Effects of Processing Variability on Thermo- Mechanical...

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015

336 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR

Effects of Processing Variability on Thermo-

Mechanical Properties of Graded Epoxy-Graphite

Composites Fabricated by Gravity Method

Tirumali Manoj1, K Balasubramanian

1*, A Kumaraswamy

2

Abstract- This experimental research article presents the

investigation carried out with graded concentration (0.5-1.0 wt%)

of graphite powder (50µm) in epoxy and the effects of the varying

parameters on thermo-mechanical properties such as the

concentration and gel / solidification time of each layer to form

the bulk epoxy-graphite graded composites. The samples of 3, 4

and 7 Layers were experimented. The graded materials have been

characterised by XRD, FESEM, TGA, Flexural & Tensile testing

and Nano-indentation. The XRD images exhibit amorphous peak

with the graphite peak indicating a shift to lower intensity. The

investigation show the effect of variability of either the

concentration, number of layers or the gel time on the 2θ value

vis-à-vis that of pure epoxy indicating a variation of 7 to 9% and

being highest for higher filler materials. The FE SEM examined

the surface morphology of fractured specimens across the cross-

section. The images indicate continuous gradation with graphite

particles being well dispersed in epoxy. The TGA results of

graded composites depict a volatile decomposition with enhanced

char yield weight loss as compared to pure epoxy. The graded

composites especially, 3 layers with respect to pure epoxy has

shown enhancement in flexural and tensile properties. The nano-

indentation analysis indicates especially for 2 and 3 layers an

increase in hardness and modulus values at the interface across the

cross-section. The experiment indicated that process variability

has an effect on thermo-mechanical properties and may be tailored

for effective end use applications especially for aerospace

requirements.

Index Terms: Fabrication; graded polymer; thermo-mechanical;

gel time; nano-indentation; gravity method

Nomenclature

σf Flexural stress, MPa

P Applied load, N

LG Span length, mm

D Maximum deflection of the center of the beam, mm

b Width of the specimen tested, mm

d Depth of the specimen tested, mm

εf Flexural strain, mm/mm

Eb Modulus of elasticity in bending, MPa

m Slope of the tangent to the initial straight-line portion of

the load-deflection curve, N/mm

Vm Volume concentration of matrix

Vp Volume concentration of particle

Ec Modulus of Elasticity of composite, MPa

Em Modulus of Elasticity of matrix, MPa

Ep Modulus of Elasticity of particle, MPa

1. INTRODUCTION

The roadmap of exploration on polymer graded materials

for their processing techniques, characterisation and bulk

production began sometime in 1990s. This evolved

soonafter the progress on Functionally Graded Materials

(FGMs) especially the work related to metals and ceramics

took successful predominance over conventional

composites especially for high temperature applications.

The basis of study on graded materials began with the

theoretical work in 1970s by Bever and Duwez [1] and

Shen et al. [2]. This was recognized that it was later

followed up for its implementation by Klingshirn et al. [3].

Later, Niino et al. [4] in 1980s described the work on

graded materials of Metals and Ceramics emphasizing its

utilization for high temperature space applications. The use

of graded polymers for high temperature applications has

been on the rise. The use of Phthalonitrile-graphite polymer

has been researched for high temperature applications for

missiles and rocket applications [5]. The other resins like

the oligomers, PMR-15, the Bismaleimides (BMI) etc have

also been explored with fillers of carbon allotropes of micro

/ nano sizes [6]. This further led to many programs as

conducted by Germany as well as Japan and other European

and Western Nations for professionalising the fabrication

technology and characterization of graded materials for cost

effective and bulk production [7]-[17]. The processing and

bulk production of metals and ceramic gradation materials

has been more or less established that the polymer graded

materials became the point of research. The polymers of

thermoplastic as well as thermosets along with the graded

reinforcements (fillers, fibres, and hybrids) became the



subjects of study. The gradation of microstructure or the

composition along one dimension are being researched with

types of polymer-reinforcement combinations for obtaining

tailored properties useful for varied science and engineering

applications. Jyongsik Yang et al. [10] have demonstrated

on the fabrication and the mechanical properties of glass

and carbon fibre polypropylene functionally graded

material which has been developed by varying the filler

concentration (Carbon fibre or Glass fibre) spatially.

Nowadays, there are several literature reviews and research

work on issues concerning studies on various aspects of

thermo-mechanical and functional properties such as

erosive wear resistant, heat transfer solutions, stress

analysis and fracture toughness, electrical conductivity &

resistivity in a material, electromagnetic shielding and UV

absorption etc. that could establish the importance of a

graded material [15], [18]-[25]. In particular, polymer

graded materials are being explored for deriving both their

primary stiffness to weight and strength to weight

advantages as well as the added contra properties within a

material due to tailored gradation of polymer matrix-

reinforcement mix.

The polymer gradation fabrication techniques are broadly

classified under either the casting or the pressing

techniques. The casting which is by gravitational method

has been seen quite economical and effective [12]-[13],

[26]-[31]. The present article is bringing out the research

work carried out in the fabrication of epoxy/ graphite

powder (Micrometer size, 50µm) composite using the

gravity method. The graphite filler concentration has been

in the order of 0.5 to 1.5wt% in steps of 0.5wt%. The

literature reviews have shown studies carried out on graded

epoxy-graphite polymers with varying graphite or other

carbon allotropes of filler concentration of micro or nano

sizes varying from 2.5wt% to 80wt% [1]-[2], [20]-[21],

[29]-[30], [31]-[34]. The materials, epoxy and graphite

have been chosen, as their source is in abundance and also

have vast applications in Science and Engineering,

especially in the aerospace applications [13]-[15]. The other

essential advantage gathered from Zurale et al. [35] has

been the cost effectiveness obtained by adding fillers such

as graphite in epoxy. The low viscosity epoxy resin with

amine curing agent that enables good crosslinking at room

temperature [36]-[38] has been taken alongwith the fine

powder graphite which has the affinity to epoxy in forming

a good bonding between each other at room temperature for

stronger crosslinking and networking during curing. The

article discusses the feasibility of fabricating a graded

epoxy-graphite composite and also the effect of variability

w.r.t. the concentration change, gel time, layer-wise

solidification time and the number of layers forming the

bulk on the thermo-mechanical properties. This research

work is a prelude to further study under progress to tailor

the resin-graded reinforcement combination for required

functional properties as applicable for aerospace

applications. The characterisation include mechanical

testing for flexural and tensile properties which is supported

by the non-destructive technique i.e. nano-indentation

technique, Field Emission Scanning Electron Microscope

(FE SEM) to study the surface morphology and dispersion

characteristics of the graded filler in the matrix. Thermo

Gravimetric Analysis (TGA) for analysing the thermal

stability status with variation of filler content or the gel

time of each layer or the number of layers and finally with

XRD technique to understand the structure of the graded

composite. The analysis has shown the preliminary

feasibility of fabricating by the gravity technique.

2. EXPERIMENTAL DETAILS

Materials

The materials used in the process for fabrication of graded

polymer are given in Table 1.

Table 1. Materials for experimentation

The materials are of analytical grade, which are used

directly without any treatment. Material key data as per

Original Equipment Manufacturer (OEM) data sheet is

mentioned in Table 2.

Table 2. Material key data

Experimental

The gradation is based on change from 100% pure polymer

(Epoxy + Hardner) to graded polymer (Epoxy + Hardener +

(0.5 / 1.0 / 1.5) wt% Gr) layerwise along the thickness

direction with the net weight equal to 100wt%. The process

by simple gravity method has been carried out mainly for

samples of 2, 3, 4 and 7 layers of graded polymer-filler



mix. The specifications of different layered gradation

samples are given in Table 3. Table 3. Specimen specification

The standard technique of uniform dispersion of graphite in

a solvent and then mixing the epoxy followed by adding the

hardner for polymerization has been carried out for each

layer of concentration of filler-polymer mix [38-39]. The

fabrication design of the open mold made of aluminum

material for tensile test specimens and flexural test

specimens has been as per the specification given in ASTM

D 638-08 (Type I) and ASTM D 790-07 (Procedure A).

The span length is taken as greater than 16:1 ratio for

ASTM D 790-07 specimens. The molds are shown in

Fig 1 (A) and (B). The flexural properties are based on the

relations as per the beams supported at large support spans.

The equations used to determine the flexural properties are

given below.

𝜎𝑓 = (3PLG)/2bd2) [1 + 6 (D/LG))

2 - 4 (d/LG)) (D/LG)] (1)

εf = 6Dd/LG2 (2)

Eb=LG3m/4bd

3 (3)

Fig 1. Aluminum molds for mechanical testing (A) Tensile

(B) Flexural

The pure epoxy composite layers (epoxy + hardner in the

ratio of 100:34) are processed as per the schematic shown

in Fig 2 (A). The fabrication of graded polymer composite

is carried out by placing layers of filler-polymer mix as per

design when each is at semi-solid state after pouring and

maintaining to a specified gel / solidification time for

partial crosslinking. The bulk which is produced after

pouring of all layers as per thickness of the specimen will

be cured at first at room temperature for 24hours and later

post cured @ 3630K for 8h as per OEM data sheet

recommendations. The overall weight of each layer of

polymer-filler mix is 100g where the concentration of filler

and Epoxy-Hardner is proportionately weighed and mixed.

The process followed in the fabrication of

Epoxy-Graphite graded polymer has taken into reference

the work carried out by Dilini et al. in the fabrication of

Graphene oxide-Epoxy Nanocomposite, [11]. The

schematic of the graded polymer process is as shown in

Fig 2 (B).

Fig 2. Schematics of graded layers process (A) Epoxy (B) Epoxy-Graphite

The bulk specimens both of tensile test and flexural test are

shown in Fig 3 (A) and (B) respectively.

Kieback et. al., [12]. and Stabik et. al. [29] have elucidated

the gradation process, which mainly consists of three

processes namely the constitutive, homogenization and

segregation. The present experiment has followed same

procedures in forming the bulk with continuous gradation.

The process of development of graded polymer as indicated

in the schematic involves several stages, first stage

involves, preparation of thermoset polymer solution by

dispersion of weighted fine graphite powder in 20ml

acetone solvent. This mixture is further subjected to

ultrasonic bath (Indian make, 20kHz pulse at 20W power)



for 10mins for uniform dispersion of graphite particles

followed by another dispersion process for 10mins in the

probe sonicator (Sonics India, 25khz, 50% amplitude). The

finely dispersed particles are then mixed with proportionate

weighted epoxy to make the total 100g as per the gradation

design of each layer. This mixture is then kept at room

temperature for 10-15mins for thorough mixing on a

magnetic stirrer which is rotated @ 220 rpm. Subsequently,

the solvent is removed by evaporation by heating the mix

@ 328-3430K for 12h on the magnetic stirrer. Soonafter

complete evaporation of solvent (acetone), the mixing of

hardner in the OEM recommended ratio of 100:34, 36

(i.e.

100 parts of epoxy: 34 parts of hardner) with the epoxy-

graphite mixture is undertaken. The polymer mix is then

de-aerated or moisture is removed in a vacuum desiccator.

The bulk mix is then weighed for pouring into each die in

the mold as per the thickness of each layer depending on

the number of layers the graded polymers is made of. The

samples of specimens with different layers prepared with

varying gel time maintained at 30mins / 1h/ 1.5h / 2h/ 4h or

a combination of these for especially 4 and 7 layer bulk

composites. Stabik et. al, 30

, has reported fabrication of

graded composite with a gel time of 33mins. The varying

gel time leads to the interfacial bonding due to

sedimentation of particles at the interface as each layer will

be partially crosslinked before the next is poured.

Fig 3. Specimens as per ASTMs (A) Tensile (D 638-08)

(B) Flexural (D 790-07)

In the current investigation, the work carried out by Stabik

et al., [26]-[30] and Ehsan et al., [31] in the development of

graded polymer was perused. The novelty is the

development of 3, 4, and 7 layers graded polymer

composite in an open mold by conventional gravity

method. The experiment demonstrates the development and

its effect on thermo-mechanical properties. The preliminary

preparation of the samples of 0.5 and 1.0wt% forming 3, 2

and 4 layers of uniform thickness and 7 layers of varying

thickness have been processed. The schematic of these

specimens are shown at Fig 4. Of these specimens, samples

of 3 layers with 0.5 and 1wt% layers as sandwich between

pure epoxy and the specimens with varying gel times were

characterised for mechanical testing, XRD, FE SEM, and

TGA, whereas, 2, 4, and 7 layers were characterised only

through FE SEM, XRD and TGA. The present process is

restricted to the laboratory work and may be extracted for

industry production in future.

Fig 4. Schematic of Graded Layers (2, 3, 4 and 7 layers)

3. CHARACTERISATION

X-ray diffraction (XRD)

XRD was used to verify the structure of the graded

composite. The XRD was performed on a Bruker D8

Advance Diffractometer with Cu Ka radiation (λ=1.541 Å)

having the slide width of 6mm operating at 40 kV and 40

mA. The scanning range was 10 - 60 with a scanning speed

of 0.1step/sec.

Flexural and tensile testing

Flexural tests were carried out on an Instron 2T capacity

Universal testing machine. The 3 point bending fixture was

mounted on this machine for the flexural tests. The

crosshead speed was maintained at 1mm/min. The tensile

tests were performed on a Tinius Olsen H25KS Mechanical

Testing Machine at a crosshead rate of 5.0 mm/min. The

flexural and tensile tests were performed as per ASTM D

790-07 and ASTM D 638-08 respectively. Minimum of 5

tests of 3 layers (0.5 and 1.0Wt%) and 3layers of 1wt% of

varying gel time (1/ 1.5 / 2 / 4h) were tested for 3 point

bending while 3layers of 1Wt% was tested for tensile

testing. The limited selection was only to ascertain the



feasibility of fabricating the graded structure and later

optimize the process for further development based on the

effects of wt% and gel time variation on properties seen in

the selected samples tested.

Microscopy

The cross-sectional surface cut with a diamond cutting tool

(ISOMET model, low speed saw, Buleher Make) or the

fractured surfaces obtained either from mechanical testing

or cryogenic by using liquid nitrogen were examined using

a Zigma Field Emission Scanning Electron Microscope

(FE SEM). The 3 and 7 layers were examined for their

surface morphology as a sample to establish the feasibility

of graded development. The diffusion of graphite particles

between layers and the interfacial bonding were also

analysed. The graded specimens for FE SEM were

prepared by gold sputtering in vacuum for 3min to avoid

charging. The FE SEM was carried out in the Secondary

Electron Mode with accelerating voltage at 5kV. Images

were captured for analysis at 49-60X and further zoomed

for 500X for distinct analysis of the surface morphology of

each layer.

Thermo gravimetric analysis (TGA)

The graded composite was examined for their thermal

stability vis-à-vis that of pure epoxy or graphite. The

investigation was carried out using M/S Perkin Elmer

Model No. FTA 6000 instrument. The TGA recording was

carried out at 20 0C/min under constant nitrogen flow of

100 ml/min from room temperature to 10730K. At least five

tests were carried out for each type of samples namely 2, 3,

4, 7 layers and also of 3 layers 0.5wt% and 3 layers 1.0wt%

of 4h gel time.

Nano indentation test

Nano-indentation tests were carried out on samples of 2, 3,

4, 7 layers and also of 3layers 0.5wt% and 3layers 1.0wt%

of 4h gel time. The tests were carried out on M/s Agilent

Nano-indenter, Model No. G 300 at a strain rate of

5mm/min. The samples were examined on their both cross-

section as well as on the top surface. The Young‘s

Modulus, hardness and strain hardening exponent have

been determined.

4. RESULTS AND DISCUSSION

XRD Measurements

XRD scans of pure epoxy and the graded materials are

shown in Fig 5 and of pure graphite powder in Fig 6. The

images of graded composites exhibit amorphous peak with

2θ ranging at 12.8060-13.163

0 corresponding to the

interlayer distance ranging from 6.92 - 7.34 Å. The graded

composite peaks exhibited are seen to be around that of

epoxy phase at 12.260 corresponding to 7.21 Å while the

pure graphite powder phase is observed at 27.60. The

graphite powder phase is observed to be close to the

reported value of 26.50 as mentioned in literature, [31],

[39]. Man Wai et al.and others, [41]-[42] have reported the

phase of epoxy resin to be of amorphous nature with

diffraction angle close to 200. The XRD scan of graphite /

epoxy graded composite is analysed taking into similarity

of the investigation made by Dilani et al., [11] and

Ding et al., [42] As per Dilani et al., [11], the diffraction

peak of graphite did not occur in the diffraction

spectrogram of graphene oxide indicating complete

oxidation of graphite to graphene oxide. The interlayer

distance is attributed to the stretching of crystal lattice

length along the axis c. Also, from Ding et al.,[42] the

observed broad peaks of amorphous nature has been

reasoned as to ZnO being encapsulated or covered by epoxy

resin and thereby weakening the ZnO peaks. This

phenomenon is seen while the ZnO is being synthesised by

in-situ method. The observation has also been associated to

the little weight concentration that has been used for the

synthesis. In the present synthesis of graded graphite-epoxy

composite, the process has been in-situ method with

graphite of little concentration mixed with epoxy. The

spectrogram of graded polymer indicates no graphitic peak

which is analogous to the scan investigated in

Dilani et al., [11]. The deduction from this analogy will be

that the graphite has undergone oxidation during the 12h

mixing process with epoxy and thereby the graphite peak

has not occurred. Also from the analogy of Ding et al.,[42]

it has been deduced that the low concentration of graphite

may have been encapsulated by epoxy matrix that the

graphite peak seems to have shifted downwards to lower

angles (12.800-13.16

0). This work also makes a reference to

the GO/epoxy composites XRD scan of Hae Kyung et al.,

[43] in which GO peaks occurs at 12.840, (001) plane. In

the present work, the investigation of XRD scan show the

effect of variability of either the concentration, number of

layers or the gel time on the 2θ value vis-à-vis that of the

pure epoxy indicating a variation of 7.47 to 9.48% and that

the intensity of higher filler material (GP7) is seen to be

highest.

Fig 5. XRD of epoxy graded composite



Fig 6. XRD of graphite powder

Surface morphology

The FE SEM examined the surface morphology of

fractured as well as cut specimens across the cross-section.

The intent was to study the surface morphology of graded

specimens and establish the continuous gradation by the

fabrication process adopted. The images shown in Fig 7 and

Fig 8 distinctly indicate the continuous gradation. The

graphite particles have dispersed well and there are less

signs of agglomeration. Nevertheless, the images need

further clarity as there could be ingress of moisture while

partial solidification. The continuous grading seen is a good

confidence measure on the process adopted to make graded

materials by gravity method. The 1.0GP31 graded polymer

composite has demonstrated a continuous gradation with

good thermo-mechanical properties. The 1.0GP34

composite though indicates a comparable property, the

gradation layers are seen to be step wise and do not show

the smooth and continuous gradation as the crosslinking

time is more and less diffusion of particles occur during this

period.

Fig 7. FE SEM of 3 graded layers of E-E+Gr-E layers

Fig 8. FE SEM of 7 layers Graded Materials of Epoxy-Graphite

Effect of filler concentrations on mechanical

properties

The flexural and tensile test results are plotted for different

filler concentrations for 3 layers, 4h gel time (0.5 and

1.0wt%) and varying gel time for 1.0wt% 3 layers graded

specimen. The results demonstrates enhancement in

flexural strength with that of pure epoxy and with increase

in filler concentration for 3 layer, 4h graded material,

as shown in Fig 9. The flexural rigidity which represents

bending stiffness is also high with increase in graphite

concentration in comparison to the pure epoxy value, as

shown in Fig 10. The flexural strain is proportionately

varying as per the modulus and strength, shown in Fig 11.

The plots of peak load over area Vs the varying wt% and

gel time, as shown in Fig 12, indicates that 1.0 wt% and 1h

gel time providing improved property. The tensile results

also indicate improved properties with gradation vis-à-vis

pure epoxy values, as shown in Fig 13 and Fig 14. This is

in close agreement with the literature reports on how

gradation has shown to improve mechanical properties with

increase in filler concentration. Subita et al.[32], Kaushik et

al. [20] and Suhermana et al., [33] have reported in their

work on the enhancement of mechanical properties with

increase in filler concentration. The elasticity of modulus

values for graded polymeric materials lies between the two

extremes which is the iso-strain based on Voigt model and

the iso-stress based on the Reuss Model, [44]-[45]. The

extreme modulus of elasticity is given below at Eq 4 and 5.

In case of any positive deviation from the extremes values

it will indicate the matrix constraints.

Ec = Vm Em + Vp Ep ---- (4)

Ec= (Em Ep) / (VmEp+VpEm) ---- (5)

The 1.0GP34 graded material has demonstrated a good

gradation and thermo-mechanical properties over that of



pure epoxy. The improvement in properties mainly

depends on the dispersion and good interfacial bonding.

The dispersion in turn depends on several factors such as

the vibration frequency for cavitation, and the timing set

which will be related to the temperature exposed to the

solution and the resulting degradation. Birgit et. al. in their

work has shown the effects of these factors on dispersion

and the corresponding effects on the mechanical properties

of the composite. The results with respect to change in gel

time seem to indicate that the 1h gel time is providing

better flexural properties than the rest as shown in Fig 15.

There seems to be an increase of 0.61% flexural strength,

26.27% flexural modulus and 23.1 % peak load /sample

area vis-à-vis that of pure epoxy. The young‘s modulus

values for the specimens are further corroborated by the

nano-indentation results which show that the average

Young‘s modulus is around 3.3 GPa and the average

hardness value is about 0.14 GPa. There seems to be no

variation in these values for any changes in terms of

concentration of fillers or number of layers or gel time.

These values measured at nano-scale level may have certain

aberrations particularly for a graded material which are

having some inhomogeneity at the interfaces across the

cross section. The results of nano-indentation have been

discussed in the subsequent section. The correlation of

mechanical testing results with that of nano-indentation

needs further analysis.

Fig 9. Av Flexural Strength Vs Wt%

Fig 10. Av. Flexural Modulus Vs Wt%

Fig 11. Av Flexural Strain Vs wt%

Fig 12. Peak load/sample area Vs Gel Time

Fig 13. Av Tensile Stress Vs Wt%

Fig 14. Av Tensile Modulus Vs wt%



Fig 15. Flexural Strength and Modulus Vs Gel Time of E-Gr Graded

Material

Thermo Gravimetric Analysis (TGA)

The TGA scans for varying concentration, number of layers

and gel time have been analysed. The scans obtained for

various specimens of epoxy-graphite graded composites as

well as of pure epoxy and graphite is shown in Fig 16. The

figure indicates the decomposition in multi-stages

especially for epoxy and 7 layers composite. The scan of

graphite powder indicates a good thermal stability after the

initial decomposition due to moisture indicated by initial

wt% loss at 5130K. The char yield is 94% at 1067

0K. The

independent scan of epoxy at Fig 17 indicates multi- phase

decomposition with the initial volatile decomposition

commencing at 5720K with a weight loss of 0.94%. The

char yield of epoxy indicates 1.21% (Fig 16 & 17) at

10700K. Hui et al., [46] and Asma et al., [39] have reported

an onset of epoxy decomposition at 6330K which is

established in the test as shown in Fig 17. The epoxy-

graphite graded composites of varying concentration based

on number of layers and filler concentration have shown

volatile decomposition commencing from a minimum of

4930K (GP2) to a maximum of 630

0K (GP3). However the

char yield weight loss of all graded composites except GP7

has shown enhanced values compared to epoxy. The

percentage of char yield varies from 0.145% (GP7) to

5.28% (GP3) and the temperature at this point is varying

from 10050K (GP7) to 1072

0K (GP3 & GP4). The increase

in char yield indicates the resistive path the matrix will

have with filler diffused in-homogenously at the interface

layers. The free movement of the matrix chain is restricted

giving rise to enhanced mechanical and thermal properties.

The extent of dispersion of particles, particle size and

saturation limits of the matrix-filler element bonding results

to the increase or decrease of thermal stability. The increase

in concentration with number of layers has seen a trend of

enhanced thermal stability up to some layers beyond which

the thermal stability reduces as noted in the case of 7 layers.

Fig 16. TGA of E-Gr graded layer

Fig 17. TGA of pure epoxy

Nano-indentation Measurement

Nakamura et al., [44] and Carmine et al., [47] have reported

micro indentation modeling work on gradation polymers

using inverse analysis method and Finite Element Method

for studying the tensile properties of a graded composite.

This paper reports the properties through nano-indentation

of graded polymers and of pure epoxy. The average

hardness and modulus of elasticity values of flat and cross

sectional surface of specimens tested are given in Table 4

and Table 5 respectively. The analysis indicates especially

for 2 and 3 layers composite an increase in hardness and

modulus values at the interface across the cross-section.

This is attributed to the effects of interface diffusion and

strengthening of bonds. The test on the cross-section is

schematically represented in Fig 18. The small triangles on

the cross section depict the location of indentation. In the

computation, Poisson‘s Ratio assumed for Epoxy was 0.30

(approx.). However, it is seen that by increasing the number

of layers of epoxy-graphite, hardness and modulus values

of the composite is getting decreased as can be seen from

the indentation test of 4 (GP4) and 7 layer (GP7)

specimens. Pop-in observed in the unloading curves shown

in Fig 19 could be due to surface cracking. Also specimen



Table 4 Nano-indentation on flat surface

Table 5 Nano-indentation on Cross-section

of epoxy and 4 layers (GP4) were sample tested with

different methods mentioned below to check whether the

results of hardness or modulus are same or not. The

methods M1 and M2 mentioned below have given the same

results for hardness as well as modulus of elasticity.

a) G-Series CSM Standard Hardness, Modulus,

and Tip Cal (M1): Continuous Stiffness

Measurement (CSM) option to return hardness (H)

and elastic modulus (E) as a continuous function

of penetration into the test surface. With the CSM

option, every indentation test returns complete

depth profiles of Young‘s modulus and hardness.

b) G-Series Basic Hardness, Modulus, Tip Cal,

and Load Control (M2): Returns hardness (H)

and elastic modulus (E) vs. penetration depth

using multiple load/unload cycles at each test site.

Constant loading rate force application. Here we

set load time, max load, and number of cycles.

Fig 18. Indentation on Cross-sectional area (W x T)

Fig 19. Load Vs Displacement of graded layers and epoxy

5. CONCLUSION

The mechanical / material characterisation results have

demonstrated the feasibility of continuous gradation. The

study of various graded specimens demonstrates 1.0GP31

(1wt%, 3 layers, 1hr gel time) composite to be preferable

comparatively due to better thermo-mechanical properties.

The nano-indentation analysis of graded polymer

composites of 2 & 3 layers indicate an increase in hardness

and modulus values at the interface across the cross-

section. The experimentation is seen to be encouraging for

tailoring functionally graded polymers for effective

aerospace applications.

Acknowledgment

The authors are grateful to the Dr Prahlada, Vice

Chancellor, DIAT (DU) and Director, MILIT, Pune for

encouragement and support. The authors extend gratitude to

R & DE (Engr), Dighy, Pune, CMTI, Bangalore, UoP,

Pune, HEMRL, Pune for support in providing raw

materials, and testing facilities. The authors are grateful to



―DIAT—NANO project EPIPR/ER/1003883/M/01/908/

2012/D (R&D)/1416‖ for support. The authors like to

acknowledge the technical staff of Material Eng., Physics

and Workshop for their technical support.

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Tirumali Manoj is a research scholar in the Department of Materials Engineering in Defence Institute of Advanced Technology (Deemed

University)., Pune, India. He received master‘s degree in mechanical

engineering (Air Armament) from University of Pune, India. He has authored a chapter in Structural Nanocomposites; Perspective for future applications,

published by Springers in 2013. He is presently in Armed Forces serving in

Indian Air Force. His active areas of research interest include the development of functionally graded and hybrid composites.

([email protected])

Dr K Balasubramanian is an Associate Professor and Head of Materials Engineering Department at Defence Institute of Advanced Technology

(Deemed University) at Pune, India. He has years of industrial experience at

UK, an eminent scholar and well acknowledged academician. Dr. K Balasubramanian obtained various prestigious fellowships including

award for Technical Excellence at UK Materials Research Institute, Selected

for ‗Hind Rattan award‘ for outstanding contribution in the area of science and technology, India and a Professional Fellow of Institute of Technology,

UK. He has published over 200 articles, patents and conference proceedings

in the field of materials science and technology. He authored a chapter in

Structural Nanocomposites; Perspective for future applications, published by

Springers in 2013. (*Corresponding author. [email protected])

Dr A Kumaraswamy is an Associate Professor in Mechanical Engineering Department at Defence Institute of Advanced Technology (Deemed

University), Pune, India. He has been a reputed academician and a visiting professor to well known engineering colleges and Institutes in India. He has

been listed in Who‘s Who in the World (29th edition), Marquis, New Jersey,

2012. Dr. A Kumaraswamy obtained various prestigious fellowships and awards including Sir Issac Newton scientific award for excellence in 2012.

He has published over 80 articles, and conference proceedings in the field of

solid mechanics, contact mechanics, FEA, Nanoindentation, Composites

and metal forming and cutting. ([email protected])

http://www.sciencedirect.com/science/article/pii/S0263224109000104

http://dx.doi.org/10.1016/j.measurement.%2001.006

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