the effect of mode-mixity and constraint in adhesively bonded composite joints

The Effect of Mode-Mixity and Constraint in

Adhesively Bonded Composite Joints

Final Year Project Report

Neville Lawless

06523587

UCD SCHOOL OF ELECTRICAL,

ELECTRONIC AND MECHANICAL ENGINEERING

BE Mechanical Engineering

University College Dublin

22 Apr 2010

Head of School: Dr D. Fitzpatrick Project Supervisor: Dr. Neal Murphy

ii

Abstract

In this investigation the effects of mode mixity and constraint in adhesively bonded composites

laminates were tested using mode I double cantilever beam (DCB), mode II end-loaded split

(ELS) and mixed mode I/II asymmetric double cantilever beam testing methods. The test

specimens were manufactured in-house in UCD labs from Hexcel Hexply 8552 composite

material with a Henkel Hysol ® EA 9895 ™ peel ply and bonded using a two part experimental

Epoxy adhesive, Hysol EA9830.05.

In attempts to attain cohesive failure, to characterize the adhesive properties, differing surface

preparation techniques were tested. These being the use of a peel ply, abrasive blasting and

plasma treatment. An investigation was also carried out on the effects obtained from the use of a

scrim cloth used for bondline thickness control.

It was shown conclusively that scrim cloths give an average increase of 94% on the value of

fracture toughness GIC. This was only displayed in DCB tests and cannot be proved for other

mode mixities.

Also shown was that although certain levels of grit blasting increase fracture toughness, this

cannot be repeated to any acceptable standard.

Finally for mode I DCB specimens it has been demonstrated that the use of a plasma treatment

in nearly all cases resulted in a minimum doubling effect on the mean GIC values.

All these results were compared with a control batch prepared using only a peel ply. From this it

is shown that the incorporation of the peel ply sets a high level of repeatability for all tests

carried out, however, along with this it sets a lower bound on toughness levels attained.

Following this, the investigation into differing mode mixities provides a good insight into the

effect each loading mode has on fracture toughness.

An increase of phase angles from 18o–40

o–63

o achieved by varying adherent thicknesses resulted

in a significant decrease in mean toughness values for each batch.

Finally, data was generated from mode I, mode II, and mode I/II from 3 similarly prepared

batches and the dramatic rise in fracture toughness mode II demonstrates over the others was

displayed. The effects of the contribution of each loading mode are made apparent for mixed

mode loading conditions with the use of a failure envelope being constructed.

iii

Table of Contents

Abstract ...................................................................................................................................... ii

Project Specification ................................................................................................................ vii

1. Introduction ........................................................................................................................ 1

2. Modes of Failure ................................................................................................................. 3

2.1 Mode I ......................................................................................................................... 3

2.2 Mode II ........................................................................................................................ 7

2.3 Mixed Mode (I/II) ...................................................................................................... 10

3. The effect of constraint on fracture toughness ............................................................... 14

4. Failure of Adhesive joints ................................................................................................ 15

4.1 Cohesive fracture ....................................................................................................... 15

4.2 Interfacial fracture...................................................................................................... 15

4.3 Other types of fracture ............................................................................................... 16

5. Specimen Manufacture .................................................................................................... 18

5.1 Materials .................................................................................................................... 18

5.2 Composite Specimen production ............................................................................... 19

5.2.1 Pre-Preg Composite Layup: ................................................................................... 19

5.2.2 Press-clave build-up and de-bulking: ..................................................................... 19

5.2.3 Peel-Ply addition: .................................................................................................. 21

5.2.4 Press Clave build-up: ............................................................................................. 21

5.2.5 Curing: ................................................................................................................... 22

5.2.6 Machining .............................................................................................................. 25

5.3 Surface Treatments .................................................................................................... 27

5.3.1 Peel Ply .................................................................................................................. 27

5.3.2 Abrasive Blasting .................................................................................................. 28

5.3.3 Plasma Treatment .................................................................................................. 29

5.3.4 Scrim Cloth............................................................................................................ 29

5.4 Preparation of DCB specimens .................................................................................. 30

5.4.1 Bonding specimens: ............................................................................................... 30

5.4.2 Finishing: ............................................................................................................... 33

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6. ELS test rig manufacture ................................................................................................. 35

6.1 CAD Drawings .......................................................................................................... 35

6.2 Rig manufacture ......................................................................................................... 37

7. Experimental procedures ................................................................................................. 40

7.1 Test methodology ...................................................................................................... 40

7.2 Testing approach ........................................................................................................ 44

7.2.1 Mode I DCB tests. ................................................................................................. 44

7.2.2 Mixed mode tests. .................................................................................................. 46

8. Beam theory analysis ........................................................................................................ 47

8.1 Mode I DCB tests ...................................................................................................... 49

8.2 Mode II ELS test ........................................................................................................ 49

8.3 Mixed mode ADCB test ............................................................................................. 50

8.4 Correction factors ...................................................................................................... 50

9. Results & Discussion ........................................................................................................ 52

9.1 Load–displacement behaviour ................................................................................... 52

9.2 Crack initiation values of G IC .................................................................................... 53

9.3 Mean crack propagation values of G IC ...................................................................... 54

9.3.1 Mode I Results. ...................................................................................................... 55

9.3.2 Mode-mixity results ............................................................................................... 65

10. Conclusion ......................................................................................................................... 70

11. Appendix ........................................................................................................................... 73

11.1 Mode I GIC Initiation values ....................................................................................... 73

11.2 Mode I GIC Mean propagation values ........................................................................ 76

11.3 Mixed mode GIC mean propagation values. ............................................................... 77

12. Acknowledgments ............................................................................................................. 81

13. References ......................................................................................................................... 82

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Table of figures

Figure 1: Schematic of the double cantilever beam specimen used for mode I testing,

load blocks can be used for the load introduction [] .......................................................... 4

Figure 2: DCB experimental setup ..................................................................................... 5

Figure 3: Indication of mode II (In-plane) fracture. ........................................................... 7

Figure 4: Schematic diagram of various mode II delamination test methods [12]. ........... 8

Figure 5: The mode II ELS delamination specimen held in the clamping fixture. ............ 9

Figure 6: Schematic of MMB test configuration. [20] ..................................................... 10

Figure 7: Illustration of the mixed-mode end loaded split test specimen: (a) for short

crack lengths and (b) for long crack lengths [25]. ........................................................... 12

Figure 8: Alternative MMELS test rigs with sliding clamped end and fixed load point

[25]. .................................................................................................................................. 13

Figure 9: Crack tip stress fields constrained by adherents. .............................................. 14

Figure 10: Cohesive fracture. ........................................................................................... 15

Figure 11: Interfacial failure ............................................................................................ 15

Figure 12: Interfacial fracture surface. Adhesive remains fully bonded on top beam. .... 16

Figure 13: Fracture with crack jumping present. ............................................................. 16

Figure 14: Crack jumping from one interface to another. ................................................ 17

Figure 15: Press clave with breather fabric showing vacuum holes. ............................... 19

Figure 16: Bagging film and Debulking tape. .................................................................. 20

Figure 17: Vacuum generator........................................................................................... 20

Figure 18: Press clave layup. ........................................................................................... 21

Figure 19: Hydraulic press. .............................................................................................. 22

Figure 20: Air compressor ............................................................................................... 23

Figure 21: Thermocouples in place in the hydraulic press. ............................................. 24

Figure 22: Temperature control system. .......................................................................... 24

Figure 23: Temperature control settings for curing composites. ..................................... 25

Figure 24: Tile Cutter. ...................................................................................................... 26

Figure 25: Respiratory facemask. .................................................................................... 26

Figure 26: Wet peel ply EA 9895 used over the course of this investigation. ................. 27

Figure 27: Grit blaster in UCD labs with close up of nozzle. .......................................... 28

Figure 28: Scrim Cloth. .................................................................................................... 29

Figure 29: Dispensing gun with mixing nozzle. .............................................................. 30

Figure 30: Specimens secured in bonding jig. ................................................................ 32

Figure 31: Bonding Jig after curing with lid still attached............................................... 32

Figure 32: 2 part adhesive dispenser for bonding loading blocks to specimens. ............. 33

Figure 33: Finished DCB specimen. ................................................................................ 34

Figure 34: Callipers (top) & Micrometer (bottom) .......................................................... 34

Figure 35: Test piece clamp. ............................................................................................ 36

Figure 36: Construction drawings. ................................................................................... 36

Figure 37: Finished model with selected material finishes. ............................................ 37

Figure 38: Vertical-turret-3-axis-milling-machine .......................................................... 38

Figure 39: Completed ELS test rig................................................................................... 39

Figure 40: Macro to record crack propagation values. .................................................... 41

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Figure 41: Macro which generates values for time corresponding to force and

displacement. .................................................................................................................... 41

Figure 42: Excel spreadsheet used for GIC calculations. .................................................. 42

Figure 43: Test area setup. ............................................................................................... 43

Figure 44: Delamination under general loading []. ......................................................... 47

Figure 45: A typical load–displacement trace for a DCB joint. ...................................... 52

Figure 46: Initiation values indicated on P vs. d graph. ................................................... 53

Figure 47: Mean GIC propagation values for 3 methods of analysis. ............................... 54

Figure 48: Effect of scrim cloth on load displacement diagram. ..................................... 56

Figure 49: Visible tearing of scrim cloth after testing. .................................................... 56

Figure 50: Effect of scrim on various surface treatments. ............................................... 57

Figure 51: Micro voids seen with scanning electron microscope. ................................... 59

Figure 52: Effect of scrim with Heavy grit blast.............................................................. 60

Figure 53: Effect of scrim with light grit blast. ................................................................ 60

Figure 54: Effect of scrim with plasma treatment. ........................................................... 61

Figure 55: Effect of scrim with no peel ply. .................................................................... 63

Figure 56: Mean fracture toughness values for differing bondline thicknesses. .............. 64

Figure 57: Mean Fracture toughness values..................................................................... 66

Figure 58: Fracture surfaces for differ mode mixity ........................................................ 68

Figure 59: Mode I and Mode II fractions of mean fracture toughness versus Mode Mix

ratio. ................................................................................................................................. 68

Figure 60: Failure envelope for the effects of scrim cloth on fracture toughness............ 69

Figure 61: Control batch J09 with peel ply. ..................................................................... 76

Table 1: Rig parts list. .............................................................................................................................. 37 Table 2: Rig cutting list. .......................................................................................................................... 38 Table 3: Control batch mean fracture toughness initiation values (M/J

2). ................................................. 73

Table 4: Heavy grit blast mean fracture toughness initiation values (M/J2). .............................................. 73

Table 5: Light grit blast mean fracture toughness initiation values (M/J2). ................................................ 74

Table 6: Plasma treated mean fracture toughness initiation values (M/J2). ................................................ 74

Table 7: Samples without peel ply mean fracture toughness initiation values (M/J2). ................................ 75

Table 8: Control batch mean fracture toughness propagation values (M/J2). ............................................. 76

Table 9: Heavy grit blast mean fracture toughness propagation values (M/J2). .......................................... 76

Table 10: Light grit blast mean fracture toughness propagation values (M/J2). ......................................... 77

Table 11: Plasma treatment mean fracture toughness propagation values (M/J2). ...................................... 77

Table 12: No peel ply mean fracture toughness propagation values (M/J2). .............................................. 77

Table 13: 18o mean fracture toughness propagation values (M/J

2). ........................................................... 77

Table 14: 18o with scrim mean fracture toughness propagation values (M/J

2)........................................... 78


2). ........................................................... 78


2). ......................................... 78


2). .......................................... 79

Table 18: Mode II with scrim mean fracture toughness propagation values (M/J2). .................................. 79

Table 19: Mode I with scrim mean fracture toughness propagation values (M/J2). .................................... 79

Table 20: Fracture toughness values for %GII ......................................................................................... 80 Table 21: Fracture toughness values with no scrim partitioned into each loading mode. ........................... 80 Table 22: Fracture toughness values with scrim partitioned into each loading mode................................. 80

vii

Project Specification

This project intends to focus on so-called secondary bonded joints, where the composite

panels are first cured and cut into specimens of the required size. The adhesive joint is

then carefully produced using a state of the art epoxy adhesive, and cured separately.

This shall be further discussed later in the review of works. An important prerequisite in

the use of these composite joints is an understanding of their fracture properties under

mixed-mode loading conditions.

In the context of fracture mechanics, tensile opening of the crack faces is termed Mode I

and shear loading is termed Mode II. One of the aims of this project is to examine the

effect of mode-mixity (the combination of tensile and shear loading) on the fracture

toughness of secondary bonded composite joints.

In addition, ongoing research in the UCD adhesives group has shown that the fracture

toughness of epoxy adhesives often varies with the thickness of the bondline itself, so

that when a thicker layer of adhesive is used, the fracture toughness appears to increase.

The second aim of this project is to investigate this effect for the secondary bonded

composite joints.

1

1. Introduction

Research of Adhesively bonded composite laminates has been an area of great interest to

the aeronautical sector in recent times. This is primarily due to the importance that

weight reduction holds in aircraft design. As a result of this, the replacement of

traditional materials such as aluminium with composites is becoming more

commonplace. Primarily due to them having high specific strengths whilst also being

lightweight. Their growth is more evident in the military area where funding for research

and maintenance is more widely available.

The use of epoxy adhesives as a method of joining materials has also numerous benefits

over other such methods. These include better resistance to fatigue because of the lack of

stress concentrators around rivet holes. This is particularly evident in thin sheet metal

encountered in aircraft manufacture. As the use of adhesives is still in its infancy in

terms of fastening, the primary structural components of aircraft are still bolted and shall

remain as so for some time to come.

Nonetheless, the performance of adhesively bonded composites still needs to be

rigorously tested before they can be put to use in working conditions.

Already, significant investment has been made in the development of standardised test

protocols for the determination of the fracture toughness of structural adhesive joints

under a variety of loading conditions

It is hoped that this investigation will serve as a guide to understand the methods

currently available and under development with regards to adhesively bonded composite

testing. With this in mind, the main aspects of the literature review will focus on papers

concerned with the experimental testing of the delamination resistance of composites

and critically comparing the methods that are currently in practice, whilst avoiding

computer modelling techniques where possible.

2

In the following sections, the different modes of loading that when applied cause

delamination and fracture to occur will be reviewed and surmised.

These being; tensile opening (Mode I), shear failure (mode II) and the main focus of this

work Mode-Mixity (Mode I/II).

Also to be included is a review of work done in the area of constraint due to the effect of

bond-line thickness.

3

2. Modes of Failure

2.1 Mode I

Current international standards for mode I (opening mode) fracture testing of composite

materials set in 2001 typically incorporate the Double Cantilever Beam (DCB) specimen

geometry [1]. This is, in most regards modelled on the test protocol set out by the

American Standards for Testing and Materials (ASTM) [2].

Mode I (opening mode) type failure, of composites is called delamination and can arise

from internal defects like cracks or contaminants in the pre-preg layup or inadequate

pressure being applied during curing. This can then cause debonding of 2 adjacent plies

if subjected to a tensile force. If delamination occurs, the specimen’s resistance to

further loading, damage and fatigue can be drastically affected.

By performing Mode I tests on DCB specimens, delamination can be achieved and a

linear elastic fracture mechanics (LEFM) approach can be taken to determine results for

the delamination resistance or energy release rate GI.

In practice this would be considered to be the most common mode of failure and so its

understanding is of paramount importance. A review was carried out by Davies et al [3]

in 1998 of the methods that were being widely used for testing. This comprehensive

review is an indication of the amount of work needed to produce working standards and

the difficulties such a challenge creates.

The Double Cantilever Beam method was chosen for its relative simplicity, both in

testing and analysis. Test specimen manufacturing, which will be further described later

in this work, is also of a simpler nature than other methods, although the procedure is

still relatively time consuming. The DCB test was initially introduced by Ripling et al.

[4] as a means to measure the fracture toughness structural bonds between metallic

substrates. From this work, an ASTM standard was published in 1973 [5].

4

A rotation factor was used to correct for rotation at the built in end and then with further

research done, [6] it was adapted to act as a beam on an elastic foundation, and

correction factors for load blocks and large displacements introduced.

The double cantilever beam shown in Figure 1, contains a thin film of PTFE,

recommended in the standards to be less than 13 microns thick. This is used to give a

point for crack initiation. The DCB specimens are fitted with loading blocks as shown

below and connected to a tensile testing machine using pin-jointed clevis grips, or piano

hinges. This ensures that there is a force acting only in the vertical direction on the

loading blocks. A load is applied with the tensile machine to cause crack initiation and

once this has propagated to 5mm the load is removed. A travelling microscope is used to

monitor crack growth.

Figure 1: Schematic of the double cantilever beam specimen used for mode I testing, load blocks

can be used for the load introduction [7]

5

The test is then carried out at quasi-static cross-head displacement rates of between 1 &

5 mm/min. [8]

Figure 2 below indicates the tensile load being applied.

Figure 2: DCB experimental setup

In recent years, the majority of research and work being carried out in the area of mode I

failure has been gathering data on the applicability of the standards to various different

material types other than carbon/glass fibre composites that are growing in use and

variety. For the scope of this investigation and the extent to which they could be applied

to this work it was deemed unnecessary that they be included.

6

Rather, the aspect of mode I delamination resistance testing here that is more applicable,

would be of composites manufactured using multi-directional layups.

At the time of writing these are not covered under International standards [1] due to the

tests being invalidated because of problems that occur due to crack branching and other

factors such as specimens becoming de-laminated away from the adhesive plane. In

1999 Choi et al, [9] carried out an assessment on this and should be referenced to for

further background on the matter. Zemcík [10] in 2008 concluded that it is reasonable to

consider the critical energy-release rate GIC value as the inter-laminar fracture

toughness in the case of the delamination of a composite with transversely oriented

fibres. If one is to follow the standards to achieve the delamination resistance by DCB

test means on cross-ply laminates it is only allowable for crack initiation values of

toughness.

De Morais [11] observed that initiation values were found to be higher in his case but

the underlying reasons are not fully understood at present.

Realistically it would be desirable to use adhesively bonded composites with multi-

directional fibres over ones with unidirectional orientations but as of yet, until standards

can be established, the use of unidirectional composites can be seen as the lower limit of

attainable toughness and strength.

7

2.2 Mode II

Mode II (In plane shear) fracture (Figure 3.) occurs when a material is subjected to a

shear stress. As with Mode I, this causes delamination with composite materials which

then leads to failure. Again a fracture mechanics approach can be employed to determine

results for fracture toughness GIIC.

As has been seen in the previous section, there has been significant progression of the

Mode I DCB test method to international standards in 2001. Surprisingly this has not

also been the case for Mode II delamination test methods as of yet.

Figure 3: Indication of mode II (In-plane) fracture.

A paper published by Davies et al [12] gives a thorough review of the four methods most

widely used for the determination of Mode II fracture toughness. These being the end -

notched flexure (ENF) test, the stabilised ENF test proposed by the Japanese industrial

standards group, the end-loaded split (ELS) test and a variant on a four-point loaded

ENF (4-ENF) test proposed by Martin et al [13]. Figure 4 shows simple free body

diagrams of all four.

8

Figure 4: Schematic diagram of various mode II delamination test methods [12].

The reason for Mode II test methods being slow to reach standardised levels is not due

to a lack of research, of which there is an extensive level carried out previously [14, 15,

16], but rather to complications that have arose in the testing area.

The ENF test makes use of a 3 point bending method which yields unstable crack

propagation and so can only be used to achieve crack initiation values.

The stabilised ENF is also deemed unfit to be standardised as it is too complex as a test

method. The reason being, it requires the input of real time crack shear displacement

values to control the loading of the specimen [14].

The remaining methods, ELS and 4-ENF are both stable and they utilize relatively

simple apparatus. So it is possible that either could be standardised at some point in the

future.

For this investigation into mode-mixity an ELS test rig (Figure 5) was manufactured,

with the intention that it could be used for both mode II and mixed mode tests.

9

This method has been used comprehensively by the European Structural Integrity

Society (ESIS), Technical Committee 4 (TC4) on polymers and composites in a round

robin [14].

The specimen preparation used is the same as that used for Mode I DCB’s and shall be

discussed in a later section. It has been shown in a publication by A.J. Brunner [17], that

on comparing the GIIC initiation values from pre-cracking, the mode I method of pre-

cracking gave more conservative fracture toughness values when compared with the

mode II method. This suggests that mode I pre-cracking should be considered over the

mode II approach. More research is needed in this area as of yet.

Figure 5: The mode II ELS delamination specimen held in the clamping fixture.

10

2.3 Mixed Mode (I/II)

The main focus of this review is to investigate the current status of mode mixity de-

lamination testing in adhesively bonded composites. A paper published by Reeder et al

[18] gives a comprehensive evaluation of methods used at that time for mode mixture

experiments. In this, the reader is also presented with the Mixed mode Bend (MMB)

test, shown in figure 6, which upon further modification formed the basis of the 2001

ASTM standard [19] method for the determination of mixed mode I/II interlaminar

fracture toughness of unidirectional fibre-reinforced polymer–matrix composites.

Figure 6: Schematic of MMB test configuration. [20]

11

The modification carried out was a non-linear analysis and redesign of the test to

account for non-linearity brought about by the large displacements occurring during

bending [20].

The MMB test is a combination of mode I Double cantilever beam and mode II End

Notched Flexure tests. The test makes use of a hinge and lever to create the two load

components by applying a single load at P. The mode mixture is achieved by varying the

dimension of c (figure 6) along the composite’s span [18].

During the development of the MMB test, Reeder proposed further modifications [21] to

provide an improved level of accuracy. Methods for careful checking of fixture

alignment were also included as de-lamination is known to occur at the corners of the

PTFE inserts. So although necessary, these provide significantly more work to be carried

out when performing the tests. A final complication that can arise, which was remedied

by Chen et al. [22] is that if the arm applying the load to the specimen approaches over

3% of the specimens weight, this can influence the value of the toughness. This suggests

that the design of the MMB test rig can be problematic. So with this and the high level

of inaccuracies that can abound, other test configurations have been considered.

One such configuration is the mixed-mode end loaded split delamination test setup. As

with the Mode I delamination of composites, the applicability of laboratory condition

experiments in the MMB test, to in-service situations is limited. This is due to the layup

arrangement, specimen thickness, curves or tapers on the component or any combination

of complications. These will all contribute to delamination growth (if it is to occur) that

can vary with the crack length along the specimen. In this case a variation of the mode

mixity will become evident and a test such as the MMB cannot encompass it whilst the

MMELS can.

Kinloch et al [23] has contributed a great deal of research towards this test method and

would be considered most relevant to this subject matter. The approach taken by

12

Kinloch is that of a beam theory one, using an analysis which was formulated by

Williams in his famous work [24].

The MMELS test, seen here in figure 7 is similar to the DCB test but only one adherent

is put under load.

Figure 7: Illustration of the mixed-mode end loaded split test specimen: (a) for short crack lengths

and (b) for long crack lengths [25].

As the crack initiates the deflection of the beam can be seen to change. While the crack

length (a) is small, the deflection is the same in both adherents, but as the crack extends,

the deflection of the lower beam approaches zero (Unloaded point). As this is occurring,

the value of GI/IIC increases until an asymptote value is reached. This model is more

pragmatic as to what would occur in real loading situations [25]. This is why for this

project, that this approach was taken.

By having this model modified as suggested by Kinloch [23], to exclude axial forces

when the load is applied as seen in Figure 8, it is believed to be a more appropriate

model to incorporate the interlaminar crack propagation of a real composite structure as

L- a decreases. [25]

13

Figure 8: Alternative MMELS test rigs with sliding clamped end and fixed load point [25].

14

3. The effect of constraint on fracture toughness

The secondary aims of this project were to investigate the effect that constraint has on

the fracture toughness of specimens.

It has been shown by C. Yan et al [26] with the use of fractography and a fracture

mechanics approach into their research that the width and thickness of the adhesive layer

effects stress and strain distributions in the adhesive. This is indicated in Figure 9 below.

Figure 9: Crack tip stress fields constrained by adherents.

A change in the adhesive layer thickness can cause a transition from small-scale yielding

to fully plastic conditions. Research has shown that a linear proportionality for

toughness values is found to exist due to the high level of constraint with small bond

thicknesses. This then reaches a critical bond thickness, where it has been found that the

toughness decreases resulting in blunting of the crack tip with loading.

To fully characterise this phenomena, cohesive failure (Section 5.1) is needed in the

adhesive layer. This was not achieved with initial experimental testing, so a variety of

surface treatments were employed with a hope that cohesive failure would result.

(Section 6.)

15

4. Failure of Adhesive joints

4.1 Cohesive fracture

Cohesive fracture is the term used when a crack propagates through the bulk of the

material.

When this type of failure occurs with adhesively bonded composites, the adhesive will

be found to remain on both of the adherents.

Cohesive fracture can occur at the centre of an adhesive layer or very near the interface.

This is referred to as “near interface cohesive fracture”. Figure 10 gives an indication of

this type of failure.

Figure 10: Cohesive fracture.

4.2 Interfacial fracture

This type of fracture occurs when the adhesive debonds from the adherents. On

inspection of the failure surface, adhesive should only remain on one of the adherents.

Interfacial failure is usually associated with the adhesive having lower fracture

toughness. Figure 11 shows this schematically.

Figure 11: Interfacial failure

16

Figure 12 indicates the types of interfacial failure experienced after testing was carried

out in UCD labs.

Figure 12: Interfacial fracture surface. Adhesive remains fully bonded on top beam.

4.3 Other types of fracture

Mixed fracture types: Occur where the adhesive can be failing cohesively and then

jumps to one of interfaces.

The alternating crack path type: Occurs if the crack jumps from one interface to the

other.

Figure 13 gives a schematic of this and Figure 14 shows the occurrence of this after

testing.

Figure 13: Fracture with crack jumping present.

17

Figure 14: Crack jumping from one interface to another.

It has also been seen that the adhesives fracture toughness can have a much higher value

than that of the bonding substrate. This can cause failure of the adherent, termed

“interlaminar failure” and is a highly undesirable form of failure.

18

5. Specimen Manufacture

As this project involves working with adhesively bonded composites, it was a requisite

to be able to manufacture carbon fibre composite specimens. This process is a lengthy

one with an associated learning curve due to the numerous components involved.

The first step was to produce high quality aero grade composite panels for delamination

resistance testing.

5.1 Materials

Composite prepreg sheets:

Hexcel Hexply 8552 /37%/46364/C.

Fibre material is Tenax HTA type 3, class 2, style 6k-135-5H.

(6k =6000 fibres/tow, 135= cured ply thickness .0135"approx, 5H = 5 harness satin

weave).

Peel ply:

Henkel Hysol ® EA 9895 ™ Peel Ply.

Adhesive:

Henkel Hysol EA9830.05.

19

5.2 Composite Specimen production

5.2.1 Pre-Preg Composite Layup:

Cut to size sheets of the Hexcel pre-preg were layered with weaves on each facing the

same directions following removal of the protective film.

Five layers in total were used with the top and bottom films being left on till removal

was necessary prior to curing.

5.2.2 Press-clave build-up and de-bulking:

PTFE film was placed on the press-clave followed by a release layer to provide a good

textured surface which let the layup be removed easily. Next the Composite layup was

placed, with a breather layer to cover it. This needed to cover the vacuum holes so that a

vacuum can be drawn evenly over the whole surface area.

Figure 15: Press clave with breather fabric showing vacuum holes.

20

De-bulking tape was then attached to the periphery of a clear plastic bagging film and

adhered to the autoclave.

Figure 16: Bagging film and Debulking tape.

The layup was then attached to a vacuum for 45 Minutes. This gives the layup a uniform

thickness over the composite area before curing and also the force involved begins to

bond the resin throughout the layers to provide a level of green strength before peel ply

addition.

Figure 17: Vacuum generator.

21

5.2.3 Peel-Ply addition:

After the layup has been removed, a peel-ply is placed on the bottom layer of the layup

after its protective film had been removed.

Peel-ply's are used to create a protective film on one side of the composite layup prior to

bonding. This will be detailed further in section 5.3.

5.2.4 Press Clave build-up:

On the press-clave, a layer of PTFE, Release layer and PTFE again were placed. The

pre-preg layup was then placed. Around this a protective dam was made with sealant

tape to prevent epoxy leakage. Another release layer was then placed to provide an

aesthetic finish and to prevent damage during curing.

A rubber pad was used here to spread the load evenly when the breather layer was laid.

The Debulking layer was again used with the sealant tape applied to the bagging film.

Finally adhesive tape was also put on the edges of the press-clave so its lid would stick

closed when attached and maintain the vacuum.

Figure 18: Press clave layup.

22

5.2.5 Curing:

The curing process has 4 components:

Hydraulic press

Compressor

Vacuum

Heat control system.

First the vacuum hose was attached to the press-clave and a vacuum drawn.

The lid was then attached and the completed layup in the press clave are all placed

carefully into the hydraulic press (figure 19).

The press is dropped with only its own weight being applied.

Figure 19: Hydraulic press.

23

The compressor (figure 20) and thermocouples (figure 21) were then attached.

Please refer to user manual for instructions.

The press was then set to rise to 30 tonnes or 500 kg, taking all necessary safety

precautions.

The Compressor was switched on and set to 30 PSI. This keeps the vacuum in place

under the bagging film as the pressure is much greater on the compressed side.

When the pressure has settled at 30 PSI the vacuum hose is then removed from the

vacuum. This needed to be ventilated to the atmosphere to get rid of volatile gases that

are released during the curing period.

The compressor was then ramped to 80 PSI to promote maximum adhesion between

composite layup plies.

The temperature control system (figure 22) was then set to run as shown in figure 23 and

started.

Prior to the press-clave being removed it is let to cool for safety concerns.

Figure 20: Air compressor

24

Figure 21: Thermocouples in place in the hydraulic press.

Figure 22: Temperature control system.

25

Figure 23: Temperature control settings for curing composites.

5.2.6 Machining

Samples were cut with a standard tile cutter (figure 24) to a width of approximately 25

mm.

Extreme caution was needed to be taken here as the dust given off is very toxic.

A full face respirator safety mask (figure 25) was used along with latex gloves and a lab

coat.

A generator was used to run a ventilation system to prevent particulates from leaving the

lab.

The finished specimens were grouped into batches according to where they lay in the

initial composite layup because the placement of the vacuum holes on the press clave

can affect the specimen thickness.

This can be investigated further if problems occur with a batch.

The specimens were then wiped clean and stored in a desiccator cabinet until the final

DCB specimens were produced to prevent moisture absorption.

26

Figure 24: Tile Cutter.

Figure 25: Respiratory facemask.

27

5.3 Surface Treatments

Over the course of this investigation cohesive failure has been a major objective that has

been strived for. It was initially aimed that having achieved this, a fracture mechanics

approach could be taken towards characterizing the properties of the adhesive being

employed.

Initial DCB tests failed in achieving this (resulting interfacial failure) so a variety of

different methods of preparing the surface to increase adhesion and encourage cohesive

fracture were investigated. The following sections shall give a brief indication of the

methods used.

5.3.1 Peel Ply

Peel plies are woven fabric sheets, usually made from Nylon or polyester. They are

heavily used in the area of composite adhesion.

They are used to create clean surfaces with a roughened texture, offering a much better

surface energy which is left on the composite after its removal. This promotes adhesion

and also offers a higher level of repeatability in experimental testing which is a requisite

particularly in the aerospace industry.

Figure 26: Wet peel ply EA 9895 used over the course of this investigation.

28

Peel plies come in two forms, wet and dry types. Wet peel plies are referred to as so

because they come pre-impregnated with a resin which cures alongside the composite

adherents. Dry peel plies use the composite’s own resin to form the improved adhesive

surface.

Peel plies are only removed immediately prior to bonding. This removes some but not

all of the risk of contamination such as release agents and particulates from machining

affecting the adhesive. The level of contamination resulting is set with tighter bounds

from the use of the peel plies.

5.3.2 Abrasive Blasting

Abrasive blasting or grit blasting as it is more commonly referred to, is a method of

treating surfaces which is widely used in all manner of surface treatment applications.

Grit blasting involves supplying compressed air through a nozzle which carries abrasive

particulates with it.

Figure 27: Grit blaster in UCD labs with close up of nozzle.

29

As grit blasting is not a very controllable method of surface preparation (unless

automated), its use is not employed to any great extent in the aerospace industry due to

the high level of standards that are in place.

5.3.3 Plasma Treatment

Plasma treatments are used widely in industry to improve the surface properties of all

manner of objects. They are used as a means of increasing the surface energy of an

object by adding functional groups to the surface of an adherent providing it with a more

hydrophilic substrate for bonding.

The plasma treatment used was:

Helium /Oxygen (He/O2)

10 l/min: 1 l/min

@ 1250W 1 pass (25s)

5.3.4 Scrim Cloth

Scrim cloths are thin woven meshes used as a method of controlling bondline thickness.

They are used in the aerospace industry as a part of the layup used when joining fuselage

skins which are bonded together.

It was hoped for the scope of this investigation that they would provide a successful

means of achieving cohesive failure in the adhesive media. Below is an SEM image of

the scrim after testing. Adhesive can be seen to remain where meshes are woven.

Figure 28: Scrim Cloth.

30

5.4 Preparation of DCB specimens

All specimens were dried in a vacuum oven prior to bonding being carried out. This step

was taken to prevent the moisture content from affecting the test results and give a more

repeatable set of conditions. The samples were placed in the oven which was then sealed

and had a vacuum drawn. They were heated for 4 hrs at 40oC and left to cool overnight

with the vacuum held.

5.4.1 Bonding specimens:

Samples were placed ready to be bonded.

A 2 part experimental Epoxy adhesive, Hysol EA9830.05, which was provided by

Henkel, is measured out by weight.

100 parts A: 53 parts B. For a 6 specimen batch, this consisted 15 grams part A and 7.95

grams part B.

This is mixed according to the user guide or is mixed using the nozzle on the dispensing

gun (figure 29).

Figure 29: Dispensing gun with mixing nozzle.

31

Moments before adhesive is applied, peel plies are removed.

Then a thin layer of epoxy adhesive is then applied to each beam of the specimen using a

spatula.

Depending on the batch, the bondline thickness can be controlled as required. This

entails the use of either thin aluminium spacers or scrim cloth to be added to the

adhesive to control the bondline.

A non stick Teflon film was placed on one of the specimens to a length of 57.50 mm for

the DCB samples or 77.5mm for the mixed mode samples.

This acts as a crack initiator prior to pre-cracking.

The specimens were then placed into a specially designed bonding jig (figure 30) which

holds them in place. Each specimen has a rubber pad and aluminium plate placed on top

and below to spread an even load over the two composite substrates.

The lid was attached using M8 bolts and a torque wrench tightened these evenly to 4

Nm. This ensures adequate pressure was applied during curing.

The specimens were cured at 80oC for 4 hours.

The jig was let cool before removal for safety reasons.

32

Figure 30: Specimens secured in bonding jig.

Figure 31: Bonding Jig after curing with lid still attached.

33

5.4.2 Finishing:

Samples were then sanded using a sanding discs to remove adhesive from the sides. This

would otherwise affect crack propagation.

Grit blasting was used to give a rough finish to one end of samples and loading blocks so

that they could be bonded successfully.

Acetone was used to clean the surface of the loading block and methanol used on the

composite specimen itself. Acetone can erode the carbon fibres so it was not used on the

composite.

Loading blocks were added using a 2 part adhesive from Henkel Locktite whish is

applied using a spatula (figure 32). A thin layer is all that is necessary. These are left to

set following the user’s guide.

A watered down Tipp-ex correction fluid was painted onto one side of the specimen and

tick marks were inscribed to produce a scaled ruler so that crack propagation could be

followed more easily. This can be seen in figure 33.

Figure 32: 2 part adhesive dispenser for bonding loading blocks to specimens.

34

Figure 33: Finished DCB specimen.

Finally 3 widths and thicknesses were measured using a micrometer and callipers (figure

34) so they could be used during calculations. These are recorded in the joint

manufacturing log book.

Figure 34: Callipers (top) & Micrometer (bottom)

35

6. ELS test rig manufacture

Concurrently whilst learning the procedure of composite specimen production, the ELS

test rig was being modelled using a computer aided design package and then

manufactured in the school workshop.

Prior to this investigation, the school of engineering in U.C.D did not have the means to carry out mode II and mixed mode testing on composite materials, so this needed to be

remedied.

This test rig concept which has been now been constructed was designed by Joseph Mohan (U.C.D. Postgraduate student 2010).

The test rig is intended to be used for both Mode II and Mixed mode tests which are described in section 3.

6.1 CAD Drawings

Having received designs of the ELS test rig the next step was to prepare a computer

model so the design could be fully visualised prior to manufacture in a bid to avoid

problems when machining each component.

This was done using Pro/ENGINEER Wildfire 4.0 cad package.

This was done in 3 steps.

Rig base

Test piece clamp

Linear rail & Bearing

The linear rail design in use was available for download off a reputable CAD component

website and the other components were designed. These were then all assembled so

engineering drawings could be produced following this.

36

Figure 35: Test piece clamp.

Figure 36: Construction drawings.

37

Figure 37: Finished model with selected material finishes.

6.2 Rig manufacture

Firstly a parts list was needed to be drawn up so parts could be sourced.

It was decided that stainless steel would be the best choice as it is strong, heavy and

durable. A strong material was needed so as not to incorporate a high level of

compliance into the system during testing. This will be detailed in later sections.

Rig Clamp Flat stainless steel Part A 80 x 68 x 25mm

Part B 80 x 68 x 25mm

Rig Base Flat stainless steel Part A 250 x 70 x 25mm

Part B 160 x 70 x 25mm

Bolts Rig Clamp 4 x M8

4 x M10

Rig Base 3x M15 Table 1: Rig parts list.

38

Next a plan for machining the stainless steel parts was formed.

Rig Clamp Flat stainless steel Part A 2 x 10mm channels milled

Part A 4 x 6.4 mm through holes drilled

Part B 4 x 6.4 mm through holes drilled

Part A 4 x 8.4 mm through holes drilled

Part B 4 x 14.5 mm holes to 8mm drilled

Rig Base Flat stainless steel Part A 1 x 16mm through hole drilled

Part A 3 x 14.5mm bore to 8mm drilled

Part B 3 x 8.4mm through hole drilled

Part B 3 x 8.4mm drilled and tapped Table 2: Rig cutting list.

Finally the test rig was machined mainly using a vertical turret 3 axis milling machine,

similar to that in figure 38 in the school workshop, under the supervision and guidance

of the workshop technicians. This was not without its problems as stainless steel is

difficult to machine and slower cutting speeds were needed to avoid damage to the

cutting tool and the work piece.

Figure 38: Vertical-turret-3-axis-milling-machine

39

The completed ELS test rig can be seen below in figure 39 which has been used to carry

out successful mode II and mixed mode delamination tests.

Figure 39: Completed ELS test rig.

40

7. Experimental procedures

7.1 Test methodology

Under the scope of this investigation, all tests (Mode I DCB, Mode II and Mixed mode

I/II) tests were carried out using a Hounsfield H50KS tensile testing machine.

A 1 KN load cell was attached by a screw to the cross-head.

Clevis grips which hold the DCB samples were attached on the top and bottom with pins

and the sample put into place also held with pins. This ensured that there was only

tensile force in the vertical direction acting on the loading blocks.

A travelling microscope and halogen lamp were positioned so that crack propagation

could be observed with a higher level of accuracy.

QMAT software was run on the PC to take readings; force (P) and displacement (d),

from the Hounsfield machine. Prior to testing the load and displacement were zeroed to

prevent errors.

The test was set to run with a displacement controlled rate of between 1mm/s - 5mm/s.

As specified in international standards. [1]

When the test was initiated, a macro (figure 40) ran simultaneously on Microsoft excel.

This was used in a stopwatch manner. As the propagating crack passed each distance on

the scale which was previously inscribed on the specimen side, the crack length (a) was

entered into the sheet and a time was recorded automatically.

Once the pre-crack reached a distance of 5 mm the test was stopped and the specimen

was released from tension.

The test was then repeated till a crack length of 65mm was achieved.

Raw data stored in QMAT was then cleaned up and added into the excel spreadsheet.

The crosshead speed was entered and another macro was run (figure 41), this calculated

41

the load for every instance of crack length. This data was used on another excel

spreadsheet (figure 41) [277] to produce results for fracture toughness and other data

used to characterize the specimens.

Figure 40: Macro to record crack propagation values.

Figure 41: Macro which generates values for time corresponding to force and displacement.

42

Figure 42: Excel spreadsheet used for GIC calculations.

43

Figure 43: Test area setup.

44

7.2 Testing approach

Due to the large amount of data to be presented, a systematic approach was taken to its

organisation.

This involves two main groupings:

Mode I DCB tests

Mixed mode tests

These were further grouped into batches represented with a “J xx” format (where xx

indicates a numerical order).

It was planned that a comparison between fracture toughness values should be made

concurrently with the surface treatments investigation and the effect of scrim cloth used

to control bondline thickness. This is used in both the Mode I DCB tests and the mode

mixity investigation.

7.2.1 Mode I DCB tests.

As explained previously, this investigation was initially concerned with achieving

cohesive failure in specimens whilst varying the bond thickness from batch to batch.

Preliminary tests indicated that without a surface treatment, this was not going to occur.

Overleaf shows the different surface preparation methods used on each batch.

All batches bar J 09 compare the effect of scrim vs. no scrim cloth.

45

J 09: Control batch. No scrim. All peel ply.

o J 09 A-L 0.15mm Bondline thickness

All specimens received no surface treatments

J 12: Heavy grit blast.

o J 12 A-C 0.15mm Bondline thickness

No scrim cloth.

o J 12 D-F 0.15mm Bondline thickness

Scrim cloth.

J 13: Light grit blast.


No scrim cloth.


Scrim cloth.

J 15: Plasma treated.


No scrim cloth.


Scrim cloth.

J 16: No Peel ply used.


No scrim cloth.


Scrim cloth.

J 17: 0.25 mm bondline samples

o J 17 A Scrim cloth

No plasma treatment

o J 17 D Scrim cloth

Plasma treated.

46

7.2.2 Mixed mode tests.

J 11: 5/9 & 9/5 ply layup.

o Lightly grit blasted. 0.15mm bondline thickness.

o J 11 A-C 5/9 ply layup

No Scrim cloth.

o J 11 D-F 9/5 ply layup

No Scrim cloth.

o J 11 G-I 5/9 ply layup

Scrim cloth.

o J 11J-L 9/5 ply layup

Scrim cloth.

J 14: All with:

o 7/7 ply layup.

o Lightly grit blasted.

o 0.15mm bondline thickness.

o Scrim

o J 14 A-D Mixed Mode

o J 14 E-H Mode II

o J 14 I-L Mode I

47

8. Beam theory analysis

For the determination of inter-laminar fracture toughness (GIC) there were two sets of

calculations that were employed. These made use of two theories, Simple beam theory

(SBT) and corrected beam theory (CBT). The reasons for this shall be discussed further

in this work.

All the calculations and explanations surmised here are drawn from a thesis [27] by the

authors supervisor which heavily influenced this project. In turn all these are from

famous works by William’s on interlaminar fracture toughness [28,29].

Williams work was intended towards the failure of laminated composites; however the

theory can be applied directly in calculations to the case of adhesively bonded

composites that have small bondline thicknesses.

Figure 44: Delamination under general loading [30].

48

The strain energy release rate (G), was first found in a generalised form:

(1)

B= Specimen Width (m)

E= Young’s modulus of substrate (Pa)

I = Second moment of area

M= Bending moments associated with each adherent

ξ = h1/2h

I = (Bh3/2h) (2)

This was then partitioned into the separate modes I and II where:

G= G1+G2 (3)

And (4)

Where: (5)

Having done this, they can be applied to the separate modes to get critical values for the

fracture resistance GC.

49

8.1 Mode I DCB tests

From symmetry in the DCB specimens

h1=h; so ξ =1/2.

Also M is the moment applied in each arm. Here M1=M2

Where MI = (P a) (load by crack length in a simple beam).

MII = 0

Applying this to equations 4 yields the Mode I fracture toughness:

(6)

8.2 Mode II ELS test

The same methodology is applied here.

The specimens contain adherents of equal thickness:

h1=h; so ξ =1/2.

However in this case, the applied load in the positive Y direction is applied to the 2

beams.

This gives: M1=-M2= (P *a)/2

Again applying this to equations 4 yields the Mode II fracture toughness:

(7)

50

8.3 Mixed mode ADCB test

The details of the ADCB calculations are of a more detailed nature due to the different

adherent thicknesses.

The reader shall again be referred to the works of Williams for a more fruitful insight.

(8)

(9)

With a phase angle of:

(10)

8.4 Correction factors

As has been mentioned already, there are two approaches to achieving figures for our

delamination results, the Simple Beam Theory and Corrected Beam Theory.

In Beam theory it is known that any displacements are considered to be infinitesimal.

This is not the case with any of the tests carried out, so corrections need to be applied to

the set of equations for GC. The load correction factor F is used to settle the irregularities

which occur.

Next is the loading block correction factor N. This is necessary because of the shape of

the loading block which causes a moment due to the stiffening of the section of beam

adhered to the loading block. Ideally a point load should exert the force but this cannot

be done practically.

51

The loading block correction factor can also accounts for the root rotation which occurs

because the beams are not fully built in.

These two correction factors are employed in the CBT analysis but not in the SBT

analysis. Along with this, the experimental compliance method (ECM) is used as a

means of comparison. The variations between SBT and CBT will become apparent in

later sections. For now it is only necessary to note that both these equations are used in

the excel spread sheets [27] used for calculation of fracture toughness and so are not

approached here due to the level of complexity.

52

9. Results & Discussion

9.1 Load–displacement behaviour

Load displacement graphs were generated for every test carried out. These simply plot

the applied load against the crosshead displacement during the test.

Figure 44 shows both the insert stage (red) where the specimen is being pre-cracked and

its unloading line, and also the reloading stage (blue) where crack propagation is

observed for the remainder of the test. This region gives the mean fracture toughness

values GIC

Close inspection of the graph indicates non linearity in the very early stages of loading.

The occurrence of this is expected due to the crosshead taking up slack in the specimen

prior to it taking up the load.

This is remedied by setting a limit on the excel spread sheet within the bounds of the

linear region which occurs prior to crack propagation. The slope of this region is

calculated and the intercept value used to re-set the initial load to zero.

0

20

40

60

80

100

120

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014

Crosshead Displacement, d [m]

Fo

rce

, P

[N

]

Figure 45: A typical load–displacement trace for a DCB joint.

53

9.2 Crack initiation values of G IC

In current ISO standards it is a requirement that the values associated with crack

initiation be determined and documented for both the initial pre-cracking from the insert

film and also for resulting pre-cracked stage [1].

This is required so that any influence caused by the insert film can be removed prior to

analysis of the test data.

The initiation values are the non-linear (NL) point, the 5% compliance value (CO 5%)

and the maximum load (MAX).

These are indicated in figure 46 below. On the insert stage, it is noted that the NL and

MAX point coincide.

These values are tabulated in the appendix in tables 3 to tables 7.

Figure 46: Initiation values indicated on P vs. d graph.

54

9.3 Mean crack propagation values of G IC

Also proposed in the ISO standards [1] is the inclusion of R-curves which are generated

by plotting the value of critical strain energy release rate GIC (fracture toughness) against

crack length. This is required so a quick visual inspection can detect the presence of

irregularities following testing. These are provided in the accompanying excel spread

sheets.

For a full comparison to be made between numerous batches, the mean GIC values for

crack propagation are collated. These are all values arising after the 3 initiation values

are determined. They are to be found in tables 8 to table 19 for all specimen batches.

In this investigation, 3 methods were employed to achieve these. Simple beam theory

(SBT), corrected beam theory (CBM) and the experimental compliance method (ECM).

An example of these is seen in figure 47 below. It should be noted that the SBT method

of analysis consistently provides values lower than the two other methods which are

highly comparable. This holds with the theory mentioned in section 8 in that the lack of

correction factors significantly hinders the values of fracture toughness achieved.

Figure 47: Mean GIC propagation values for 3 methods of analysis.

55

9.3.1 Mode I Results.

9.3.1.1 The effect of scrim cloths

The first comparison that has been made in this investigation is the effect of the scrim

cloth addition as a means of controlling bondline thickness.

It has been found conclusively from a comparison of tables 9, 10, 11, and 12 in the

appendix that the scrim cloth significantly increases the value of the specimens fracture

toughness when used to control the bondline thickness to a better level of repeatability.

It was observed during testing that the scrim cloth acts in a large number of cases in the

same manner as the fibres in composite materials when fibre bridging occurs. During

testing the scrim appears to branch in a manner not unlike a cobweb when crack jumping

occurs. This can be seen from the load displacement diagram (figure 48).

The region which typically falls off with standard DCB P vs. d diagrams once crack

propagation begins is seen to hold at a constant value till the fibres snap. The resulting

load is then too great to be held by the specimen so a dramatic drop off is encountered.

The load then begins to recover towards the region where it would usually be, had scrim

cloth effects not taken over.

Visual inspection of the specimen (figure 49) following testing shows that the scrim

maintains an attachment to one of the adherents until the applied tensile load reaches a

level too great for the specimen to maintain. Tearing of the scrim is evident and there

appears to be a sense of proportionality in the manner of its tearing with that of the

reloading line on the graph until final failure of the specimen is reached.

However this plateauing does not occur in every specimen, so one cannot say in all

certainty that this is the sole cause.

56

Figure 48: Effect of scrim cloth on load displacement diagram.

Figure 49: Visible tearing of scrim cloth after testing.

0

20

40

60

80

100

120

0.000 0.005 0.010 0.015 0.020 0.025

Crosshead Displacement, d [m]

Force,

P [N]

57

Figure 50 below gives a good visual indication of the dramatic effect of the use of scrim

cloths on the mean fracture toughness values for crack propagation in the various

batches tested. It was calculated that a mean increase of 94% was achieved by the use of

a scrim cloth. This alone indicates the use of a scrim as a bondline thickness control has

a major secondary benefit.

The effects of scrim cloth addition on mode-mixity will be discussed in later sections.

Figure 50: Effect of scrim on various surface treatments.

J09 Control J12 Heavy GB J13 Light GB J15 Plasma

treatmentJ16 No peel ply J17 .25mm

bond

0

100

200

300

400

500

600

700

800

900

1000

Fracture

toughness

GIC

(J/M^2)

Variation of GIC with treatment type

NO SCRIM SCRIM

58

J09 Control Batch

For this initial batch 12 samples were tested which used a peel ply to give accurate

repeatable results for the mean GIC crack propagation values.

It was found that this was 193 +/- 24 J/M2 having a standard deviation of 12% with this

being calculated from the mean of each value (table 8).

It can be said that as a control this batch provides a satisfactory means of comparison.

9.3.1.2 The effects of grit blasting as a surface treatment

Two methods of grit blasting were employed in this investigation. A heavy grit blaster

and light grit blaster with finer grain sizes were used for the purposes.

For an unbiased means of comparison all values will be compared to the batch J 09

(table 8). Also the specimens without scrim cloths are the only ones being considered as

the use of this would not give satisfactory statistics.

Visual inspection of figure 52 and figure 53 suggests that the use of both methods of grit

blasting is an unnecessary step, as the results do not to appear to deviate far from the

control region.

A further inspection of tables 9 & 10 show the mean value for a heavy grit blast (HGB)

is 217J/M2 while for the LGB is 141 J/M

2. This suggests that the HGB was successful in

increasing fracture toughness by increasing the surface energy of the adherent, if even

not to a significant level.

The interesting finding is the LGB actually decreases the mean GIC value which is

surprising. The reasons for this are not yet known, however it is likely that the abrasive

nature of the grit blast was sufficient to damage the peel ply interface with the substrate

it was bonded to without actually removing the peel ply.

Another aspect could be the fine nature of the abrasive particles, which although the

sample was cleaned with methanol prior to bonding, could be still present at microscopic

levels.

By the use of scanning electron microscopy it was found that there were indeed voids

and some particulates left in the adhesive following testing. The scope of this

investigation did not permit further research but it is felt that the sizes evident in figure

59

51 might be in a range too small to represent abrasive grit. However these voids could

play a role in lowering GIC values so a further study might warrant this.

Figure 51: Micro voids seen with scanning electron microscope.

60

Effects of scrim cloth on mean GIC values

0

100

200

300

400

500

600

700

Batch J12

Fra

ctu

re t

ou

gh

ne

ss

(J

/M^

2)

NoScrim

Scrim

Figure 52: Effect of scrim with Heavy grit blast.


0

50

100

150

200

250

300

350

400

450

Batch J13

Fra

ctu

re t

ou

gh

ne

ss

(J

/M^

2)

NoScrim

Scrim

Figure 53: Effect of scrim with light grit blast.

61

9.3.1.3 The effect of plasma treatment as a surface treatment

It has been shown successfully in this work that the use of a plasma treatment gives a

significant increase of fracture toughness in all specimens.

In nearly all cases there was a minimum doubling effect on the mean GIC values.

From table 11 it is clear that even with the lowest mean propagation value being 352+/-

141 J/M2 this still yields a lower bounding value of 211J/M

2. On comparison again with

table 8, (factoring out sample L as this appears to have an irregularly high value) the

maximum possible value still falls slightly below the lowest value for plasma treated

samples.

The maximum fracture toughness reached by specimens without peel ply was 1325

J/M2. In achieving this, most of these failed with interlaminar behaviour. This indicates

that the adhesive performs better than the composite substrate to which it was bonded.

Figure 54: Effect of scrim with plasma treatment.


0

200

400

600

800

1000

1200

1400

1600

Batch J15

Fra

ctu

re t

ou

gh

ne

ss

(J

/M^

2)

NoScrim

Scrim

62

9.3.1.4 The effect on fracture toughness without use of a peel ply

For this stage of the investigation it was again hoped to achieve cohesive failure of the

adhesive by performing the mode I DCB tests without the use of a peel ply.

Inspecting figure 61 shows that the peel ply does in fact provide a lower bound value for

fracture toughness. The average propagation value being found in table 12 for mean

toughness values without peel plies is seen to have a value of 433 J/M2. Again this is

providing a near doubling compared to that of the control batch.

Repeatability has been mentioned before with regards to peel plies. This investigation

shows that there is a high level of associated repeatability in this case. Figure 55 has six

mean values all having a sizeable standard deviation from the mean; these values are

spread over a range in the region of 300 J/M2. On making a comparison with the peel ply

treated samples in figure 61 it can also been seen that deviation from the mean is

nowhere near as great, with these falling into a much smaller spread of about 50/60

J/M2.

Finally, it needs also to be mentioned that on further study of the associated tables for

this batch it is evident that even with the presence of a scrim cloth; the value for G IC can

in some cases be less than that of those with the peel ply. This is a point which needs to

be noted as it indicates that errors can occur with samples and that results can deviate

from the norm.

63


0

100

200

300

400

500

600

700

800

900

Batch J16

Fra

ctu

re t

ou

gh

ne

ss

(J

/M^

2)

NoScrim

Scrim

Figure 55: Effect of scrim with no peel ply.

9.3.1.5 The effect of bondline thickness on fracture toughness

Having initially envisaged that cohesive failure would be achieved and a full

investigation carried out into bondline thickness effects it has been felt that a section on

this point is necessary for finality.

Two samples were tested using a bondline thickness of 0.25mm as opposed to the

0.15mm thickness used for all other tests. One of these being grit blasted and the other

plasma treated. It was found that the resulting mean fracture toughness values were:

J17 A- No plasma: 394.1+/-44.8 J/m2

J17 D- Plasma treatment: 966.7+/-392.2 J/m2.

On first inspection of these values, there seems to be no apparent significance held here

and the trend, as seen with a plasma treatment before is followed, but on comparison of

J17 A with the J12 batch which had identical surface preparations there is an interesting

64

point to be made. There seems to be a decrease in mean fracture toughness for the

specimen with a bigger bondline. This would go against other published literature in the

area of constraint. Figure 56 shows the mean toughness values for the specimens and the

difference is over 100J/M2, a value which is too large to ignore as experimental error.

An Inspection of the fracture surfaces once again reveals interfacial failure to have

occurred. It can be justifiably argued from this that although the 0.15mm bondline

specimen has a greater GIC value, this is not just a characteristic of the adhesive, but of

the interface between it and the adherent.

It is a known fact that a crack will propagate in the plane of least resistance and consume

the least energy, so deducing from this, it can be said that the 0.25mm specimen’s

adhesive layer could still well hold a higher fracture toughness due to constraint effects

and so cause crack propagation to jump to the interface sooner than for the 0.15mm

because this offers less resistance to fracture. This analogy, it is felt could possibly stand

as an explanation for such unexpected behaviour.

Figure 56: Mean fracture toughness values for differing bondline thicknesses.

65

9.3.2 Mode-mixity results

The effect of phase angle Ψ on fracture toughness of adhesively bonded composite has

been shown to have a significant impact on the attainable levels of toughness.

The results from the investigation were found to agree with literature on this area [31].

The phase angles of each type of mode mixity were calculated using equation 10

(section 8.3).

The 3 mode mixtures were a 5/9 ply layup, a 7/7 ply layup and a 9/5 ply layup (the

leading number being the loaded arm). These resulted in phase angles of 18o 40

o and 63

o

respectively.

Figure 57 constructed from table 13 to table 19 gives a good visual indication of the

mean fracture toughness for each mode mixity.

The first notable point to be considered is the effect of the scrim on these mixed loadings

is not as drastic when compared with that of the mode I DCB specimens. On inspecting

the test specimens after testing, it is seen that the failure type is in nearly all cases

primarily interfacial when the scrim cloth is present. Without scrim there is a significant

increase in cracks jumping from one interface to the other.

This perhaps may be the reason for smaller variations of GI/IIC values for samples with

and without scrim cloths. As has been discussed previously, and visible in figure 49,

there was a visible tearing or fibre bridging of the scrim cloth in numerous mode I DCB

samples, without this tearing or bridging the scrim only serves as a control on the

bondline thickness.

This observation could be a valuable one, as the presence of shear forces occurring due

to the influence of the mode II partition in the mixed mode tests appears to affect this.

Mixed mode loading is a more accurate modelling of the forces that can occur in service

conditions. For instance, the oscillations of an aircraft wing with a high velocity airflow

over them can give rise to mixed mode loading. In these cases, it should be taken into

66

consideration that the use of a scrim not be used to provide any functional increase of

mean GIC values.

The next results to be discussed are the mean GI/IIC values resulting from the different

phase angles discussed above.

The mean values achieved for crack propagation (table 13 to table 17) were:

334+/-103 M/J2 without scrim and 389+/-110 M/J

2 with

scrim for the 18

o phase,

221+/-48 M/J2 with scrim for the 40

o phase and,

213+/-35M/J2 without scrim and 383+/-79 M/J

2 with

scrim for the 63

o phase.

Immediately it is clear that there is an associated trend between the values of fracture

toughness attained and the phase angle at which they were achieved.

The mean fracture toughness values are seen to decrease with an increasing phase angle

from 18-63o. This correlates well with results published from other researchers in this

area.

Figure 57: Mean Fracture toughness values

67

This decreasing toughness can be partly answered with the increasing stiffness of the

adherent being loaded. The type of failure in the adhesive bond tends to change with

different layups. Figure 58 gives a good indication of this. The 5/9 ply layup tended to

result in an alternating crack propagation whilst the 9/5 ply layup resulted in purely

interfacial failure in nearly all instances. The energy required to cause crack jumping

appears to give rise to increased fracture toughness in this case

The results obtained were then consolidated into their associated GI and GII partitions.

These values and their standard deviations, over the range of mode mixities developed

and also the mode II ELS and mode I DCB batch are presented in tables 20, 21 and 22.

Figure 59 indicates the mode I and mode II fractions as a function of mode mix ratio.

Again here, the dominance of fracture toughness obtained from the use of a scrim cloth

is very much apparent. It is clear to be seen that the Mode II specimens result in GIIC

values well above those obtained by all other test methods. This shall be discussed in

more detail.

Figure 59 displays the failure envelope constructed from the mode I and mode II

partitions. The failure envelope encompasses the region outside which failure of the

specimen shall occur. Values of fracture toughness reached which fall below the arrest

values dictated by the lower bounding standard deviations from the mean will

experience no crack propagation. Once these values are reached, propagation shall occur

in a stable manner and then failure occurs once the limit of the envelope is reached.

68

Figure 58: Fracture surfaces for differ mode mixity

Figure 59: Mode I and Mode II fractions of mean fracture toughness versus Mode Mix ratio.

69

Figure 60: Failure envelope for the effects of scrim cloth on fracture toughness.

It has been said by thouless et al. [32] that the energy release rates associated with crack

propagation in uniaxial loading are not affected by the bonding interfaces. But with the

incorporation of shear forces, the effects of different mechanisms which hinder crack

propagation become more evident. For this investigation this is immediately seen in

figure 57 again. The mode II fracture toughness is of a significantly higher value for all

tests carried out. It was found that the fracture toughness in this case reaches a value of

2026.6+/-866.2M/J2 which is an increase of over 3.5 times when compared with mode I

and mixed mode tests for the same specimen layup.

This can arise from any number of reasons. The main one being felt is due to mechanical

interlocking between the interfaces. The maximum stress for materials in shear occurs at

an angle of 45o

to the surface which shear is acting along. This results in the crack

attempting to propagate in this direction. It can be seen during testing with the use of a

microscope that crack propagation (or branching of crack tips) tend to occur at an angle

close to this before the main crack jumps forward again. This could indicate why, in

figure 60 that the mode II toughness values seem to be a lot higher than would be

expected if a trend line is followed from the other results.

70

10. Conclusion

On completion of this investigation into the effects of constraint and mode mixity in

adhesively bonded composites joints, there have been a number of interesting findings

which have been reported. Initially the effects of constraint were to be investigated but

initial testing proved fruitless in achieving cohesive failure to characterize adhesive

bondline properties. Various surface treatments and methods were decided upon to attain

cohesive failure, but again these did not provide the results required. However, in this

process a number of findings were made on the effects of these treatments on the

composite specimens fracture toughness values.

The first of which was the significant impact obtained from the usage of a scrim cloth as

a means of controlling bondline thickness. It was found that the fracture toughness GIC

in the majority of cases for DCB specimens attained a value much higher (an average

94% increase) than that of a specimen which received the same surface preparation but

without a scrim cloth being used.

In the case of this comparison being made with asymmetric double cantilever beams and

loading modes II and mode mixity (I/II), this increase of GI/IIC was not encountered to

the same significant level. It was observed that the shear component of these modes

takes away from the contribution that the scrim cloth provides to fracture toughness in

pure tensile loading.

As components in service conditions undergo various modes of loading, this means of

bondline control is an area that further research into could benefit greatly.

A plasma treatment was then applied to increase the surface energy and wettability of

the bonding substrate to improve performance. This was achieved with a dramatic

increase of fracture toughness being found. Interlaminar failure was the resulting trend,

leading to an outperformance of the tested adhesive over the composite specimens.

71

As a means of comparison, samples were tested initially using only peel plies as a

control batch. It has been shown that these provide a high level of repeatability over

large numbers of samples. This provided the platform against which all other specimens

were compared against. For completeness, other samples were compared with these

having no peel ply and just a roughened surface. These were shown to have a much

higher mean toughness value but a much higher standard deviation and unrepeatable

results.

Having established the use of peel plies as a positive method of control, it would be

highly recommended that further work is done on improving the adhesion mechanisms

employed by these. The motive is that the peel ply sets the lower limit of fracture

toughness for the components being bonded. If this lower bound can be raised then

designers can be safe in the knowledge that this improvement can positively affect the

fracture toughness and life of such components in service.

The final conclusion to be drawn from mode I DCB tests carried out is that there is

unsubstantial evidence for the necessity of abrasive blasting to promote adhesion from

scope of this investigation.

It has been seen that the use of a heavy grit blast on peel ply surfaces increases the mean

fracture toughness by minimal amounts and the resulting values were seen to deviate

greatly from the mean. This highly suggests that this method of surface treatment is

unsuitable in applications as highly controlled as the military and aerospace.

Also a light grit blast was performed on peel ply surfaces to again promote adhesion. It

was found from testing that this caused a decrease in mean fracture toughness below the

level achieved from using peel plies alone. This suggests to the author that there was

enough considerable exposure to the abrasive as to cause damage to the peel ply, but not

remove it totally, as may have been achieved with the heavy grit blast. This could be the

primary means of causing it to fail prematurely at a GIC value less than that of the

control batch.

Finally the effects of differing mode mixities on the fracture toughness of composite

specimens were studied using the asymmetric DCB and ELS method. Although the

72

ADCB method is not preferred over the mixed mode bend test as this has reached an

international status, it was found that the results were in a range which was predictable

and the standard deviation from the mean was small.

Following this, tests were carried out on the mode II fracture toughness of similar

specimens and again these resulted in values being entirely greater than those from mode

I and mixed mode, as has been documented in other literature. The mean toughness

values were found to lie a lot higher than anticipated and it is believed that various

mechanisms including mechanical interlocking are the cause.

Failure envelopes for the range of mode mixities were then established which

characterize the attainable fracture toughness values for the type of specimens in

question.

Having achieved these successful results from all mode mixities it can be finally

concluded that the design and construction of the ELS rig documented in this work was

a success and can be employed for further testing in the future.

73

11. Appendix

11.1 Mode I GIC Initiation values

Table 3: Control batch mean fracture toughness initiation values (M/J

2).

Table 4: Heavy grit blast mean fracture toughness initiation values (M/J

2).

74

Table 5: Light grit blast mean fracture toughness initiation values (M/J

2).

Table 6: Plasma treated mean fracture toughness initiation values (M/J

2).

75

Table 7: Samples without peel ply mean fracture toughness initiation values (M/J

2).

76

11.2 Mode I GIC Mean propagation values

Control batch with peel ply

0

50

100

150

200

250

300

350

400

450

Me

an

GIC

va

lue

s (

J/M

^2

)

Figure 61: Control batch J09 with peel ply.

Table 8: Control batch mean fracture toughness propagation values (M/J

2).

Table 9: Heavy grit blast mean fracture toughness propagation values (M/J

2).

77

Table 10: Light grit blast mean fracture toughness propagation values (M/J

2).

Table 11: Plasma treatment mean fracture toughness propagation values (M/J

2).

Table 12: No peel ply mean fracture toughness propagation values (M/J

2).

11.3 Mixed mode GIC mean propagation values.

NO SCRIM 5/9 ply

J11 18o Phase

.15mm Bondline

A B C Totals

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

MEAN 369 41 410 229 25 255 307 34 341 300 33 334

SD 111 13 124 62 7 69 25 3 28 93 10 103

CoV 30 31 30 27 28 27 8 8 8 31 31 31


2).

78

SCRIM 5/9 ply

J11 18o Phase

.15mm Bondline

A B C Totals

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

MEAN 186 20 207 292 32 324 421 47 468 351 39 389

SD 26 3 29 70 8 77 44 5 49 99 11 110

CoV 14 14 14 24 23 24 10 11 11 28 28 28


2).

NO SCRIM 9/5 ply

63o Phase

J11

.15mm

Bondline

A B C Totals

GIC

CBT

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

MEAN 44 166 210 47 175 221 44 166 210 45 168 213

SD 5 20 26 11 43 54 5 21 26 7 28 35

CoV 12 12 12 24 25 24 12 12 12 16 16 16


2).

SCRIM 9/5 ply

63o Phase

J11

.15mm

Bondline

A B C Totals

GIC

CBT

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

GIC

(CBT)

GIIC

(CBT)

GI/IIC

(CBT)

MEAN 90 340 429 77 292 370 80 300 380 80 303 383

SD 13 51 64 7 28 36 19 73 92 16 63 79

CoV 14 15 15 10 10 10 24 24 24 20 21 21


2).

79

Table 17: 40

o with scrim mean fracture toughness propagation values (M/J

2).

Table 18: Mode II with scrim mean fracture toughness propagation values (M/J

2).

Table 19: Mode I with scrim mean fracture toughness propagation values (M/J

2).

80

%GII Scrim SD No scrim SD

0 521.865776 77.5403366 196.563563 40.371801

11 389.342455 109.580374 333.632255 103.109248

43 221.284934 48.2697603 80 382.623025 79.2700826 213.132171 34.6275973

100 2026.63292 866.268184

Table 20: Fracture toughness values for %GII

No scrim GI GII SD GI SD GII

196.563563 0 40.371801 0

300.450412 33.1818427 92.6817408 10.4301994

44.7907093 168.341461 7.12394516 27.5082529

Table 21: Fracture toughness values with no scrim partitioned into each loading mode.

Scrim GI GII SD GI SD GII

521.865776 0 77.5403366 0

350.604035 38.7384196 98.6181348 10.9672152

130.142416 91.142518 28.4994726 19.7788703

80.1177998 302.505225 16.3435207 62.9327573

0 2026.63292 0 866.268184

Table 22: Fracture toughness values with scrim partitioned into each loading mode.

81

12. Acknowledgments

I would sincerely like to thank Mr Joe Mohan for his continual help and patience over

the course of this project. Without his advice and guidance this investigation could not

have happened.

I would furthermore like to thank Dr Neal Murphy for all his help and time put into

making the final year projects a success.

Big thanks are also necessary to all the technicians in the UCD workshop who worked

with me giving me good practical experience in the workshop and help in other design

areas where needed.

Finally I would like to thank all my family and friends who have supported me over the

course of the final year.

Cheers,

Neville.

82

13. References

1 Fibre-reinforced plastic composites – determination of mode I interlaminar fracture toughness, G IC for

unidirectionally reinforced materials, 15024, International Organisation for Standardisation, ISO;

2001.

2 Standard test method for mode I interlaminar fracture toughness of unidirectional fibre-

reinforced polymer–matrix composites, D5528, American Society for Testing and Materials

International, ASTM; 1994.

3 Davies P, Blackman BRK, Brunner AJ. Standard test methods for delamination resistance of composite

materials: current status .Appl Compos Mater 1998; 5(6):345–64.

4 Ripling EJ, Mostovoy S, Patrick RL, Measuring fracture toughness of adhesive joints. Materials

research and Standards. ASTM, vol. 4, March 1964.

5 ASTM D3433, in “Annual Book of ASTM standards. Adhesives section 15” Philadelphia, PA: ASTM;

1990.

6 Olsson R. A simplified improved beam analysis of the DCB specimen. Comput Sci Technol 1992;

43:329.

7 A.J. Brunner, B.R.K. Blackman, P. Davies. A status report on delamination resistance testing of

polymer–matrix composites. Engineering Fracture Mechanics 75 (2008) 2779–2794

8 Determination of the mode I adhesive fracture energy, GIC, of structural adhesives using the double

cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens. British standards BS

7991:2001

9 Choi NS, Kinloch. AJ. Williams. JG. Delamination fracture of multidirectional carbon-fiber/epoxy

composites under mode I, mode II and mixed mode I/II loading. J Compos Mater 1999;33(1):73–100

10 Zemcík, Robert, and Vladislav Las. Materials and technology. Numerical and experimental analyses of

the delamination of cross-ply laminates. 42 (2008): 171-174.

11 De Morais AB, de Moura MF, Marques AT, de Castro PT. Mode-I interlaminar fracture of

carbon/epoxy cross-ply composites. Compos Sci Technol 2002; 62(5):679–86.

83

12 Davies P, Sims GD, Blackman BRK, Brunner AJ, Kageyama K, Hojo M, et al. Comparison of test

configurations for determination of mode II interlaminar fracture toughness results, from

international collaborative test programme. Plast Rubber Compos1999;28(9):432–7.

13 Martin RH, Davidson BD. Mode II fracture toughness evaluation using a four point bend end notched

flexure test. In: Fourth International conference on deformation and fracture of composites,

Manchester, 1997.

14 Blackman, B.R.K., Brunner, A.J., Williams, J.G. Mode II fracture testing of composites: a new look at

an old problem (2006). Engineering Fracture Mechanics, 73 (16), pp. 2443-2455.

15 Blackman BRK, Brunner AJ, Cartier DDR. Determination of apparent Mode II interlaminar

fracture toughness, GIIC, for unidirectionally reinforced composite laminates. In: ESIS TC4 Protocol,

2006.

16 Blackman BRK, Kinloch AJ, Paraschi M. The determination of the mode II adhesive fracture energy,

GIIC, of structural adhesive joints: an effective crack length approach. Engng Fracture Mech 2005;

72:877–97.

17 Brunner AJ. Experimental aspects of mode I and mode II fracture toughness testing of fibre-reinforced

polymer-matrix composites. Comput Meth Appl Mech Engng 2000; 185:161–72.

18 Reeder J. Crews J. Mixed-Mode Bending Method for Delamination Testing. AIAA Journal, 28(7),

1270-1276. (1990)

19 Standard test method for mixed mode I-mode II interlaminar fracture toughness of unidirectional fibre-

reinforced polymer–matrix composites, D6671/D6671M. American Society for Testing and Materials,

2006.

20 Reeder JR, Crews Jr. Non-linear analysis and redesign of the mixed mode bending delamination

test. NASA Technical Memorandum 102777; 1991.

21 Reeder JR. Refinements to the mixed mode bending test for delamination toughness. J Compos Technol

Res 2003;25(4):191–5.

22 Chen JH, Sernow R, Schulz E, Hinrichsen G. A modification of the mixed-mode bending test

apparatus. Compos Part A: Appl Sci 1999;30(7):871–7.

84

23 Kinloch AJ, Wang Y, Williams JG, Yayla P. The mixed-mode delamination of fibre

composite materials. Compos Sci Technol 1993;47(3):225–37.

24 Williams JG. On the calculation of energy release rates for cracked laminates. Int J Fract

1988;36:101–19.

25 Blanco. N and M. Gamstedt. Analysis of the mixed-mode end load split delamination test. Composite

Structures 76 (2006): 14-20.

26 Yan, Cheng; Mai, Yiu-Wing; Ye, Lin. Effect of Bond Thickness on Fracture Behaviour in Adhesive

Joints. The Journal of Adhesion 75.1 (2001). 14 Apr. 2010

27 J. Mohan, “An investigation into composite to composite adhesive bonding.” PhD thesis, University

College Dublin, 2010.

28 S. Hashemi, A. J. Kinloch, and J. G. Williams. The analysis of interlaminar fracture in uniaxial fibre-

polymer composites. Proceedings of the Royal Society of London, 427:173–199, 1990.

29 J. G. Williams. The fracture mechanics of delamination tests. Journal of strain analysis, 24(4):207–214,

1989.

30 Villaverde, Norbert. Variable Mixed Mode Delamination in Composite Laminates under Fatigue

conditions: Testing and Analysis. PhD thesis, Universitat De Girona, 2004. Web. 10 May 2010.

<http://www.tdr.cesca.es/TESIS_UdG/AVAILABLE/TDX-0207105-122952//tnbv.pdf>.

31 Fei Xiao, Chung-Yuen Hui, Junichiro Washiyama, Edward J. Kramer. Phase Angle Effects on Fracture

Toughness of Polymer Interfaces Reinforced with Block Copolymers Macromolecules, 1994, 27 (15),

pp 4382–4390.

32 J. P. Parmigiani and M. D. Thouless, The Effects of Cohesive Strength and Toughness on Mixed-mode

Delamination of Beam-Like Geometries. Engineering Fracture Mechanics, 74, 2675-2699 (2007).

http://www-personal.umich.edu/~thouless/Parmigiani(2007).pdf

http://www-personal.umich.edu/~thouless/Parmigiani(2007).pdf

the effect of mode-mixity and constraint in adhesively bonded composite joints

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