Investigating Mode I and II Crack Propagation in GFRP Composite Laminates

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MSc DEGREE IN Advanced Product Design Engineering (MSc)

Sathya Senadheera

K1135912

Investigating Mode I and II Crack Propagation in

GFRP Composite Materials

Date: 16th October 2015

Project Supervisor:

Dr Homayoun Hadavinia

School of Mechanical and Automotive Engineering

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Investigating Mode I and II Crack

Propagation in GFRP Composite

Laminates

Sathya Senadheera

K1135912

Advanced Product Design Engineering (MSc)

Project Supervisor:

Dr Homayoun Hadavinia

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ABSTRACT

Experimental investigations were conducted to characterize Mode I and Mode II

interlaminar fracture and delamination growth behaviors of a glass fibre reinforced

polymer composite material (GFRP) in accordance to ASTM standard testing

methods. Reduction methods and results yielded by these methods will be

compared. Samples were fabricated using a unidirectional GFRP prepreg material

TenCate E722. Materials were cured by means of a vacuum bag curing technique. 3

types of GFRP laminates layups were used with a total of 30 layers each. Layup

types used include and

Laminate layers between the initial Teflon induced cracks have been varied to

angles of ±45o depending on the sample layup type. Volume fraction sets are also

performed in accordance to ASTM standards and compared. Feature to fabricating

samples (laser etched scale), and data gathering methods (recording all tests

digitally) are introduced to minimise human error.

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ACKNOWLEDGEMENTS

I would like to express my deepest gratitude to Dr. Homayoun Hadavinia,

project supervisor and mentor for the research done in this report, and Dr Andy

Lung for the guidance given to me. I also extend my sincere gratefulness to the

Engineering Technical Officers at the Roehampton Vale Campus, Mr Dean

Wells, Mr Dave Haskell and Mr John MacBean as well as Mr Alex Vine of the

Penhryn Road Campus, without whom the work presented herewith would not

have become a success.

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Contents ABSTRACT .................................................................................................................. i

ACKNOWLEDGEMENTS............................................................................................ ii

TABLE OF FIGURES .................................................................................................. v

LIST OF TABLES ...................................................................................................... vii

1. INTRODUCTION ............................................................................................... 1

1.1 Composite Materials ....................................................................................... 1

1.2 Problem Statement......................................................................................... 2

1.3 Objectives ...................................................................................................... 2

2. LITERATURE REVIEW ..................................................................................... 3

2.1 Types of Composites and Their Properties .................................................... 3

2.1.1 Natural Composites ............................................................................................ 3

2.1.2 Major Modern Composite Classes ..................................................................... 3

2.2 Fibre Reinforcement ....................................................................................... 8

2.3 Other Reinforcement Types ......................................................................... 10

2.4 Matrices ........................................................................................................ 11

2.4.1 Thermoplastic Matrices ................................................................................ 11

2.4.2 Thermoset Resin Polymers .......................................................................... 12

2.5 Fracture Modes ............................................................................................ 13

2.7 Tensile Tests ................................................................................................ 14

2.8 Interlaminar Fracture Toughness Testing ..................................................... 15

2.8.1 Mode I Tests ................................................................................................ 15

2.8.2 Mode II Tests ............................................................................................... 17

2.9 Standard Test Methods for Constituent Content of Composite Materials ..... 20

2.9.1 Fibre Volume and Weight Ratio Relationship ............................................... 22

3. METHODOLOGY ............................................................................................ 23

3.1 Materials and Equipment .............................................................................. 23

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3.1.1 Zwick-Roell ProLine Universal Testing Machine .......................................... 23

2.1.2 HPC LaserCutter .......................................................................................... 23

3.1.3 Prepreg Material ........................................................................................... 24

3.1.4 Specimens ................................................................................................... 25

3.2 Manufacture ................................................................................................. 27

3.2.1 Laminate Preparation and Curing ................................................................ 27

3.2.2 Problems at Curing Stage ............................................................................ 28

3.2.3 Finishing ....................................................................................................... 30

4 EXPERIMENTATION ...................................................................................... 33

4.1 Specimen Data ............................................................................................. 33

4.2 Mode I Tensile Testing ................................................................................. 35

Load vs. Displacement Graphs for DCB Tests .......................................................... 37

4.3 Mode II Tensile Testing ................................................................................ 39

Load vs. Displacement Graphs for ENF Tests .......................................................... 41

4.4 Volume Fraction Tests ................................................................................. 44

5 ANALYSIS ....................................................................................................... 46

5.1 Mode I Fracture Toughness Analysis ........................................................... 46

5.2 Mode II Fracture Toughness Analysis .......................................................... 52

4.2 Discussion .................................................................................................... 57

6 CONCLUSIONS .............................................................................................. 60

7 REFERENCES ................................................................................................ 62

8 APPENDICES ................................................................................................. 65

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TABLE OF FIGURES

Fig No Description Page

2.1 Specific strength vs. stiffness properties of advanced fibres 4

2.3 Composite Reinforcement Types 10

2.4 General Characteristics of Thermoset and Thermoplastic Matrices 11

2.5 Types of crack propagation modes 13

2.8.1 DCB sample setup with loading blocks for mode I tests 15

2.8.2 3ENF sample setup for Mode II tests 18

3.1.1 Universal Testing Machine 23

3.1.2 HPC Laser engraver/cutter 23

3.1.3 Properties of the E722 Laminate 24

3.1.4 Layup Dimensions 25

3.1.5 Layup Specifications (fibre angles of layers) 26

3.2.1 Cutting layers from Prepreg 27

3.2.2 Layups with Teflon insert 27

3.2.3 Vacuum bagging 27

3.2.4 Curing procedure 27

3.2.5 Excess resin leakage 28

3.2.6 Batch (5) and (6) 29

3.2.7 Loading block preparation 30

3.2.8 Loading block dimensions 30

3.2.9 Applying epoxy adhesive 30

3.2.10 Finished DCB Samples 30

3.2.11 Laser etching 31

3.2.12 Finished DCB Specimen 31

3.2.13 Finished 3ENF Specimen 31

4.2.1 Tensile Test Rig setup for DCB samples 35

4.2.2 Nature of Mode I crack propagation. 36

4.2.3 Load vs. displacement for DCB4 Specimen 37



4.3.1 Tensile Test Rig setup for 3ENF samples 39

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4.3.2 Nature of Mode II crack propagation 40

4.3.3 Load vs. displacement for ENF1 Specimen 41



5.1.1 Mode I GIC calculations for DCB4-1 46



5.1.4 Comparing Mode I; MBT, CC, and MCC graphs for all DCB4 Specimen 47




5.1.8 Comparing Mode I; MBT, CC, & MCC graphs for all DCB2 Specimen 49




5.1.12 Comparing Mode I; MBT, CC, & MCC graphs for all DCB2 Specimen 51

5.2.1 Mode II GIIC calculations by MBT method for ENF1 specimens 52

5.2.2 Mode II GIIC calculations by CC method for ENF1 specimens 52

5.2.3 Comparison of CC and MBT method for ENF1 specimens 53







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LIST OF TABLES

Table Ref Description Page

2.2 Mechanical properties of engineering fibres 9

2.7.1 Material Characterisation properties obtained

from volume fraction and tensile tests 14

2.7.2 Experimentally obtained mechanical properties

of GFRP Composite Laminates 15

2.9.2 Typical fibre volume fractions 22

4.1.1 Specimen data for DCB samples in Mode I testing 33

4.1.2 Specimen data for 3ENF samples in Mode II testing 34

4.4 Weight Fraction Calculations 45

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1. INTRODUCTION

1.1 Composite Materials

By definition of the RSC (Royal Society of Chemistry, 2015) a composite

material is produced with the combination of more than one material, typically with

materials of different or contrasting properties and they do not dissolve or blend with

each other. The combination of such properties adds unique characteristics to the

resulting composite material. At present it is more common for composites to contain

2 materials: the matrix/binder that bonds or surrounds together fibres of other

material fragments which are referred to as the reinforcement.

The original concept of composite materials can be traced far into history, for

example Mongolian horn bow arcs (a re-curved composite bow) circa 2000 BCE,

these were made with the combination of wood and cow tendons glued together to

result in high strength. The first example of modern mass-produced composites were

fibreglass (1930s) and is still widely used for the construction of things such as sports

gear, boat hulls, racing car bodies, etc. Other advanced composites include carbon

fibres and carbon nanotubes that are being successfully used as the reinforcement

material. Generally the major advantage of modern day composite use is that it is

lightweight and very strong. The choice and combination of the matrix and

reinforcement elements of the composite can be varies in order to meet specific

requirements of an application, a composite also possess the ability to be applied to

a mould and cured.

Fibre Reinforced Polymer Materials (FRP) is used extensively in industry today

and among them are GFRPs: glass fibres reinforced with a polymeric matrix.

According to a case study by Bath University (Williams et al., 2015); GFRPs offer

advantages over metals in the field of structural manufacture such as low weight,

ease of manufacture, low cost and strength. One of their main disadvantages are low

stiffness and like any other material GFRPs are subjected to gradual deterioration

especially as a result of crack propagation or delamination due to interlaminar stress

concentrations. This can initiate at the manufacture stage as well as later into the

material’s lifetime in use. In addition to typical material specifications GFRP

properties depend greatly on fibre direction, as each ply is anisotropic laminates can

be reinforced by stacking layers with varying fibre angles together.

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1.2 Problem Statement

Composite materials are known for their range of vast applications, versatility, in

being relatively light weight, strong and easy to use. However, composites are not

immune to damage and failure. This may occur during manufacture as well as

prolonged use. One important failure model is delamination and crack propagation,

which will be explored in this project (mode I and mode II), specifically in Glass Fibre

Reinforced Polymer Materials

1.3 Objectives

a) Investigate Mode I and Mode II crack propagation and fracture toughness of

GFRP (Glass fibre reinforced polymer) composite material – TenCate 722-02

b) Experimentation to be done in accordance to ASTM (American society for testing

and materials) standards, based on DCB (double cantilever beam) and 3ENF (3

end notch flexure tests) tensile testing

c) Conduct volume fraction test on the composite material used in order to

determine fibre and matrix ratios for the material used.

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2. LITERATURE REVIEW

2.1 Types of Composites and Their Properties

2.1.1 Natural Composites

These types of composites are found in plants and animals and are naturally

occurring, based on the principle of fibres (reinforcement material) surrounded by a

matrix material. For example:

Wood – made of cellulose fibres (polymer material) bonded together a much weaker

substance named lignin. This combination of cellulose and lignin form a stronger

composite.

Bone – made of hydroxyapatite (a hard, brittle material that is mostly calcium

phosphate) and a collagen matrix, a flexible protein. (Royal Society of Chemistry,

2015)

2.1.2 Major Modern Composite Classes

The main classes of modern day composites are polymer matrix composites (PCMs)

metal matric composites (MMCs), intermetallic composites (IMCs), ceramic matrix

composites (CMCs), carbon-carbon composites (CCCs) and hybrid composites,

(Schwartz, 1997). The scope of this project will investigate a PCM material (TenCate

E722)

PCMs

Most developed and researched on type of composites in existence with numerous

applications, consisting of high performance fibres and matrices. Fibres induce a high

strength and modulus, the matrix spreads loads uniformly and resists

weathering/corrosion. PMC strength is proportional to strength of fibres and inversely

proportional to stiffness, therefore strength and stiffness depend heavily on fibre

manufacture as seen in Fig 2.1 Differences in flexibility of fibres in certain regions

eventually lead to shear stresses, the stress concentrations caused lead to fatigue

cracking.

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Fig 2.1 - Specific strength vs. stiffness properties of advanced fibres (Schwartz, 1997).

In comparison with most composites PCMs have an excellent service record, but in

most practical applications the technology has surpassed the basic understanding of

these materials. According to Advanced Materials by Design, 1988; in order to create

optimised composites and improve material properties of PCMs the following four

basic issues need to be addressed:

Processing Science – A thorough scientific understanding must be established in

order to identify how variations in manufacture processes can affect the end

properties of the material. This in turn will allow for developments in low-cost

manufacture methods for PCMs

Impact Resistance – This is a property that is critical to establishing durability and

reliability of PCM structures.

Delamination – Growing scientific evidence suggests that delamination is the

main, if not one of the root modes of damage propagation in PCM materials,

especially in those with laminar structures.

Interphase – Behaviour if the interphase regions (region between the fibres and

the matrix) influence the overall behaviour of PCMs. The interphase is generally

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the area where loads are transferred between the matrix and reinforcement, the

amount of interaction between the two is a property that needs to be studied in

order for it to be controlled as a design variable.

MMCs

Metal Matrix composites consist mainly of metal alloys that are reinforced with

a continuous fibre support system. Whiskers and particulates may also be used as a

type if reinforcement for these types of composites (will be discussed in section 2.3 of

this report). Since the matrix material present in these composites are of a metals

type MMCs generally have higher levels of resistance to temperatures in comparison

to PCMs however this also means that the added drawback to MMCs is their greater

weight. MSSCs are not used as widely as PCMs however MMCs are used in the field

of civil construction where withstanding high temperatures is the dominating

requirement, other more niche applications include light alloy MMCs being used in

space structures, engines and airframes. MMCs also have a high cost of production

therefore in order to increase the use of MMCs in industry applications there is a

need to optimise manufacturing and processing methods. Other advantages of using

metal matrices as opposed to polymer matrices are: their high tensile strength and

shear modulus, smaller expansion coefficients, moisture resistance, dimensional

stability, they are quite easy to join or bond together and MMCs are ductile and are

not as prone to brittle failure.

According to Schwartz, 1997; there isn’t a great deal of research and

development done for MMCs that are fibre or particulate reinforced; in order to

optimise their strengths and stiffnesses. This is mainly because other non-reinforced

alloys are preferred in industry, and also MMCs containing graphite reinforcements

have very poor bonds in its interphase layers when used with metal matrices

containing such elements as aluminium or magnesium.

CMCs

Ceramic Matrix composites tend to have high temperature resistance qualities

however they also pose fundamental structural limitations as ceramics have natural

tendency towards brittle fracture as ceramics cannot undergo shear deformation like

seen in the case of metals. Ceramic fibre reinforcements are used in order to reduce

these limitations by reducing the tendency for crack formation in the ceramic matrix.

Structural CMCs in use at present comprise most commonly of SiC fibre

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reinforcement coupled with ceramic matrices such as Si3N, Al2O3 or SiC. Fibre

reinforcements can be continuous, chopped or even small discontinuous whisker

platelets or particulates.

In addition CMCs have average to low fracture toughness’s but provide an

advantage over the use of superalloys in industries CMCs possess up to 70% lower

density and maximum use temperature that is higher than of superalloys by up to

500oC (Schwartz, 1997). CMCs are used in turbine engine components and

hypervelocity flight structures and other aerospace industry applications.

CCCs

Carbon-carbon composites as the name suggests are made up of carbon

based reinforcements embedded in carbon based matrix materials, initially CCC

processing is similar to that of PCMs however the original matrix used is an organic

compound and the heating up of this compound eventually converts it to carbon. CC

are used widely in applications where there is a greater need for resistance to high

temperatures and thermal shock. However systems must be put in place to prevent

or slow down oxidation rates when using CCCs, in addition CCCs also have high

costs of manufacturing and therefore not very widely used in industry.

According to Mazumdar (Mazumdar, 2002), the usages of CCCs are generally

divided to two categories: non-structural composites (used mainly in commercial and

military use for example aircraft brakes, rocket nozzles, etc.); and structural

composites that have very high payoff applications (especially when used with

engine or missile parts, satellite structures, etc.), they are not in production in high

volumes and still require significant research and development.

IMCs

There is a great deal of research that goes into intermetallic composites

(IMCs) today as they hold potential in vast applications in the field of future gas

turbine engines with high temperature, efficiency and performance characteristics, for

both commercial and military use. IMCs contain low strength intermetallic matrices

however with the addition of high strength fibre reinforcements this effect can be

negated.

According to Schwartz (Schwartz, 1997), one of the major problems in the

production of successful IMCs is compatibility issues that arise between the fibre

reinforcements and the matrices both in terms in chemical properties and

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mismatches in their corresponding thermal expansion coefficients. To overcome

these issues studies are being done on kinetic properties of reactions with materials

used in IMCs and methodologies are being developed to identify appropriate fibres to

be used with IMC fabrication.

Hybrid Composites

Hybrid Composite materials or HMCs are another type of composite that is still

largely being researched into. HMCs aim to hybridise composite materials in

existence with other materials (there could be other composites or basic unreinforced

materials) as well as composites with multiple forms of reinforcements; in order to

yield multiple structural applications and optimise material properties to higher

standards. Development areas for HMCs can be subdivided into mainly of 5 types:

selective reinforcements, thermal management, smart skins and structures, ultra-

lightweight materials, and hybrid composite materials.

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2.2 Fibre Reinforcement

Fibres are the main constituent on a fibre reinforced composite material, and is

the most common types of reinforcement in use today; they take up the most volume

and bear a larger portion of the loads applied to the material. Selection of fibre type is

important as they affect the following properties of the material (Schwartz, 1997):

Specific gravity

Compressive strength and modulus

Tensile strength and modulus

Fatigue strength and fatigue failure properties

Cost

Thermal and electric conductivity

Glass fibres – This is the most common type of reinforcing PMCs (polymer matrix

composites) in industry. E-glass is available as a continuous filament and is suitable

for most methods of resin impregnation. S-glass was originally developed for aircraft

parts and missile exteriors, it is more expensive the E-glass and of all glass fibres

used this has the highest tensile strength. S-2 glass is a less costly version of S-

glass. Glass fibres are also available in woven form, however this weave that cerates

rubbing within fibres contribute to higher levels of strength degradation as surface

flaws increase under cyclic loading. Tensile strength of glass fibres are also reduced

under water and static loads/fatigue.

Aramid fibres – They are based on rigid, linear, rod-shaped chains of polymers

made of para-linked aromatic amides with prominent anisotropic properties. An

example of is Kevlar 49, it is a highly crystalline aramid fibre that have the lowest

specific gravity and highest tensile strength/weight ratio of current reinforcing fibres.

Kevlar 49 has a density lower than of Glass or Carbon fibres, resulting in high

specific strength values.

Carbon fibres – they have high tensile strength/weight ratios and tensile

modulus/weight ratios, very low coefficients of thermal expansion. Disadvantages of

carbon fibres are their low impact resistance, high electric conductivity and cost,

however in most industrial applications the weight savings negate the cost factor.

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Polyethylene (PE) Fibres – They have the highest strength/weight ratios most

commercially used fibres. Other features include low moisture absorption rates and

high resistance to abrasion. However they have a very low upper temperature limit

(approximately 120oC) where the fibres lose strength, at low temperatures PE fibres

provide high impact resistance.

Boron fibres – They cost more than carbon fibre composites, they possess high

stiffness and strength properties but low densities. In comparison to carbon fibres

Boron fibres also possess high moduli, and are largely used in aerospace

applications or state of the art sporting/medical structural components.

Carbon Nanotubes – According to Nanocyl.com, 2015; these are tube like

structures made of carbon atoms, with diameters ranging from <1nm to 50nm. In

addition to high stiffness and strength properties they also have high thermal an

electrical conductivity. Carbon nanotube composites have a vast range of

applications ranging from conductive plastics and gas storage structures to technical

textiles. Table 2.2 shows a comparison of density, Young’s modulus, strength and

critical strain in various fibre reinforcement types including High speed steel fibres.

Table 2.2 – Mechanical properties of engineering fibres (Nanocyl.com, 2015)

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2.3 Other Reinforcement Types

Other types of notable composite reinforcements (as illustrated in fig 2.3) include

Whiskers, Particulates and Wires.

Fig 2.3 - Composite Reinforcement Types (Advanced Materials by Design, 1988)

Whiskers – These materials are fibrous in nature and are often characterised by

their single-crystal structures that contain nearly no crystalline defects Various

materials such as metals, metal oxides, carbides and halides, as well as some

organic materials can be prepared into whiskers under strictly controlled

environments. Whiskers generally contain a singular dislocation that runs along its

structure at its central axis, in addition the relative yield strength of whiskers are

similar to that of the compound/ element that it is made of (Homeny et al., 1990).

Particulates – Particulates (powders) are the cheapest reinforcement material

available and therefore its production and use is most common, especially since it is

widely used with MMCs. Particulates help produce isotropic properties in MMCs

which is desirable in structural applications (Advanced Materials by Design, 1988).

According to Mazumdar (Mazumdar, 2002); currently the use of particulates have

aided in the fabrication of composites with some of the highest levels of

reinforcement volume fractions for various composites containing ceramic elements

(i.e; oxides, carbides, nitrides)

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Wires – Wires are a common name given to reinforcements that are metallic

filaments with high elastic moduli by definition (Mazumdar, 2002); among the most

common types are tungsten and molybdenum filaments, stainless steel filaments are

is also used in some applications. The major disadvantage of metallic filaments is

that they have higher densities, especially in comparison to ceramic whiskers.

However wires contain better ductile properties that ceramic whiskers and any other

fibres. According to Schwartz (Schwartz, 1997), research and development is being

carried out to use metallic filaments to be used in composites to carry high tensile

loads and possess higher toughness levels.

2.4 Matrices

There are two major types of composite matrices that are in use in industry today,

namely thermoplastic matrices and thermoset resin matrices (Advanced Materials by

Design, 1988). Figure 2.4 displays general features of examples of both classes of

matrices, in terms of their processing characteristics, usage, resistance to solvents

and toughness qualities.

Fig 2.4 – General Characteristics of Thermoset and Thermoplastic Matrices (Advanced

Materials by Design, 1988)

2.4.1 Thermoplastic Matrices

Thermoplastic resins are also known as engineering plastics they contain

some polyesters, polyphenylene sulphide, poly-etherimide, polyamid, polyether-

ether-ketone (or PEEK), and other types of liquid crystal polymers. These

thermoplastic resins contain molecules that are long and discrete, which melts to a

viscous liquid at approximately around 500” to 700” F (260° to 3710 C) processing

temperature. Then the formed viscous liquid is cooled to an amorphous, semi-

crystalline, or crystalline solid. Properties of the final matrix are mostly affected by the

material’s crystallising ability. Thermoplastics processing is a reversible process

unlike the curing process of thermosetting resins. Just by only reheating up to the

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process temperature the resin could be shaped in a different way as desired.

Although Thermoplastics at higher temperature practically are inferior to thermoses,

chemical stability and strength, crack resistance and impact to damage is higher.

However, current research has developed high performing thermoplastics for

example PEEK, which contain a semi-crystalline microstructure which results an

excellent strength and solvent resistance at high temperatures.

Thermoplastics have a great potential in future manufacturing point of view,

simply because it is easy and quick to heat and cool a material than it is to cure it.

Simply because of this reason thermoplastic matrices are very attractive for example,

in automotive industry where high volumes are involved. Presently, thermoplastics

are mainly used with discontinuous fibre reinforcements for example, chopped glass

or carbon/graphite. Potential for high performing thermoplastics reinforced with

continuous fibre is immense. As an example, in next generation fighter aircrafts

thermoplastics could replace instead of epoxies in the composite structure

2.4.2 Thermoset Resin Polymers

Thermosetting resins generally comprise of polyesters, vinylesters, epoxies,

bismaleimides, and polyamides. In fibre reinforced plastics and epoxies

thermosetting polyesters are usually used, this makes up most of the advanced

composite resins that are current in the market today. These types of resin

substances initially have low viscosities but once thermoset resins undergo chemical

reactions or heat treatments, polymer chains in the material will become cross-linked

and bond the whole matrix together as an embedded 3D-netowrk .The

aforementioned process is referred to as ‘Curing’ . As a result of their 3D cross-linked

structures, thermoset resin matrices possess high dimensional stability and a good

resistance to temperatures and solvents. Improvements on the toughness and raising

the max operational temperature levels of thermoset resins are two of the major

focuses on their research and development areas.

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2.5 Fracture Modes

Composite fracture occurs at three modes, depending on three loading

scenarios as shown in Fig 2.5 below. Mode I is an opening or tensile mode, mode II

fractures consist of a sliding or shear effect and mode III fractures occur due to a

tearing or shear loading effect. This report will only be looking at fracture modes of

type I and II. In practical occurrences of material fracture caused by cracking,

especially in metals, mode I type of fracture is predominantly present.

Fig 2.5- Types of crack propagation modes (Prasad et al., 2011)

In addition to polymeric, these fracture modes (by its singularity or as a mixed

mode type of fracture) can also be observed in material that contains glass or

ceramics (brittle materials). Fibre reinforced polymer composites (FRPs) usually

show a mixture of brittle and ductile failure modes. In polymer composites features

observed with crack propagation include interlaminar fracture due to delamination or

matrix cracking and fibre deboning, breakage, or pull-out (Dharan, 1978). The matrix

or polymer resin of FRPs absorbs energy in failing and the fibre reinforcement

undergoes brittle breakage (Silva et al., 2006). It is also possible to visibly see

debonded fibres that have been pulled out from the polymer matrix on the fracture

surfaces of FRPs. If there is a change in direction of the crack propagation within the

material this is a result of deflections caused by fibres as they tilt, twist or move out of

place. As a result of the fibre pull-out effect on the material energy is dissipated along

the material in the form of friction and in some instances heat (eg: when materials are

subjected to dynamic loads), this dissipation of energy slows down crack growth by

reducing the stress concentrations at the crack tip.

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2.7 Tensile Tests

In order to identify mechanical characteristics of a composite material there are a set

of standard tests that must be performed before investigating delamination of the

material. ASTM D3039, ASTM D5083 and BS EN 2747 define the procedures for

these tensile tests. Tables 2.7.1 and 2.7.2 both show material properties that were

obtained experimentally for the unidirectional GFRP prepreg TenCate E722 (epoxy

matrix material). In each of the 2 cases mechanical properties of the GFRP material

were obtained by performing tensile and compression tests according to ASTM

standards.

Values such as Young’s are modulus and shear modulus will be necessary for

subsequent fracture toughness calculations. After volume fraction tests are

performed those values can be compared to the experimental results attributed

below

Property Value

E11 35.1 GPa

E22 11.6 GPa

G12 4.45 GPa

nu12 0.23

nu21 0.05

Tensile strength in fibre direction (Xt) 723 MPa

Tensile Strength normal to fibre direction (Yt) 75.2 MPa

Shear Strength 114 MPa

Volume Fraction 40.4%

Table 2.7.1 - Material Characterisation properties obtained from volume fraction and tensile tests (Bryars, 2010)

Where, E is strain (a vector quantity dependant on direction), G is shear modulus, v

is poisson’s ratio and nu is tensile stress (a vector quantity dependant on direction),

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Table 2.7.2 – Experimentally obtained mechanical properties of GFRP Composite

Laminates (Al-Khudairi et al., 2015)

2.8 Interlaminar Fracture Toughness Testing

2.8.1 Mode I Tests

According ASTM methods and standards, the setup for mode I interlaminar

fracture testing is by using a double cantilever beam (DCB), where an elongated

block of composite material (usually rectangular in shape) with a non-varying

thickness is used as shown in figure 2.8.1 below.

Fig 2.8.1 – DCB sample setup with loading blocks for mode I tests (Prasad et al., 2011)

Where, with regard to figure 2.8.1; a is crack length from line of application in of

force, b is specimen width, h is the thickness of the specimen, and L is total length of

the DCB specimen.

A prerequisite element of a DCB specimen is to contain a crack prior to the

start of the test; this is created by the means of a Teflon insert that is added into the

samples at their manufacturing stage. According to Prasad et al., 2011; there are

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3methods to determine energy release rate (GI), they are the Modified beam theory

(MBT) method, Compliance calibration (CC) method and the Modified compliance

calibration (MCC) method (Prasad et al., 2011)

MBT Method:

(1)

Equation 1 is always an overestimate due to rotation that may occur at delamination

plane; this is corrected by using equation 3, where Δ is obtained using a plot of C1/3

vs. delamination in mm. Compliance (C) is described as δ/P

(2)

CC Method:

(3)

A graph of log (δi/Pi) vs. log (a) is plotted using data gathered and a best fit line is

drawn where n is its gradient.

MCC Method:

⁄

(4)

A graph of a/h vs. C1/3 is plotted (where a/h represents delamination length of

specimen normalised by its thickness), where S1 is the slope if its best fit line

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2.8.2 Mode II Tests

In general composite materials that are designed to contain high performance

characteristics tend to have some of the highest levels of in-plane strength and

stiffness (Prasad et al., 2011). Weakening of the interlaminar regions of composites

can be observed when increasing shear and tensile stresses are applied to it, this in

turn gradually deteriorates the strength between laminate layers. Delamination is

observed as a result of this occurrence, in practice failure that is instigated by

delamination is a result of both compressive and bending stresses, bending stresses

are formed as delaminated layers tend to create a buckling effect on the plane

(Jones et al., 1985).

In accordance with ASTM standards the End-notched flexure (ENF) tests is

one of the methods designed to measure the interlaminar fracture toughness

characterises of a material under mode II fracture (or in-plane shear deformation

mode). According to Brunner (Brunner et al., 2008); materials testing authorities such

as ASTM, JIS and ESIS have put forward several types of ENF test methods in order

to investigate fracture toughness under mode II fracture. However it must be noted

that ENF tests are quite unstable (in comparison to the DCB test methods used for

Mode I testing) and crack propagation is not easily observable. As a result on the

unstable fractures the stability of the test can be evaluated by comparing strain

energy release rate against fracture growth rate . Determination of crack initiation

values is therefore not straightforward, however plotting resistance curves can be

easily managed. According to studies done by O'Brien (O'Brien, 1998); friction is

identified to be a contributing factor to in fracture toughness of a composite material,

this results in the question, whether or not ENF tests provide conclusive material data

(as the ENF tests does not take the effect of friction into account)

18

Fig 2.8.2 - 3ENF sample setup for Mode II tests (Prasad et al., 2011)

Figure 2.8.2 above shows the basic setup of a 3ENF (3 end-notched flexure) test that

is in accordance to testing standards, that can be used for mode II tests. Where, a-

crack length form specimen edge (note: this is the edge of the ENF test area of the

specimen, and not the edge of the material), L is specimen length, 2t is specimen

thickness and F is the force applied on the specimen. According to De Moura, M. and

De Morais, A. (2008), there are several methods of evaluation test data and fracture

toughness properties for mode II tests, namely; the Modified beam theory method

(MBT), Compliance calibration method (CCM), Direct beam theory (DBT), corrected

beam theory with effective crack length (CBTECL) and the Compliance based beam

method (CBBM).

As both Mode II and Mode II test data will be evaluated in this report the MBT and

CCM methods will be followed through with the 3ENF tests, as similar theories are

used in the fracture toughness calculations of Mode II tests (with the exception of the

Modified Compliance Calibration Method for Mode I tests)

19

Modified Beam Theory Method (MBT)

According to Prasad, Venkatesha and Jayaraju (2011) the initial crack length

vs. load deflection graph is obtained from the test in order to find the highest load-

deflection level. The interlaminar fracture toughness for mode II (GII) is obtained quite

straightforwardly from equation 5. Where is the displacement (mm), a is crack

length, L is specimen length, F is the force applied (N) and B is the specimen width

(mm)

⁄ (5)

Compliance Calibration Method (CC)

De Moura, M. and De Morais, A. (2008)’s method for using the CC method

Mode II test evaluation makes use equation 6, below. Where is the longitudinal

modulus (determined by tensile tests), is the transverse shear correction factor, a

is crack length, L is specimen half-length, F is the force applied (N) and h is the

specimen half-width (mm)

(6)

The compliance method suggested by Gwo-Chung Tsai (Tsai, 2006),

suggests in the use of equation 7 (G - strain energy release rate) for the

determination of interlaminar fracture toughness ENF tests.

(7)

Where,

(8)

20

Substituting equation 8 in to equation 7, results in the following simplified

formula (equation 9) for calculating GII.

(9)

2.9 Standard Test Methods for Constituent Content of

Composite Materials

According to ASTM Standards (2007), there are a several test methods defined so

far to determine the constituents of composites. There are two main approaches:

(a) Physically eliminating the matrix, by either chemical digestion or by ignition. The

fibre reinforcement is left undamaged as a result and allows calculation of either

the fibre of matrix content (by weight or volume). Percentage void volume can

also be calculated.

(b) Calculating reinforcement/matrix content (by weight or volume) and cured ply

thickness using measured thickness of the laminate. (This method is only used

for laminates of known fibre areal weight) in this case percentage void volume

cannot be calculated

6 out of the 7 test in this method concern the chemical digestion of the matrix

whereas the other method (Procedure G of ASTM D3171-06) involves an ignition

(burnoff) procedure in a muffled oven. Using the following formula in equation 10,

fibre volume ratio (Vf) of the specimen can be found:

(10)

21

Where,

- (fibre weight/composite weight)

- (matrix volume/composite volume)

, -volume density, fibre density of e-glass

= 1-

= 1- - (assuming that void volumes are zero)

Theoretically fibre percentage of FRP materials can exist up to 90.8%, the highest

reached in practice is about 70%, but typically in accordance with manufacturing

parameters fibre percentages of FRP’s are of 50-65% (Bryars, 2010)

22

2.9.1 Fibre Volume and Weight Ratio Relationship

According to Heslehurst (Heslehurst, 2015); when manufacturing composite

structures prepreg materials, correct ratios of fibre to resin weights must be used.

The fibre- volume ratios play a crucial role in the laminar properties of a composite

material, Burn-of tests are usually performed on composite materials that contain

reinforcements such as glass or ceramic (these must not be affected by high

temperatures) or carbon where temperature can be controlled such that the carbon

fibres do not incinerate)

Void volume can affect the composite’s mechanical behaviour; higher void volume

indicates lower fatigue resistance and greater likelihood of moisture degradation and

weathering. It also leads to scattered or some irregular strength properties. Void

volume is an indicator of the quality of a composite. However

Table 2.9.2 - Typical GFRP fibre volume fractions (Heslehurst, 2015)

Table 2.9.2 gives typical values of experimentally obtained fibre volume fractions in

polymer composites. Ranges are stated in here because; specific values for fibre

volume fractions in composites will depend upon the processes used to manufacture

the samples.

23

3. METHODOLOGY

3.1 Materials and Equipment

3.1.1 Zwick-Roell ProLine Universal Testing Machine

Other hardware and software related to tensile testing were also used such as the

DynoCam (to digitally record the crack propagation videos) and the Travelling

Microscope. Testing machine speed is independent of the test load; therefore the

speed of the test can be lowered so that crack propagation is seen clearly.

Fig 3.1.1 – Universal Testing Machine

2.1.2 HPC LaserCutter

Fig 3.1.2 – HPC Laser engraver/cutter

In order to better view the crack propagation along the test specimen the machine in

Figure 3.1.2, is used to create an etched scale (ranging from 0.5-1mm divisions)

along each DCB and ENF Sample.

24

3.1.3 Prepreg Material

The type of composite material used is TenCate E722-02; it is a mid-temperature

curing modified epoxy component prepreg (Technical Data TenCate E722, 2013).

Prepreg is short for pre-impregnated denotes that the material contains glass fibres

in a toughened epoxy resin system.

Beneficial features of E722 include: easy drapeability into complex shapes, good

surface finish, medium tack level to easily laminate into mold surfaces, low volatile

content, has a 60 day shelf life at ambient temperature, autoclave vacuum bag or

press curable. Fig 3.1.3 shows the typical material properties for E722 cured at

120oC for 1 hour

Fig 3.1.3 – Properties of the E722 Laminate (Technical Data TenCate E722, 2013).

25

3.1.4 Specimens

Specimen were manufactured to ASTM standards using the GFRP prepreg material

TenCate E722-02, the dimensions used for the specimen are show in figure 3.1.4, a

PTFE layer was inserted at the halfway point of each layup as specified in figure

3.1.4 to create the initial crack.

Fig 3.1.4 - Layup Dimensions

There were 3 types of layups made to be used with each DCB and 3ENF tests:

Two sets of each of the tree layup types are to manufactured as one set would be

used for Mode I testing and the other for Mode II tests. Further detail of the

specifications of these layups can be seen in figure 3.1.5

26

Fig 3.1.5 – Layup Specifications (fibre angles of layers)

The ‘//’ notation for the DCB and the ENF layups represent the position in the layup

where the artificial crack will be created at the manufacture stage (point where the

Teflon insert or PTFE Layer is placed). All end samples for DCB and ENF testing will

have a total length of a 140mm and a width of 20mm, the thickness of each of the

specimen will vary depending on the layers used, and the quality of manufacture.

Once the specimens have been finished, thickness will be measured in several

places (about 3) in order to determine an average thickness value for each

specimen.

Fig 3.1.6 – DCB test specimen

Fig 3.1.7 – 3ENF test specimen

Figure 3.1.6 above illustrates a completed specimen for DCB testing and figure 3.1.7

shows a specimen for ENF testing, where b is the specimen’s average thickness

(mm), h is specimen width (mm), F is Force applied (N), L is sample length for DCB

tests (mm) and a is the initial crack length (mm) sample. These are be the

measurements used in calculations and recording of data in this report.

27

3.2 Manufacture

3.2.1 Laminate Preparation and Curing

Fig 3.2.1- Cutting layers from Prepreg

Fig 3.2.2 – Layups with Teflon insert

Fig 3.2.3 – Vacuum bagging

Fig 3.2.4 – Curing procedure

Initially the prepreg was cut into 140x140mm sections, using an initial template

for each layer pertaining to the required fibre angle in 90, 0 and 45 degree fibre

angles (see fig 3.2.1). A total of 180 layers or ply was cut out of the prepreg material;

cut-outs that were not used immediately were refrigerated and stored for later use (to

temperatures specified by prepreg manufacturer).

The bottom half of the layup is positioned on an aluminium plate covered with

protective PTFE layer (see fig 3.2.2). An additional 55mm PTFE (thin) section is

added to create the initial crack at manufacture. Layups must be positioned such that

adequate space is left between them to allow resin leakage at the edges, this resin

leak can be minimised by using heat-proof tape to seal off the layup boundaries prior

to curing.

28

Finished layup topped with a second aluminium plate (with protective PTFE

over), this step is needed to maintain the cured sample is smooth and to minimise

trapped air within the layup. The layups (between AL sheets) are then enveloped in a

breather material, nylon sheet, thick vacuum bag respectively, and then sealed with

tape (airtight seal). A nozzle is left on the top centre to ensure there is equal pressure

applied on the setup due to the vacuum suction (See fig 3.2.3 for vacuum bagging

assembly)

The air extraction unit in the oven ensures that a continuous vacuum

environment is maintained throughout the entire curing process. The Cure

temperature used is 130 oC for 1 hour (see fig 3.2.4). Then the cured samples were

left to cool for 10-15 minutes before removing from vacuum bag setup.

3.2.2 Problems at Curing Stage

One of the problems encountered at the curing stage was the loss of pressure

midway through curing process. Excessive resin leak/runoff was observed (see fig

3.2.5 leading to very dry samples. These samples were unusable as they were of

poor quality. Fibres were clearly visible on the top surface of the cured samples as

not enough polymer matrix material was present at the top (see fig 3.2.6).

Fig 3.2.5 – Excess resin leakage

29

Fig 3.2.6 – Batch (5) and (6)

Subsequently batch (5) and (6) laminates had to be discarded. At this point

the GFRP material stored in the lab had run out, therefore extra specimen from

earlier batches (made for DCB tests) were used for ENF tests with the

and layup specifications.

Precautionary Measures

In the future the following measures can be followed in order to prevent or minimise

problems faced with premature loss of vacuum.

Checking the seal of the bag in the oven about 5 minutes into vacuum curing

process for loss of pressure, if the hissing sound of escaping air is heard, the

process can be stopped and a new vacuum bag setup can be applied.

A wider heat proof tape can be used to seal the lay-up edges, before curing, in

order to better contain the seeping resin.

Layups must be set in the middle of the Al trays, and the trays used must have

similar surface dimensions to that of the layup surface. This ensures that the

pressure applied on top of the assembly creates equal forces on all parts of

the layup. This measure would prevent unevenness in the surface of the layup

and yield more consistent thickness for each individual test specimen.

Another precautionary measure is, allowing enough tolerances for cutting, to make

up for material loss due the band saw blade.

30

3.2.3 Finishing

The cured laminates were subsequently marked out with the dimensions for

each test specimen as detail in earlier sections. The cutting tool used is the band

saw, with the thinnest possible blade in order to minimise loss of material. The

drawback to using a very fine blade is that it will have to be replaced partway thought

the cutting process. As the blade wears out and friction between it and the material

increases, this may have a significant impact on the initial crack (to propagate). On

the other hand a very course blade might have a similar adverse effect on the initial

crack an over al quality of the sample. Therefore a compromise must be made on

blade size to minimise any damages that may result at manufacture. Alternatively the

vertical mill can be used with a narrow saw of a thickness of approximately 1.5mm.

The setup of the equipment takes longer, however there is minimal material wastage,

and good finish, straighter cut, cleaner cut means less delamination growth while

manufacture. The cut edges were then deburred in the belt sander for a finishing

effect.

Loading Blocks

Fig 3.2.7 – Loading block preparation

Fig 3.2.8 – Loading block dimensions

Fig 3.2.9 – Applying epoxy adhesive

Fig 3.2.10- Finished DCB Samples

31

For DCB tests it was necessary to manufacture loading blocks with the

dimensions specified in figure 3.2.8. An Aluminium rod with a cross section of

10x10mm was selected and 24 mm sections were cut from it. The additional 4mm is

kept to account for loss of length when deburring to even out the surfaces. The

rotating 4-jaw chuck is used to create the 5mm diameter hole in the loading blocks

(see fig 3.2.7)

All samples cleaned with acetone. DP460 epoxy adhesive is used to attach

the loading blocks to the DCB samples (fig 3.2.9) the surfaces to be bonded are

lightly abraded with sandpaper and coated with a thin film of adhesive. Once the

loading blocks were clamped into place on each specimen (fig 3.2.9), they can be

oven cured at 50oC for 30 minutes as per Scotch Weld Specifications, for adhesive

curing there is no need to use a vacuumed environment.

Laser Etching

Fig 3.2.11 – Laser etching

Fig 3.2.12 - Finished DCB Specimen

Fig 3.2.13 – Finished 3ENF Specimen

32

In previous fracture mode tests performed with DCB and ENF samples, the crack

propagation was observed by painting the side of the samples with a white correction

fluid and pencil-marking 1mm divisions by hand. Or by making several pencil marks

at certain predetermined points such that the data can be collected once the crack

reaches it. But due to crack jumps, especially evident with ENF tests, this method

does not allow for much accuracy. Therefore a scale was created in SolidWorks to be

etched on the sides of the specimens using the HPC LazerCutter machine (see

figure 3.2.11).

Specimen sides were painted with correction fluid and sanded down by hand to

further smoothen the finish, in order to better observe crack propagation while testing

and to etch the a measurement scale on (as the LazerCutter cannot etch on the

GFRP material itself. Scale added using a laser cut (laser does not cut into GFRP

thereby creating an etching effect)

Figures 3.2.12 and 3.2.13 show DCB and ENF specimen respectively with the

scale added to them. The 1st 10mm of the scale (on the cracked end) was given

0.5mm divisions (to give a finer reading of the position of the initial crack), then

follows on with 1mm divisions. Scales start approximately 50mm as accurately as

possible from the edge of the cracked end of the specimen, but a0 distances were re-

measured before testing (as crack lengths are measured from the point of application

of loads and not the edge of the material)

33

4 EXPERIMENTATION

4.1 Specimen Data

Table 4.1.1 – Specimen data for DCB samples in Mode I testing

There were 9 DCB specimens to be tested, 3 specimens for each of the 3

types of layups. Table 4.1.1 shows the names given to each specimen and their

corresponding layup type. h refers to the width of each specimen and b corresponds

to thickness of specimen. 3 values of b and h are measured for each specimen using

the Vernier callipers and averaged out to be used in subsequent calculations. The

crack tips are measured from the point of application of load (or the centre of the hole

in the loading block). ao refers to the initially observed crack top point from point of

application of load, and a1 refers to the corrected crack top point, this is because

once the DCB samples were loaded into the test setup, previously hidden sections if

the initial crack (beneath the white paint layer) were more visible.

A Significantly low maximum load is observed with DCB3-2, in comparison to

the other 2 samples of its type of fibre layup. It is also observed that Maximum loads

for the DCB2 type of layup have greatly varying Maximum load values.

34

Table 4.1.2– Specimen data for 3ENF samples in Mode II testing

Similarly table 4.1.2 above displays these corresponding details for all the

ENF test specimens. However in this case L refers to the 50mm half-length of the

specimen (a 100mm wide setup is used for the ENF setup). Length of the crack tip

must also be corrected, a refers to the length of the crack tip from the edge of the

material of each specimen. Once a specimen was loaded into the test setup,

corrected values for the actual crack tip length (a0) could be found. The start point of

the a0 is the edge of the 100mm mark of the 2L specimen length (see fig 3.1.7).

A notably low maximum load are observed with ENF1-3 specimen, almost all

of the other maximum load data recorded do not vary too much from each other

(within their respective Layup types)

35

4.2 Mode I Tensile Testing

Double Cantilever beam (DCB) tests were carried out in accordance to ASTM

standards as specified in section D5528-01, as detailed in section 2.8.1 of this report.

9 specimens were tested. Figure 4.2.1 below shows the test rig setup that the DCB

tests were carried out in. In addition to setting up the jaws of the Universal testing

machine to accommodate the DCB test, additional equipment such as the travelling

microscope and data recording devices and software (camera, laptop, and screen

recorder) are also set up. All tests are carried out in standard room temperature; this

was not a factor that was measured as the effect of temperature effects on crack

propagation were not taken into account.

Fig 4.2.1 – Tensile Test Rig setup for DCB samples

Fibre pull-out is not a strong phenomenon observed, only minor visible fibre-

matrix de-bonding seen in the layup, crack propagation is very clear and

the crack tip growth was easy to follow as seen in figure 4.2.2 below. A greater level

of fibre pull out, ply separation and some crack branching were observed with the

remaining 2 layup types.

36

Fig 4.2.2 – Nature of Mode I crack propagation.

The TestXpertII records the Force (N) and the Cross head Displacement (mm)

values throughout the progress of the test. In order to perform fracture toughness

calculations it is necessary to record the corresponding force and displacement

values at each observable crack tip position. To avoid a very large error margin

caused by human error all data is recorded, as crack progression may be hard to

follow, and this may pose a risk where count of each mark may be lost.

The TestXpertII test progression video is recorded using a screen record tool,

in the same instant the test start button is pressed, the video stream recording of the

crack propagation is started on. (Using the DinoCam camera, connected to a laptop

with its recording software, the camera is mounted on to the travelling microscope).

Since the 2 videos for each test can be played back together in synch, the rest can

be reviewed and readings can be taken more than once in the event that a crack

jump occurs.

As the crack progresses the markings on the scale can be counted along.

Since the start position of the crack is already measured, crack length at each

reading is known. The speed that the tests were carried out is at 2mm/min.

37

Load vs. Displacement Graphs for DCB Tests

Figure 4.2.3 shows the results of tensile tests on the DCB specimen (Load vs.

Displacement graphs) of the layup type, Figure 4.2.4 shows the same

data for the layup type and Figure 4.2.5 shows that of the

layup type. The transition from a linear region to non-linear

denotes start point of crack tip propagation.

Fig 4.2.3 – Load vs. displacement for DCB4 Specimen

38

Fig 4.2.4 - Load vs. displacement for DCB2 Specimen

Fig 4.2.5 - Load vs. displacement for DCB3 Specimen

Higher levels of fluctuations in force data after the point of non-linearity has been

surpassed, is a characteristic that points to unstable crack growth in the sample. The

graphs for DCB4 layups have a steeper drop in force readings after the point of non-

linearity has been passed (in comparison to DCB2 and DCB3 layups) this may

suggest that the DCB4 samples show more brittle properties in comparison to

samples of the other 2 layups.

39

4.3 Mode II Tensile Testing

Fig 4.3.1 – Tensile Test Rig setup for 3ENF samples

10 ENF samples were tested, Figure 4.3.1 shows typical unstable crack propagation

in 3ENF samples, numerous crack jumps are observed, it is difficult to keep track of

crack tip on the travelling microscope. Recordings taken with time stamps ensure that

test progression can be revisited once complete, in order to gather information. The

ENF tests performed are position controlled experiments, where a continuous quasi

static downward force is applied at a constant speed of 2mm/min. Originally the rest

was stopped once it reached to a max displacement of 6mm, however it was found

better to leave the end conditions to a max Force of +50kN, then stop the rest once

enough data has been recorded. A travelling microscope was used to observe crack

top propagation on both cases

40

Fig 4.3.2 – Nature of Mode II crack propagation (Left: initial stage, right: crack

progression)

Figure 4.3.2 shows the nature of crack propagation in Mode II ENF tests there are

numerous crack jumps that occur in the direction of the main crack propagation.

Taking recordings manually in this case using the travelling microscope while the test

is running is especially difficult. To overcome this, video recordings were made

similar to the DCB tests. When the recordings are played in synch, they can be

paused once a crack jump occurs to avoid missing the crack tip position.

41

Load vs. Displacement Graphs for ENF Tests

Figure 4.3.3 shows the results of Mode II tests on the ENF specimens (Load vs.

Displacement graphs) of the layup type, Figure 4.3.4 shows the same

data for the layup type and Figure 4.3.5 shows that of the

layup type. Similarly, to the DCB data, the transition from a

linear region to non-linear denotes start point of crack tip propagation.

Fig 4.3.3 - Load vs. displacement for ENF1 Specimen

42


43


Delamination initiation points are notoriously difficult to ascertain by initial visual

inspection of a sample while DCB or ENF test are being carried out. The only clear

method to determine the start delamination is by observation of the points where the

region of nonlinearity ends in Load vs displacement graphs.

44

4.4 Volume Fraction Tests

Procedure

In order to carry out volume fraction tests in accordance to standards set by

ASTM D3171, sections of composite material used were cut out, 3 samples (A, B and

C) were taken for the purpose of this test. A length of 10mm was cut off from the end

of a test specimen. Each sample is then thoroughly cleaned off any foreign material

that isn’t the GFRP (i.e the correction fluid) using acetone. Using a micrometer, the

dimensions of each sample was measured, averaged and recorded; as seen in Table

4.4 below. This data is necessary to determine an approximate average volume for

each sample.

Each sample was then placed in a ceramic crucible. The weights of each of

the crucibles and the collective crucible + sample weights was measured and

recorded. The difference in weight determines sample weight and thereby allows the

composites density to be calculated. In order for the fibre to be weighed, the matrix

must be separated from the sample. The matrix-burnoff method is used to achieve

this. The samples are placed carefully into an oven that was preheated to a

temperature of 500oC using crucible tongs, and left to be burnt off (heated) for 5

hours. Once the oven was turned off, the samples were taken out and left to cool off

at room temperature for about 5-10 minutes. These crucible + burnt off samples were

then weighed, the change in weight accounts for the weight of the epoxy matrix that

has been lost, the new weight subtracted by the corresponding crucible weight yields

the fibre weight. It is already known that E-Glass fibre has a density of 2.55 g/cm3,

therefore the fibre volume can now be calculated. By using the methodology detailed

in section 2.9 of this report, calculations were made to determine the overall fibre

volume fraction as a percentage (see Table 4.4 below for all data)

45

Specimen

A B C

Length (cm)

L1 0.97 0.89 1.09

L2 1.02 0.91 0.94

L3 0.93 0.94 0.95

Width (cm)

w1 2.14 2.17 2.16

w2 2.11 2.18 2.12

w3 2.03 2.16 2.13

Thickness (cm)

b1 0.63 0.67 0.66

b2 0.65 0.61 0.63

b3 0.61 0.59 0.65

Average Values

Lavg 0.97 0.99 0.99

wavg 2.09 2.17 2.14

bavg 0.63 0.62 0.65

Average Volume (cm3) V 1.29 1.34 1.37

Crucible weight (g) 30.0181 30.0243 30.1021

Start weight (g) 31.8634 31.9662 31.9375

Sample weight (g) W 1.8453 1.9419 1.8354

End weight (g) 30.9173 30.9257 31.0752

Fibre weight (g) Wfibre 0.8992 0.9014 0.9731

Fibre weight ratio (%) Wf 48.7292 46.4185 53.0184

Resin weight (g) Wresin 0.9461 1.0405 0.8623

TenCate E722 resin density (g/cm3) ρr 1.21 1.21 1.21

Fibre (E-Glass) Density (g/cm3) ρf 2.55 2.55 2.55

Fibre Volume (cm3) Vfibre 0.35 0.35 0.38

fibre/resin wt ratio 0.95 0.87 1.13

fibre/resin density ratio 2.11 2.11 2.11

fibre/resin Volume ratio 2.00 1.83 2.38

matrix volume ratio Vm 0.33 0.35 0.30

Fibre Volume ratio Vf 0.67 0.65 0.70

Fibre Volume fraction (%) 66.70 64.61 70.40

Average Fibre Volume Fraction (%) 67.24

Table 4.4 - Weight Fraction Calculations

The calculated overall fibre volume fraction was found to be 67.24% this is very

significantly higher than the volume fraction of 40.4% recorded by Bryars (2010),

however it is not too far off from the volume fraction of 61±2% recorded by Al-

Khudairi et al., (2015). According to Heslehurst (2015), table 2.9.2 values show that

the possible range of GFRP fibre volume fractions can lie between the ranges of 50-

70%. Therefore the calculated overall fibre volume fraction of 67.24% found satisfies

requirements of literature review.

46

5 ANALYSIS

5.1 Mode I Fracture Toughness Analysis

DCB4 Samples

Fig 5.1.1 – Mode I GIC calculations for DCB4-1

Fig 5.1.2 - Mode I GIC calculations for DCB4-3


47

Fig 5.1.4 – Comparing Mode I; MBT, CC, and MCC graphs for all DCB4 Specimen

48

DCB2 Samples




49

Fig 5.1.8 - Comparing Mode I; MBT, CC, and MCC graphs for all DCB2 Specimen

50

DCB3 Samples




51

Fig 5.1.12 - Comparing Mode I; MBT, CC, and MCC graphs for all DCB3 Specimen

Fracture toughness (GIC ) values for MBT, CC and MCC methods for most layup

types were very closely plotted with very less variations. However large deviations of

GIC values are seen in in fig 5.1.2 (DCB4-3) and some deviations are observed in fig

5.1.9 (DCB3-1). By comparison of GIC data within a single layup type it is apparent

that the DCB4 and DCB3 layup types showed higher

levels of variation between samples. Refer to Appendix A for a complete listing of

recorded data and Mode I fracture toughness calculations.

52

5.2 Mode II Fracture Toughness Analysis

ENF1 Samples

Fig 5.2.1 - Mode II GIIC calculations by MBT method for ENF1 specimens

Fig 5.2.2 - Mode II GIIC calculations by CC method for ENF1 specimens

53

Fig 5.2.3 – Comparison of CC and MBT method for ENF1 specimens

ENF2 Samples


54


Fig 5.2.6 - Comparison of CC and MBT method for ENF2 specimens

55

ENF3 Samples



56

Fig 5.2.9 - Comparison of CC and MBT method for ENF3 specimens

In mode II fracture toughness results, it is observed that there is a greater trend for

GIIC values obtained to be greater that those obtained with the CC method as evident

in CC and MBT comparison graphs in figures 5.2.3, 5.2.6 and 5.2.9 above. Some of

the specimen data in these CC and MBT comparison graphs have been omitted (in

figures 5.2.3 and 5.2.9) as insufficient data was recorded in some cases. This is due

to the quick progression of unstable delamination propagation in ENF tests, even

when the speed of tests was halved to 1mm/min, it posed a challenge to keep up

with the crack tip on the travelling microscope.

Refer to Appendix B for a complete listing of recorded data and Mode II fracture

toughness calculations.

57

4.2 Discussion

Table 4.2.1 – DCB non linearity points

(NL) and NL + 10%

Table 4.2.2 – ENF non linearity points (NL)

and NL + 10%

Using the load and displacement values that were gathered from the graphs in

figures 4.2.3 – 4.2.5 for DCB Tests and figures 4.3.3 – 4.3.5 for ENF test, values for

the initiation of the points of non-linearity (NL in mm) for each specimen can be

found, as well as the corresponding, in addition values for a 10% increase of the

point of non-linearity is also found (NL+10% in mm). Tables 4.2.1 and 4.2.2 show the

corresponding crosshead displacement values for each tested specimen at their

respective NL and NL+10% points.

With the fracture toughness data that were calculated (refer to Appendix A for

all Mode I, and appendix B for all Mode II fracture toughness calculations) as seen in

the graphs of section 5 (Analysis) of this report, Corresponding fracture toughness

(GIC and GIIC) values are obtained for Mode I and II tests, in order to characterise

their interlaminar fracture properties.

Mode I fracture toughness (GIC) data make use of 3 methods of calculating

GIC, interlaminar fracture properties for all 3 types of layups are shown in table 4.2.3

below. Fracture toughness (GIC) values are obtained in table 4.2.3 for Modified beam

theory (MBT) method, Compliance calibration (CC) method and the Modified

compliance calibration (MCC).

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Mode I Interlaminar Fracture Properties

Laminate Layup

Specimen

Point of non-linearity GIC (J/m2)

Point of non-linearity +10% GIC (J/m2)

MBT CC MCC MBT CC MCC

DCB4-1 555.04 562.12 518.08 581.81 587.27 547.33

DCB4-3 571.17 680.12 300.53 687.54 811.31 361.67

DCB4-4 799.63 808.35 804.12 903.37 909.67 908.97

DCB2-1 149.11 154.48 149.11 207.86 214.57 207.10

DCB2-2 374.72 384.97 363.20 546.89 559.43 531.61

DCB2-3 480.27 498.86 484.24 560.84 580.18 565.70

DCB3-1 517.70 475.6 447.87 626.99 570.61 545.80

DCB3-2 774.62 773.40 787.38 822.28 820.97 836.53

DCB3-3 430.97 448.48 435.95 591.98 614.39 597.13

Table 4.2.3 - Mode I Interlaminar Fracture Properties

Mode II Interlaminar Fracture Properties

Laminate Layup

Specimen Point of non-linearity

GIIC (J/m2) Point of non-linearity +10%

GIIC (J/m2)

MBT CC MBT CC

ENF1-1 3015.85 879.15 2749.67 1083.41

ENF1-4 2725.91 1191.96 2700.28 1313.11

ENF1-5 2811.08 950.87 3074.17 1162.99

ENF1-6 2109.90 926.21 3069.38 1496.76

ENF2-1 2792.88 914.45 2934.51 1036.45

ENF2-2 2413.20 810.48 2971.61 943.35

ENF2-3 2740.33 755.63 3361.76 985.61

ENF3-1 2288.39 405.85 3498.48 750.58

ENF3-2 2400.25 509.70 3013.01 730.28

ENF3-3 3010.08 627.01 3840.86 899.93

Table 4.2.4 - Mode II Interlaminar Fracture Properties

Similarly interlaminar fracture properties for all samples of Mode II tests are shown in

table 4.2.4. Two methods of calculation are used in the case of mode II GIIC values

are Modified beam theory (MBT) method, Compliance calibration (CC) method.

From table 4.2.4, it is noted that there is a very large difference in GIIC values for the

MBT method of data reduction, however the MBT values of GIC in mode I tests are do

not show a very significant variation to the CC or the MCC reduction methods used.

59

According to information found by reviewing literature on the development of

interlaminar fracture toughness testing methods of materials, there is a need new

developments and testing techniques to be introduced. At present, although there are

standardised methods for Mode I tests in unidirectional fibres, there are difficulties in

testing multi-directional composites. Multidirectional composites under opening loads

result in very high amounts of crack branching, most often the crack plane is moves

away from its original position, according to Prasad et al (2011) ASTM do not

recognise multidirectional composites to be used with their standard Mode I DCB

test methods.

In the case of Mode II testing standards, they have been set in joint co-

operation with such authorities as JIS, ASTM and ESIS. Although ASTM standards

are widely used (and also used in this report for mode II tests), according to

Hodgkinson (2000) the ASTM standards (they are national standards) are not in

concession with other international authorities..

Much less research has been put forward for mode III tests (one of the

reasons why mode II tests are not covered in the scope if this report, in addition to

time limitations).

Other issues that need to addressed is the effect of friction and thermal effects

in the materials being tested, as well as the data reduction method. Each reduction

method will produce a different result, as seen in Table 4.2.3 although there are

smaller variations with some samples; others show a very high variation. In fracture

toughness values observed in Table 4.2.4 the 2 data reduction methods have a very

large variation. In order for meaningful fracture toughness testing methods to evolve,

more research is necessary.

60

6 CONCLUSIONS

Composite manufacturing methods play a key role in its mechanical properties as

well as tensile testing machine setup and recording times. Initially measured visible

crack is never the actual crack tip (it can be found by following crack in the first few

seconds) especially 3ENF 3-2 even with recording true start point of crack is difficult

to determine More Mode II testing should be done with multi directional laminates.

Crack branching that was somewhat similar to ENF tests was observed even in Mode

I especially when ply between the crack propagation planes contained opposing fibre

angles.

An area of concern is the fracture toughness (GIIC) results obtained using the MBT

method for Mode II ENF tests; they were very high in comparison to the GIIC values

obtained using the CC method, this could be due to incorrect readings (example: if

the videos recorded were not synched properly), values that were not corrected

properly or a calculation error. In future repetitions of the work carried out in this

project it is necessary to take care with data recording and calculations.

61

Contributions

Modifications and improvements made to past MSc research in this subject area

(Fracture Mode Testing) previously conducted at Kingston University at Postgraduate

Level:

(a) Laser cut precision scale

(b) Using other different fibre angle layers between the Teflon (PTFE) induced

crack plane to observe its effect on crack propagation

(c) Updated Data recording method

Future Research

(a) A test rig for Mixed mode testing can be manufactured and tested

(b) It is seen that less experimental analysis is done for mode 3 failure, so further

research can be done to find better standardised methods for Mode II fracture

testing.

(c) Effect of temperature and moisture of fibre matrix structure can be considered

(d) Modifications to manufacturing techniques can be made (especially the DCB

rig, to lessen effects of rotation)

(e) Modifications to the Mode II tests setup can be made (in accordance to ASTM

Standards) as there tends to be a slight sliding motion along the bottom jaw of

the tensile testing machine. Although length of the sample is considered to be

a uniform 100mm in each case, this slip may affect the initial crack length

values recoded.

62

7 REFERENCES

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8 APPENDICES

Appendix A – Mode I Fracture Toughness Calculations

For Double Cantilever Beam Data – 3 specimens from layup [015//015]

Table A1 – Mode I calculations for Specimen 4-1

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67


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For Double Cantilever Beam Data – 3 specimens from layup


69



70

For Double Cantilever Beam Data – 3 specimens from layup



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Table A10 – Constants used in DCB Calculations

Constants DCB4 - Specimen DCB2 - Specimen DCB3 - Specimen

1 3 4 1 2 3 1 2 3

Delamination

length

Correction

Parameter - Δ

12.612 16.72 16.8679 13.067 7.699 16.96 13.14 0.0091 8.57

n 2.4715 2.3356 2.7031 2.502 2.704 2.391 2.24 2.99 2.68

A1 28.195 29.403 25.833 24.46 21.56 25.91 29.71 20.14 23.21

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Appendix B – Mode II Fracture Toughness Calculations

For 3End-Notch Flexure Data – 4 specimens from layup [015//015]

Specimen 1-3

Specimen 1-4

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Specimen 1-5

Specimen 1-6

Investigating Mode I and II Crack Propagation in GFRP Composite Laminates

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