project report

MAXIMISING THE STRENGTH OF CONTINUOUS GLASSFIBRE REINFORCED NYLON

FABRICATED BY ADDITIVE MANUFACTURING

NATIONAL UNIVERSITY OF IRELANDUNIVERSITY COLLEGE DUBLIN

Final ReportMEEN30120

Mechanical Engineering ProjectUCD School of Mechanical and Materials Engineering

by

GAVIN KELLY

April 2016

Prof. Michael D. GilchristHead of School

Prof. Denis DowlingProject Supervisor

Abstract

Parts manufactured by additive manufacturing (AM) tend to have poor mechanical propertiesdue to the limitations of the materials used, typically polymers. AM has huge potential in manyareas as it can produce extremely complex shaped parts and design changes can be implementedeasily thus it is very customisable. This has led to growing interest in the AM of composites. Byembedding the polymer parts with fibres, the mechanical properties can be enhanced. A newtype of 3-D printer produced by Markforged enables the AM of continuous fibre compositesand is the only commercial printer capable of this. An investigation was made to maximisethe strength of composites fabricated using Markforged’s printer. Four different fibre layoutswere used which included perpendicular, quasi-isotropic, concentric and anisotropic layouts. Foreach layout, the position of the glass fibre layers within the composite were arranged in twomanners. Type 1 specimens were made by printing nylon and fibre layers alternately. For Type 2specimens, the fibre layers were positioned at the outer surfaces with no nylon layers between themto form a sandwich panel. Tensile and 3-point bending tests were carried out on all specimens.Anisotropic specimens had the highest tensile strength (304 MPa) and flexural strength (297MPa). Perpendicular specimens had the lowest tensile strength (40MPa) and flexural strength (58MPa). Type 2 specimens showed an increase in modulus values, particularly for flexural moduluswhich increased by roughly 60% compared to type 1 specimens. The concentric specimen hadthe highest flexural modulus (10.15 GPa) and perpendicular had the lowest (1.5 GPa). The use offibre resulted in a tensile toughness that was around 9 times less than that of a pure nylon specimen.However, it increased the flexural toughness. The anisotropic specimen had a flexural toughnessaround 7 times greater than the nylon specimen. Type 2 specimens showed more delamination oflayers after failure than type 1 specimens. Overall, type 2 specimens had a superior combinationof strength and stiffness for both tensile and flexural loading than type 1. The particular fibrelayout should be chosen depending on the direction of the applied load.

Acknowledgements

I would like to express my gratitude to Prof. Denis Dowling for the ongoing support and adviceoffered while completing this project.

I would also like to thank Andrew Dickson for his continuous support and guidance on theequipment used in the fabrication and testing of the materials.

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Contents

Abstract i

Acknowledgements ii

Contents iii

List of Figures v

List of Tables vii

1 Introduction 11.1 Project Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Report Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Literature Review 42.1 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2.1 Classification, Properties and Applications . . . . . . . . . . . . . . . 42.2.2 Fibre Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.3 Position of Fibre Layers . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.4 Composite Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.5 Characteristics of Fibres . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Additive Manufacturing (AM) . . . . . . . . . . . . . . . . . . . . . . . . . . 142.3.1 Introduction to AM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.3.2 Fused Deposition Modelling (FDM) of Polymers . . . . . . . . . . . . 152.3.3 FDM of Short Fibre Composites . . . . . . . . . . . . . . . . . . . . . 162.3.4 FDM of Continuous Fibre Composites . . . . . . . . . . . . . . . . . . 182.3.5 FDM of Continuous Fibre Composites using the MarkForged 3-D Printer 20

2.4 Literature Review Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Experimental Methods 22

3.1 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2.1 Markforged Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2.2 Tensile and 3-Point Bending Specimen Details . . . . . . . . . . . . . 22

3.3 3D Printing of Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.4 Tensile and 3-Point Bending tests . . . . . . . . . . . . . . . . . . . . . . . . . 25

iii

Contents iv

4 Results and Discussion 274.1 Design of Test Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.1.1 Fibre Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1.2 Type 1 and Type 2 Specimens . . . . . . . . . . . . . . . . . . . . . . 29

4.2 Effect of Specimen Design on Tensile Properties . . . . . . . . . . . . . . . . 304.3 Effect of Specimen Design on Flexural Properties . . . . . . . . . . . . . . . . 354.4 Fracture Surface Observations . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5 Conclusion and Future Work 445.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

A Stress-Strain Curves for Tensile and Bending Tests 47A.1 Tensile Test Stress-Strain Curves for Type 1 Specimens . . . . . . . . . . . . . 48A.2 Tensile Stress-Strain Curves for Type 2 Specimens . . . . . . . . . . . . . . . 51A.3 3-Point Bending Test Stress-Strain Curves for Type 1 Specimens . . . . . . . . 53A.4 3-Point Bending Test Stress-Strain Curves for Type 2 Specimens . . . . . . . . 56

Bibliography 58

List of Figures

2.1 Two types of composite classification. (a) based on the matrix materials, (b)based on the reinforcement materials [39] . . . . . . . . . . . . . . . . . . . . 6

2.2 Effect of fibre orientation on strength of the composite [19] . . . . . . . . . . . 82.3 (a) unidirectional layered composite. (b) Quasi-isotropic layered composite [3] 82.4 Tensile strength of neat polyester and polyester/kenaf composites [40] . . . . . 92.5 Relationship between Young’s modulus and fibre orientation angle [37] . . . . 92.6 Impact strength of neat polyester and polyester/kenaf composites [40] . . . . . 102.7 Flexural modulus of neat polyester and polyester/kenaf composites [40] . . . . 102.8 Schematic of the FDM process [30] . . . . . . . . . . . . . . . . . . . . . . . 162.9 Schematic of the 3D printer head developed byMatsuzaki et al. [26] for producing

CFRPs using an FDM in-nozzle impregnation technique. . . . . . . . . . . . . 193.1 ASTM-D638 tensile test specimen dimensions (note: the specimen height is 3.2mm) 233.2 ASTM-D790 3-point bending test specimen dimensions (note: the specimen

height is 3.2mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3 Markforged 3-D printer [24] . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.2 Illustration of specimen fibre layouts . . . . . . . . . . . . . . . . . . . . . . . 284.3 Cross-section showing the arrangement of layers within type 1 specimens (yellow

represents nylon and grey represents glass fibre) . . . . . . . . . . . . . . . . . 294.4 Cross-section showing the arrangement of layers within the type 2 specimens

(yellow represents nylon and grey represents glass fibre) . . . . . . . . . . . . . 294.5 Typical tensile stress-strain curves for type 1 specimens . . . . . . . . . . . . . 314.6 Typical tensile stress-strain curves for type 2 specimens . . . . . . . . . . . . . 314.7 Max tensile strength for type 1 and type 2 specimens . . . . . . . . . . . . . . 334.8 Tensile modulus for type 1 and type 2 specimens . . . . . . . . . . . . . . . . 344.9 Tensile toughness for type 1 and type 2 specimens . . . . . . . . . . . . . . . . 354.10 Typical flexural stress-strain curves for type 1 specimens . . . . . . . . . . . . 364.11 Typical flexural stress-strain curves for type 2 specimens . . . . . . . . . . . . 364.12 Max flexural strength for type 1 and type 2 specimens . . . . . . . . . . . . . . 374.13 Flexural modulus for type 1 and type 2 specimens . . . . . . . . . . . . . . . . 394.14 Flexural toughness for type 1 and type 2 specimens . . . . . . . . . . . . . . . 404.15 The different modes of failure leading to damage of a laminate [19] . . . . . . . 414.16 From left to right for both (a) and (b): Nylon, perpendicular, quasi-isotropic,

concentric and anisotropic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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List of Figures vi

A.1 Stress-strain curves from the results of tensile tests carried out on 5 specimensconsisting of 100% nylon (no-fibre). . . . . . . . . . . . . . . . . . . . . . . . 48

A.2 Stress-strain curves from the results of tensile tests carried out on 5 perp#1specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

A.3 Stress-strain curves from the results of tensile tests carried out on 5 quasi-iso#1specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

A.4 Stress-strain curves from the results of tensile tests carried out on 5 conc#1specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

A.5 Stress-strain curves from the results of tensile tests carried out on 5 aniso#1specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

A.6 Stress-strain curves from the results of tensile tests carried out on 5 perp#2specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

A.7 Stress-strain curves from the results of tensile tests carried out on 5 quasi-iso#2specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

A.8 Stress-strain curves from the results of tensile tests carried out on 5 conc#2specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

A.9 Stress-strain curves from the results of tensile tests carried out on 5 aniso#2specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

A.10 Stress-strain curves from the results of 3-point bending tests carried out on 5specimens consisting of 100% nylon (no-fibre). . . . . . . . . . . . . . . . . . 53

A.11 Stress-strain curves from the results of 3-point bending tests carried out on 5perp#1 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

A.12 Stress-strain curves from the results of 3-point bending tests carried out on 5quasi-iso#1 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

A.13 Stress-strain curves from the results of 3-point bending tests carried out on 5conc#1 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

A.14 Stress-strain curves from the results of 3-point bending tests carried out on 5aniso#1 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

A.15 Stress-strain curves from the results of 3-point bending tests carried out on 5perp#2 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

A.16 Stress-strain curves from the results of 3-point bending tests carried out on 5quasi-iso#2 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

A.17 Stress-strain curves from the results of 3-point bending tests carried out on 5conc#2 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

A.18 Stress-strain curves from the results of 3-point bending tests carried out on 5aniso#2 specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

List of Tables

3.1 Tensile test specimen nylon and glass fibre volume fractions . . . . . . . . . . 233.2 3-point bending test specimen nylon and glass fibre volume fractions . . . . . . 233.3 Hounsfield tensile test settings . . . . . . . . . . . . . . . . . . . . . . . . . . 253.4 Instron 100 kN test settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

vii

Chapter 1

Introduction

Additive manufacturing (AM) has traditionally been used for the production of prototypes butthese parts tended to have poor mechanical properties due to the limitations of the materials used,typically polymers. AM has huge potential in many areas as it can produce extremely complexshaped parts and design changes can be implemented easily thus it is very customisable. Theseopportunities could be exploited if the strength limitations associated with AM products could beovercome. This has led to growing interest in the AM of composites. By embedding the polymerparts with fibres, the strength can be greatly increased. Fibres can be short or long (continuous)and a range of different materials such as glass, carbon or kevlar. Generally, the longer the fibres,the stronger the part. The AM of continuous fibre reinforced composites is a new technologyand the Mark One 3-D printer being used in this project is the first printer of its kind capable ofprinting such composites. Thus very little research has been conducted in this area and so it is ofgreat interest to further investigate how the strength of composites produced by the Mark Oneprinter can be maximised.

Fibres can be arranged in various ways within a polymer matrix. Different arrangements changethe mechanical properties of the composite depending on the direction the fibres are facing withrespect to the applied load. The focus of this project is to investigate the effect of altering fibrelayout, orientation and layer position on the mechanical properties of continuous glass fibrecomposites fabricated by additive manufacturing. Currently there is no published literature onthis topic for parts manufactured by AM. The overall aim is to maximise the strength of thesecomposites.

1

Chapter 1. Introduction 2

1.1 Project Objectives

• To determine the optimum fibre layout and layer positioning for maximising the tensileproperties of continuous glass fibre composites fabricated by AM. Mechanical performancewill be evaluated based on tensile strength, tensile modulus and tensile toughness.

• To determine the optimum fibre layout and layer positioning for maximising flexuralproperties of these composites. Again, mechanical performance will be evaluated based onflexural strength, flexural modulus and flexural toughness.

• To determine what design provides a good combination of tensile and flexural properties.

1.2 Report Outline

The literature review begins with an introduction to composites, describing what a composite isand their advantages over other materials. Different types of composites are explained along withsome examples of their applications. Details are given about the effect of fibre orientation onthe mechanical properties of a composite. The effect of fibre volume content and the position ofthe fibre layers within the composite is also discussed. Some background composite theory isgiven, in particular an explanation of the rule of mixtures for binary composites equation whichenables the calculation of fibre and matrix volume fractions along with theoretical values forelastic moduli. The impact on composite structures due to the fibre aspect ratio and critical fibrelength are then discussed.

The second part of the literature review discusses additive manufacturing (AM). Details are givenof the process of AM, different AM methods and their advantages. Particular emphasis is put onfused deposition modelling (FDM) which is employed by the machine being used in this research.The strength limitations associated with traditional additive manufactured parts leads to the nextsection, which is additive manufacture of composites. Details are given of previous research inattempting to maximise the strength of composites fabricated by FDM including FDM of shortfibre reinforced polymers and attempts at FDM of continuous fibre reinforced composites alongwith the issues encountered. The MarkForged Mark One continuous fibre composite printer isthen discussed with reference to one previous study which which tested carbon fibre specimensprinted by the Mark One.

Chapter 1. Introduction 3

Chapter 3 discusses the materials used by the Markforged for the printing of the specimens anddetails of the tensile and flexural test specimens are given. The procedure followed to print thespecimens is then described. The tensile and 3-point bending test procedures are then outlined.

Chapter 4 begins by explaining the 4 fibre layouts used in this study. The anisotropic, quasi-isotropic and perpendicular layouts were chosen based on section 2.2.2 of the literature reviewand the work done by Yong et al. [40] with these layouts. A concentric layout was also usedhowever no literature was found which made use of this layout. The type 1 design was based on asuggestion by Van Der Klift et al. [36] who tested carbon fibre samples printed by the Markforged.It was noted that an interesting area of research would be to study the effect of printing fibre andnylon layers alternately. The type 2 design was chosen based on section 2.2.3 of the literaturereview which indicated that to maximise flexural rigidity, fibre reinforcement should be located atthe outer surfaces.

The results of the tensile and flexural tests are then given. The effects of the fibre layouts for type1 and type 2 designs on tensile strength, tensile modulus and tensile toughness are discussed andsimilarly for flexural strength, modulus and toughness. Fracture surface observations are thendiscussed with regard to delamination, voids and fibre pull-out.

Chapter 5 sums up the with the main conclusions to be drawn from this study.

Chapter 2

Literature Review

2.1 Chapter Overview

The literature review begins by outlining composites, the different types and their constituentmaterials. The effect of fibre orientation on the mechanical properties of composites is discussedwith reference to particular studies. Composite theory is then explained and how the the elasticmodulus of a composite can be determined using the rule of mixtures for binary composites

equation. The impact on composite structures due to the fibre aspect ratio and critical fibrelength are then discussed. The second part looks at additive manufacturing with a focus on FDM.Reference is made to previous studies investigating FDM of polymers and composites includingshort and long fibre composites. The Mark One printer is then discussed, referencing the onlystudy that’s currently available involving testing of materials made by the Mark One 3-D printer.

2.2 Composite Materials

2.2.1 Classification, Properties and Applications

According to Smith and Hashemi [34], "A composite material may be defined as two or morematerials (phases or constituents) integrated to form a new one. The constituents keep theirproperties, and the overall composite will have properties different than each of them". They areusually composed of a reinforcing material and a resin binder or matrix. Different combinations

4

Chapter 2. Literature Review 5

of reinforcement and matrix materials can be used to bring about desired mechanical properties[34].

Composites can be classified based on the matrix or reinforcement material. These two typesof classification systems are illustrated in figure 2.1. With regards to matrix material, generallyspeaking, the three types of composites used in engineering applications are [39]:

• Polymer-matrix composites (PMC)

• Metal-matrix composites (MMC)

• Ceramic-matrix composites (CMC)

A polymer-matrix composite is used in this study. According to Painter and Coleman [31],"polymers are very large molecules (macromolecules) that are comprised or built up of smallerunits or monometers". They are divided into two groups, namely thermoplastics and thermosets.Thermoplastics flow more easily when subjected to external forces, usually at elevated tempera-tures. They can be re-heated and re-moulded, and the majority of them hold their shape at roomtemperature. Thermosets initially flow and can be moulded but once cured, they become set intheir shape. Re-heating cured thermoset will cause damage and degradation to the specimen [31].

With regards to the reinforcement materials, composite materials can be classified into:

• Particulate composites

• Fibre-reinforced composites

• Structural composites

This research focuses on fibre-reinforced polymer-matrix composites (PMC). Three main types offibre are used in these composites which are Kevlar, glass and carbon fibres. Kevlar has excellentimpact and abrasion resistance, glass offers high strength and is the very cost-effective and carbonis very stiff and has an extremely high strength to weight ratio [23].

Composite materials offer many advantages over traditional materials such as metals. They enablea 30−40% decrease in the overall weight of components when compared to metals like aluminiumor steel. This in turn leads to a number of other benefits including reduced operating costs, reducedemissions and improved fuel efficiency. The life-time cost of composite components is lower


FIGURE 2.1: Two types of composite classification. (a) based on the matrix materials, (b) basedon the reinforcement materials [39]

than competing materials because of longer life span, superior quality of parts and structures,lower assembly and installation costs along with lower maintenance costs due to durability andcorrosion resistance [3, 13, 39].

A disadvantages of composites are the costs of raw materials and fabrication. When comparedto their metal counterparts, composites are expensive due to the high price of glass, Kevlar andcarbon fibres, and the thermosetting and thermoplastic resins. The tooling and capital costs ofcomposite part manufacturing tend to be higher than that of metals [13]

Despite their high cost, due to their high mechanical performance, fibre reinforced compositesare used in a wide range of applications. For example, they are used in the aerospace industryfor the manufacture of parts for airplanes, rockets and missiles [27, 27, 33]. The automotiveindustry which is the largest application sector uses composites for components such as body


panels, chassis and interiors [39]. The use of composites for sports and recreation equipment israpidly increasing most notably with high-performance sporting goods such as racing boats, tennisrackets, fishing rods and golf clubs [27]. The light weight and corrosion resistance characteristicsof composites make them ideal for marine applications such as power boats, recreational yachts,passenger ferries, naval ships and offshore structures which are exposed to harsh environments[21, 27, 28]. In the renewable energy industry, composites are used for the fabrication of windturbine blades due to their long fatigue life, low density which reduces gravity forces and highmaterial stiffness to achieve optimal aerodynamic performance [8].

2.2.2 Fibre Orientation

The strength of fibre reinforced composites is related to the orientation of the fibres [34] (seefigure 2.2). Composite laminates are made by stacking layers of unidirectional fibre on top of eachother. A fundamental advantage of laminates is the ability to control and adapt the orientation offibres so that the overall composite can best resist loadings. The contribution of individual layersto the laminate resistance will change depending on their relative orientation with respect to theloading direction [19].

Quasi-isotropic parts can be fabricated by stacking layers of unidirectional fibre at varyingorientation angles (see figure 2.3). When fibres are orientated parallel to the applied load,maximum strength is achieved. Any deviation in the orientation of fibres from parallel alignmentdecreases the mechanical strength [34]. Yong et al. [40] investigated the effect of fibre orientationwithin a polymer on the the mechanical properties of the composite. Layers of kenaf fibrewere sandwiched between two layers of polyester films. The final composite was five layers inheight containing three layers of polyester resin and two layers of kenaf fibres. Three differentpolyester/kenaf composites were prepared with fibres orientated perpendicularly, anisotropicallyand quasi-isotropically. Tensile, Izod impact and flexural tests were performed on cut specimensof the composites. Results of tensile strength tests are shown in figure 2.4. The composite withfibres orientated in perpendicular directions had the lowest tensile strength. The composite withfibres in quasi-isotropic arrangement had a 6% higher tensile strength than the composite withfibres in perpendicular directions. The composite with fibres in anisotropic arrangement hadthe highest tensile strength, 55% higher than that of the composite with fibres in perpendiculararrangement.


FIGURE 2.2: Effect of fibre orientation on strength of the composite [19]

FIGURE 2.3: (a) unidirectional layered composite. (b) Quasi-isotropic layered composite [3].

It was reported that alignment of fibres parallel to the direction (anistropic arrangement) of theapplied load resulted in stiffer materials thus improving the Young’s modulus [40]. A similar resultwas obtained by Wang et al. [37], who used numerical simulations to examine the relationshipbetween fibre orientation and Young’s modulus of glass fibre reinforced polymers. Cells weresingle-ply and contained 20% fibre fraction which were uniformly distributed at different angles.


FIGURE 2.4: Tensile strength of neat polyester and polyester/kenaf composites [40]

A strain of 10% was applied to the cells in the vertical direction. The maximum Young’s moduluswas obtained at a fibre orientation angle of 0 deg such that loading was applied along the fibredirection. Young’s modulus then decreased with increasing fibre orientation angle until around60 deg, where it began to level off. This can be seen in figure 2.5.

FIGURE 2.5: Relationship between Young’s modulus and fibre orientation angle [37]

The Izod impact test of the polyester/kenaf composites carried out by Yong et al. [40] showedanisotropic fibre arrangement to have over 50% greater impact strength than quasi-isotropic andperpendicular orientations. For the composite with fibre orientated in a perpendicular direction,the crack propagated easily along the fibre axial direction. For the composite with fibre orientatedin an anisotropic direction, the crack tip normally propagated through the polymer matrix andthen stopped at the next fibre layer. This arrangement formed strong bridges over cracks. The


quasi-isotropic composite displayed intermediate failure mechanism. The results can be seen infigure 2.6.

FIGURE 2.6: Impact strength of neat polyester and polyester/kenaf composites [40]

The flexural modulus of the polyester was improved as a result of quasi-isotropic and anisotropickenaf fibre layers but the perpendicular orientation reduced the flexural modulus. The anisotropicfibre arrangement showed the highest flexural modulus and it was concluded that the flexuralmodulus decreased as the fibre orientation angle increased with a minimum at 90 deg and amaximum at 0 deg. Therefore high bending resistance is evident when the fibre is in an anisotropicorientation. The results can be seen in figure 2.7.

FIGURE 2.7: Flexural modulus of neat polyester and polyester/kenaf composites [40]

Wang et al. [37], used numerical simulations to examine the relationship between fibre volumecontent and Young’s modulus. Cells containing three layers of fibres uniformly distributed withdifferent fibre volume contents were tested. A strain of 10% was applied to the cells in thevertical direction. Almost linear relationship between Young’s modulus and fibre volume content.Thus theoretically, the more fibre in the composite, the higher the Young’s modulus. However,according to [3] fibres can no longer be fully surrounded by the matrix material if the volume


fraction is greater than around 80%. The matrix material transfers the load to the fibres, whichcarry most of the load applied to the composite structure.

2.2.3 Position of Fibre Layers

The overall focus of this research is based on the layout and positioning of fibres within the 3-Dprinted composite. The Markforged printer builds parts by depositing layers of material andthe layers containing fibre or nylon can be individually selected. Thus the position of the layerscontaining fibre may have an effect on the mechanical performance of the composite.

According to Rathnakar.G and Shivanand.H [32], the stresses in a composite due to flexural loadingvary through its thickness. The flexural stresses are zero at the neutral axis and a maximum atthe outer surfaces. In pure bending, failure initiates at either the compressive or tensile side,depending on which is weaker. The stress on an individual layer depends on its distance fromthe neutral axis and the stiffness of that layer. To maximise flexural rigidity, fibre reinforcementshould be located at the outer surfaces. This positioning of fibre layers is known as a sandwichstructure [19].

2.2.4 Composite Theory

In this section, the rule of mixtures for binary composites equation is discussed which enables thecalculation of fibre and matrix volume fractions along with theoretical values for elastic moduli.Since the materials supplied by Markforged are patented and their properties cannot be disclosed,this equation can be used to determine the type of fibre used once the fibre volume content isknown. This method was used by Van Der Klift et al. [36] to determine the type of carbon fibreused by Markforged.

In the following equations, the subscripts c, f and m refer to the composite, fibre and matrix,respectively. For isostrain conditions, the elastic modulus of unidirectional continuous fibrereinforced composites can be calculated if the elastic moduli of the fibre and matrix and theirvolume percentages are known. In the direction parallel to the fibres, the elastic modulus can bedetermined using the rule of mixtures for binary composites equation 2.1:

Ec = EfVf + EmVm (2.1)


where E is the elastic modulus and V is the volume percentage [3, 34].

However, in cases when the applied stress is very large, deformations occur in the matrix materialand consequently, it contributes little to the overall stiffness of the composite. The stress-straincurve is no longer linear and the elastic modulus can be approximated by equation 2.2 [3]:

Ec = EfVf (2.2)

When the applied load is in the direction perpendicular to the fibres, the individual components ofthe composite act independently and the elastic modulus is now calculated using equation 2.3 [3]:

1Ec

=VmEm

+VfEf

(2.3)

The rule of mixtures can be used as an approximation to the tensile strength of a compositeconsisting of parallel, continuous fibres. However, in reality the strength strongly depends on thebonding between the fibres and the matrix.

�c = Vf�f + Vm�m (2.4)

where �m is not the actual tensile strength of the matrix, but the stress acting on the matrix whenthe fibre fails [3].

For a binary composite under isostrain conditions, the ratio of the loads on the fibre and matrixregions can be calculated using the relation shown in equation 2.5:

P = �A (2.5)

where P is the load, � is the stress and A is the fractional area. Therefore, since � = E� and�f = �m [34],

PfPm

=�fAf�mAm

=Ef �fAfEm�mAm

=EfAfEmAm

=EfVfEmVm

(2.6)


Under isostrain conditions, if the total load on a composite is known, then equation 2.7 applies[34]:

Pc = Pf + Pm (2.7)

The load on each fibre and matrix region can then be calculated by combining 2.6 and 2.7 and ifEf , Em, Vf , Vm, and Pc are known [34].

2.2.5 Characteristics of Fibres

The characteristics of fibres are extremely important when aiming to maximise strength. Thissection explains the fibre aspect ratio. The composite strength increases with increasing aspectratio.

Fibre dimensions can be characterized by their aspect ratio 2.8:

ld

(2.8)

where l is the fibre length and d is the diameter. Surface imperfections on fibres can lead tofracture under load. Thus the diameter should be kept as small as possible to minimise the surfacearea. Also, the ends of the fibres do not carry as much load and so the more ends in a composite,the lower its load-carrying capability. The fibre critical length is determined from equation 2.9:

lc =�fd2�i

(2.9)

where lc is the critical fibre length, �f is the strength of the fibre, d is the fibre diameter and �i isthe stress where the matrix begins to deform[3].

The fibre acts almost as if it were continuous if l > 15lc . An estimate of the composite strengthcan be determined using equation 2.10:

�c = Vf�f(

1 −lc2l

)

+ Vm�m (2.10)

where �m is the matrix stress when the fibre fails.


2.3 Additive Manufacturing (AM)

This section of the literature review discusses additive manufacturing (AM). Details are given ofthe process of AM, different AM methods and their advantages. Particular emphasis is put onfused deposition modelling (FDM) which is employed by the machine being used in this research.The strength limitations associated with traditional additive manufactured parts leads to the nextsection, which is additive manufacture of composites. Details are given of previous research inattempting to maximise the strength of composites fabricated by FDM including FDM of shortfibre reinforced polymers and attempts at FDM of continuous fibre reinforced composites alongwith the issues encountered. The MarkForged Mark One continuous fibre composite printer isthen discussed with reference to one previous study which tested carbon fibre specimens printedby the Mark One.

2.3.1 Introduction to AM

Additive manufacturing (AM), more commonly known as 3-D printing, is an additive processwhereby materials are applied in successive layers in order to fabricate a three-dimensional solidobject from a digital model. A representation of the part geometry is created using computeraided design or solid modeling software which generates a file (eg STL file format). This is thensliced into individual cross-sectional layers (eg CLI file format) [7, 41]. This layer-wise data isconverted into numerical control code (eg CNC G- and M-codes) [41] and used to generate thecuring or binding paths followed by an AM machine. The machine solidifies or binds lines ofmaterial until an individual layer is completed. A new layer is then built on the previous oneand this process is repeated until the part is fully built [38]. There are a number of different AMtechniques including stereolithography (SLA), selective laser sintering (SLS), laminated objectmanufacturing (LOM), photo-masking and fused deposition modelling (FDM). The 3-D printerused in this project employs FDM which will be described in detail in the next section.

AM offers a number of advantages when compared to traditional manufacturing methods suchas injection moulding or machining/subtractive technologies. Parts of virtually any shape withcomplex geometry and intricate details can be produced by AM [23]. Injection moulding requiresexpensive moulds, tools, forms and punches which are not used in AM processes. Subtractivetechnologies use multi-axis cutting machines to remove material from parts to create a desiredshape. This process results in high levels of waste material whereas there is considerably less


waste material with AM. Initial products can be produced much more quickly using AM since noset up time is required and design changes can be implemented easily. Thus it is extremely usefulfor small production runs, customised products and prototyping [7].

Polymeric parts made by injection moulding tend have isotropic properties. A disadvantageassociated with parts made by AM is that they tend to have anisotropic properties. This is becauseof void formation and the raster orientation of the deposited material i.e. the material is strongerin the direction in which it was deposited. This can greatly affect the mechanical strength of partsproduced by AM [1].

2.3.2 Fused Deposition Modelling (FDM) of Polymers

Fused deposition modelling is one of a number of different additive manufacturing systems[20] which builds parts from the deposition of extruded thermoplastic materials (see figure 2.8).Thermoplastic is used due to their re-heating and re-moulding properties unlike thermosets. Aheated extrusion print head is fed by a spool of thermoplastic filament which is melted beforebeing deposited on the workpiece[38]. The filament is moved into a channel of the nozzle bymotor driven rollers and heated near the nozzle tip to become melted. The solid filament pushesthe melted portion out through the nozzle onto the workpiece [41]. A computer controls themovement of the print head in the X-Y axis directions corresponding to each cross-sectional layerof the part [38]. A layer consists of small, adjacent lines of material, layed in a particular pattern[20]. When one layer is complete, the extrusion head moves up a preset distance in the Z-axisdirection (or the print bed moves down) and the next layer is bonded to the previous layer throughthermal heating. Post-curing is not required for the part when printing is complete [38].


FIGURE 2.8: Schematic of the FDM process [30]

The 3-D printer used in this research employs a FDM process or more specifically, a fused filamentfabrication (FFF) process. In order for FFF to produce parts with good mechanical properties,the polymer used must have a proper melting and solidification temperature range. The meltingtemperature should be low enough to prevent excessively high processing temperatures but itshould not be too low so that the material has a high heat distortion temperature. To improveprecision and dimensional accuracy, the material must have a low coefficient of thermal expansion.Linear shrinkage in a part should be less than 1%. Also, the material must have good strength,stiffness, flexibility and ductility. Thus a thermoplastic polymer is ideal for use in FDM [41].

Common materials used in FDM include acrylonitrile-butadiene-styrene copolymer (ABS), anylon copolymer and investment casting wax. The mechanical properties of these materials islimited and thus many parts fabricated by FDM can only be used as geometric replicas [20]. Thisgreatly restricts the wide range of potential applications for FDM technology [41] and has led tointerest in developing new FDM materials with improved mechanical properties [20].

2.3.3 FDM of Short Fibre Composites

In order to increase the strength of parts made by additive manufacturing, research has beenconducted on the development of composite materials to be used by FDM machines. It isknown that composite strength increases with the use of longer fibres. The use of continuousfibre dramatically increases the strength and stiffness of parts when compared to short fibrecomposites [23]. A number of studies investigated the use of short and long fibre reinforced


polymer composites as feedstocks for FDM. Various tests were performed to obtain the mechanicalproperties of printed specimens and evaluations were made.

Tekinalp et al. [35] prepared short fibre, ABS-carbon fibre composites . ASTM D638 type-Vdog-bone samples were produced using a Solidoodle 3 commercial desktop FDM unit. Thematerial deposition direction was parallel to the tensile loading direction. Dramatic increasesin both tensile strength and modulus were observed using ABS-carbon fibre when compared toneat-ABS (no fibre). Scanning electron microscope (SEM) micrographs of the fracture surfacesshowed relatively large triangular voids orientated similarly across the neat-ABS specimen. Thesewere gaps between deposited beads of material or inter-bead pores and were not expected tosignificantly effect the specimen’s mechanical performance as they formed channels which ranparallel to the direction of loading. The addition of short carbon fibres reduced the size of thesechannels but caused voids to form inside the beads (inner-bead pores). This could cause failure atlower stresses due to stress concentration points. Poor interfacial adhesion between the matrixand the fibres was noted as a result of fibre pull out. It was also observed that fibres were mainlyorientated in the load bearing direction due to the nature in which the material was deposited bythe nozzle. At 40 wt% carbon fibre loading, printing could not be completed because of nozzleclogging. Therefore high fibre loading could not be achieved using this method [35].

A more comprehensive study was carried out by Ning et al. [30] who tested carbon fibre reinforcedpolymer (CFRP) composites consisting of varying carbon fibre lengths and content. Similarly toTekinalp et al. [35], Ning et al. [30] made composite filaments by extrusion to be used in FDM.The filaments consisted of ABS thermoplastic pellets mixed with carbon fibre powders of twodifferent average carbon fibre lengths (150�m and 100�m). The filaments were extruded twice tohelp distribute the carbon fibres more equally. ASTM D638-10 standard was followed for thetensile test and ASTM D790-10 for the flexural test. An increase in tensile strength and Young’smodulus was achieved with the use of carbon fibres and further increased when using the longer150�m fibres. However, average values of toughness, yield strength and ductility had a tendencyto decrease with the addition of carbon fibres. The maximum increase in tensile strength was22.5% using 5 wt% carbon fibre and the maximum increase in Young’s modulus was 30.5% using7.5 wt% carbon fibre. Flexural stress, modulus and toughness all increased by 11.82%, 16.82%and 21.86% respectively. Void formation was very evident in the 10 wt% carbon fibre samplesand resulted in poor mechanical properties [30].


2.3.4 FDM of Continuous Fibre Composites

Namiki et al. [29] developed a method for printing continuous carbon fibre reinforced polymer(CFRP) composites by FDM. The thermoplastic matrix material used was poly-lactic acid (PLA).The printer head of the 3D printer was modified to allow for in site impregnation of the continuouscarbon fibre into the PLA filament before being extruded through the nozzle. The diameter of thenozzle was 1.4mm which is relatively large compared to conventional FDM methods. This wasneeded to prevent clogging of the the carbon fibres but resulted in poor layer resolution. Tensiletest specimens with continuous fibres orientated in the loading direction and a fibre volumefraction of 1.0% were printed. Continuous fibre reinforcement resulted in a ≈ 60% increase intensile strength and a ≈ 70% increase in Young’s modulus.A scanning electron microscope imageof the fracture surface showed fibre pull out and poor impregnation of fibres to the matrix. Largevoid formation occurred between PLA filaments because of the low layer resolution [29].

A study carried out by Gray et al. [20] noted that it may not be possible to build dimensionallyprecise parts by extruding pre-impregnated carbon or glass fibres through a nozzle. A high aspectratio of fibre (L/D > 100) combined with a small diameter nozzle is required for high resolution.Gray et al. [20] opted for thermotropic liquid crystalline polymers (TLCPs) which have fibrildiameters of about one order of magnitude smaller than that of carbon or glass fibres. Thus higherresolution could be achieved when using TLCPs as continuous fibre feedstock for FDM. Greatermechanical properties were obtained when using long fibre TLCPs as compared to short fibreTLCPs.

Matsuzaki et al. [26] developed a method which enabled the printing of continuous fibre reinforcedthermoplastics based on FDM. The print heads of two commercially available printers weremodified. The first was from a blade-1 3D printer with a preheating system which was usedfor carbon fibre reinforced thermoplastics. The second was from a FlashForge 3D printer withno preheating system which was used for jute fibre reinforced thermoplastics. The continuousfibre and thermoplastic resin filament are supplied separately to the printer head. The resinfilament is heated by a heater in the print head. A motor feeds the reinforcing fibre into thenozzle where it is combined with resin filament. The reinforcing fibres are pre-heated using anichrome wire to enhance the permeation of the thermoplastic resin with the fibre bundles. Theresin-reinforced fibres are then extruded through the nozzle and deposited on a hot print bed.The layer resolution was relatively low as a large nozzle diameter was used to prevent clogging.Scanning electron microscopy was used to determine the fibre volume fraction of the extruded


filaments. The volume fraction of carbon fibre and jute fibre was 6.6% and 6.1% respectively.Tensile test specimens using PLA thermoplastic as the matrix were printed with fibres aligned inthe loading direction for both continuous carbon and jute fibres. Neat PLA samples were alsoprinted for comparison. The tensile strength and tensile modulus for carbon fibre samples were185.2MPa and 19.5 GPa respectively, which were 435% and 599% of those of PLA samples. Forthe jute fibre samples, the tensile strength and modulus were 57.1MPa and 5.11 GPa respectivelywhich were not significant improvements but corresponded to 134% and 157% of those of thePLA samples. With increasing applied stress, the fibre-matrix bonding degraded which resultedin a lack of improvement of tensile strength and a decrease in tensile modulus. It was noted thatthere was no tension applied to the jute fibre when it was being fed to the print nozzle. Thiscaused non-uniform fibre configuration due to twisted fibres which may have resulted in weakpoints in the printed specimen. SEM images of the fracture surfaces showed fibre pull-out whichindicated insufficient adhesion between the thermoplastic resin and fibres for both carbon andjute fibres. It was noted that treatment of fibres would help to achieve greater improvements inmechanical properties. Voids between deposited filaments occurred due to the extruded filamentassuming an elliptical shape. An estimated upper limit of fibre volume fraction which could beachieved using this method was 40 − 50%, however typical carbon composites used in aerospaceapplications have a fibre volume fraction of ∼ 67%.

FIGURE 2.9: Schematic of the 3D printer head developed by Matsuzaki et al. [26] for producingCFRPs using an FDM in-nozzle impregnation technique.


2.3.5 FDM of Continuous Fibre Composites using the MarkForged 3-D Printer

The MarkForged Mark One 3-D printer is capable of printing complex shaped thermoplastic partsreinforced with continuous fibres. It does this by using a dual print head, one prints thermoplasticfilaments and the other embeds the thermoplastic with continuous fibre filaments [23]. Thisovercomes the resolution issues encountered by Namiki et al. [29]. The Mark One’s print headpositioning system has a resolution of 6.25 micron [23].

A study has been conducted by Van Der Klift et al. [36] using the Mark One 3-D printer toevaluate it’s effectiveness and capabilities in printing continuous fibre reinforced thermoplastic.The matrix material was MarkForged’s specially tuned nylon FFF (fused filament fabrication) andthe continuous fibre filament used was carbon. It was noted that due to the printing process andpattern, a small discontinuity occurs in each of the fibre layers. It was suggested that this could beto prevent the possibility of the print head becoming stuck if it tried to print a continuous square,as it operates very close to the surface of the specimen. Three types of tensile test samples wereprinted, the first had 10 layers of nylon and no carbon fibre, the second had 4 layers of nylon and6 layers of carbon fibre and the third had 8 layers of nylon and 2 layers of carbon fibre. The fibrewas orientated unidirectionally (anisotropic) in all samples. The tensile strength of the sampleswith 2 layers of carbon fibre varied between 126 MPa and 171 MPa which was around one thirdof the tensile strength results for the samples with 6 layers of carbon. It was noted that failureoccurred at the clamping locations on the samples and also at points where discontinuities in thefibre were present. Also, there was a larger amount of void formation in the samples containing 6layers of carbon fibre than those containing 2 layers. The more carbon layers in a sample, themore the measured elastic modulus deviated from the estimated value using the rule of mixturesequation 2.1. Increased void formation was a contributing factor to this [36].

2.4 Literature Review Conclusion

It is evident from the literature that very little research involving continuous fibre reinforcedcomposites fabricated by AM has been carried out. The one paper available making use of theMark One printer [36] only investigates the effect of changing the number of layers of carbonfibre within the tensile test specimen and indicates that further research should be carried out onthe effect of the position of the fibre layers on the mechanical properties of the composite. This


study also uses carbon fibre which can only be printed in a concentric layout with the Markforgeddue to it’s high stiffness.

Glass fibre is used in this research which can be printed in more layouts than carbon fibre. Thusthe fibres can be tailored to maximise strength for particular loading directions. Based on section2.2.2 of the literature review and the work done by Yong et al. [40] with these layouts, fibrelayouts used in this study will be perpendicular, quasi-isotropic and anisotropic. A concentriclayout will also be used however no literature was found which made use of this layout. Specimendesigns will also be based on a suggestion by Van Der Klift et al. [36] who tested carbon fibresamples printed by the Markforged. It was noted that an interesting area of research would be tostudy the effect of printing fibre and nylon layers alternately. Specimen design will also be basedon on section 2.2.3 of the literature review which indicated that to maximise flexural rigidity,fibre reinforcement should be located at the outer surfaces.

Chapter 3

Experimental Methods

3.1 Chapter Overview

3.2 Materials

3.2.1 Markforged Materials

Only materials supplied by Markforged can be used as feedstock for the Mark One printer. Thetwo materials used were nylon thermoplastic filament and glass fibre filament. These materialsare patented by MarkForged and their properties cannot be disclosed. Van Der Klift et al. [36]suggested that the nylon used in the Mark One printer had similar properties to Nylon-6 orNylon-66. The carbon fibre volume fraction was determined by evaporating the matrix materialaccording to JIS K 7075 which enabled the calculation of a theoretical value for the elasticmodulus using the rule of mixtures equation. This helped to determine the type of fibres that wereused. A similar procedure would need to be followed to determine the type of glass fibre used.

3.2.2 Tensile and 3-Point Bending Specimen Details

Tensile tests were conducted according to the ASTM D638 − 10 standard [4]. Proper dimensionswere selected according to to this standard and are shown in figure 3.1.

22

Chapter 3. Experimental Methods 23

180mm

19mm

13mm

FIGURE 3.1: ASTM-D638 tensile test specimen dimensions (note: the specimen height is3.2mm)

Fibre Layout Nylon volume (cm3) Glass Fibre volume (cm3) Nylon % Glass Fibre %Anisotropic 6.71 2.66 71.61 28.39Concentric 7.42 2.03 78.52 21.48Quasi-isotropic 6.25 3.17 66.35 33.65Perpendicular 6.32 3.42 65.91 34.09Nylon 9.63 0 100 0

TABLE 3.1: Tensile test specimen nylon and glass fibre volume fractions

3-point bending tests were conducted according to the ASTMD790−10 standard [5]. Dimensionswere selected according to to this standard and are shown in figure 3.2.

123mm

12.7mm

FIGURE 3.2: ASTM-D790 3-point bending test specimen dimensions (note: the specimen heightis 3.2mm)

Fibre Layout Nylon volume (cm3) Glass Fibre volume (cm3) Nylon % Glass Fibre %Anisotropic 3.42 1.72 66.54 33.46Concentric 3.82 1.34 74.03 25.97Quasi-isotropic 1.79 1.79 50 50Perpendicular 3.42 1.81 65.39 34.61Nylon 5.01 0 100 0

TABLE 3.2: 3-point bending test specimen nylon and glass fibre volume fractions

3.3 3D Printing of Specimens

The Markforged printer can be seen in figure 3.3. The process followed to print the specimenswas as follows. CAD drawings were made of the tensile and bending test specimens, The drawing.STL file was imported to MarkForged’s "Eiger" program which enabled fibre orientations andlayers to be specified. The Mark One printer was set up before printing. Its dual print head is fed


FIGURE 3.3: Markforged 3-D printer [24]

by a nylon filament and a separate glass fibre filament. The nylon spool is kept in an externalsealed drybox along with desiccant pouch to prevent absorption of moisture. The spool wasplaced on the drybox idler and the magnetic retention cap was attached. The spool and idler werethen placed in the drybox and the nylon was fed into the filament feed tube ensuring not to allowthe sprung spool to unwind. On the printer touchscreen, "Load Nylon", was selected and thefilament was inserted into the back of the extruder.

A glass fibre spool was then inserted into the fibre inlet tube ensuring not to let it unwind. Thespool was placed on the spool holder and secured with the spool holder cap. On the printertouchscreen, "Load Fibre" was selected and the fibre was fed in until it was gripped by the fibreextruder. The excess fibre was cut and removed.

On the printer touchscreen, "Level Print Bed", was selected and the instructions on the screenwere followed. The three thumbscrews underneath the print bed were loosened until the print bedwas in its lowest position. The shim labeled "PLASTIC SHIM 100uM" was placed beteen thenylon nozzle and the print bed and the thumbscrew was adjusted until a slight resistance was feltwhile sliding the shim. This was repeated for the other thumbscrews. The fibre nozzle was alsoadjusted until a slight resistance was felt while sliding the shim using a 2.5mm allen key.

The above procedures do not need to be done often as there is a lot of filament on the spools andthe print bed remains level for many prints but should be checked occasionally.

The print bed was cleaned with warm water and dried with a paper towel. A thin layer of "Elmer’swashable glue", was then applied to keep the part attached to the bed during the printing process.The anisotropic specimen was selected on "Eiger", and "Add Part", was pressed until there were


5 specimens on the print preview. The print button was selected. It took approximately 9 hours toprint the 5 specimens. The print bed was removed from the Mark One and placed on a table. Thespecimens were pried off using the bed scraper at a shallow angle to prevent gouging the printbed. This was then repeated for the concentric, isotropic, perpendicular and nylon specimens. Inthe end there were 5 of each type of specimen, ready for testing.

3.4 Tensile and 3-Point Bending tests

The Hounsfield H50KS screw-drive materials testing machine was used to perform the tensiletesting. A 50kN load cell was used along with two grips (one fixed and one moveable) to clampthe specimens. Test speed was set to 100mm∕min for the nylon samples due to their ductileproperties resulting in greater elongation before failure. Test speed was set to 5mm∕min for thecomposite samples. The parameters were set as shown in table 3.3. Care was taken to ensure eachtensile test specimen was clamped evenly and centered in the clamps. When a test was completed,the fractured parts of the specimen were removed from the clamps and numbered. Microsoftexcel files containing force vs extension graphs were produced by the QMAT software and laterconverted to stress-strain curves.

Load Range (N) 10000 (nylon) / 50000 (fibre)Extension Range (mm) 2000Test speed (mm/min) 100 (nylon) / 5 (fibre)Sample Length (mm) 100Preload (N) 0

TABLE 3.3: Hounsfield tensile test settings

The 3-point bending tests were performed with an Instron 100 kN test machine using two supportsand a midway loading nose. The testing parameters are shown in table 3.4. The TestXpertsoftware produced excel files containing stress-strain data which was used to produce graphs.

Test speed (mm/min) 2.2Support span (mm) 51.2Preload (MPa) 0.1

TABLE 3.4: Instron 100 kN test settings

For every specimen design, 5 samples were tested to obtain average values. Tensile moduli weredetermined by looking at the slope of the linear region of the stress-strain curves from the tensiletests. The TestXpert software automatically calculated the flexural modulus. Both tensile and


flexural toughness were calculated by integrating the area under the stress-strain curves up to thefailure strain.

Chapter 4

Results and Discussion

4.1 Design of Test Specimens

4.1.1 Fibre Orientation

As described in the literature review conclusion, four specimens were designed containing differentfibre layouts based on previous studies by Yong et al. [40] and Van Der Klift et al. [36]. Theselayouts were perpendicular, quasi-isotropic, concentric and anisotropic. 100% nylon specimenswere used for comparison. These specimen designs are illustrated in figure 4.2.

27

Chapter 4. Results and Discussion 28

Nylon

fibre = 0°

fibre = 0°

fibre = 0°

Nylon

fibre = 0°

fibre = 0°

fibre = 0°

Nylon

fibre = 0°

fibre = 0°

fibre = 0°

Nylon

fibre = 0°

fibre = 0°

fibre = 0°

(A) Anisotropic Specimen

Nylon

fibre = 90°

fibre = 90°

fibre = 90°

Nylon

fibre = 90°

fibre = 90°

fibre = 90°

Nylon

fibre = 90°

fibre = 90°

fibre = 90°

Nylon

fibre = 90°

fibre = 90°

fibre = 90°

(B) Perpendicular Specimen

Nylon

fibre = Conc

fibre = Conc

fibre = Conc

Nylon

fibre = Conc

fibre = Conc

fibre = Conc

Nylon

fibre = Conc

fibre = Conc

fibre = Conc

Nylon

fibre = Conc

fibre = Conc

fibre = Conc

(C) Concentric Specimen

Nylon

fibre = 0°

fibre = 90°

fibre = 135°

fibre = 45°

Nylon

fibre = 0°

fibre = 90°

fibre = 135°

fibre = 45°

Nylon

fibre = 0°

fibre = 90°

fibre = 135°

fibre = 45°

Nylon

fibre = 0°

fibre = 90°

fibre = 135°

fibre = 45°

Nylon

fibre = 0°

fibre = 90°

fibre = 135°

fibre = 45°

(D) Quasi-Isotropic Specimen

x

z

y

FIGURE 4.2: Illustration of specimen fibre layouts


4.1.2 Type 1 and Type 2 Specimens

Layer thickness resolution on the Mark One printer is 0.1mm. Thus at 3.2mm in height, thespecimens contained 32 layers. 20 layers of nylon and 12 layers of glass fibre. For each fibredesign illustrated in figure 4.2, the position of the glass fibre layers were arranged in two manners.The first was "type 1" specimens where the first 3 outer layers of fibre were separated by 1 layer ofnylon and the remaining fibre layers separated by 2 nylon layers (see figure 4.3). The second was"type 2" specimens which was a sandwich panel construction. The first layer was nylon, layers2 − 7 contained glass fibre, layers 8 − 25 were nylon, layers 26 − 31 contained glass fibre andlayer 32 was nylon (see figure 4.4). For each design, type 1 and type 2 specimens contained thesame fibre volume fraction and orientation, just different layer positioning.

For the remaining sections, perpendicular, quasi-isotropic, concentric and anisotropic specimenswill be denoted by the prefixes Perp, Quasi-iso, Conc and Aniso respectively followed by #1 or#2 to distinguish between type 1 and type 2 specimens.

FIGURE 4.3: Cross-section showing the arrangement of layers within type 1 specimens (yellowrepresents nylon and grey represents glass fibre)

FIGURE 4.4: Cross-section showing the arrangement of layers within the type 2 specimens(yellow represents nylon and grey represents glass fibre)


4.2 Effect of Specimen Design on Tensile Properties

Typical tensile stress-strain curves for type 1 and type 2 specimen designs are shown in figure 4.5and figure 4.6 respectively. The curves were selected based on the curve that best representedthe mean tensile values from the results of 5 tests. Nylon specimens were left out of the graphsbecause of much higher strain values. The effect of specimen design on the tensile propertiesincluding tensile strength, tensile modulus and tensile toughness are shown in figures 4.7, 4.8and 4.9 respectively. Bar charts were used to represent the mean values of tensile properties.Each bar has an individual error bar to express the standard deviation of the results. A large errorbar indicates a large standard deviation and thus inconsistent results for that particular specimendesign. It can be noted that tensile loading was applied in the x-direction and flexural supportswere aligned along the x-axis for all specimens (see figure 4.2)


0

50

100

150

200

250

300

350

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain curves for Type 1 Specimens

Aniso#1

Conc#1

Quasi-iso#1

Perp#1

FIGURE 4.5: Typical tensile stress-strain curves for type 1 specimens.

0

50

100

150

200

250

300

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Type 2 Specimens

Aniso#2

Conc#2

Quasi-iso#2

Perp#2

FIGURE 4.6: Typical tensile stress-strain curves for type 2 specimens.

Tensile Strength:


Figure 4.7 shows that tensile strength increased with increasing amount of glass fibre orientationtowards the loading direction. For the perpendicular specimens, the glass fibres were alignedperpendicularly to the loading direction and therefore did not provide reinforcement. This gave riseto low strength as only the nylon matrix was supporting the load. The tensile strength increasedgreatly for the quasi-isotropic specimens. 2 of the glass fibre layers contained fibre angled at 45°tothe loading direction and 2 layers contained fibre angled at 135°which provided some strength. 4of the glass fibre layers contained fibre facing in the loading direction which provided maximumstrength. The remaining 4 glass fibre layers faced in the perpendicular direction and provided nostrength. The strength increased further for the concentric specimens as more glass fibres facedin the loading direction. The glass fibres curve slightly in the narrowing region of the tensiletest specimen. At the ends of the tensile test specimen the glass fibres face perpendicularly tothe loading direction but these sections were contained within the clamps of the Hounsfield testmachine. The anisotropic specimens had the highest tensile strength as all the glass fibres facedin the loading direction.

All of the type 1 specimens had a higher tensile strength than the type 2 specimens except for theconcentric design. The standard deviation for for Conc#1 was larger than that of Conc#2. Thiswas due test 5 for Conc#1 which was 199MPa and significantly lower than the other tests (seefigure A.4 in appendix A). The Aniso#1 specimen showed the highest tensile strength of 304MPa.

Perp#1 had the same strength as nylon but Perp#2 showed reduced strength. This must be takeninto consideration when trying to maximise the tensile strength of a particular part. If the tensileforce applied to the part is in one direction then the anisotropic specimens would be optimalhowever if there are multi-directional tensile forces then this design could potentially be extremelyweak as exposed by the results of the perpendicular specimens. A good compromise would be touse the quasi-isotropic design which has fibres orientated in the 0°, 45°, 90°and 135°directions.


44 44

167

224

304

40

160

232

274

0

50

100

150

200

250

300

350

Nylon Perpendicular Quasi-isotropic Concentric Anisotropic

(MPa

)

Maximum Tensile Strength

Type 1 Type 2

FIGURE 4.7: Max tensile strength for type 1 and type 2 specimens.

Tensile Modulus:

Figure 4.8 shows that the tensile modulus of elasticity generally increased with increasing glassfibre orientation towards the loading direction. An exception to this was for the Quasi-iso#2specimen which had a slightly higher modulus than the Conc#1 specimen. However, the standarddeviation associated with all of the type 2 specimens was larger than that of the type 1 specimens.

The Aniso#2 specimen had the highest tensile modulus which was 7.1 GPa.

The perpendicular specimens resulted in an increase in modulus when compared to the nylonspecimen.


0.9

2.7

3.6

4.2

6

2.4

4.5

5.6

7.1

0

1

2

3

4

5

6

7

8


(GPa

)

Tensile Modulus

Type 1 Type 2

FIGURE 4.8: Tensile modulus for type 1 and type 2 specimens.

Tensile Toughness:

Figure 4.9 shows that the tensile toughness decreases significantly with the addition of glassfibre to the nylon. The largest toughness value was found in the nylon specimen which was 74.2J*m−3*104. Test 4 for the nylon specimen strained to 3.66% whereas the average strain for theother tests was 2.09%. Thus it was disregarded and the toughness was calculated from the averageof the other 4 tests. (See figure A.1 in appendix A).

The toughness for specimens containing glass fibre tended to increase with increasing fibreorientation towards the loading direction. The difference in toughness between type 1 and type2 specimens was not significant and the standard deviation values were small. The Aniso#1specimen had the highest toughness of the specimens containing glass fibre which was 11.8J*m−3*104.


74.2

2.76.8 8.4

11.8

4.5 6.29.2 10.5

0

10

20

30

40

50

60

70

80

90


(MJ/

m3)

Tensile Toughness

Type 1 Type 2

FIGURE 4.9: Tensile toughness for type 1 and type 2 specimens.

4.3 Effect of Specimen Design on Flexural Properties

According to the ASTM D790 − 10 standard [5], the flexural test should be terminated when themaximum strain in the outer surface of the test specimen reaches 5% or when the fracture occursprior to reaching the maximum strain. The specimens tested in this investigation did not fracturewithin the 5% strain limit. Typical flexural stress-strain curves for type 1 and type 2 specimendesigns are shown in figure 4.10 and figure 4.11 respectively. These curves were selected usingthe same methodology as for the tensile stress-strain curves. It is evident that the flexural stress ata given strain for the specimens containing glass fibre was larger than that of the nylon specimen.There is a noticeable difference in flexural properties between type 1 and type 2 specimens. Asthe strain increased, the type 2 specimens reached their maximum flexural stress first, with theexception of the perpendicular specimens.


0

50

100

150

200

250

300

350

0 2 4 6 8 10

Stre

ss, M

Pa

Strain, %

Flexural Stress-Strain Curves for Type 1 Specimens

Aniso#1

Conc#1

Quasi-iso#1

Perp#1

Nylon

FIGURE 4.10: Typical flexural stress-strain curves for type 1 specimens.

0

50

100

150

200

250

300

350

0 2 4 6 8 10

Stre

ss, M

Pa

Strain, %

Flexural Stress-Strain Curves for Type 2 Specimens

Aniso#2

Conc#2

Quasi-iso#2

Perp#2

Nylon

FIGURE 4.11: Typical flexural stress-strain curves for type 2 specimens.

The effect of specimen design on the flexural properties including flexural strength, flexuralmodulus and flexural toughness are shown in figures 4.12, 4.13 and 4.14 respectively. Bar charts


were defined in the same way as for the tensile tests.

Flexural Strength:

With regards to the 3-point bending test, a fibre angle of 0°means that the fibres are parallelto the direction between the test sample supports and an angle of 90°means that the fibres areperpendicular to this direction. It can be seen in figure 4.12 that the use of glass fibre reinforcementcan greatly increase the flexural strength. All specimens showed greater flexural strength than thenylon specimen. Type 1 samples displayed greater strength than type 2 samples with the exceptionof the perpendicular specimens. There was a relatively large difference in strength between theQuasi-iso#1 and Quasi-iso#2 specimens. Upon observation of the original test samples, it wasnoticed that some glass fibre reinforcement was missing from the Quasi-iso#2 test samples. Thisindicates that the fibre jammed or ran out prior to finishing the print. Thus the strength value forQuasi-iso#2 should have been higher. The Aniso#1 specimen had the highest flexural strengthwhich was 297 MPa. Nylon showed the lowest strength at 42 MPa.

4258

192

229

297

76

136

224

286

0

50

100

150

200

250

300

350


(MPa

)

Maximum Flexural Strength

Type 1 Type 2

FIGURE 4.12: Max flexural strength for type 1 and type 2 specimens.

Flexural Modulus:


Figure 4.13 shows that the use of glass fibre can significantly increase the flexural modulusof elasticity. Both the concentric and anisotropic specimens displayed high moduli. In thesespecimens all fibre under load were orientated at 0°i.e. they bridged the gap between the supportpoints. The concentric specimens did contain some fibre in the perpendicular direction at theends but these sections were not under load. The anisotropic specimens contained 13 rowsof continuous glass fibre in each fibre layer whereas the concentric specimens contained 10.Therefore it would be expected that the moduli of the anisotropic specimens would be higherhowever this was not the case. The moduli of the Aniso#1 and Conc#1 specimens were 5.93 GPaand 6.48 GPa respectively. The moduli of the Aniso#2 and Conc#2 specimens were 9.58 and10.15 respectively. The standard deviation bars are much greater for the anisotropic specimensthan the concentric specimens. This was because of test 5 for Aniso#1 (see figure A.14 in appendixA) and test 1 for Aniso#2 (see figure A.18 in appendix A) which significantly reduced the meanvalues and increased the standard deviation. Had these outliers been omitted the mean modulivalues for the anisotropic specimens would have been slightly higher than those of the concentricspecimens at 6.6 MPa and 10.38 MPa for Aniso#1 and Aniso#2 respectively.

Conc#2 showed an increase in modulus of 57% compared to Conc#1 and Aniso#2 showed anincrease in modulus of 62% compared to Aniso#1. The slightly larger increase for the anisotropicspecimens was due to the greater number of glass fibre rows per fibre layer (13 instead of 10).

The flexural modulus for Quasi-iso#2 was 4.14 GPa and only increased slightly to 4.37 GPa forQuasi-iso#2. The average increase in flexural modulus between type 1 and type 2 specimens forthe other designs was 70%. Had a printing error not occured for Quasi-iso#2 it’s modulus mayhave been in the region of 7 GPa.


1.061.5

4.14

6.485.93

2.88

4.37

10.159.58

0

2

4

6

8

10

12

14


(GPa

)

Flexural Modulus

Type 1 Type 2

FIGURE 4.13: Flexural modulus for type 1 and type 2 specimens.

Flexural Toughness:

In figure 4.14 it can be seen that the use of glass fibre increases the flexural toughness for allspecimen designs. It is evident that the toughness increases with decreasing fibre orientationangle i.e glass fibres at an angle of 0°contribute to high toughness.

The toughness of Conc#1was 921 J*m−3*104 and increased by 71% for Conc#2 to 1578 J*m−3*104.This occurred due to a number of factors. With reference to figure A.13 and figure A.17 in ap-pendix A, the higher modulus of Conc#2 caused it to reach maximum stress at a lower strain thanConc#1. In addition, the flexural stress did not drop significantly after the maximum stress pointfor Conc#2 but did for Conc#1. Also, the average strain to failure for Conc#2 was 8.8% and 6.2%for Conc#1. Since the toughness is obtained by integrating the area under the stress-strain curves,these factors resulted in a much higher toughness for Conc#2.

Aniso#2 showed the greatest toughness value which was 1878 J*m−3*104. This was 80% higherthan that of Aniso#1. This was due to the same factors as described above for the concentricspecimens. (See figure A.14 and figure A.18 in appendix A).


Quasi-iso#1 had a toughness of 1091 J*m−3*104 which reduced to 879 J*m−3*104 for Quasi-iso#2. Both specimens had a strain to failure of 8.8% so this decrease was due to the lower flexuralstrength of Quasi-iso#2. (See figure A.12 and figure A.16 in appendix A).

The nylon specimen had the lowest flexural toughness which was 262 J*m−3*104. This increasedto 365 J*m−3*104 for Perp#1. Perp#2 had a higher toughness than Perp#1 due to a higher flexuralstrength and modulus.

262365

1091

9211046

524

879

1578

1878

0

500

1000

1500

2000

2500


(J*m

-3*1

04)

Flexural Toughness

Type 1 Type 2

FIGURE 4.14: Flexural toughness for type 1 and type 2 specimens.

4.4 Fracture Surface Observations

This section looks at the fracture surface of specimens to get a better understanding of the typesof failure. Figure 4.16 shows type 1 tensile and flexural specimens after failure. For the tensilespecimens, fracture often occurred at the narrowing region. This may have partly been due todiscontinuities in the glass fibre at that location.


The different modes of failure of a laminate are shown in figure 4.15. Delamination may occurdue to tensile or compressive forces. Forces perpendicular to the fibres may cause matrix ruptureand parallel forces may cause fibre rupture [19].

FIGURE 4.15: The different modes of failure leading to damage of a laminate [19]

Much greater delamination occurred with type 2 specimens than type 1. This can be seen in Figure4.17. Poor adhesion between the glass fibres and nylon may have caused the slight decreases intensile and flexural strength for type 2 specimens over type 1 specimens (with the exception ofthe concentric design which had a slightly higher strength for type 2 specimens). This level ofdelamination must be taken into account when designing a part as it results in catastrophic failure.This was not the case with Type 1 samples.

Figure 4.18 (A) shows fibre pull-out which also indicates poor fibre/matrix bonding. (B) Showsvoids in the cross section of a type 1 specimen.


(A) Type 1 tensile test specimens after failure (B) Type 1 flexural test specimens after failureFIGURE 4.16: From left to right for both (a) and (b): Nylon, perpendicular, quasi-isotropic,

concentric and anisotropic

(A) Separation of glass fibre layers from nylon corefor type 2 tensile test specimen

(B) Microscope image of type 2 tensile test specimencross-section showing delamination between glass

fibre and nylonFIGURE 4.17


(A) Microscope image of tensile test specimenshowing fracture surface and fibre pull-out

(B) Microscope image of type 1 tensile test spec-imen cross-section showing voids

FIGURE 4.18

Chapter 5

Conclusion and Future Work

5.1 Conclusion

In this study, CFRTP samples were prepared using Markforged’s Mark One 3-D printer whichcombines nylon and continuous glass fibre filaments. Experimental investigations were conductedto determine the effect different fibre layouts (perpendicular, quasi-isotropic, concentric andanisotropic) and arrangement of fibre layers within the specimen (type 1 and type 2) had onthe mechanical properties of the composite with a view to maximising strength. Effects ontensile properties (including tensile strength, tensile modulus and tensile toughness) and flexuralproperties (including flexural strength, flexural modulus and flexural toughness) were investigated.Observations were made on the fracture surface and cross-section of specimens after testing usinga digital microscope. The following conclusions were drawn from this study.

(i) Compared to the nylon specimen, adding glass fibre reinforcement increased tensile strengthand tensile modulus but decreased tensile toughness. It increased flexural strength, flexuralmodulus and flexural toughness.

(ii) Overall, type 1 specimens had a higher tensile strength than type 2 due to superior bondingbetween the glass fibre and nylon matrix which reduced layer delamination.

(iii) Anisotropic layout had the highest tensile strength (304MPa) and perpendicular had thelowest (40 MPa) which was less than nylon (44 MPa). Thus the quasi-isotropic specimenshould be used for multi-directional strength.

44

Chapter 5. Conclusion and Future Work 45

(iv) Type 2 specimens had a higher tensile modulus than type 1 specimens. If stiffness is amore important requirement than strength, then type 2 specimens should be used.

(v) The tensile toughness values between type 1 and type 2 specimens for each fibre layout werevery similar. By choosing a fibre design that maximises strength or modulus depending onthe requirements, the resulting tensile toughness value would not alter significantly.

(vi) Similarly to the tensile strength results, type 1 specimens had a higher flexural strength thantype 2 specimens. Again this increase in strength was due to superior bonding betweenthe glass fibre and nylon matrix which reduced layer delamination. The quasi-isotropicspecimens should be used for multi-directional bending strength.

(vii) If flexural strength is required the type 1 specimens should be used but if flexural stiffnessis required, type 2 specimens should be used.

(viii) The concentric specimens achieved higher flexural moduli with more reliable results thanthe anisotropic specimens. Also the concentric design uses less glass fibre and so is morecost effective.

(ix) Unlike for the tensile toughness, the flexural toughness increased with the use of glass fibrecompared to nylon. Type 2 specimens should be chosen to maximise flexural toughness.

(x) Overall, for combinations of high strength and stiffness, type 2 specimens should be used.The slight reduction in tensile and flexural strength is more than offset by the large increasein tensile and flexural moduli. In addition, the tensile toughness values for type 1 and type 2specimens are very similar but the flexural toughness for type 2 specimens is much greater.

5.2 Future Work

• Fill pattern and density: Markforged’s Eiger software allows for triangular, hexagonal andrectangular fill patterns for the nylon matrix along with the fill density. In this study filldensity was set to 100% for all tests. The effect of fill pattern and density on mechanicalproperties and specimen weight could be investigated.

• Plasma treatment of fibres: Attempts could be made to improve the bonding between thefibres and matrix by plasma treating the fibres to enhance surface energy. This could furtherimprove the mechanical properties.

Chapter 5. Conclusion and Future Work 46

• Fibre volume content: In this study all specimens contained 12 layers of glass fibre. Re-search could be carried out on the effect of altering the number of layers containing fibreon the mechanical properties of the composite.

Appendix A

Stress-Strain Curves for Tensile and

Bending Tests

47

Appendix A. 48

A.1 Tensile Test Stress-Strain Curves for Type 1 Specimens

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5 4

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Nylon Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.1: Stress-strain curves from the results of tensile tests carried out on 5 specimensconsisting of 100% nylon (no-fibre).

0

5

10

15

20

25

30

35

40

45

50

0 0.02 0.04 0.06 0.08 0.1 0.12

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Perp#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.2: Stress-strain curves from the results of tensile tests carried out on 5 perp#1specimens.

Appendix A. 49

0

20

40

60

80

100

120

140

160

180

200

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Quasi-iso#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.3: Stress-strain curves from the results of tensile tests carried out on 5 quasi-iso#1specimens.

0

50

100

150

200

250

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Conc#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.4: Stress-strain curves from the results of tensile tests carried out on 5 conc#1specimens.

Appendix A. 50

0

50

100

150

200

250

300

350

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Aniso#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.5: Stress-strain curves from the results of tensile tests carried out on 5 aniso#1specimens.

Appendix A. 51

A.2 Tensile Stress-Strain Curves for Type 2 Specimens

0

5

10

15

20

25

30

35

40

45

0 0.05 0.1 0.15 0.2

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Perp#2 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.6: Stress-strain curves from the results of tensile tests carried out on 5 perp#2specimens.

0

20

40

60

80

100

120

140

160

180

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Quasi-iso#2 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.7: Stress-strain curves from the results of tensile tests carried out on 5 quasi-iso#2specimens.

Appendix A. 52

0

50

100

150

200

250

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Conc#2 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.8: Stress-strain curves from the results of tensile tests carried out on 5 conc#2specimens.

0

50

100

150

200

250

300

0 0.02 0.04 0.06 0.08 0.1

Stre

ss, M

Pa

Strain, %

Tensile Stress-Strain Curves for Aniso#2 Specimen

Test1

Test 2

Test 3

Test 4

Test 5

FIGURE A.9: Stress-strain curves from the results of tensile tests carried out on 5 aniso#2specimens.

Appendix A. 53

A.3 3-Point BendingTest Stress-StrainCurves for Type 1 Specimens

0

5

10

15

20

25

30

35

40

45

-2 0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Nylon Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.10: Stress-strain curves from the results of 3-point bending tests carried out on 5specimens consisting of 100% nylon (no-fibre).

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Perp#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.11: Stress-strain curves from the results of 3-point bending tests carried out on 5perp#1 specimens.

Appendix A. 54

0

50

100

150

200

250

0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Quasi-iso#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.12: Stress-strain curves from the results of 3-point bending tests carried out on 5quasi-iso#1 specimens.

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Conc#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.13: Stress-strain curves from the results of 3-point bending tests carried out on 5conc#1 specimens.

Appendix A. 55

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Aniso#1 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.14: Stress-strain curves from the results of 3-point bending tests carried out on 5aniso#1 specimens.

Appendix A. 56

A.4 3-Point BendingTest Stress-StrainCurves for Type 2 Specimens

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Perp#2 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.15: Stress-strain curves from the results of 3-point bending tests carried out on 5perp#2 specimens.

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Quasi-iso#2 Specimen

Test 1

Test 2

Test 3

Test 4

test 5

FIGURE A.16: Stress-strain curves from the results of 3-point bending tests carried out on 5quasi-iso#2 specimens.

Appendix A. 57

0

50

100

150

200

250

0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Conc#2 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.17: Stress-strain curves from the results of 3-point bending tests carried out on 5conc#2 specimens.

0

50

100

150

200

250

300

350

0 2 4 6 8 10

Flex

ura

l Str

ess,

MPa

Flexural Strain, %

3-Point Bending Stress-Strain Curves for Aniso#2 Specimen

Test 1

Test 2

Test 3

Test 4

Test 5

FIGURE A.18: Stress-strain curves from the results of 3-point bending tests carried out on 5aniso#2 specimens.

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