max = ± 2 2 σ x σ y 2 τ + xy [2.4] 2τ 2φ = tan -1 xy σ σ x y [2.5] where φ is the angle between the principal planes and the x-y planes. When φ is positive, the principal planes are clockwise of the x and y planes. Where the principal

40 17 stresses are known and it is desired to find the stresses acting on a plane oriented at angle φ from the principal plane number 1, the equations are ; σ φ = σ 1 + σ 2 2 σ 1 σ + 2 cos 2φ [2.6] 2 τ φ = σ 1 σ 2 sin 2φ [2.7] 2 In Equation [2.7], τ φ represents a shear stress acting on the φ plane and directed 0 90 from the φ plane. When plotted on σ -τ coordinates, equation [2.6] and [2.7] produce a circle which is symmetric about the σ axis. Because of the 2φ in the equations, angle measured on the circle are twice those measured at the stress element. It is the basis for Mohr s circle, an extremely useful graphic technique for stress analysis. Fig. 2.6 and 2.7 shows how Mohr s circle illustrates equation [2.3] to [2.7]. Fig. 2.6 : Mohr s circle illustrating equation 2.3 to 2.5 [19]

41 18 Fig. 2.7 : Mohr s circle illustrating equation 2.6 and 2.7 [19] Strain Any physical bodies subjected to forces such as stresses, deforms under the action of these forces. Strain is the elongation per unit of length [4]. The term strain can also be describe as the direction and intensity of the deformation at any given point with respect to a specific plane passing through that point. Thus, state of strain is a tensor and is analogous to state of stress. Strains are always resolved into normal component ε (epsilon) and shear component,γ (gamma). With reference to Fig. 2.8 and Fig. 2.9, normal strain may be defined as ; ε x = lim { x 0 dx x ε y = lim { y 0 dy y ε z = lim { z 0 dz z [2.8] With reference to Fig. 2.9, shear strain may be defined as ; lim γ yx = { y 0 dx = tan θ 0 radian [2.9] y where angleθ represent the deviation from an initial right angle. If the total elongation

42 19 is in a given original length, L, thus elongation per unit length,ε (epsilon) is ; ε = L L [2.10] It is a dimensionless quantity. The quantity ε is very small, except for a few materials such as rubber. If the strain is known, the total deformation of an axially loaded bar is ε L. The subscript notations for strains correspond to that used with stresses and also the sign conventions for strains follows directly from those stress. Fig. 2.8 : Strain of cubical element subjected to uniaxial tension (a) Three dimensional view (b) Plane view [5] Fig. 2.9 : Cubical element subjected to plane shear strain (a) Three dimensional view (b) Plane view [5]

43 Principal Strains and Planes Principal strains in the x-y plane, the maximum shear strain in the xy plane, and the orientation of the principal axes relative to the x-y axes are given by equation [2.11] to [2.14]. ε 1, ε 2 = ε x + ε y 2 ± ε x ε y γ xy [2.11] γ max = ± ε x ε y γ xy [2.12] γ xy 2φ = tan -1 ε ε [2.13] x y Where φ is the angle between the principal planes and the x and y planes. Similarly, the strain counterparts of stress are ; ε φ = ε 1 + ε ε1 ε 2 2 cos 2φ [2.14] From knowledge of the three principle strains, a Mohr s circle representation of the state of strain can be made, as shown in Fig Note that the coordinates of the Mohr plot are normal strain and half of shear strain. The circle between points 1 and 2 represent the strain component on all planes containing the 3 axis ; the circle between 1 and 3 represents strains on all planes containing the 2 axis ; the circle between 2 and 3 represents strains on all planes containing the 1 axis. Strain components for all planes containing none of the axes are represented by the shaded area between circles. As when representing stresses, the largest of the three circles is referred to as the principle circle.

44 21 Fig : Mohr s circle representation of state of strain [5] Stress-Strain Curve In the study of the properties of materials, it is customary to plot diagrams on which a relationship between stress and strain is reported. Experimentally determined stress-strain diagram differ considerably for different materials. One type is shown in Fig. 2.11, which is for mild steel, a ductile material widely used in construction. The brittle type is shown in Fig

45 22 Fig : Normal stress-strain curve for a ductile materials (i.e. mild steel) [5] Fig : Normal stress-strain curve for a brittle type materials (i.e. concrete) [5] Hooke s Law for Shearing Stress and Strain In the previous sections it was shown that in an element of a body the shearing stress must occur in parallel plane acting on mutually perpendicular planes. When only these stresses occur, the element is said to be in pure shear. Such a system of stresses distorts an element of an elastic type shown in Fig (a). The diagonals OA and BC are axes of symmetry for a distorted element.

46 23 Fig : An element of a body in pure shear [5] If attention is confined to the study of small deformations, and further, if behavior of an element is considered only in its elastic range, it is again found experimentally that there is a linear relationship between the shearing stress and the angle γ (gamma) shown in Fig (c). It must be noted that an extension in any one direction will result in producing lateral contractions in the other two directions. This effect is called Poisson s effect and v is the parameter termed Poisson s ratio which takes this effect into account. Therefore, the strain in the x, y, z directions can be related to the stresses by the following equations ; 1 ε = ( ) E [ σ ν σ + σ ] x x y z 1 ε y = E [ σ ν ( σ + σ )] y z x 1 ε = ( ) E [ σ ν σ + σ ] z z [2.15] x y These equations may be solved to obtain stress components as functions of strains, and for plane stress conditions, we have ;

47 24 σ x = E [ 1 ν ) ε + ν ( ε + ε ) ] ( 1+ν )(1 2ν ) ( x y z σ y = E [ 1 ν ) ε + ν ( ε + ε ) ] ( 1+ν )(1 2ν ) ( y z x σ z = E [ 1 ν ) ε + ν ( ε + ε ) ] ( 1+ν )(1 2ν ) ( z x y [2.16] For the special case in which the x, y, and z, axes are coincident with principal axes 1, 2, and 3, equations [2.16] are simplified by virtue of all shear stresses and shear strains being equal to zero ; 1 ε 1= E [ σ ν ( σ + )] 1 2 σ 3 1 ε 2 = E [ σ ν ( σ + )] 2 3 σ 1 1 ε 3 = E [ σ ν ( σ + )] [2.17] 3 1 σ 2 σ 1 = E ( 1+ν )(1 2ν ) [ 1 ν ) ε + ν ( ε + )] ( 1 2 ε 3 σ 2 = E ( 1+ν )(1 2ν ) [ 1 ν ) ε + ν ( ε + )] ( 2 3 ε1 σ 3 = E ( 1+ν )(1 2ν ) [ 1 ν ) ε + ν ( ε + )] ( 3 1 ε 2 [2.18]

48 25 For the commonly encountered biaxial-stress state, one of the principal stresses is zero (e.g. σ 3 = 0), the equations become ; 1 ε 1= ( σ 1- ν σ 2 ) E 1 ε 2 = ( σ 2 - ν σ 1) E ν ε 3 = - ( σ 1 + ν σ 2 ) [2.19] E Substitution of this expression in equation [2.18] gives E ( ε 1 + νε 2 ) 1 ν σ 1 = 2 E ( ε 2 + νε 1) 1 ν σ 2 = 2 σ 3 = 0 [2.20] Relationship Between Young s Modulus and Shear Modulus Figure 2.14 illustrates an element subjected to pure plane shear stress. The four intercepts on the Mohr circle represent stress coordinates on x, y, 1, and 2 planes, which are planes perpendicular to these respective axes. The Fig represents the resultant strain present in the same element.

49 26 Fig : Three representations of a state of pure plane shear stress [5] Fig : Three representations of a state of pure plane shear strain [5] modulus ; From equations [2.19], we can obtain the Shear Modulus in terms of the Young s 1 ε 1 = ( σ 1- ν σ 2 ) E 1 ε 2 = ( σ 2 - ν σ 1) E and, because the τ xy in the same direction to the principal stress, thus ; also ; 1 ε = ( τ + ) = E 1 xy ντ xy τ xy (1+ν ) E 1 ε = (- τ ) = - E 2 xy ντ xy τ xy (1+ν ) [2.21] E

50 27 From the Mohr strain circle in Fig.2.15 ; γ xy 2 = ε 1 = τ xy (1+ν ) E and from the relationship between G, E, and ν ; G = E 2(1 + ν ) [2.22] Rearrange equations 2.16, finally, G = τ γ xy xy = E 2(1 + ν ) [2.23] where G is a constant of proportionality called the shearing modulus of elasticity or the modulus of rigidity. The G value is a constant for a given material and measured in the same unit as E, while γ is measured in radian. The expression for the three different sets of shearing strains can be stated as follows ; τ xy = Gγ xy τ yz = Gγ yz τ zx = Gγ zx [2.24] The shear stress in equation [2.24] can be determined from a thin tube subjected to a torsion or torque. From previous experiments it is known that the appearance of τ -γ curve is similar to that of the σ -ε diagrams of a tension test for the same material. In this project, the Arcan test method is used in order to obtain similar properties of shear stress when the material is subjected to pure shear.

51 Adhesives Adhesives were first used many thousands of years ago, and most were derived from naturally occurring vegetable, animal, or mineral substance. Synthetic polymeric adhesives displaced many of these early products due to stronger adhesion and greater resistance to operating environments. The classification of adhesives is shown in Fig Fig : Classifications of an adhesive [3]

52 29 An adhesive is a substance capable of holding substrates (adherends) together by surface attachment. A material merely conforming to this definition does not necessarily ensure success in an assembly process. For adhesives to be useful it must not only hold materials together but also withstand operating loads and able to transfer stresses uniformly. The successful application of adhesives depends on many factors. Adhesives face a complex task of selecting the proper and correct processing conditions that allow the bond to form. One must also determine the substrate-surface treatment which will permit an acceptable degree of permanence and bond strength. The adhesive joint must be correctly design and applied in order to avoid stresses within the joint that could cause premature failure [2] Epoxy Adhesive The term epoxy has come to mean superglue, which bonds almost anything to anything. Although this is certainly an exaggeration, epoxy adhesive will bond to a wide variety of materials without requiring heat or pressure in most cases. Epoxies are available in a great variety of forms ; some will cure at room temperature and some require heat curing; most are two-component liquid or paste systems that require no mixing; some are available in supported or unsupported films, and others are available in pre-impregnated tapes or granules and powders. Epoxy resins are generally used in adhesive applications in a two part system in which a hardener must be added to the resin just before application. The hardener, (usually an amine or anhydride) forms crosslink between the epoxy resin molecules, thus converting the liquid resin into a hard matrix. This curing is achieved through an epoxy ring opening reaction with primary or secondary amines or acid anhydrides as hardeners.

53 30 Epoxies, because of their polar groups, display very high adhesives strength to metals, ceramics, glass, and other polymers. Epoxies are famous for their versatility. They can be used to bond most any two surfaces no matter how different. Also epoxies, after reacting with a hardener, give very high cohesive strength as well. Other advantages of epoxies are that they are highly reactive and do not give off high levels of volatile reaction by-products (some amine hardeners are volatile, however, and do flash evaporate during curing). Epoxies shrinkage is much less than other reactive polymers such as polyester, for example. The additions of fillers to the adhesive also reduce shrinkage even further. Because of the toxic vapors which may be given off when working with epoxies, all work with these resins should be done in areas having adequate ventilation. When epoxy adhesives set, they form a very hard, strong and tough glue line. In fact, for some applications, epoxies may be too hard. Unmodified epoxies lack flexibility and have poor strength through wide temperature variations, special epoxy alloys are formulated with NBR, urethane, acrylics, and nylon. The special alloy formulations are now used extensively in aircraft construction adhesives. Fig shows the idealized chemical structure of a typical epoxy. Fig : Epoxy chemical structure [6]

54 Adhesive Compositions Modern day adhesives are often fairly complex formulations of components that perform specialty functions. The adhesive base or binder is the primary component of an adhesive. The binder is generally the resinous component from which the name of the adhesive is derived. For example, an epoxy adhesive may have many components, but the primary material is epoxy resin. A hardener is a substance added to an adhesive formulation to initiate the curing reaction and take part in it. Two-component adhesive systems have one component, which is the base, and a second component, which is the hardener. Upon mixing, a chemical reaction ensues that causes the adhesive to solidify. A catalyst is sometimes incorporated into an adhesive formulation to speed the reaction between base and hardener. A catalyst is a substance which markedly speeds up the cure of an adhesive when added in a minor quantity as compared with the amounts of primary reactants [3]. Solvents are sometimes needed to lower viscosity or to disperse the adhesive to a spread able consistency. Often, a mixture of solvents is required to achieve the desired properties. A reactive ingredient added to a adhesive to reduce the concentration of binder is called a diluent. Diluents are principally used to lower viscosity and modify processing conditions of some adhesives. Diluents react with the binder during cure, become part of the product, and do not evaporate as does as solvent. Fillers such as calcium carbonate and catalyst are generally inorganic particulates added to the adhesive to improve working properties, strength, permanence, or other qualities. Fillers are also used to reduce material cost. By selective use of fillers, the properties of an adhesive can be changed tremendously. Thermal expansion, electrical and thermal conduction, shrinkage, viscosity, and thermal resistance are only a few properties that can be modified by use of selective fillers.

55 Conclusion The basic definition of brittle material was presented in this chapter to define their characteristic such as their mode of failure and fracture surface. Apart from that, the stress, strain, and shear stress relation briefly stated in order to relate and explain how the basic theory of engineering was used to produce the Arcan fixture and Arcan test method.

56 33 CHAPTER 3 ARCAN TEST METHOD EVOLUTION 3.1 Introduction Over the years, there has been considerable interest in the development of a suitable specimen and loading configuration for determining the shear deformation and failure of composite materials with an accurate result to fit with real application. Although torsional testing of thin-walled tubes is generally regarded as providing a uniform and pure state of shear, grip-related failure can arise, and specimen fabrication is more tedious than other alternative method. The Arcan test method was first introduced and developed in 1978 [1]. The method was tried to overcome problem such as existence of other stress component (i.e. tensile stress) besides the shear stress on final measurement. To produce a uniform state of plane-stress for the solid specimen, Arcan et al. (1978) developed this method to determine mechanical properties of isotropic as well as orthotropic composite materials under uniform plane stress conditions by means of a specially designed butterfly-shaped specimen. Advantage of Arcan test method compared to cylinder-torsion test method was their fixture can provide a state of uniform pure shear stress in an area known as a

57 34 significant section using a plane stress loading. Apart from that, this test method also gives advantages that small specimen or anisotropic specimen can be tested with high degree of accuracy and reliability. 3.2 The Evolution of the Arcan Test Fixture and Specimen Arcan et al. proposed a biaxial fixture, commonly known as the Arcan fixture, to produce biaxial states of stress. The compact nature of the Arcan fixture enables obtaining the shear properties in any in-plane directions in a relatively manner. The Arcan fixture can be used to apply shear force to the test specimen. Arcan and Voloshin [1] used this method to determine the longitudinal and through-thickness shear modulus of a unidirectional laminated CFRP composite. Their results compared favorably to those obtained from cylinder-torsion tests. The early design concept of the Arcan fixture is shown in Fig Fig. 3.1 : The early concept of Arcan test method [2]

58 35 The test fixture specimen is manufactured from the material to be tested. In the configuration, the testing is performed in shear mode. The hatched area denotes the deformation zone. By applying the force, F, in different directions, combinations of tension and shear loading are possible to produce. This method includes pure shear as a special case, when the angle α = The principle behind the geometry of the specimen is that in the pure shear zone, the isostatics will intersect the sheared cross-section (AB in Fig. 3.2) at an angle of α = ± 45 degree. Fig. 3.2 : Significant section of the Arcan s butterfly specimen [1] In 1980, Arcan and Voloshin have modified the test method by bonded the test specimen on the aluminum circular plane with the anti symmetric cut-outs, as shown in Fig Fig. 3.3 : Butterfly specimen bonded to aluminum circular plane [1]

59 36 The test fixture development continuously made by Yen et al. [14] in order to eliminate the use of adhesive. The modified Arcan fixture was made of two pairs of stainless steel parts, each pair equivalent to one half of the original Arcan fixture. A butterfly shape cutout was machined to half the thickness in each part to house the specimen. Three holes were drilled at each part to allow tightening the two parts together with screws. The butterfly specimen which is joined on either side of two half circular grips as in Fig. 3.4 are connected to a universal testing machine at the top and bottom, respectively. The grips together with the butterfly specimen formed a circular disk with two anti-symmetric cut-outs. Fig. 3.4 : Test fixture set-up and butterfly specimen modified by Yen et al. [14] The modified-arcan fixture and its butterfly specimen are designed to determine the shear moduli, non linear-stress strain response, and strength of thick section pultruded composites under shear combined with different biaxial stress conditions. The modification proposed by Yen et al. includes bolting a butterfly shaped specimen between two identical halves of the Arcan fixture.

60 37 By using mechanical fastening and trapezoid cut-outs, specimens with different thicknesses can be accommodated without the use of adhesives. The fixture used in this study does not include a trapezoidal cut-out. Instead, a large number of bolts are used to connect the butterfly specimen to the steel fixture. The load is applied to the fixture using clevis pins to minimize out-of-plane forces and moments. Fig. 3.5 shows a schematic of the modified Arcan fixture with the butterfly specimen. Fig. 3.5 : Arcan fixture and butterfly specimen modified by Yen et al. [14] The fixture is flexible to accommodate the pultruded specimens with various thicknesses. The butterfly specimen design is shown in Fig 3.6. Six units of 6.4 mm (0.25in) diameter sleeve bolts are used to transfer the load from the steel fixture to each side of the specimen and the bolts are hand-tightened. The holes used in Arcan fixture modified by Yen et al. to grip the butterfly specimen then eliminated by using the clamped aluminium circular plane. The specimen is plane circular with anti symmetric cutouts. The significant section of the specimen AB must be designed in such way that the state of stress on AB shall be uniform as possible.

61 38 Fig. 3.6 : Butterfly specimen used by Yen et al. [14] 3.3 Theoretical Analysis Background The modified Arcan fixture and its butterfly specimen can be used for pure shear and biaxial stress conditions testing, as illustrated in Fig The shear response, in the presence of various biaxial stress states, can be obtained in a relatively simple manner by varying the angle (α ) at which the load is applied. A case of pure shear is produced in section AB when α = 90 degree. The basic concept behind both configurations is that the Arcan test set-up has a well-defined section, usually referred as significant section, where the stresses are assumed to be uniform. Fig. 3.8 shows the significant section as a bold line at the center of the butterfly specimen. This uniformity is a result of an appropriate choice of the geometrical parameters of the butterfly specimen in accordance with the tested material and the biaxial loading angle. Another outcome of the butterfly type geometry is the stresses at the significant section are the highest and thus, failure or initial yield is more likely to occur within the section.

62 39 Fig. 3.7 : Arcan fixture for shear testing with different loading configurations [20] Fig. 3.8 : Internal mean shear and normal stresses along the significant section [20]

63 40 Assuming a uniform stress distribution, both shear and axial forces are applied to the tested specimen by loading the sample as previously shown in Fig The mean shear stress, τ xy, and the mean normal stress, σ y, at the significant section are defined in a local coordinate system, where the x-axis is parallel and the y-axis is perpendicular to the significant section. Both components can be directly determined from the forces that are transmitted by the joints between the Arcan grips and the testing machine, as previously shown in Fig.3.7. The forces that act along in the positive axis of the universal testing machine referred as the vertical applied force, P y. The force perpendicular to the vertical one is referred to as the horizontal force, P x. The angle between the fixed axis of the testing machined (vertical axis) and the direction of the significant section (local x-axis) is referred as loading angle, α. Finally, A denotes the cross-sectional area of the specimen significant section, (i.e. thickness x width). From Fig. 3.8, the known force applied to the rig will produced shear and normal stress at section AB. In order to determine the normal stress σ x and the shearing stress τ xy exerted on the face perpendicular to the x-axis, a prismatic element with faces respectively perpendicular to the x and y axes shall be considered. It can be observed, if the area of the oblique face is denoted by A, the areas of the vertical and horizontal faces are respectively equal to A cosθ and A sinθ. It follows that the forces exerted on the three faces are as shown in Fig. 3.9 (No forces are exerted on the triangular faces of the element, since the corresponding normal and shearing stresses have been assumed equal to zero in z-direction).

64 41 (a) (b) Fig. 3.9 : Prismatic element in state of equilibrium [19] By using components along the x and y axes from Fig (b), the following equilibrium equations were obtained ; F x = 0 : σ x A - σ x ( A cosθ )cosθ - τ xy ( A cosθ )sinθ - σ y ( A sinθ )sinθ - τ xy ( A sinθ )cosθ = 0 [3.1] F y = 0 τ x y A + σ x ( A cosθ )sinθ - τ xy ( A cosθ ) cosθ - σ y ( A sinθ )cosθ + τ xy ( A sinθ )sinθ = 0 [3.2] By solving the first and second equation for σ x and τ x y, σ x = σ x cos 2 θ + σ y sin 2 θ + 2τ xy sinθ cosθ [3.3] τ x y = -(σ x - σ y ) sinθ cosθ + τ xy (cos 2 θ - sin 2 θ ) [3.4] Recalling the trigonometric relations ; And ; sin 2θ = 2sinθ cosθ cos 2θ = cos 2 θ - sin 2 θ [3.5]

65 42 cos 2 θ = 1+ cos 2θ 2 sin 2 θ = 1 cos2θ 2 [3.6] By substituting these trigonometric relations, we can write Eqn. [3.3] as follows ; σ x + σ y σ x = 2 σ x σ y + 2 cos 2θ + τ xy sin 2θ [3.7] Using the relations Eqn. [5], we write Eqn. [4.8] as ; σ x σ y τ x y = - 2 sin2θ + τ xy cos 2θ [3.8] The expression for the normal stress, σ y is obtained by replacing θ in Eqn. [5.2] by the angle (θ +90 ) that the y axis forms with the x axis. Since cos (2θ ) = -cos 2θ and sin (2θ ) = -sin2θ, we have ; σ x + σ y σ y = 2 σ x σ y - 2 cos 2θ - τ xy sin2θ [3.9] Adding Eqn. [3.7] and [3.8], the below equation is obtained ; σ x + σ y = σ x + σ y [3.10] The equations [3.9] and [3.10] obtain previously are the parametric equations of a circle. This mean that a set of rectangular axes and plot a point M of abscissa σ x and ordinate τ x y was choose for any given value of the parameter θ, all the points obtained will lie on a circle, as illustrated in Fig.3.10.

66 43 Fig : Mohr s circle due to stress analysis [19] To establish this property, the θ is eliminate from Equations [3.9] and [3.10] ; this is done by first transposing (σ x + σ y ) / 2 in Eqn. [5.1] and squaring both members of the equation, then squaring both members of Eqn. [5.2], and finally adding both equation. Therefore, 2 σ x σ y 2 σ x + τ x' y' = 2 σ 2 x σ y 2 + τ 2 xy [3.11] σ x + σ y and, σ ave = 2 σ x σ y 2 and R = + τ xy 2 2 [3.12] Then, Eqn. [3.11] and Eqn. [3.12] can be written in the form of equation of a circle. (σ x - σ ave ) τ x' y' = R 2 [3.13]

67 44 From Fig. 3.9, if the stresses on the significant section AB are uniform, it follows from the previous equilibrium analysis that on significant section AB as shown in Fig. 3.8 ; σ xx = P sinα and τ xy = A o P cosα [3.14] A o Where A o = the area of the significant section AB The rectilinear portions of the cut-outs are oriented at 0 ± 45 and, therefore, the principal stresses in the vicinity are also in these directions. It follows that τ xy on AB as given by Eqn. [3.14] is a principal shear stress. Therefore on AB, σ xx = σ yy = P sinα [3.15] A o and the principal stresses are ; σ 1 = σ xx + τ xy = P (sinα + cosα ) A o σ 2 = σ xx - τ xy = P (sinα - cosα ) [3.16] A o 3.4 Shear Analysis on Specimen From the previous analysis, the maximum normal stress acted at from the horizontal axis. Therefore, strains can be measured by applying a rosette type strain gauge at 0 ± 45 0 ± 45 at the center of section AB measured from horizontal axis. An analysis

68 45 has been done to find the relationship between normal strain and shear strain. For an elastic material, when the element is subjected to shear and normal stresses, it will deform to a new shape as show in Fig Fig : Element deformation due to shear [19] Firstly, the expression is derived for the normal strain ε(θ ) along a line AB forming an arbitrary angle θ with the x axis. To do so, the right triangle ABC is considered, which has hypotenuse (Fig. 3.11(a)) and the oblique triangle A B C into which triangle ABC is deformed (Fig. 3.11(b)). Denoting by s the length of AB, we express the A B length as s [1 + ε(θ )]. Similarly, denoting by x and y the length of sides AC and CB, we express the length of A C and C B as x(1 + ε x ) and y(1 + ε y ), respectively. The right angle at C in Fig deforms into an angle equal to π + γ xy in Fig. 3.11(b), and applying the law of cosines to triangle A B C ; thus 2

69 46 (A B ) 2 = (A C ) 2 + (C B ) - 2 (A C)(C B ) cos ( s) 2 [1 + ε(θ )] 2 = ( x) 2 (1 + ε x ) 2 + ( y) 2 (1 + ε y ) 2 π 2 + γ xy -2( x)(1 + ε x )( y)(1 + ε y )cos π 2 + γ xy [3.17] From Fig. 3.11(a), the relation between x and y is known, thus ; x = ( s)cosθ y = ( s)sinθ [3.18] And since γ xy is very small and can be neglected, therefore ; cos π 2 + γ xy = - sin γ xy γ xy [3.19] Substitute from Eqn. [3.18] and [3.19] into Eqn. [3.17], and recalling that cos 2 θ + sin 2 θ = 1, also neglecting second-order terms in ε(θ ),ε x,ε y, and γ xy ; ε(θ ) = ε x cos 2 θ + ε y sin 2 θ + γ xy sinθ cos θ [3.20] Equation [3.20] is enable to determine the normal strain ε(θ ) in any direction AB in terms of the strain components ε x, ε y, γ xy and the angle θ that AB forms with the x axis. As an example, for the theta values, θ = 0, Eqn. [3.20] yields ε (0) = ε x and 0 0 that, for θ = 90, it yields ε (90 ) = ε y. By using the trigonometric relations from Eqn. [3.5] and [3.6], we can write Eqn. [3.20] in the alternative form of ; ε x + ε ε y x = 2 ε x ε y + 2 cos 2θ + γ xy 2 sin 2θ [3.21]

70 47 0 Replacing the (θ ) value with (θ +90 ), the normal strain along the y axis can 0 0 be obtained. Since cos (2θ ) = -cos 2θ and sin (2θ ) = - sin 2θ ; ε y = ε x + ε y 2 ε x ε y - 2 cos 2θ - γ xy 2 sin 2θ [3.22] By adding Eqn. [3.21] and Eqn. [3.22], ε x + ε y = ε x + ε y [3.23] From the stress and strain relation, the shear stress strain relation can be obtained, where the shear strain in the ± 45 0 is ; γ xy = ε 45 o - ε- 45 o [3.24] Which can be written as ; γ xy = 2ε 0 [3.25] 45 Therefore, the in-plane Shear modulus, G, is ; G xy = ε τ xy [3.26] ε G xy = τ xy [3.27] 0 2ε 45 And finally, G xy = τ γ xy xy [3.28]

71 Recent Research and Development Using Arcan Test Method a) S. C. Yen, J.N. Craddock and K.T. Teh S. C. Yen et al. had used a modified Arcan fixture for the in-plane shear test of materials such as aluminum, Plexiglas and composite material (i.e. Graphite/PEEK) [14]. In general, they had modified the Arcan fixture used by M. Arcan et al. [1], as previously stated in subtitle 3.2. The objective of their study was to modify and improve the Arcan fixture for better control shear test data. i. Stress-Strain Relationship From the strain and loading data from the test, the relationship between the applied shear stress and the strain in ± 45 0 direction was established. The stress-strain data for the Graphite/PEEK composite specimen is shown in Fig a. based on the Von-Mises criterion b. based on the Tresca criterion c. from the [90] 16 specimen Fig : The strain results of a Graphite/PEEK specimen in shear [14]

72 49 The strain results of a Graphite/PEEK specimen in shear had verify that a state of pure shear was present during the experiment. This is deduced from the fact that the straining transverse to the applied load was practically zero, thus indicating no normal stress in that direction. The principal strains in the -45-deg and +45-deg directions were then used to calculate the shear strain. This was done by subtracting the strain in the -45 degree direction from that +45 degree direction. As a result, a shear strain relation for each gauge specimen were obtained. The calculated shear moduli of aluminum, Plexiglas and Graphite/PEEK are given in Table 3.1. Table 3.1 : Average shear modulus and shear strength test results of various materials [14] Material Average Shear Modulus, G (GPa) Average Shear Strength, τ (MPa) Theory Experiment Theory Experiment Aluminum Plexiglas 42 a 36 Composite (Graphite/PEEK) The standard shear modulus and shear strength data of aluminum (6061-T6) were obtained from a mechanics of material textbook [18] and were used to compare the test results obtained from their research. For Plexiglas, only the tensile strength was found from the literature published by their vendors. The data in Table 3.1 were then used to calculate the shear strength of Plexiglas based on the Von-Mises and Tresca criteria. For the thermoplastic composite material, the elastic shear modulus and strength obtained from the tensile test of the [45] 2s laminate was used for comparison. It was found that the shear properties obtained from the Arcan shear test method was agreed with reference data provided by M. Arcan [1].

73 50 ii. Fracture Surface The fracture surface for the aluminum and thermoplastic composite were found parallel to the direction of the applied load, as shown in Fig and Fig This appearance indicates failure mechanism due to a state of shear stress. It should be pointed out that the fracture of graphite/peek specimen occurred at a location slightly away from the gauge section. This may be due to misalignment between the loading axis and the gauge section or suspected due to initiation of sharp edges or formation of crack within the region. Fig : Shear failure of an aluminum specimen [14] Fig : Shear failure of a Graphite/PEEK specimen [14]

74 51 The fracture mechanism of the Plexiglas initiate from the notch roots of the specimen. As a result, the fracture surface was generated and found 45 0 from the loading axis, which is the direction of the tensile principal stress corresponding to the state of pure shear, as shown in Fig This fracture mechanism supports the fact that brittle materials generally fail in a tensile mode, which was also found in the Iosipescu shear test of vinyl-ester conducted by Sullivan et al. Fig : Failure mode of a Plexiglas specimen [14] b) Rani El-Hajjar and Rami Haj-Ali From the experiment conducted by Rani El-Hajjar and Rami Haj-Ali [20], the Arcan fixture with butterfly specimen are used to measure the in-plane shear properties of thick-section pultruded FRP composites. The main objectives of their experiment are to analyze the effect of notch radius on the shear properties study, the strains profiles along the AB section, and to determine the materials shear modulus, G. i. Effect of Notch Radius There were three notch radii selected to determine the most appropriate radius 1.27 mm, 2.54 mm and 5.05 mm. Fig shows the effect of the notch radius on the

75 52 shear stress profile along the gauge section for axial roving orientation tested by them. A normalized stress profile near to 1.0 was found near the center for the specimen with a notch radius of 2.54 mm. Fig : Effect of notch radius on shear stress profile along gage section [20] On the other hand the simulation by isotropic assumption and orthotropic value shows that the blunted notch results in a lower stress concentration near the notch tip, with a more gradual stress build up compared to the sharp notch as shown in Fig The stress profile is uniform near the blunted notch tip and resulting in a normalized shear stress closer to 1.0.

76 53 Fig : Effect of sharp notch on shear stress along the gage section [20] ii. Stress-Stain Relation of Arcan Test Method From their test data, a stress-strain curve was plotted and it can be noted that all specimens was perfectly failed in brittle manner. The shear stress versus shear strain curves shown in Fig has verified that a state of pure shear was present during the testing because the curve is linearly propagated. This is also deduced from the fact that the straining transverse to the applied load was practically zero, thus indicating no normal stress in that direction [14].

77 54 Fig : Shear stress strain response from Arcan shear test [20] Both strains, ε 45 and ε 45 linearly propagated and almost symmetry along x-axis as shown in Fig This indicates that this testing method is reliable as the strain data obtained is balance in each direction, and can be used to determine the shear properties, shear modulus, and shear strain of brittle materials, especially for composites. Fig : Measured strain profiles at center of transverse butterfly specimen during pure shear test [20]

78 Conclusion In this chapter the Arcan fixture development process was shown, which is including the time-line of Arcan fixture, how the rig works, advantages of significant section on butterfly specimen and the reliability of the Arcan test result. As a conclusion, the Arcan test method can be used to determine the mechanical properties of composite material such as shear strength and shear modulus.

79 56 CHAPTER 4 SPECIMEN PREPARATION, EXPERIMENTATION AND TESTING 4.1 Introduction The main topics that will be discussed in this chapter are the test specimen preparation and the testing procedure using the Arcan test method. The closed mould method was used to produce the butterfly specimens. The specimens were exposed to their designated conditions, namely ; Lab Control (LB-control), Plain Water (PWwet/dry), Salt Water (SW-wet/dry), and Outdoor (OD). The salt water, plain water and outdoor specimens are exposed to their respective conditions for 7 days wet and 7 days dry (i.e. alternate cycle) for duration of 6 months. After that, the specimens were tested in order to investigate any sign of exposure conditions effects. The shear test was carried by using the Instron Universal Testing Machine Series IX Model 4206 with Arcan test rig (fixtures). The fracture surface of the test samples of each exposure condition was then undergone microstructure analysis in order to investigate factors or elements that contribute to the failure. The Arcan test fixture used in this project was almost similar to the fixture used by Rani El. Hajjar and Rami Haj-Ali [9]. The applied load to the rig was in tensile but the loading mode was transferred or imposed onto the specimen in the form of pure shear.

80 Material Details In this study, the epoxy adhesive namely Resifix-31, supplied by Exchem, United Kingdom was used. The properties of Resifix-31 are achieved by blending/mixing a modified epoxy resin and inorganic fillers to form a base component, which is activated by a thixotropic formulated amine hardener. Resifix-31 epoxy is smooth, very viscous and light grey (almost white) in colour paste. Resifix-31 is harder to mould due to its high viscosity but fast cured. It took only 24 hours for Resifix-31 epoxy to cure, which depends on the ambient laboratory condition. These structural epoxy adhesives consist of part A and B and their chemical formulation are listed in Table 4.1. The adhesive materials, part A and B must be mixed by a mixture ratio of 3:1, by following the supplier specification. The parts A and B of structural adhesives Resifix-31 are shown in Fig. 4.1 and the properties of Resifix-31 are shown in Table 4.2. Table 4.1 : Chemical formulation of Resifix-31 structural adhesives Material Chemical Formulation Colour Resifix-31 Part A (Epoxy) : pentaethylenehexamine, m- phenylenobis (methylamine),4,4- isopropylidene diphenol and poly(oxy(methyl-1,2-ethandyl)),alpa- (2-aminemethylethyl) omega-(2-amine) White Part B (Hardener) : Epoxy Constituent Dark Grey

81 58 Table 4.2 : Product data of Resifix-31 epoxy adhesive * Product Data Value Shear strength (MPa) 22 Tensile strength (MPa) 28 Shear modulus (GPa) 3.8 Compressive strength (MPa) 75 Poisson s ratio 0.28 Thermal expansion 33 x 10-6 / C Pot life (minutes) 45 (slow grade) Service time (hours) 24 *As supplied by Exchem EPC Group, United Kingdom (a) (b) Fig. 4.1 : Two parts structural adhesives of Resifix-31 (a) Part A (b) Part B

82 Specimen Preparation The specimen size of 60 mm length by 45 mm width with average thickness in range of 4.0 mm ~ 5.0 mm was used for the experimentation and study, as shown in Fig The 90 degree notches was formed at the centre of 60 mm length side by side such that the distance between notches was left about 10 mm at the middle to introduce shear field on the significant area (AB). A radius of 1.5mm was made in order to eliminate stress concentration point and produce a uniform shear stress distribution along AB by following Rani El. Hajjar and Rami Haj-Ali [20] specimen size. ASTM D 5379, however, specifies a notch radius of 1.3mm in order to minimize the shear stress concentration at the notch radius and the use of large radii is not entirely successful in promoting a uniform shear stress distribution [17]. All specimens were 0 0 conditioned in room environment ranging from 25 C - 33 C temperature and 70% - 90% relative humidity for 7 to 14 days before left for experimentation and testing (i.e. for controlled sample) to ensure specimens were fully cured. At least five specimens for each group of sample were selected for the study. The flow chart of the specimen preparation process is shown in Fig. B1 in the Appendix B section. * All dimensions in mm Fig. 4.2 : The butterfly specimen geometry

83 60 The butterfly shape cast epoxy specimens were prepared by mixing two parts of adhesive system consists of epoxy and hardener with the ratio of 3:1. Butterfly shape specimens were produce by casting the mixture onto a female mould. To produce five (5) pieces of butterfly shape specimen, 5 sets of the mixture (consist of weight ratio of 210 gram epoxy and 70 gram hardener) were prepared as shown in Fig Fig. 4.3 : Adhesive system Part A/epoxy (white) and Part B/hardener (dark grey) A low speed electric mixer was then used to mix the materials until it turns soft grey in colour, which means the colour of the mixture turns from white, dark grey to soft grey colour as shown in Fig Due to short pot life, the mixture was mix with small quantity by using plastic container. The mixing process was done in the laboratory control room where the temperature and relative humidity was in range of 24 C to 26 C and 40% to 55% (i.e. by depending on the ambient laboratory condition). Fig 4.4 : A mixing process using slow speed electric mixer

84 61 The butterfly shape specimens were cast by using a mild steel mould which consists of male part (top) and female part (base) as shown in Fig The mould are assemble together to produce a butterfly shape specimens, as shown in Fig (a) (b) Fig. 4.5 : Male and female parts of the butterfly mould (a) Male (b) Female Fig. 4.6 : A complete assembly of mould parts The moulds surfaces were clean by using soft cloth and Carnauba wax to ensure it is free from dirt and dust before used for casting. This mould parts cleaning process is shown in Fig. 4.7.

85 62 Fig. 4.7 : Surface cleaning process using soft cloth with Carnauba wax After cleaning process, two flat plates (27.5 cm x 6.4 cm) are attached to cover the back part of the female mould by screws as shown in Fig. 4.8 (a) and (b). (a) (b) Fig 4.8 : Flat plate attached to male part by screws (a) Flat plate before installation (b) Flat plate after installation The epoxy casting process starts by casting the epoxy into the female part mould. The casting process was carefully done by using metal scrapper in order to ensure minimum air trapped in the mixture, as shown in Fig It is important to make sure that the epoxy adhesives fill the volume of the butterfly cast-shape and not overfilled.

86 63 Fig. 4.9 : Casting process by applying epoxy mixture into the female mould The male mould part was then attached to the female part, which means the male part applied compression load onto the adhesives to form a solid butterfly shape as shown in Fig About 10 kg of weight was placed onto the top side of the mould to applied an extra uniform pressure onto the male-female mould assembly, as shown in Fig Finally the specimens were leave in a room temperature condition ranging from 23 0 C to 33 0 C for 24 hours for full chemical reaction (i.e. curing) before the de-moulding process. Fig 4.10 : Male part attached to the female part

87 64 Fig 4.11 : A 10 kg mass used to press the female part from top side In de-moulding process, the specimens were carefully de-mould by applying soft knocking the butterfly shape Teflon block with wood hammer as shown in Fig The polishing process was done onto the selected specimens by using Mecapol P255 U Polish Machine. This process was done to smoother and round sharp edges on the specimen as shown in Fig. 4.13(a) and Fig. 4.13(b). Fig : De-moulding process of specimen using Teflon block and wood hammer

88 65 (a) (b) Fig : Surface grinding and polishing process of the specimen a) Mecapol P255 U polishing machine b) Manual polishing technique In final stage, prior exposure to nominated conditions, the specimens were checked on their quality, established their code name and date of manufactured, as shown in Fig The width, thickness and area of every sample are shown in Appendix A2. Fig : Specimens ready to be exposed

89 66 A conclusion has been made that the average specimen width, thickness and cross sectional area were in range of mm ~ mm, 4.35 mm ~ 4.51 mm and mm 2 ~ mm 2, as shown in Table 4.3. Table 4.3 : Specimen average width, thickness and significant area Sample Code Average width (mm) Average thickness (mm) Average significant area (mm 2 ) ESST-CO (Control) ESLT-LB (Laboratory) ESLT-OD (Outdoor) ESLT-PW (Plain water) ESLT-SW (Salt water) Experimentation The samples were exposed to four different types of environmental conditions that reflect the tropical climatic conditions, namely; laboratory condition (LB), outdoor condition (OD), and exposure to plain and salt water under wet/dry cycles. The wet and dry cycles were 7 days wetting and 7 days drying respectively and was experimented in two controlled laboratory rooms whereby and the room for salt water condition was separated Environmental Exposures The test matrix of environmental durability exposure conditions designed for this research programme is given in Table 4.4. The number of test specimen per sample for each candidate condition and their number of cycles was clearly indicated. The individual effects of each exposure condition are being evaluated. In this programme,

90 67 climatic tropical exposures include: laboratory condition, plain water wet/dry cycles resistance, salt water wet/dry cycles resistance and weathering (natural) resistance were established. For the laboratory (LB) exposure condition, the sample was experienced 75% to 90% relative humidity (RH) and 23 C to 33 C of room temperature. The test sample was placed onto the aluminium angle bar supported by mild steel rack and held in a horizontal position. The sample exposed surface was weekly rotated. The immersion test was selected for plain water and salt water to test the effects of prolonged immersion in tropical climate and ocean tropical water. Normal plain (tap) water was used for water resistance exposure condition. Substitute ocean tropical water prepared following standard manufacturer specification (i.e. tropical ocean salt for aquarium) was used for the salt water resistance exposure. Immersion of specimens in plain water and salt water was done by using the 50 liters capacity of GFRP cylindrical tank. The water quality for each condition is shown in Table 4.5 while the selected test condition is shown in Table 4.6. Table 4.4 : Exposure Conditions for resifix-31 epoxy specimens Tank code Specimen code Exposure duration (month) ph Water temp. ( C) Control room temp. ( C) Relative humidity (%) Conductivity A (plain water) ESLT PW (W/D01-W/D05) B (Plain watercontrol) C (salt water) _ ESLT SW (W/D01-W/D05) D Salt water (control) 8 liter plain water + 300g sea salt Sg =

91 68 Table 4.5 : Water quality measurement tested in environmental laboratory, Faculty of Civil Engineering, UTM Plain Plain water Salt Salt water PARAMETER water control water control Calcium (Ca) Magnesium (Mg) Chloride (Cl) Alkalinity Acidity Suspended Solid (SS) ph Table 4.6 : Plain Water and Salt Water Experimentation Data Conditions (measured in research laboratory control room) Environmental Durability Test Conditions Test Duration Test Laboratory 70% to 90% RH at 22 C to 33 C 6 months Plain Water Resistance ph = at 80% to 90% RH 6 months (24 cycles) Water temperature at 25 C to 26 C Wet/Dry Cyles Salt Water Resistance ph = at 80% RH 6 months (24 cycles) Water temperature at 25 C to 26 C Wet/Dry Cyles Outdoor Resistance (Natural Weather) 70% to 90% RH Temperature at 22 C to 35 C 6 months

92 69 The specimen preparation process for each conditions of exposure is shown in Fig to Fig Firstly the cylindrical tanks were filled with plain (tap) water and salt by 1: 27.5 ratios, which means 1 liter of water added with 27.5g tropical ocean salt, as listed in Table 4.4. For an outdoor exposure conditions the specimens were placed on single aluminium rail and the specimens surface were alternately rotate (weekly) respected to their test duration. In the ends of the exposure durations, the specimens were brought to the laboratory and prepared for final load test within two weeks time. Fig : Adding salt in plain water Fig : Mixing ocean tropical salt and plain (tap) water Fig : Specimens in plain water condition Fig : Specimens in salt water condition Fig : Specimens in control room condition Fig : Specimens exposed to outdoor condition

93 Strain Gauge Installation A rosette type strain gauge, TML FCA-1-11 with 1 mm gauge length was installed onto one side of the butterfly specimen at the center of the significant area (A-B) as shown in Fig The gauge was attached on one surface and the direction is 0 ± 45 from the horizontal axis (x). Before installation, the surface of the significant area was polished with 1000 grade grain size sand paper prior cleaned by using liquid acetone to remove grease, dust or dirt. Then the strain gauge was attached onto the specimen by referring to standard installation procedure. The complete gauge attachment onto butterfly specimen is shown in Fig and important parameters of the gauge specification are shown in Table 4.7. Fig : Rosette type strain gauge installation at ± 45 0 and at the centre of AB line Fig : Complete gauge installation onto the butterfly specimen

94 71 Table 4.7 : Rossete type strain gauge specifications Manufacturer Gauge Type Tokyo Sokki Kenkyujo Co, Ltd Japan TML FCA-1-11 Gauge Factor 1=2.08, 2=2.08 ± 1% Insulation Resistance Coefficient of Thermal Expansion Tolerance Temperature Coefficient of Gauge Factor Less than 50V 11.8 x 10-6 / 0 C ± 0.85 [( µ m/m) / 0 C +0.1 ± 0.05% / 10 0 C After the specimen was completely installed with the strain gauge, an adhesive film thickness of 0.1mm was bonded onto both male and female Arcan fixture (grips) to ensure the butterfly specimen perfectly fixed and prevent from slip during loading. This process is shown in Fig and Fig Next, the male grip is used to clamp the butterfly specimen and four screws are used to tighten them together at the upper side and the lower side diagonally as shown in Fig to Fig These screws are carefully tightened to make sure the specimen outer surface does not overstressed. Fig : The adhesive film bonded onto specimen area Fig : Butterfly specimen mounted to grip

95 72 Fig : Arcan male grip attached onto female grip Fig : Complete assembly of butterfly specimen Fig : Screws tightening process The soldering technique was used to joint the strain gauge lead wire to the terminal (i.e. previously bonded onto the Arcan fixture). This process was carefully done in order to prevent the terminals from disconnected (over tensioned) during loading, as shown in Fig

96 73 Fig : Soldering lead wire to terminals 4.5 Arcan Fixture Set-Up The Arcan test fixture consists of a pair of male and female parts, as shown in Fig The exact shape and size of specimen was mounted into the female part follows by the male part. Both parts were tightened by screws to ensure the specimen was tightly gripped between the fixtures to prevent from slippage and misalignment during loading. The complete assembly of the fixtures was attached to the holder at lower and upper part accordingly prior attached to the Instron Universal Testing Machine, as shown in Fig The tensile loads were applied onto the Arcan fixture through holders before the loading configuration changed to shear mode that finally imposed onto the specimen. The direction of principal shear then will act in the direction of ± 45 0 as referred to the horizontal axis of specimen significant section.

97 74 Fig : Arcan fixture set-up Fig : Assembly of Arcan fixture

98 Arcan Fixture Installation The fixture holders are attached to the loading machine, namely Universal Testing Machine (UTM) by two pins with a diameter of 15mm as shown in Fig After the holder attached to the UTM, the complete Arcan grip fixture consisting the butterfly specimen was connected to the holder by using two pins at the upper and lower section of the Arcan plate. All the pins are carefully checked to make sure the connection was in perfect adjustment before applying the load tight, as shown in Fig Fig : Attachment grip to holder by pin Fig : Complete Arcan fixture attachment

99 Instrumentation and Measurement The complete instrumentation and measurement system is shows in Fig and consist the following important features ; Fig : Instrumentation set-up 1. A load frame where the Arcan fixture was installed and loaded in tensile. 2. A control panel that control the testing parameters such as loading rate. 3. A computer for the user to key in the information and the properties of the specimen and set the format for the plotting of graph and results. 4. A data logger to record and print out the strain readings (i.e. test strain data) Testing Procedure The specimen was loaded in tension by using an Instron Universal Testing Machine Series IX Model 4206 instrumented with 5 kn load cell. The loading rate was

100 77 set-up to 1 mm/min and the specimen was loaded up to failure Specimen principal strains (i.e. ± 45 0 ) were measured at every 0.1 kn load increment and this was manually recorded by TDS 303 data logger until near to failure. The applied load and machine displacement were recorded automatically by the computer. Apart from that, a stopwatch was use to obtain time of failure for each specimen. As a result, the relationship between the applied shear stress and shear strain in each gauge of the strain rosette can be established by dividing applied load on cross-sectional area of significant section. The testing process is shown in Fig to Fig Fig : Arcan test rig ready to be t est Fig : Strain reading initialization Fig : Test data collection during testing

101 78 From the carried out test, the following data were recorded ; i. Load at/near to failure, (kn) ii. Strain in ± 45 0 at significant area for every 0.1 kn load, (µε) iii. Failure mode (i.e. cracking or fracture patterns) iv. Time to failure (s) Maximum load, strain near to brittle failure and mode of failure were recorded and observed for every single specimen. Finally, the following equations were used to determine the shear stress, τ, average shear strength, average shear strain, γ, and shear modulus, G a, at the significant area. a) Shear stress, τ τ = P/ b s h s, where ; τ = shear stress (N/mm 2 ) P = load (N) b s = length of significant section (mm) h s = thickness of significant section (mm) b) Shear strain, ε γ = 2 (ε x ) θ = 45, where ; γ = shear strain ( µε ) (ε x ) θ=45 = strain at 45 angle to the horizontal axis of the sample (ε ) c) Shear modulus, G a G a = τ/ ε, where ; G f elasticity (N/mm 2 a = shear modulus o ) τ/ ε = slope of the plot of shear stress as a function of shear strain within the linear portion of the curve (N/mm 2 )

102 79 The relations between a, b, and c is shown in Fig Fig : Relationship between shear stress, shear strain and shear modulus shown by graph of τ (vs) ε 4.7 Field-emission Scanning Electron Microscope (FESEM) After the test, the surface fracture and microstructure analysis of each sample was done by using Field-Emission Scanning Electron Microscope (FESEM). FESEM is a microscope that works with electrons (particles with a negative charge) instead of light. These electrons are liberated by a field emission source. The object is scanned by electrons according to a zig-zag pattern. Electrons are liberated from a field emission source and accelerated in a high electrical field gradient. Within the high vacuum column these so-called primary electrons are focused and deflected by electronic lenses to produce a narrow scan beam that bombards the object. As a result secondary electrons are emitted from each spot on the object. The angle and velocity of these secondary electrons relates to the surface structure of the object. A detector catches the secondary electrons and produces an electronic signal. This signal is amplified and transformed to a video scan-image that can be seen on a monitor or to a digital image and processed for further analysis.

103 80 FESEM was used in this project to visualize very small topographic details on the surface of fractioned objects and also used to determine the elemental composition measurement of each sample. The FESEM is shown in Fig consist of the following ; 1. Cryo Unit - A cylindrical column that is mounted on a desk and used to hosts the electron beam. 2. Container with Liquid Nitrogen - Maintain the vacuum and the temperature in the instrument and the cryo-unit. 3. Vacuum Chamber Unit - Small chamber where the specimen been put, washed and dried to produce fine structures. 4. Control Panel - The object can be observed on the large screen. 5. Second Control Panel - The small screen serves to watch the object chamber while it is scanned. Fig : Field-emission Scanning Electron Microscope (FESEM)

104 Conclusion As a conclusion, this chapter explained clearly about how the butterfly specimen was produced from raw material into complete specimen, the experimentation and test set-up such as Arcan rig installation, specimen testing procedure, and data analysis from the tested experimentation. It is important to make sure the specimen and test set-up done carefully in order to minimize the error.

105 82 CHAPTER 5 RESULTS AND DISCUSSION 5.1 Introduction The results and discussion focuses on the experimental data analysis for control and exposures samples. The determination of shear properties of Resifix-31 epoxy adhesive specimens due to tropical exposure conditions through Arcan test method is the main parameter to be discussed. The shear stress versus shear strain (τ vs γ ) represented by graph has verified that a state of pure shear was produced during final load test. Then the fracture surface of the test specimens of each exposure condition undergoes microstructure analysis in order to investigate the sign of failure mechanism. 5.2 Results All test samples show a brittle failure characteristic for each exposure conditions. The fracture line occurred at about 45 0 angle measured from the specimen principal axis (x) to the gauge length vertical line.

106 83 This confirmed that the specimens failed in the direction of principal stresses (i.e. failure characteristic of brittle material). This is consistent with the theoretical analysis, where the direction of principal stresses is at 45 o from pure shear stress plane (gauge section) as shown in Fig By visual inspection on the fractured surface of specimens, it can be seen that the Resifix-31 epoxy failure occurs almost perfectly at 45 o angle. The principal strains in the 45º and +45º directions were then used to calculate the shear strain. By using equation [3.24] as stated in chapter 5.2.2, the shear stressstrain relations for each gauge specimen can be obtained. This was done by subtracting the strain in the 45º direction from the +45º direction. As a result, the shear stress-strain relations for each gauge specimen were obtained. The elastic shear modulus was then determined using the least square fit (i.e. curve gradient) of the earlier stage of the shear stress-strain curve Result Discussion There are four main topics that were discussed in this chapter that relates to the objectives of the study, which are summarize as follows ; i) Graph discussion discuss the curve pattern of the sample for every exposure conditions, including the controlled sample. ii) Discussion - discussion on the effect of exposure for every sample/condition according to the plotted graph and relate with the microstructure analysis outcomes. iii) Comparison - comparison of the experimentation results of every sample related to their exposure conditions. iv) Mode of failure - discussion of the failure mode due to pure shear load and fracture pattern.

107 Test Data Sample Calculation From the obtained Arcan test results, the shear strength, shear strain and the shear modulus were calculated. In this calculation, we assumed that the shear stress distribution along significant section, AB, is uniform. A sample calculation for Resifix- 31 ESLT-LB01 with notch radius of 1.5mm specimen based on Fig. 5.1 at 1000N load is given as follows ; Fig. 5.1 : Specimen geometry for sample calculation Specimen thickness, t = 4.43 mm Specimen significant gauge length, h = mm Load carried by the specimen at the moment, F = 1000 N Principal strain in +45 o direction, ε 4112µε o = 45 Principal strain in -45 o direction, ε = 3646µε o 45 The average shear stress, τ ave, and shear strains, γ, are determined as follows ; Specimen cross sectional area, A o = t h A o = (4.43 x 10.93) mm A o = mm 2

108 85 i. Average shear stress, τ ave, τ ave = F A o Force = Area [3.14] x10 τ ave = 6 = MPa ii. Shear strain, γ, γ = ε o ε o [3.24] γ = 4112 ( 3646)µε γ = 7758µε The alignment of the stress element can be determined from equation 3.22 as follows ; ε o + ε o ε ave = [3.22] ε ave = 2 ε ave = 233.0µε The state o f strains of the stress element can be seen from a Mohr s circle constructed based on the above strains values as shown in Fig ε Centre of circle, C = ε = x 45 o + ε 2 = 233.0µε 45 o Radius of circle, R = γ 7758µε = 2 2 = µε Diameter of circle, φ = 2R = 7758µε

109 86 Fig. 5.2 : Mohr s circle constructed based on principal strains of Resifix-31 ESLT-LB01 at 1000N loading condition The Mohr s circle shows that the pure shear condition did not occur exactly at the mid section of the specimen where the principal strain is zero. Instead, the pure shear area was slightly shifted to the right side. However, this result is still acceptable as the shifting is about 8.5% from the circle radius. On the other hand, practically it is almost impossible to obtain zero error in experiments compared to theoretical analysis. Therefore, the Arcan testing method which was used to determine shear properties of brittle epoxy specimen can be considered as reliable and justified. The result shows that the shifting of the stress element as indicated by average strain value increases as the loading increases. This is due to the nature of brittle materials, where the micro cracks will typically propagate rapidly through the specimen as the loading increased. However, the misalignment of the pure shear area is still less than 9% [17]. The result obtained also has been affected by the alignment of the test rig. When the butterfly specimen attached in the Arcan rig, all the 4 parts of the rig must be

110 87 aligned carefully. Since the installation of the rig was normally done without any measurement instrument, therefore it was difficult to ensure all the four parts are perfectly align Experimental Results of Control Specimen (ESST-CO) There are five (5) specimens for the controlled samples. All specimens, ESST-C01 to ESST-C05 has shown a brittle characteristic of failure, which occurred 45 0 from the principal axis (x) and without necking (i.e. cup and cone) during experimentation. Example of raw data for ESST-C01 specimen is shown in Table 5.1 and the complete test data for the controlled sample, which was calculated from previously stated equations, is shown in Table 5.2 while the rest of graph for the remain specimens are included in the Appendix B section. Table 5.1 : Raw experimental data for Resifix-31 ESST-C01 Load (N) Principal strain at +45 o, ε (µε) +45 Principal strain at -45 o, ε (µε) 45 Average shear stress τ ave (MPa) Average Strain, ε x (µε) Shear strain, γ (µε)

111 88 Table 5.2 : Testing results for ESST-CO sample Specimen Ult Load (kn) Strain near to failure Shear Mod (GPa) Shear strength τ (MPa) Shear strain, γ ( µε ) Time to Failure (sec) Weight gain (%) Weight reduction (%) ESST-C ESST-C ESST-C ESST-C ESST-C Average From Table 5.2, all specimens experienced no weight gain or weight reduction as they were kept in plastic container after the de-moulding process. The highest ultimate failure load for the controlled sample is kn and the lowest is kn. For this controlled sample, the average ultimate failure load is kn. The average shear modulus, G, and shear strength, τ, is 2.97 GPa and MPa. As compared to manufacturer s specifications, the shear modulus and shear strength for Resifix-31 are 3.8 GPa and 22.0 MPa. The value of shear modulus and shear strength obtained from the Arcan test method for controlled sample are 21.84% lower and 33.05% highe r from the manufacturer s value (i.e. previously stated). Average time to fail for most of the specimens is about 66.4 seconds while the total average of shear strain is µε. Th e shear stres s and shear strain data in Table 5.2 was then used to plot the strain-shear stres s curve as shown in Fig It has verified that a state of pure shear was present during the testing. This is deduced from the fact that the straining transverse to the applied load was practical ly zero, thus ind icating no normal stress in that direction. The stress strain curves show that strain on each specimen like a linear propagated. The different recorded strain value between stra ins gauges +45 and -45º is about 7%. [17], which shows that loading misalignment as little as 1 0 from normal may cause a 6% difference in shear property.

112 89 Fig. 5.3 : Shear stress-strain curve for Resifix-31 ESST-C01 During testing, it can be observed ESST-C01 specimen linearly gained shear loads from the start until up to failure, and had show brittle type failure mode as shown in Fig The fracture mechanism of the epoxy appeared to initiate from the notch roots of the specimen. This fracture surfaces was found at 45º to the loading axis, which is the direction of the tensile principal stress corresponding to the state of the pure shear as shown in Fig. 5.5 (a) and (b). Such failure mechanism was also found in the Iosipescu method on vinylester specimen conducted by Sullivan et al. (1984), and Arcan test method done by Yen S.C. et al. on Plexiglas specimen [14]. Fig. 5.4 : Specimen ESST-C01 after failed

113 90 (a) (b) Fig. 5.5 : Brittle failure of ESST-C01 a) ESST-C01 specimen in female rig b) Failure occurred at ± Experimental Results of Laboratory Exposure For laboratory exposure condition, only four (4) specimens in the sample were tested because specimen ESLT-LB05 was broken during experimentation. In this sample, all test specimens, ESLT-LB01 to ESLT-LB04 has shown a brittle characteristic of failure. Example of raw data for ESLT-LB01 specimen is shown in Table 5.3 and the complete test data for the laboratory sample, which was calculated from previously stated equations, is shown in Table 5.4.

114 91 Table 5.3 : Raw experimental data for Resifix-31 ESLT-LB01 Load (N) Average shear Principal Principal strain at +45 o, strain at -45 o stress, τ ε +45 ( µε) ε 45 ( µε) ave (MPa) Average Strain, ε x (µε) Shear strain, γ (µε) Table 5.4 : Testing results for ESLT-LB sample Specimen Ult Load (kn) Strain near to failure Shear Mod (GPa) Shear strength τ (MPa) Shear strain, γ ( µε ) Time to Failure (sec) Weight gain (%) Weight reduction (%) ESLT-LB ESLT-LB ESLT-LB ESLT-LB Average From Table 5.4, all specimens experienced weight gain when exposed in the laboratory condition. The average weight gain for each specimen is about %. This weight gain was suspected due to moistures absorption from surroundings in controlled room, as the specimens were experienced 75% to 90% relative humidity (RH) in

115 92 23 C to 33 C room temperature during six months of exposure duration. The highest ultimate failure load in the sample is kn and the lowest is kn. For this laboratory sample, the average ultimate failure load is kn. The average shear modu lus, G, and shear strength, τ, is 2.59 GPa and MPa. These shear modulus and shear strength for laboratory exposure are 12. 8% and 25.34% lower as compared to controlled sample. Average time to fail for most of the specimens is about seconds while the total average of shear strain is µε. The shear strain versus shear stress curve, as shown in Fig. 5.6 was then plot to show the relation between them. It can be seen th at the ESLT-LB specimens linearly gained shear load s from the start until up to failu re, but the curve was i nitially nonlinear which may due to slipped of specimen during the testing. However, it can be neglected as it was too small. Fig. 5.6 : Shear stress-strain curve for Resifix-31 ESLT-LB01 From the stress-strain curves in Fig. 5.6 it can be noted that all specimens were perfectly failed in brittle manner. The fracture surfaces for laboratory sample was also found almost 45º to the loading axis, which is the direction of the tensile principal stress corresponding to the state of the pure shear as shown in Fig. 5.7 and Fig. 5.8.

116 93 Fig. 5.7 : Specimen ESLT-LB01 after failed Fig. 5.8 : Brittle failure of ESLT-LB01 specimen Experimental Results of Outdoor Exposure There are four (4) specimens for the outdoor sample because specimen ESLT-OD05 was also broken during experimentation. Specimens ESLT-OD01 to ESLT-OD0is shown has also shown a brittle characteristic of failure. During installation of butterfly specimen onto the Arcan female grip, the specimens colour had change from light grey white to light yellow (yellowish) colour, due to oxidation process during early exposure to outdoor condition. Example of raw data for ESLT-OD01 specimen in Table 5.5 and the complete test data for the laboratory sample is shown in Table 5.6.

117 94 Table 5.5 : Raw experimental data for Resifix-31 ESLT-OD01 Load (N) Average shear Principal Principal strain at +45 o, strain at -45 o stress, τ ε +45 ( µε) ε 45 ( µε) ave (MPa) Average Strain, ε x (µε) Shear strain, γ (µε) Table 5.6 : Testing results for ESLT-OD sample Specimen Ult Load (kn) Strain near to failure Shear Mod (GPa) Shear strength τ ( MPa) Shear strain, γ ( µε ) Time to Failure (sec) Weight gain (%) Weight reduction (%) ESLT-OD ESLT-OD ESLT-OD ESLT-OD Average In this sample, all specimens experienced weight reduction in percentage as listed in Table 5.5. This is because the specimens are exposed to tropical conditions as hot, humid, rain, day and night for duration of 6 months. The average specimen weight reduction occurred for ESLT-OD sample is 0.098%. Yellowish effect occurred and can be seen as the specimens colour changed and some dust been detected on both surfaces of the specimens.

118 95 The highest ultimate failure load in the sample is kn and the lowest is kn. For outdoor sample, the average ultimate failure load is kn. The average shear modulus, G, and shear strength, τ, is 2.70 GPa and MPa. Shear modulus and shear strength for outdoor sampl e a re 18.4% and 9.09% lower as compared to controlled sample (as previously stated). Average time to fail is about seconds. The highest shear strain is 9308 µε for specimen ESLT-OD03 and the average shear strain value for this sample is 8435 µε. During th e test, it can be seen that the ESLT-OD specimens linearly gained shear loads from the start until up to failure. However, for the outdoor exposure sample ESLT-OD, the graph was not perfectly linear, which may due to h ighly brittleness, suspected from porosities that o ccurred at the significant section. Apart from that, the specimen cross sectional area is the lowest compared to other exposure conditions. This may be the effect of oxidation, as the size of the specimens became smaller (i.e. shrinkage happened) compared to the controlled specimens. The shear stress and shear strain data in Table 5.6 then used to plot the strain-shear stress curve as shown in Fig Fig. 5.9 : Shear stress-strain curve for Resifix-31 ESLT-OD01

119 96 From Fig. 5.9, the plotted stress-strain curve was not perfectly linear, which may due to highly brittleness/yellowish effect due to oxidation or due to the specimen slippage in the Arcan rig. Errors may happen due to misalignment of Arcan fixture during tightening of screws process and there was a stress concentration point at the notch radius. However, the strain-shear stress curve shown in Fig. 5.9 has verified that a state of pure shear was present during the testing. The yellowish effect could break polymer chain which finally encourages the specimen to fail faster. Other than that, suspected porosities may occurred at the significant section (AB) that lead cracks to initiate faster from the notch roots of the specimen, which is 45º to the direction of the tensile principal stress (i.e. loading axis) as shown in Fig and Fig Fig : Specimen ESLT-OD01 after failed Fig : Brittle failure of ESLT-OD01 specimen

120 Experimental Results of Plain Water Wet-Dry Exposure Sample For plain water wet-dry exposure condition, five (5) specimens have been tested. All specimens, ESLT-PW01 to ESLT-PW05 have shown a brittle characteristic of failure. Example of raw data for ESLT-PW01 specimen is shown in Table 5.7 and the complete test data for the plain water (w/d) sample is shown in Table 5.8. Table 5.7 : Raw experimental data for Resifix-31 ESLT-PW01 Load (N) Average shear Principal Principal strain at +45 o, strain at -45 o stress, τ ε +45 ( µε) ε 45 ( µε) ave (MPa) Average Strain, ε x (µε) Shear strain, γ (µε) Table 5.8 : Testing results for ESLT-PW sample Specimen Ult Load (kn) Strain near to failure Shear Mod (GPa) Shear strength τ (MPa) Shear strain, γ ( µε ) Time to Failure (sec) Weight gain (%) Weight reduction (%) ESLT-PW ESLT-PW ESLT-PW ESLT-PW ESLT-PW Average

121 98 According to Table 5.8, there was an increment in weight for all specimens as they experienced wet/dry cycles for the duration of sixth months in plain water. The average weight gain for plain water exposure sample is 0.245%. The highest ultimate failure load in the sample is 1.10 kn and the lowest is kn. For this sample their average ultimate failure load is kn. The average shear modulus, G, and shear strength, τ, is 2.64 GPa and MPa. The shear modulus and shear strength for plain water (w/d) sample are 11.1% and 32.01% lower as compared to controlled sample. Average time to fail for most of the specimens is about seconds while the highest shear strain is 8721 µε, which occurred on specimen ESLT-PW04. Apart from that, the average shear strain for this sample is 7426 µε. The shear strain vers us shear stress curve, as shown in Fig was then plot to show the relation between shear stress and shear strain. It can be seen that the ESLT-PW specim ens linearly gained shear loads from the start until up to failure, bu t the curve was not perfectly linear at the end of the failure, suspected from porosities that occurred at the significant section. Fig : Shear stress-strain curve for Resifix-31 ESLT-PW01

122 99 All the specimens in the plain water wet-dry exposure have shown a brittle failure characteristic under load. This fracture surfaces was found at 45º to the loading axis, which is the direction of the tensile principal stress corresponding to the state of the pure shear as shown in Fig and Fig Fig : Specimen ESLT-PW01 after failed Fig : Brittle failure of ESLT-PW Experimental Results of Salt Water Wet-Dry Exposure Sample There are five (5) specimens in the sample of salt water wet-dry exposure condition. All specimens, ESLT-SW01 to ESLT-SW05 have shown a brittle characteristic of failure. Example of raw data for ESLT-SW01 specimen is shown in Table 5.9 and the complete test data for the plain water (w/d) sample is shown in Table 5.10.

123 100 Table 5.9 : Raw experimental data for Resifix-31 ESLT-SW01 Load (N) Principal strain at +45 o, ε ( µε) +45 Principal strain at -45 o, ε ( µε) 45 Average shear stress τ ave (MPa) Average Strain, ε x (µε) Shear strain, γ (µε) Table 5.10 : Testing results for ESLT-SW sample Specimen Ult Load (kn) Strain near to failure Shear Mod (GPa) Shear strength τ (MPa) Shear strain, γ ( µε ) Time to Failure (sec) Weight gain (%) Weight reduction (%) ESLT-SW ESLT-SW ESLT-SW ESLT-SW ESLT-SW Average By referring to testing results as listed in Table 5.10, all specimens experienced weight gain about 2.674%. This weight gain occurred due to salt water absorption process during six months of exposure. The highest ultimate failure load in the sample is kn and the lowest is kn. The average ultimate failure load is kn.

124 101 For the shear properties, an average shear modulus, G, and shear stren gth, τ, is 2.54 GPa and MPa. The average value s of shear modulus and shear strength obtained from the salt water exposure condition are 14.48% and 33.05% lower compared t o controlled sample. An average time to fail for most of the specim ens is about seconds while the average of shear strain for the sample is 8277 µε. Th e shear stress-strain curves graph for ESLT-SW01 is shown in Fig The stress strain curves show a linear relationship between shear stress and shear strain, thus the shear modulus, G was obtained from the curve gradient. During testing, it can be observed ESST-SW01 specimen linearly gained shear loads from the start until up to failure, and had show brittle type failure mode. Fig : Shear stress-strain curve for Resifix-31 ESLT-SW01 The fracture surface s for salt water sample was also found almost 45º to the loading axis, which is the direction of the tensile principal stress corresponding to the state of the pure shear as shown in Fig and Fig However, the fracture shape was not same for salt water sample, as there were many sharp edges occurred. This may be caused by the chemical contents in the specimen after been exposed in salt water.

125 102 Fig : Specimen ESLT-SW01 after failed Fig : Brittle failure of ESLT-SW Result Analysis By using the test results from the experimentation, the shear strain versus shear stress graph was plot in order to show the relationship between strain on each specimen side, both ε 45 and ε 45. The curve linear propagated and almost symmetry to the horizontal axis for both directions. The strain differences value between in the direction of +45 and -45º is about 7% [17] and can be neglected as it was too small. The shear strains versus shear stress graph for every sample are shown in Fig to Fig

126 103 Fig : Shear strain versus shear stress for ESST-CO01 Fig : Shear strain versus shear stress for ESLT-LB01

127 104 Fig : Shear strain versus shear stress for ESLT-OD01 Fig : Shear strain versus shear stress for ESLT-PW01

128 105 Fig : Shear strain versus shear stress for ESLT-SW01 From the test result obtained for all exposure conditions, a comparison has been made between their shear strength, τ with the weight gain and reduction factors, as shown in Table Table 5.11 : Comparison of average shear strength, τ, and average shear modulus, G of Resifix-31 exposure samples Exposure Condition Controlled Average Shear strength τ (MPa) Average Shear modulus G(GPa) Average Weight Gain (%) Average Weight Reduction (%) Average time to failure (s) (ESST-C) Laboratory (ESLT-LB) Outdoor (ESLT-OD) Plain Water (ESLT-PW) Salt Water (ESLT-SW)

129 106 From Table 5.11, it shows the ESLT-OD sample has the highest shear strength value, which is MPa, followed by ESLT-LB, MPa, ESLT-SW, MPa and finally the ESLT-PW, MPa. Base on the results, a conclusion has been made that when the specimens experienced wet/dry cycles in plain or salt water, their shear strength had decreased and this could happened, suspected by the absorption of ion in the water, which was about 0.25% to 2.67% in weight gain. The chemical chain of the epoxy, as shown previously in Fig may be weak as the water has taken part between the epoxy bonds. Other factor that influenced and affected the shear strength is the oxidation process for ESLT-OD sample as the weight was reduced about 0.10% by means of yellowish effect on specimens as they were exposed to wet/dry outdoor condition. The Resifix-31 epoxy shear strength versus exposure conditions is shown in Fig The shear strength for ESLT-LB, ESLT-OD, ESLT-PW and ESLT-SW are lower than the controlled sample, ESST-CO. ESLT-PW sample has the lowest shear strength value, which is 32% lower than the controlled sample, followed by ESLT-SW, 26.6%, ESLT-LB, 25.4% and finally the ESLT-OD, 18.4%. Fig : Shear strength versus exposure condition for all test samples

130 107 The Resifix-31 epoxy shear modulus versus exposure conditions is shown in Fig The shear modulus for ESLT-LB, ESLT-OD, ESLT-PW and ESLT-SW are also lower than the controlled sample, ESST-CO. ESLT-SW sample has the lowest shear modulus value, which is 14.5% lower than the controlled sample, followed by ESLT-LB, 12.8%, ESLT-PW, 25.4% and finally the ESLT-OD, 9.0%. Fig : Shear Modulus, G, versus sample exposure conditions for all test samples By comparison, the shear modulus obtained through Arcan test method is about 30% lower than the calculated value obtained from tensile test [9]. From the theory, it is assumed that the material is homogeneous and isotropic. Unfortunately, this condition does not exist in Resifix-31 epoxy due to defect such as porosity. The testing method and procedure also plays important roles in obtaining an accurate test results. As an example, bonding of the specimen to the Arcan rig and the test rig alignment also gave a significant effect to the experimental result [14]. The shear modulus, G, given by the manufacturer for Resifix-31 is 3.8 GPa which is about 21.84% higher than the test result for ESST-CO. This is very much dependent on the manufacturer s definition and procedu re of testing. Each manufacturer has their own testing specification, which may be differ from each other. More importantly, their testing method depends very much on the materials on site application.

131 Microstructure Analysis All the Resifix-31 epoxy specimens in this experimentation programme failed in brittle manner under final load test. The shear strength, τ, and shear modulus obtained were lower (previously stated) than the manufacturer s specification. One of the factors that influenced the accuracy of the test data is porosity, as shown in Fig Porosity was occurred upon the butterfly specimen shape preparation. This could happened due to mixing speed using electric mixer during mixing process of adhesives, part A and part B. If much porosity occurred along the significant section, the specimen will failed much faster than the expected value. These porosities were hard to be detected by visual inspection, thus a Fieldemission Scanning Electron Microscope (FESEM) was used to detect and analyze the size of the porosities (i.e. air bubble). For every sample, ESST-CO, ESLT-LB, ESLT-OD, ESLT-PW and ESLT-SW were scanned at three different places. The image was scanned by using 200 x 10 3 magnification scale. Then the porosities size is measured by using scale of 1 cm = 80 µ. Fig : Fracture surface of Resifix-31 epoxy for ESST-CO sample

132 109 Porosities also occurred in the ESLT-LB, ESLT-OD, ESLT-PW and ESLT-SW, as shown in Fig to Fig Fig : Porosities in ESLT-LB sample Fig : Porosities in ESLT-OD sample Fig : Porosities in ESLT-PW sample Fig : Porosities in ESLT-SW sample The average porosity size measured from the ESST-CO sample is about µ, as shown in Fig As previously stated, these porosities encourage the specimens failed faster than expected.

133 110 Fig : Porosities size in Resifix-31 epoxy ESST-CO sample 5.5 Chemical Elements in Resifix-31 Epoxy Adhesive Determined by FESEM After the samples exposed to designated conditions, selected specimen undergoes scanning process by using the FESEM microscope. The main objective is to determine the change of base elements in each sample. The selected specimen was chosen according to their shear strength and shear modulus test data consistency. The base element for each sample is presented in Table Table 5.12 : Chemical element in Resifix-31 epoxy for experimentation purposes Exposure Elements Weight (%) Condition Specimen O C Si Ti Mg Cl Controlled ESST-C Laboratory ESLT-LB Outdoor ESLT-OD Plain Water ESLT-PW Salt Water ESLT-SW

134 111 From Table 5.12, an assumption has been made that the shear strength and shear modulus of each exposure conditions depends on the contents of Silicon, Si, Magnesium, Mg, Titanium, Ti, and Carbon, C. Silicon is the main filler in the hardener (Part B) which was added to enhance the bond properties. However, the compositions of silicon in each specimen may be un-uniform due to mixing process and the chemical reaction during exposure. The highest magnesium contents are in ESLT-OD03, followed by ESLT-C02, ESLT-SW03, and finally ESLT-PW05. Other elements contents such as oxide, chlorine and silicone in specimens is not much differ for each exposure conditions. As a conclusion, if the specimen has high presence of silicon, titanium, carbon and magnesium contents produced much better shear stress and shear modulus value. The test data for the scanning process is included in Appendix D Conclusion From the Arcan test result, the outdoor exposure sample had the highest shear strengt h and shear modulus, followed by the laboratory, plain water and finally the salt water exposure condition. These results had show that if Resifix-31 epoxy adhesive submerged in plain or salt water, there was a weight gain experienced by the specimens, as the water decrease their shear strength and shear modulus value.

135 112 CHAPTER 6 CONCLUSION AND RECOMMENDATION 6.1 Conclusion This project objective was successfully achieved by using the Arcan test method in order to determine the shear properties of Resifix-31 epoxy adhesives. The significant section of the butterfly specimen (previously stated) had proved that the Arcan test method was reliable, as the shear stress strain relation is linearly propagated. Even though the strain in both directions was not perfectly symmetry, the test result is still a ccepted as the difference value (i.e. 7% ) is small and can be neglected. Main factor that influenced the test data is porosity (i.e. air bubbles). Porosity was occurred upon butterfly specimen preparation due to mixing and casting process of Resifix-31. The specimen failed much faster when so much porosity occurred. All specimens in this experimentation failed in brittle manner under final load test. The shear strength and shear modulus were 21.84% higher and 33.05% lower from the manufacturers quoted data as compared to the controlled specimen. Apart from that, the shear strength and shear modulus value were different as they were exposed to various conditions. This differences happened due to some reasons, such as ;

136 113 a) The butterfly specimen was not properly fit in the Arcan fixture housing as they can slip during the testing process. b) The specimen surface was not flat, when the specimen is removed from the butterfly mould. This can affected the specimen shape and strength. c) So much porosity occurred in the specimens due to improper handling when the mixing and casting process took part. The porosity (i.e. air bubbles) reduced the strength of the specimen, as they directly made the specimen failed faster. d) Misalignment occurred when the tightening of the butterfly specimen onto the housing. This was hard to detect by visual inspection. 6.2 Recommendation In order to obtain a better test result in the future, some recommendation have been made, such as ; a) The number of specimen is increase to obtain a much accurate shear strain data on the exposure condition. b) A new butterfly mould has to be create to eliminate and decrease the porosity. c) A simulation study carried out by using finite element software such as Msc. Nastran to make a comparison between test data and software simulation. d) An advance electronic device used to detect the misalignment during the tightening of fixture and Arcan grip installation process.

137 114 REFERENCES [1] M. Arcan, Z. Hashin and A Voloshin, A Method to Produce Uniform Plane-stress States With Application to Fiber-Reinforced Materials, 1978 [2] D. Mohr, M. Dodoyo, Analysis of the Arcan Apparatus in the Clamped Configuration, 2002 [3] I.H. Marshall, Handbook of Polymer and Elastomers, fourth edition, McGraw Hill, London, 2002 [4] Ayal De S, Jayatilaka, Fracture Of Engineering Brittle Materials, Applied Science Publisher, London, 1979 [5] Popov, E.P, Mechanics of Materials, Prentice Hall Inc, New Jersey, 1976 [6] Chong, Ching Lee, Interlaminar Fracture Testing of Glass Fiber Thermoset Based Resin In Mode II Condition, Tesis Sarjana Muda, Universiti Teknologi Malaysia, 2002 [7] Chan, Kok Hong, Vibration : Properties of Structural FRP Composites Plate, Tesis Sarjana Muda, Universiti Teknologi Malaysia, 2001 [8] Frank A. McClintock, Mechanical Behavior of Materials, Addison-Wesley Publishing Company, Massachusetts, USA, 1966

138 115 [9] Marini Sawawi, Evaluation Of Adhesive Epoxy Materials In A Composite/Concrete System Under Shear Loading, Tesis Sarjana, Universiti Teknologi Malaysia, 2005 [10] Teng J.G et al. FRP Strengthened RC Structures, USA : John-Wiley & Sons Inc [11] Shukur et al. Experimental Study of Cast Epoxy Shear Properties Using Arcan Test Method, Universiti Teknologi Malaysia. Unpublished, 2005 [12] C. H. Turner, T. Wang, D. B. Burr, Shear Strength and Fatigue Properties of Human Cortical Bone Determined from Pure Shear Tests, 2001 [13] Cagle C.V., Handbook of Adhesive Bonding, USA : Mc Graw-Hill Inc.1973 [14] Yen S.C., Craddock J.N. and Teh K.T., Evaluation of a Modified Arcan Fixture for In-Plane Shear Test of Materials, Experimental Techniques, December, 1988 [15] Sullivan J.L., Kao B.G. and Oene H. V, Shear Properties and A Stress Analysis obtained from Vinyl-ester Iosipescu specimens, Experimental Mechanics, September, 1984 [16] Liu J.Y., Flach D.D., Ross R.J. and Rammer D. R., Improved Arcan Shear Test for Wood, International Wood Engineering Conference, October 28-31, New Orleans, LA. Baton Rouge, LA: Louisiana State University: Vol. 2: 85-90, 1996 [17] Hongkinson J.M., Mechanical Testing of Advanced Fibre Composites, Woodhead Publishing Limited, Cambridge, England, pp: , 2000

139 116 [18] Arcan M., Hashin Z. and Voloshin A, A Method to Produce Uniform Plane-Stress States with Applications to Fibre-Reinforced Materials, Experimental Techniques, 18(40, , April 1978) [19] Hibbeler R.C. Mechanics of Material, 5 th Edition, USA : Pearson Education Inc 2003 [20] Rani El-Hajjar and Rami Haj-Ali. In Plane Shear Testing of Thick Pultruded FRP Composites Using Modified Arcan Fixture Composites Part B, Volume (35) : pg , 2004

140 APPENDIX 117

141 118 Appendix A1 Adhesives Classification Table A1 : Adhesives classified by chemical composition Table A2 : Thermosetting based adhesives

142 Table A2 : Thermosetting based adhesives (continue) 119 Table A3 : Properties of Thermosetting Resins

143 120 Appendix A2 Table A4 : Experimental measured dimension data of butterfly specimens for adhesive epoxy Resifix-31 before exposure (Notch radius, R=1.5mm). Specimen Average Width (mm) Average Thickness (mm) Significant Area (mm 2 ) Control ESST ESST ESST ESST ESST Laboratory ESLT-LB ESLT-LB ESLT-LB ESLT-LB Outdoor ESLT-OD ESLT-OD ESLT-OD ESLT-OD Plain Water ESLT-PW ESLT-PW ESLT-PW ESLT-PW ESLT-PW Salt Water ESLT-SW ESLT-SW ESLT-SW ESLT-SW ESLT-SW

144 121 A PPENDIX B1 Arcan Shear Test Result for Resifix-31 Control Specimen (ESST-CO) Fig. B1 : Shear stress versus shear strain f or ESST-C02 Fig. B2 : Shear stress versus shear strain for ESST-C03 Fig. B3 : Shear stress versus shear strain for ESST-C04

145 122 Fig. B4 : Shear stress versus shear strain for ESST-C05 Fig. B5 : Shear stress versus shear strain for ESLT-LB02 Fig. B6 : Shear stress versus shear strain for ESLT-LB03

146 123 Fig. B7 : Shear stress versus shear strain for ESLT-LB04 Fig. B8 : Shear stress versus shear strain for ESLT-OD02 Fig. B9 : Shear stress versus shear strain for ESLT-OD03

147 124 Fig. B10 : Shear stress versus shear strain for ESLT-OD04 Fig. B11 : Shear stress versus shear strain for ESLT-PW02 Fig. B12 : Shear stress versus shear strain for ESLT-PW03

148 125 Fig. B13 : Shear stress versus shear strain for ESLT-PW04 Fig. B14 : Shear stress versus shear strain for ESLT-PW05 Fig. B15 : Shear stress versus shear strain for ESLT-SW02

149 126 Fig. B16 : Shear stress versus shear strain for ESLT-SW03 Fig. B17 : Shear stress versus shear strain for ESLT-SW04 Fig. B18 : Shear stress versus shear strain for ESLT-SW 05

150 127 Fig. B19 : Shear strain versus shear stress for ESST-C02 Fig. B20 : Shear strain versus shear stress for ESST-C03 Fig. B21 : Shear strain versus shear stress for ESST-C04

151 128 Fig. B22 : Shear strain versus shear stress for ESST-C05 Fig. B23 : Shear strain versus shear stress for ESLT-LB02 Fig. B24 : Shear strain versus shear stress for ESLT-LB03

152 129 Fig. B25 : Shear strain versus shear stress for ESLT-LB04 Fig. B26 : Shear strain versus shear stress for ESLT-OD02 Fig. B27 : Shear strain versus shear stress for ESLT-OD03

153 130 Fig. B28 : Shear strain versus shear stress for ESLT-OD04 Fig. B29 : Shear strain versus shear stress for ESLT-PW02 Fig. B30 : Shear strain versus shear stress for ESLT-PW03

154 131 Fig. B31 : Shear strain versus shear stress for ESLT-PW04 Fig. B32 : Shear strain versus shear stress for ESLT-PW05 Fig. B33 : Shear strain versus shear stress for ESLT-SW02

155 132 Fig. B34 : Shear strain versus shear stress for ESLT-SW03 Fig. B35 : Shear strain versus shear stress for ESLT-SW04 Fig. B36 : Shear strain versus shear stress for ESLT-SW05 APPENDIX B2

156 133 Resifix-31 Epoxy Adhesive Part A (epoxy) Ratio : 3 (i.e. 210g) Part b (hardener) Ratio : 1 (i.e. 70g) Mixing Process Casting Process Curing Process De-moulding Process Cleaning Process Checking process Specimen Ready to Be Exposed Fig. B37 : Specimen preparation flow chart

157 134 APPENDIX C1 Microstructure of Fracture Surface of Control Specimen (ESST-CO) Fig. C1 : Fracture surface of ESST-C02 under 200x magnification Microstructure of Fracture Surface of Laboratory Specimen (ESLT-LB) Fig. C2 : Fracture surface of ESLT-LB01 under 200x magnification Microstructure of Fracture Surface of Outdoor Specimen (ESLT-OD)

158 135 Fig. C3 : Fracture surface of ESLT-OD03 under 200x magnification Microstructure of Fracture Surface of Plain Water Specimen (ESLT-PW) Fig. C4 : Fracture surface of ESLT-PW05 under 200x magnification Microstructure of Fracture Surface of Salt Water Specimen (ESLT-SW) Fig. C5 : Fracture surface of ESLT-SW03 under 200x magnification