Materials

Materials and Design 25 (2004) 289–296

www.elsevier.com/locate/matdes

&Design

On the tensile and shear strength of nano-reinforcedcomposite interfaces

S.A. Meguid *, Y. Sun

Engineering Mechanics and Design Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto,

5 King�s College Road, Toronto, Ont., Canada M5S 3 G8

Received 9 June 2003; accepted 29 October 2003

Abstract

The tensile debonding and shear properties of composite interfaces reinforced by two different homogeneously dispersed

nanofillers, carbon nanotubes and alumina nanopowder, are investigated. The composite adherends used are made of carbon fibre/

epoxy laminate and aluminium alloy 6061-T6. The results reveal that varying the weight percentage of the nanofillers into the epoxy

matrix adhesive favourably influences the debonding and shear characteristics of the interface. The results also indicate that in-

creasing the amount of the nanofillers beyond a certain weight fraction of the adhesive reduces the interface strength. SEM and

TEM images of the fillers are presented and used in support of a proposed strengthening/weakening mechanism(s).

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Nano-reinforced interfaces; Carbon nanotubes; Alumina nanopowder; Mechanical behaviour; Hybrid structures

1. Introduction

In view of their importance and utility in aerospace,

automotive and communication fields, carbon fibre re-

inforced plastics (CFRP) are currently being extensively

studied. This is because this class of materials possesses

admirable properties low weight, high fracture tough-

ness with relatively high strength [1–5] that makes them

suitable for such applications. Recent research efforts inthis field have focused not only on the determination of

the mechanical properties and interface morphology of

carbon fibre reinforced plastics but also on the proper-

ties of hybrid reinforced composites in which dispersed

second phase nanoparticles are added to epoxy adhe-

sives [6–10]. However, the influence of the incorporation

of nanoscale particles into epoxy adhesives for the

purpose of joining two dissimilar composite materialshas not been given its due attention. This may be due to

the large variance in function, intricacy of geometry,

incompatible materials and operating conditions. In-

deed, joining of dissimilar materials requires consider-

* Corresponding author. Fax: +1-416-978-7753.

E-mail address: meguid@mie.utronto.ca (S.A. Meguid).

0261-3069/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2003.10.018

ation of the compositional compatibility of interfaces[11–16].

Structural bonded joints can fail at different locations

and by a variety of failure modes. Failure can occur or

initiate in the adhesive or in the adherend, depending on

the geometrical configuration, the materials of the

adherends, the adhesive as well as the manufacturing

procedure. It is difficult to describe and define all the

possible failure modes of adhesive bonded joints. Weclassify the failure modes of adhesive bonded joints into

the following four general categories: (i) adherend fail-

ure due to tension, (ii) interfacial failure due to shear,

(iii) debonding cohesive failure, and (iv) out-of-plane

failure due to delamination in composite adherends [17].

However, the presence of the interface in adhesively

bonded joints governs the strength of that joint.

In many practical applications, high stress within theinterfacial region, which is typically semi-brittle, may

result in rapid crack propagation through composite

interfaces. Therefore, a strong interface, which possesses

high toughness, is highly desirable. Gadakaree [18] im-

proved the mechanical properties of fibre reinforced

composites by improving the toughness of the glass

matrix with micro-scale particle fillers. Krishnamoorti

290 S.A. Meguid, Y. Sun / Materials and Design 25 (2004) 289–296

discussed the shear properties of polymer-layered sili-

cate nanocomposites in [19]. Sklep [20] successfully

prepared a series of polymer/clay nanocomposites with

layered silicate clays as an inorganic dispersed phase via

intercalation polymerisation. Soanoudakis and Young[21] and Wang et al. [22] also studied the effect of filler

particles in reinforced resin composite systems and ob-

served the toughness improvement from the addition of

filler particles. Recently, Hussain et al. [23] studied the

improvement in the mechanical properties of carbon

fibre reinforced epoxy composites by nanofiller disper-

sion. In this paper, we focus our attention to the influence

of the homogeneous dispersion of carbon nanotubes andalumina nanoparticles to epoxy adhesive for the purpose

of enhancing its interface properties.

2. Experimental procedure

2.1. Materials used and their preparation

2.1.1. Epoxy adhesive

In this study, we used epoxy-based adhesive – Dexter

Hysol EA 9330 produced by Loctite Aerospace Limited.

This adhesive is typically used for aerospace, automotive

and electronics applications. It is a two-component

paste adhesive, which is easily mixed at room tempera-

ture and has high peel strength. This adhesive system

has an excellent environmental resistance and bonds to avariety of substrates. The original properties and com-

position as provided by the manufacturer are summar-

ised in Tables 1 and 2.

2.1.2. Carbon fibre and epoxy resin laminate substrate

Carbon fabric (unidirectional) G1157/EFP 100 pro-

vided by Hexcel Fabrics was used for one of the sub-

Table 1

Physical and chemical properties of epoxy adhesive as provided by

manufacturer

Properties Value

Specific gravity 1.15

Viscosity 1000–3000 Poise

Brookfield 5 Spdl

Volatile organic content <10 g /1

Vapour density >1

Table 2

Composition of epoxy adhesive as provided by manufacturer

No. Composition of adhesive Wt%

1 Bisphenol A/Epichlorohydrin/Epoxy resin 85–90

2 Butadiene – Acrylonitrile Copolymer 3–4

3 Epichloro – Hydrin – polyglycol copolymer 2–4

4 Aluminium oxide 3–5

strates. The carbon fabric has nominal weight per unit

area of 290 g/m2, while the epoxy resin density is 1.25 g/

cm3. The unidirectional carbon fibre laminate has fibre

volume fraction of 55%. The ultimate tensile strength of

the laminate ¼ 1200 MPa, compressive strength ¼ 155MPa, shear strength ¼ 57 MPa, tensile modulus E11 ¼180 GPa and E22 ¼ 12 GPa, shear modulus G12 ¼ G13 ¼28 GPa, and Poisson�s ratio ¼ 0.263.

2.1.3. Aluminium substrate

Aluminium 6061-T6 is widely used in aircraft and

automotive parts as well as in electrical communications

and connectors. It has excellent joining characteristics,good acceptance of coatings, combines relative high

strength, good workability and high resistance to cor-

rosion. It has a density of 2.7 g/cm3, ultimate tensile

strength of 310 MPa, tensile yield strength of 275 MPa,

and a modulus of elasticity of 69 GPa and a Poisson�sratio of 0.28.

2.1.4. Single walled carbon nanotubes

The manufacturer�s instructions indicate that the rawsingle walled carbon nanotubes were synthesised by the

electric arc technique. In this case, a composite rod

(Ni:Y:C) is vaporised under helium atmosphere. The

pristine material contains SWNTs ropes coexisting with

metallic nanoparticles and other forms of carbon

(graphite, amorphous nanoparticles, nanohorn). The

purification involved the treatment of the carbon na-notubes with nitric acid followed by a cross flow filtra-

tion and an annealing under nitrogen atmosphere at

1600 �C. Fig. 1(a) and (b) show SEM images of single

wall carbon nanotube at two different amplifications. It

can be seen that the nanotubes have diameters between

30 and 60 nm and are a few hundred nanometers long.

The tubes appear like ropes, which will be homoge-

neously dispersed and embedded in the epoxy adhesivesupon proper mixing.

2.1.5. Nanoalumina fibre powder

The influence of nanoscale alumina fibre powder with

approximately 2–4 nm in diameter and a few hundred

nanometers in length has also been investigated. The

surface area of the alumina nanopowder ranges from

300–700 square meters per gram. Individual fibres areproduced by the electro-explosion of metal wire, ap-

peared linear and co-mingled in a bundle under TEM

microscope.

The fibres have unique sorption properties, cationic

and anionic chemisorption properties, such as scaveng-

ing precious and heavy metals from water. Fig. 2(a) and

(b) show TEM images of the fibre powder. As can be

seen, they are agglomerated in the longitudinal direc-tion. It can also be seen that the alumina nanofibres

have aspect ratios greater than 20 and many of the fibres

appear to be hundreds of nanometers long. Detail of

Fig. 2. TEM micrographs of (a) alumina nanofibre powder noting

Fig. 1. SEM images of (a) single wall carbon nanotubes and (b) en-

larged single wall carbon nanotubes.

S.A. Meguid, Y. Sun / Materials and Design 25 (2004) 289–296 291

some of the pertinent properties of carbon nanotubesand alumina nanofibre powder is provided in Table 3.

Table 3

Comparison between carbon nanotube and alumina nanopowder

Properties Carbon nanotube Alumina

nanopowder

Size 30–60 nm in diameter 2–4 nm in

diameter

Young�s modulus 1 TPa (in-plane) 300 GPa

Tensile strength 200 GPa (in-plane)

0.1–0.5 MPa (out-of-plane)

2000 MPa

Aspect ratio 10–30 20–80

Surface area 300–600 m2/g 300–700 m2/g

absence of particles and (b) nanofibre powder noting fibres in focus in

the lower foreground.

2.1.6. Preparation of adherends

The two dissimilar aluminium and carbon fibre adh-

erends were machined from their respective sheets. The

carbon fibre reinforced laminates were fabricated by

autoclave at specified pressure and temperature. The

surface was sand blasted, cleaned by chemical solventand dried before bonding. Carbon nanotubes and alu-

mina nanopowders were respectively dispersed into the

above-mentioned epoxy adhesive. The dispersion was

carried out by stirring the mixture for 30 min at 50 �C.This was then followed by adding and blending the re-

maining components of the adhesive. This ensured the

homogeneous dispersion of the nanofillers into the ad-

hesive. Fig. 3(a) shows typical micrographs of the epoxyresin with uniformly dispersed carbon nanotubes, while

Fig. 3(b) shows the corresponding figure with alumina

nanopowder. The weight concentration in these two

cases was 2.5%. Additional nanomixtures with nanofil-

lers at nominal weight percentages of 1.5%, 5%, 7.5%,

10%, 12.5% and 15% were also prepared. The substrates

were then bonded together and carefully cured at a

controlled room temperature for seven days.

2.2. Details of mechanical testing

Fig. 4(a) and (b) show schematics of the test set ups

used for the tensile debonding and shear testing of the

Fig. 3. Nanoimages of epoxy adhesive after dispersed carbon na-

notubes and alumina nanopowder: (a) SEM image of epoxy adhesive

with carbon nanotube and (b) TEM image of epoxy adhesive with

alumina nanopowder.

Fig. 4. Schematics of the test setups used: (a) tensile test and (b) single-

lap shear test.

292 S.A. Meguid, Y. Sun / Materials and Design 25 (2004) 289–296

new interface. Fig. 4(a) was employed for determining

the tensile stress–strain characteristics, modulus of

elasticity and ultimate tensile strength of the nano-

reinforced interface. The loading was applied perpen-

dicularly to the bonding plane, and the test procedure

was carried out as per ASTM 4541-95 standard re-

quirement. A plane bonding area of 10 mm� 10 mmwas used in our work. The tests were carried out using

an electro-hydraulic servo-controlled testing machine at

a crosshead speed of 1.0 mm/min. It is worth noting that

the load co-axiality was ensured using special crosshead

adapters and strain gauged samples.

The single-lap test set up shown in Fig. 4(b) was

employed for determining the shear modulus and shear

strength of the nano-reinforced interface. The length of

overlap of the bonding region was 12.7 mm. The

bonding area had a constant thickness of 0.1 mm and a

constant width of 25.4 mm. The adherends were securely

tightened to an adapter, which in turn was secured in the

grips. This arrangement ensured that no slip would take

place during testing. Analogous to the tensile test, the

loading train was vertically aligned prior to testing. Thetesting was carried out as per ASTM D3162-92 standard

requirement using the same electro-hydraulic testing

machine and the same loading rates. The influence of the

stiffness of the loading bars, the adherends and the

machine crosshead was considered during the testing of

the interface. The testing results show a maximum dis-

placement of 0.035 mm at a tensile load of 2.5 kN, and

0.053 mm at a shear load of 5.0 kN, respectively. Thecompliance of the machine as well as loading train was

subtracted from the load-deformation results reported

in this work.

3. Results and discussion

Fig. 5(a) shows tensile load–displacement for threedifferent cases: epoxy adhesive with alumina nanopow-

der (EANP), epoxy adhesive with carbon nanotube

(EANT) and epoxy adhesive (EA). The figure indicates

that the bonding strength increases as a result of the

presence of uniformly dispersed carbon naotubes and

alumina nanoparticles. This is evident in the increase in

the critical load, with the critical load for EANT being

1.2 times EA and EANP being 1.4 times EA.The load–displacement data of Fig. 5(a), which are

averages of a minimum of three tests, were used to ob-

tain the mechanical properties of EANP and EANT.

Fig. 5(b) and (c) show the remarkable improvement in

Young�s modulus as well as the ultimate tensile strength

Fig. 6. Test results showing (a) typical shear load–displacement dia-

grams for nano-reinforced interfaces, (b) effect of weight percentage of

nanoparticles on ultimate shear strength and (c) effect of weight per-

centage of nanoparticles on shear modulus.

Fig. 5. Controlled test results showing (a) typical tensile load–dis-

placement diagrams for nanoreinforced interfaces, (b) effect of weight

percentage of nanoparticles on ultimate tensile strength and (c) effect

of weight percentage of nanoparticles on tensile modulus.

S.A. Meguid, Y. Sun / Materials and Design 25 (2004) 289–296 293

for the cases involving different weight fractions of ho-

mogeneously dispersed nanofillers used in the study. The

increase continues with the increase in the weight per-

centage of the nanofillers. However, for percentages

above 10%, the properties degrade to below the EA,

indicative of the sensitivity of the epoxy to the strength

of the concentration of the dispersed nanofillers.

Fig. 6(a) shows load–displacement data resulting

from the shear lap tests for three different cases: EANP,

EANT and EA. Again, the weight fraction ratio of the

filler concentration was 2.5% for EANP and EANT.

The figure also reveals that there exists strengthening ofshear resistance as a result of dispersed carbon na-

notubes and alumina nanoparticles, as evidenced by the

increase in the critical shear loads with the critical load

for EANT being 1.1 times EA and EANP being 1.3

Fig. 7. Microscopy of surface at interface of CFRP specimen, showing

the failure surface with different weight percentage of nanopowder: (a)

surface with 2.5% weight of nanopowder has larger areas of fibres

pulled out and exposed, (b) surface with 7.5% weight nanopowder

shows a moderate area of fibres is exposed and (c) surface with 15%

weight nanopowder shows less areas of fibre exposed.

294 S.A. Meguid, Y. Sun / Materials and Design 25 (2004) 289–296

times EA. Fig. 6(b) and (c) show the remarkable im-

provement in the shear modulus as well as the shear

strength for the cases involving different concentrations

of dispersed alumina nanopowders and carbon na-

notubes. Analogues to the tensile tests, the results alsoreveal the sensitivity of the shear properties to the con-

centration of the nanofillers. An increase in the weight

fraction of the nanofillers beyond 7–8% results in a re-

duction in the shear properties of the adhesive.

Fig. 7 shows three SEM micrographs of the failure

surface of the carbon CFRP composite. They provide

useful information about the strength of the interface as

a result of the dispersion of the carbon nanopowder. Forexample, Fig. 7(a) shows the surface failure at the in-

terface from the CFRP side due to shear loading for a

weight fraction of 2.5% nanodispersed alumina nano-

powder. Two distinct regions exist: the first is the dark

region showing exposed fibres of the CFRP substrate,

while the second contains remnants of the nano-

reinforced adhesive. The rough areas of the CFRP sug-

gest a stronger bond exists at the interface as a result ofthe strengthening effect resulting from the dispersed

nanofillers. Fig. 7(b) and (c), with their respective

nanopowder weight concentrations of 7.5% and 15%,

show reduced areas of the exposed fibres of CFRP.

Fig. 7(b) shows clearly fibre pull out at the interface of a

section of the bare CFRP. However, larger areas are

covered by the epoxy adhesive indicative of the reduc-

tion in the strength in comparison with Fig. 7(a) for the2.5% case. For carbon nanotubes reinforced interfaces,

Fig. 8(a)–(c) show typical failures of three different in-

terfaces corresponding nanotube weight concentration

of 2.5%, 7.5% and 15%, respectively. Again, it is ob-

served that for the 2.5% case, large areas of carbon fibres

of the substrate are exposed due to the strong adhesive

bonding (Fig. 8(a)). In contrast, SEM analysis shows

that smaller areas of carbon fibres appear for weightfractions exceeding 7–8% (Fig. 8(c)).

It is worth noting that at a given weight (volume)

percent, the presence of nanoparticles plays a major

role in determining the strength of the interface. We

attribute that dependence of the tensile and shear

properties of nano-reinforced interface to the follow-

ing. The nanosized fillers are characterised by large

surface areas per unit gram. As the number of adhe-sively joined points increases, the cohesive strength of

the epoxy increases leading to a higher mechanical

strength of the interface. Our experimental results

show that there is a limit to the number of dispersed

nanofillers beyond which a drop in the properties is

observed. Once the nanoparticles fully fill the gaps

and porosities and all contact points are established,

the additional particles could not interact effectivelywithin the epoxy adhesive and consequently poor

matrix infiltration occurs. The additional particles may

force the polymer molecules to take up a strained

conformation and thereby considerably modify mo-lecular structures of polymer (OCH2, CH2O) and in-

terfaces that can be easily debond. It is also believed

that agglomeration of the nanoparticles could act as

failure initiation sites, which could result in lowering

the strength and stiffness of the adhesive.

We attribute that dependence of the tensile and

shear properties of nano-reinforced interface to the

Fig. 8. Microscopy of surface at interface of CFRP specimen, showing

the failure surface with different weight percentage of nanotubes: (a)

surface with 2.5% weight of nanotubes has larger areas of fibres torn

and pulled out, (b) surface with 7.5% weight nanotubes has moderate

areas of fibres exposed and (c) surface with 15% weight nanotubes

shows less areas of fibre exposed.

S.A. Meguid, Y. Sun / Materials and Design 25 (2004) 289–296 295

followings. The nanosized fillers are characterised bylarge surface areas per unit gram as the number of

adhesively joined points increases, the cohesive

strength of the epoxy increases leading to a higher

mechanical strength of the interface. This is due to (i)

the ability of the nanofillers to fill any microscopic

gaps present in the epoxy as a result of their disper-

sion, and (ii) mechanical interlocking resulting from

the presence of extensive contact regions between the

fillers and the adhesive.

4. Conclusions

In this study, we focused our attention to the deter-

mination of the influence of the homogeneous dispersion

of nanofillers in a special epoxy adhesive for the purpose

of increasing its interfacial strength and properties. Twodifferent types of nanofillers were used; namely, carbon

nanotubes and alumina nanopowder. The work con-

centrated on the experimental determination of the ten-

sile and shear properties of the nano-reinforced interface.

The results reveal that at a given weight (volume) per-

cent, the presence of nanoparticles plays a major role in

determining the strength of the interface. We attribute

that dependence of the tensile and shear properties ofnano-reinforced interface to the large surface area and

the consequent interlocking of the nanofillers with the

epoxy adhesive. Our experimental results also show that

there is a limit to the number of dispersed nanofillers

beyond which a drop in the properties is observed.

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