A novel microbond bundle pull out technique to evaluate the interfacial properties of fibre composite systems..

01/30/1601/30/16 INCCOM-VSSC 13, NOV- 14 and 15, Trivandrum City.INCCOM-VSSC 13, NOV- 14 and 15, Trivandrum City. 11
Prof. K. PadmanabhanProf. K. Padmanabhan
Centre for Excellence in NanocompositesCentre for Excellence in Nanocomposites
School of Mechanical and Building SciencesSchool of Mechanical and Building Sciences
VIT-University, VelloreVIT-University, Vellore
Email: padmanabhan.k@vit.ac.inEmail: padmanabhan.k@vit.ac.in
A Novel Microbond Bundle PulloutA Novel Microbond Bundle Pullout
Technique To Evaluate The InterfacialTechnique To Evaluate The Interfacial
Properties Of Fibre Reinforced PlasticProperties Of Fibre Reinforced Plastic
CompositesComposites

01/30/16 INCCOM-VSSC 13, NOV- 14 and 15, Trivandrum City.2
Eastern Bottom up andEastern Bottom up and
Western Top down Philosophy !

ContentsContents
 General Design Considerations for CompositesGeneral Design Considerations for Composites
 The Interface Tests in CompositesThe Interface Tests in Composites
 Microbond Tests-Single and MultipleMicrobond Tests-Single and Multiple
 Concept, Conduct and Evaluation of Multiple FibreConcept, Conduct and Evaluation of Multiple Fibre
Microbond TestsMicrobond Tests
 Fractography and FeedbackFractography and Feedback
 Comparison with Existing MethodsComparison with Existing Methods
 Conclusions and SummaryConclusions and Summary

Information is whereInformation is where
design begins !design begins !
Design is defined
as the complete
information
required to
produce a
product or
render service
-
Anonymous

General Design RequirementsGeneral Design Requirements
 Tensile, Compressive, Shear , Flexure and CreepTensile, Compressive, Shear , Flexure and Creep
 Load Control, Position Control and Strain Control.Load Control, Position Control and Strain Control.
 Ductility and Plastic StrainDuctility and Plastic Strain
 Buckling, Collapsibility and CrushingBuckling, Collapsibility and Crushing
 Modularity and FlexibilityModularity and Flexibility
 Crumple Zones and CrashworthinessCrumple Zones and Crashworthiness
 Static and Dynamic Fracture EnergyStatic and Dynamic Fracture Energy
 Impact Strength and ToughnessImpact Strength and Toughness
 Noise Vibrations and HarshnessNoise Vibrations and Harshness
 Fatigue , Hot Wet Fatigue, Acoustic Fatigue, FatigueFatigue , Hot Wet Fatigue, Acoustic Fatigue, Fatigue
after DBTafter DBT
 Statics and Dynamics of Structures and AssembliesStatics and Dynamics of Structures and Assemblies
 Failure Mode Interactions, Multiple Causes of FailureFailure Mode Interactions, Multiple Causes of Failure
 Multi FunctionalityMulti Functionality

Interface TestsInterface Tests
 Single fibre pullout testSingle fibre pullout test
 Single fibre push out test, brittle materialsSingle fibre push out test, brittle materials
 Micro bond single fibre pullout testMicro bond single fibre pullout test
 Fibre Fragmentation testFibre Fragmentation test
 Multiple fibre pullout test and Micro bondMultiple fibre pullout test and Micro bond
multiple fibre pullout test, Statisticallymultiple fibre pullout test, Statistically
averaged results, More precise, Lessaveraged results, More precise, Less
difficult, Mesomechanical in nature.difficult, Mesomechanical in nature.
 Interlaminar shear strength test, ILSS.Interlaminar shear strength test, ILSS.

The Fibre-Resin MicroThe Fibre-Resin Micro
bondbond
A drop of the cured resin on the
fibre surface showing adhesion
Fibre pullout test through a micro
vise design fixture
Ref: Bernad Miller, Umesh Gaur and Douglas E. Hirt, Composites Sci. Tech.,
42, pp 207-219, (1991). A classic !

Micro bond Bundle Pull-outMicro bond Bundle Pull-out
TestTest
Carbon Fibre/ Epoxy Drop

Multiple Fibre PulloutMultiple Fibre Pullout
Ref: K. Padmanabhan , Toyobo Confidentiality Report, 2002.
Micro bond Matrix slab
Ref: C. Y. Yue and K. Padmanabhan , Composites B, 30(1999) p205. A Classic !

Present Advantages andPresent Advantages and
LimitationsLimitations It is easy to formulate and evaluate compared to any knownIt is easy to formulate and evaluate compared to any known
interfacial test method.interfacial test method.
 It requires no tooling or shaping. It is less cumbersome toIt requires no tooling or shaping. It is less cumbersome to
perform and evaluate than the single fibre microbond test orperform and evaluate than the single fibre microbond test or
the bundle pullout test .the bundle pullout test .
 It has more physical relevance as it is mesomechancial inIt has more physical relevance as it is mesomechancial in
nature, well averaged in results due to a fibre bundle and morenature, well averaged in results due to a fibre bundle and more
precise than the microbond single fibre pullout test .precise than the microbond single fibre pullout test .
 It addresses the issue of volume fraction similar to the bundleIt addresses the issue of volume fraction similar to the bundle
pullout test which the single fibre pullout tests cannot give.pullout test which the single fibre pullout tests cannot give.
 It is limited only by a consistent drop size and wetting angleIt is limited only by a consistent drop size and wetting angle
and may not be applicable to thicker fibre bundles that are usedand may not be applicable to thicker fibre bundles that are used
in high GSM ( grams per square metre ) fabric materials.in high GSM ( grams per square metre ) fabric materials.
 Pressure is never considered in this test but most of thePressure is never considered in this test but most of the
laminates are fabricated with about 1-20 bars of pressure .laminates are fabricated with about 1-20 bars of pressure .
 The resin slab technique described elsewhere can useThe resin slab technique described elsewhere can use
pressurized conditions and the influence of external pressurepressurized conditions and the influence of external pressure
on the interfacial shear properties of laminated composites canon the interfacial shear properties of laminated composites can
be evaluated.So, the estimates of the present technique arebe evaluated.So, the estimates of the present technique are
conservative as the positive influence of compacting pressureconservative as the positive influence of compacting pressure
on the interfacial properties , is never considered.on the interfacial properties , is never considered.

Modelling and AnalysisModelling and Analysis
 Carbon Fibre /epoxy Matrix MicrobondCarbon Fibre /epoxy Matrix Microbond
Embedded Composite SystemEmbedded Composite System
Cylindrical Assemblage
Matrix Microbond
Pull Out Through a Micro vise
• Micromechanics
le

Non-linear QuasistaticNon-linear Quasistatic
AnalysisAnalysis
 Surface to surfaceSurface to surface
contactcontact
 CA is anisotropic andCA is anisotropic and
matrix is isotropicmatrix is isotropic
 Only failure and postOnly failure and post
failure conditions werefailure conditions were
considered due toconsidered due to
relevance and precisionrelevance and precision
 Model has dihedralModel has dihedral
symmetrysymmetry
Solid 8 noded brick 185 element
Hex swept volumes

Contact Angle and EmbeddedContact Angle and Embedded
Length of ResinLength of Resin
Fig . Schematic of the microbonds of resin
droplets on the fibre bundle showing the angles of the cured droplet
θ
θ
θ
θ

The EDM Drilled MicroviseThe EDM Drilled Microvise
Figs. Optical micrographs of EDM drilled
microvises
with approximate diameters of 0.5 and 1mm .

TABLE I
Features Of The Test Specimens And
Methods
Type of fibre: Carbon fibre industrial grade .
Tensile strength= 2.36 GPa and modulus Ef
= 120 GPa
No: of filaments in a bundle = 600
Resin : High Perf. epoxy adhesive
Elastic modulus of cured resin, Em = 2.2 GPa
Vf
in the bundle section = 0.14
Ave. bundle Ф in composite = 0.428 mm
Radius of bundle, r = 0.214 mm
Microvise Ф = 465 +/- 5 μm and 1mm
Average diameter and length of the resin drop, variable

Pull Out ForcePull Out Force vs.vs. DisplacementDisplacement
Displacement
Fig . A schematic pull out force vs displacement plot for a
multiple fibre microbond pull out test.

Microbond Bundle Pullout Test
Specimen
ID
(mm)
Drop
Length (l)
mm
Contact
Angle
(θ, deg )
Maximum Force for Fibre Pull
Out (Newtons)
1
2
3
4.
5.
6.
2.25
4.42
3.17
2.97
2.9
2.84
24.25
12.5
30.5
20.5
20.25
22
22.97
64.85
75.65
81.25
59.85
32.52
Average 3.09 mm 21.7 degrees 56.18 NThe evaluated intrinsic bond strength, τ, = 13.52 MPa at Vf
=
0.14
The evaluated interfacial shear stress at failure, IFSS = 56.23
MPa.

Interfacial ParametersInterfacial Parameters
 Intrinsic bond strength,Intrinsic bond strength, ττ
 Interfacial shear strength,Interfacial shear strength, ττss
 Interfacial frictional stress, τInterfacial frictional stress, τff
 Fibre/matrix coefficient of friction, µFibre/matrix coefficient of friction, µ
 Matrix shrinkage pressure, PMatrix shrinkage pressure, Po=o= ττf/f/ µµ

Interfacial Property EvaluationsInterfacial Property Evaluations
τ = F/ πФ l ---------------------- (1)
where F is the peak debonding force in newtons
minus the initial frictional force, if any. In the
present case the microvise was optimally
designed not to produce any frictional trace with
the fibres or the adhering resin. Ф is the average
diameter of the fibre bundle in mm and l is the
embedded length of the resin droplet in the fibre
bundle in mm. The interfacial intrinsic bond
strength is given by the peak debonding force
divided by the available outer interfacial area.

Interfacial EvaluationsInterfacial Evaluations
A drop profile is illustrated in Fig. The wetting contact angleA drop profile is illustrated in Fig. The wetting contact angle
produces a force that compresses the resin against the fibreproduces a force that compresses the resin against the fibre
end acting against the shearing process. This force , Fc, shouldend acting against the shearing process. This force , Fc, should
be equal to F cot θ, F being the simultaneous direct force thatbe equal to F cot θ, F being the simultaneous direct force that
produces the compressive component. This can beproduces the compressive component. This can be
debonding /adhesive, tack or frictional at any tentativedebonding /adhesive, tack or frictional at any tentative
location in the load deflection plot that has be understood withlocation in the load deflection plot that has be understood with
care and the component should not be considered as due tocare and the component should not be considered as due to
only the maximum force . It manifests on frictional onset as aonly the maximum force . It manifests on frictional onset as a
second component .Thus we can writesecond component .Thus we can write
F = Fad + μFc --------------------------------------F = Fad + μFc --------------------------------------
(2)(2)
Where ,Where ,
Fad = Fmax = τA . ---------------------------------- (3)Fad = Fmax = τA . ---------------------------------- (3)
A being the considered interfacial cylindrical area. In this caseA being the considered interfacial cylindrical area. In this case
the maximum force is also the force required for debonding.the maximum force is also the force required for debonding.
However, the peak pullout force can occur after debonding dueHowever, the peak pullout force can occur after debonding due
to build up of frictional stress which can again be addressed byto build up of frictional stress which can again be addressed by
the equation 2.the equation 2.

Interfacial EvaluationsInterfacial Evaluations
So,So,
F = τA + μFF = τA + μFff cot θ ----------------------------------- (4)cot θ ----------------------------------- (4)
Here, FHere, Fff , is a variable force down the load deflection plot unlike, is a variable force down the load deflection plot unlike
the peak debonding force that is singular, θ is the angle ofthe peak debonding force that is singular, θ is the angle of
wetting and contact of the fibre bundle with the resin dropletwetting and contact of the fibre bundle with the resin droplet
and μ is the static coefficient of friction as the pull out occurs atand μ is the static coefficient of friction as the pull out occurs at
a rate of 1-5 mm/min. It is evident from the load deflection plota rate of 1-5 mm/min. It is evident from the load deflection plot
( see Figure) that the value of the coefficient of friction is likely( see Figure) that the value of the coefficient of friction is likely
to change from the onset of frictional sliding to a steady value,to change from the onset of frictional sliding to a steady value,
if the frictional sliding persists. The various stages of theif the frictional sliding persists. The various stages of the
failure process are already discussed in the schematic sketchfailure process are already discussed in the schematic sketch
shown in Fig. The interfacial shear stress (IFSS) is generatedshown in Fig. The interfacial shear stress (IFSS) is generated
by dividing the equation (4) with A that also gives us theby dividing the equation (4) with A that also gives us the
frictional stress values. Hence,frictional stress values. Hence,
F/A = IFSS= τ+ μτF/A = IFSS= τ+ μτff cot θ -------------------------------- (5)cot θ -------------------------------- (5)

Fig . Drop length vs. Pull out force

Fig . IFSS vs. Volume
fraction

FractographyFractography
Fig. The fibre bundle after the shear pull out of the bond. b) SEM picture
of the resin microbond droplet sheared out of the fibre bundle. Notice
that the drop is still intact in this case.

FractographyFractography
Fig. : SEM fractograph of the microbond sheared through the fibre
bundle. Notice the micro vise mark on the resin droplet due to
compressive stresses near the exit. b) A completely sheared out intact
drop from the bundle of fibres due to pull out.

Conclusions
1. The interfacial properties of the Carbon fibre /epoxy
composite, like the interfacial frictional stress, τf , the static
coefficient of friction, μ, the instantaneous interfacial shear
stress, the intrinsic bond strength ,τ, and the maximum
interfacial shear strength, τi, were predicted and evaluated
with the aid of micromechanical theories and the newly
developed microbond bundle pullout technique.
2. The methods involved provide us with the interfacial
properties evaluated that are consistent within the
experimental allowables. The average interfacial shear
strength is 56.23 MPa from the bundle microbond pullout
test performed at a Vf of 0.14. Higher volume fraction
predictions are also made as practical impossibilities of
achieving higher volume fractions of fibre bundles exist in the
microbond tests due to the lack of pressure in forming the
resin drops.

ConclusionsConclusions
3. An important finding of this investigation is that the
mesomechanical evaluation of the interfacial properties is
more reliable than the micromechanical predictions or
their single fibre experimental evaluations due to their
comparable values with the laminate level properties,
raising new hopes on the precision (but not accuracy) of
the strength based small specimen testing approaches.
4. There is a mesomechanical domain which appears
to be different from the micro or the macromechanical
domain. This domain is significant as most of the
macromechanical properties depend on mesomechanical
fracture features like craze, crack tip plasticity, fibre
bundle debonding from matrix and damage tolerance, to
mention a few.
5. The fractographic investigations substantiate the
results obtained thereby throwing more light on the
associated fracture mechanisms and the tools to be
adopted to improve the techniques after understanding
the fracture and failure modes .

AcknowledgementsAcknowledgements
The AR and DB, New Delhi, are thankedThe AR and DB, New Delhi, are thanked
for the funded project bearing numberfor the funded project bearing number
1650/2012. The author thanks the1650/2012. The author thanks the
CAMPT-VIT, University ,Vellore, a FIST-CAMPT-VIT, University ,Vellore, a FIST-
DST facility for the Instron 8801 testing ofDST facility for the Instron 8801 testing of
the carbon/epoxy specimens. He alsothe carbon/epoxy specimens. He also
thanks M. Ramya for the assistance.thanks M. Ramya for the assistance.

ReferencesReferences
1.Akio Shindo, R Fuji and M. Sengoku and Japanese Bureau of
Industrial Technics, Japanese Patent 4405/1962 ( 13.6.62) .
2. J.D.H. Hughes, The carbon fibre/epoxy interface- A review,
Composites Science and Technology,Volume 41, Issue 1,
Pages 13–45, 1991 .
3. K. Padmanabhan, Interfacial properties of Zylon AS/epoxy
composites, Toyobo Confidentiality Agreement Report,
( Toyobo Co. Ltd, Japan), May, 2002.
4. K. Padmanabhan and CY Yue, Interfacial properties of Zylon
AS/epoxy composites by the multiple fibre pull out technique-A
case study, Proc-ACUN-4, UNSW, Sydney, Australia- 21-25,
pp238-245, 2002,
5. Bernad Miller, Umesh Gaur and Douglas E. Hirt, Composites
Sci. Tech., 42, pp 207-219, (1991).
6. J.K. Kim and YW Mai, Engineered Interfaces in Fibre
reinforced Composites, Elsevier, Amsterdam , 1998.
7. Ever J. Barbero, Chapter 4, Micromechancics, in Introduction
to Composite Materials Design, CADEC, Taylor and Francis, PA,
(1999).

ReferencesReferences
8. C.Y. Yue and Padmanabhan. K, Interfacial Studies on Surface
Modified Kevlar Fibre/Epoxy Matrix Composites,
Composites:B, Vol : 30, pp 205-217, 1999.
9. K. Padmanabhan, The Singapore- MIT Alliance Report ,
Macrostructural property evaluation of fibre/polymer composites
from the micromechanical behaviour through the mesomechanical
route-Part , Innovation in Manufacturing Systems and
Technology, NTU, Singapore, November 2003,
10. M.R. Piggott, Mesostructures and mesomechanics in fibre
composites, Composites Science and Technology, Vol:5, Issue
2, pp 121-252, (1995).
Thank you !

Hygrothermal BehaviourHygrothermal Behaviour
 Fibre reinforced plastics are known forFibre reinforced plastics are known for
environmental attacks that reduce their function.environmental attacks that reduce their function.
 Mechanical properties degrade over time !Mechanical properties degrade over time !
 Moisture plays havoc at elevated temperatures,Moisture plays havoc at elevated temperatures,
in the presence of voids, defects and in low Tin the presence of voids, defects and in low Tgg
plastics .plastics .
 Diffusion and osmotic pressure are the drivingDiffusion and osmotic pressure are the driving
mechanisms for hygrothermal attackmechanisms for hygrothermal attack
 Evaluation methods and surface preservationEvaluation methods and surface preservation
 Important in marine, biomedical, aeronautical,Important in marine, biomedical, aeronautical,
electronic and automobile applicationselectronic and automobile applications

Multi Scale Composites forMulti Scale Composites for
DentistryDentistry
 Bis GMA, UDMA, Methacrylic Esters containBis GMA, UDMA, Methacrylic Esters contain
glassy particles that are mostly less than 1glassy particles that are mostly less than 1
micron in sizemicron in size
 Esters and acrylates/ceramic filler (bariumEsters and acrylates/ceramic filler (barium
alumina silica glass, glassy microfillers, 0.1 toalumina silica glass, glassy microfillers, 0.1 to
10 microns size) restorative composites.10 microns size) restorative composites.
 Multi scale composites are also useful asMulti scale composites are also useful as
luting cements, crown and bridge materialsluting cements, crown and bridge materials
and cements and veneer materialsand cements and veneer materials..

Composites in ProsthodonticsComposites in Prosthodontics
Tooth is a functionally graded
composite material with enamel
and dentin. In the third maxillary
molar the occlusal stress can
be 2-3 MPa.
The masticatory heavy chewing
stress will be around 193 MPa.
A composite restorative must with
stand this with an FOS and with
constant hygrothermal attack.

Composites in MicroelectronicComposites in Microelectronic
PackagingPackaging
The BOM includes Copper lead frame,
Gold wires for bonding, Silver –epoxy
for die attach, Silicon die and Epoxy
mould composite with Phenolics, Fused
silica powder and Carbon black powder
as the encapsulant materials.

ASTM STP D 5229 M RuleASTM STP D 5229 M Rule
 The MOT( Maximum Operating Temperature)The MOT( Maximum Operating Temperature)
of the material, device/component should beof the material, device/component should be
at least 25at least 25 ºº Celsius lower than the lowest TgCelsius lower than the lowest Tg
(normally wet) of the material attained after(normally wet) of the material attained after
hygrothermal equillibration. All the polymerichygrothermal equillibration. All the polymeric
materials and their composites must satisfymaterials and their composites must satisfy
this rule in order to qualify for certification forthis rule in order to qualify for certification for
reliability and durability.reliability and durability.

That one coming being,That one coming being,
Was covered with void,Was covered with void,
That arose through the powerThat arose through the power
of heatof heat
-The Rig Veda ( The Existence-10.129.03)-The Rig Veda ( The Existence-10.129.03)

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