A new generation fibre reinforced polymer applications Master of Technology

A new generation fibre reinforced polymer applications Master of Technology
A new generation fibre reinforced polymer
composites for low and cryogenic temperature
applications
A thesis submitted in partial fulfilment of the requirements for the degree of
Master of Technology
In
Metallurgical and Materials Engineering
By
DEVALINGAM SANTHOSH KUMAR
(213MM1470)
Department of Metallurgical and Materials Engineering
National Institute of Technology Rourkela
2015
i
A new generation fibre reinforced polymer
composites for low and cryogenic temperature
applications
A thesis submitted in partial fulfilment of the requirements for the degree of
Master of Technology
In
Metallurgical and Materials Engineering
By
DEVALINGAM SANTHOSH KUMAR
(213MM1474)
Under the guidance of
Prof. Rajesh Kumar Prusty
&
Prof. Bankim Chandra Ray
Department of Metallurgical and Materials Engineering
National Institute of Technology Rourkela
2015
ii
National Institute of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled, “A new generation fibre reinforced polymer
composites for low and cryogenic temperature applications” submitted by Mr. Devalingam
Santhosh Kumar in partial fulfilment of the requirements for the award of Master of
Technology in Metallurgical and Materials Engineering at National Institute of Technology,
Rourkela is an authentic work carried out by him under our supervision and guidance. To the
best of our knowledge, the matter embodied in the thesis has not been submitted to any other
university/institute for the award of any Degree or Diploma.
Date:
Prof. R.K. Prusty
Prof. B.C. Ray
Department of Metallurgical and Materials Engineering
National Institute of Technology, Rourkela, Odisha -769008
iii
ACKNOWLEDGEMENT
I gladly take this opportunity out to express my deepest gratitude and sincere regards to my
Project guides Prof. R.K. Prusty and Prof. B.C. Ray, Department of Metallurgical & Materials
Engineering, National Institute of Technology, Rourkela for their continuous guidance, expert
advice, invaluable support and incomparable motivation throughout the project work. This
opportunity of working under their guidance is no less than a privilege. I am also thankful to
Prof. S.C. Mishra, Head of Department of Metallurgical and Materials Engineering, National
Institute of Technology, Rourkela.
I am obliged to thank Mr. Dinesh Kumar Rathore (Ph.D. Scholar) and Mr. Kishore Kumar
Mahato (Ph.D. Scholar) for rendering support and guidance throughout the project work. I
further wish to extend by thanks to Mr. Rajesh Pattnaik for his cooperation during my
experimental work.
Lastly, I would like to thank all my dear friends for their generous support. To name a few, I
would mention Meet Jayesh Shukla, Rajneesh and Pranav.
DEVALINGAM SANTHOSH KUMAR
(213MM1470)
Department of Metallurgical and Materials Engineering
National Institute of Technology, Rourkela
iv
List of Figures
Figure 1.1: Classification scheme for different composite types
Figure 1.2: 3K plain weave (a) glass fibre (b) carbon fibre
Figure 1.3: Classification carbon nanotubes
Figure 1.4: Applications of (a) Fibre reinforced composites (b) carbon nanotubes.
Figure 1.5: Sectional view of compressed hydrogen tank.
Figure 2.1: Schematic of Matrix cracking at cryogenic temperature i.e.77K.
Figure 2.2: Tensile stress vs. strain curves of pure epoxy resin (ER), BCP-ER, SWCNT/ER, and
SWCNT/BCP/ER
Figure 2.3: Vickers’s hardness of pristine MWCNTs/epoxy and GPTMS-MWCNTs/epoxy.
Figure 2.4 Stress vs. strain curves epoxy and MWCNTs/epoxy nano composites at (a) room
temperature (b) 77K
Figure 3.1: Dispersion of CNT in epoxy and further fabrication of laminated composite.
Figure 3.2: Experimental set up of UTM-Instron 5967.
Figure 3.3: Experimental set up for dynamic mechanical thermal analyser (DMTA).
Figure 4.1: Flexural stress vs. strain curves for GE and CNT-GE composites for (a) -80⁰C (b) 20⁰C
Figure 4.2: Variation in (a) Flexural strength (b) Flexural modulus with testing temperatures for
GE and CNT-GE
Figure 4.3: Variation in (a) Eʹ, (b) Eʺ and (c) tanδ with temperature for GE and CNT-GE
composites.
Figure 4.4: Weibull fitting of experimental for GE and CNT-GE at (a) -80⁰ C and (b) 20⁰ C.
Figure 4.5: Comparison between experimental and simulated stress vs. strain for GE and CNTGE composites at (a) -80⁰C and (b) room temperature (20⁰C).
Figure 4.6: SEM images of fractured surfaces of GE composites of after flexural testing at (a, b)
v
-80⁰C and (c, d) room temperature (20⁰C).
Figure 4.7: SEM images of the fractured surfaces of CNT – GE composites for after flexural
testing at (a, b) -80⁰C and (c, d) room temperature (20⁰C).
Figure 4.8: SEM images of the fractured surfaces of CNT-GE composite after flexural testing at
room temperature (20⁰C) showing (a) distribution of MWCNTs in the epoxy (b) crack bridging
by MWCNTs.
Figure 5.1: Flexural stress vs. strain curves for GE and CNT-GE composites conditioned in
liquid nitrogen for (a) 0hr (b) 0.25 hr (c) 1 hr and (d) 4hr.
Figure 5.2: flexural strength for GE and CNT-GE composites with conditioning time in liquid
nitrogen.
Figure 5.3: Variation in flexural modulus for GE and CNT-GE composites with conditioning
time in liquid nitrogen.
Figure 5.4: Variation in failure strain for GE and CNT-GE composites with liquid conditioning
time.
Figure 5.5: Weibull fitting for experimental GE and CNT (0.1%)-GE composite conditioned in
liquid nitrogen (a) 0hr (b)0.25hr (c) 1 hr and (d) 4hr.
Figure 5.6: Variation in Weibull shape parameter and scale parameter with varying of CNT
content and conditioning time.
Figure 5.7: Comparison between experimental and simulated flexural stress vs. strain curves for
GE and CNT-GE composites.
List of Tables
Table 3.1: Properties of epoxy, glass fiber and MWCNTs.
Table-3.1: Weibull scale (σo) and shape (β) parameters for GE and CNT-GE composites at
various temperatures
vi
Table of Contents
Certificate ................................................................................................................................. iii
Acknowledgement .................................................................................................................... iv
List of Figures ............................................................................................................................ v
List of Tables ...........................................................................................................................vii
Abstract ...................................................................................................................................... x
Chapter 1
Introduction ................................................................................................................................ 1
1.1
Introduction to Composites materials ............................................................................. 2
1.2
polymer composite materials .......................................................................................... 2
1.3
The matrix phase ............................................................................................................. 3
1.3.1
Thermo setting polymers ......................................................................................... 3
1.3.2
Thermo plastic polymers ......................................................................................... 3
1.4 Fibre reinforced phase.......................................................................................................... 3
1.4.1 Glass fibre ...................................................................................................................... 4
1.4.2 Carbon fibre................................................................................................................... 4
1.5. Nano reinforcement ............................................................................................................ 4
1.5.1 Carbon nano tubes ......................................................................................................... 5
1.6 Applications of FRP composites .......................................................................................... 5
1.7 Motivation for current project .............................................................................................. 6
1.8 Objective of the present work ............................................................................................. 6
References .................................................................................................................................. 7
Chapter 2
Literature Survey…………………………………………………………………………………….....8
2.1 Literature Survey ................................................................................................................. 8
vii
References ................................................................................................................................ 14
Chapter 3
Experimental procedure……………………………………………………………………………….15
3.1 Materials ............................................................................................................................ 15
3.2 Fabrication process ............................................................................................................ 16
3.2.1 Dispersion of MWCNTs into epoxy. .......................................................................... 16
3.2.2 Fabrication of fibre reinforced nano composite .......................................................... 17
3.3 Material characterization ................................................................................................... 18
3.3.1 Mechanical characterization for low temperature conditioning……………………...18
3.3.2 Mechanical characterization cryogenic temperature conditioning..................................
3.3.3 Dynamic mechanical thermal analyser……………………………………………………….19
3.3.4 Fractographic analysis………………………………………………………………………..19
Chapter 4
Low temperature performance of CNT-GE composite ........................................................... 20
4.1 Introduction ........................................................................................................................ 20
4.2 Results and discussion ....................................................................................................... 20
4.2.1 Flexural behaviour at various temperatures ................................................................ 20
4.2.2 Dynamic mechanical thermal analyser (DMTA) ....................................................... 23
4.2.3 Constitutive flexural deformation model ................................................................... 24
4.2.4 Fractography .............................................................................................................. 27
4.3 Conclusion ......................................................................................................................... 30
References ............................................................................................................................... 31
Chapter 5
Cryogenic temperature performance of CNT-GE composite…………………………………………32
5.1 Introduction ..................................................................................................................... 33
5.2 Results and discussion.................................................................................................... 33
viii
5.2.1 Flexural performance after cryogenic treatment ......................................................... 33
5.2.2 Damage constitutive failure model ............................................................................. 36
5.3 Fractography ...................................................................................................................... 39
5.4 Conclusion ......................................................................................................................... 42
References ................................................................................................................................ 42
ix
Abstract
In the present investigation, alteration in flexural performance of glass-epoxy (GE) composite
and CNT (0.3%)-GE for low temperature and glass-epoxy (GE) composite and CNT (0.1%,
0.3% and 0.5%)-GE composites due to liquid nitrogen conditioning is studied for various time
lengths. The epoxy resin is first modified by 0.1% ,0.3% and 0.5% MWCNT, which is then used
along with E-glass fibres to fabricate laminate.Flexural strength and modulus were evaluted by
3-point bend test. For low temperature conditioning, addition of 0.3 wt. % MWCNT into GE
composite significantly lowered the Tg by 12 °C due to hindrance in crosslink formations. The
reinforcement efficiency (relative change in modulus) due to CNT incorporation in GE
composite is as high as 30%, when the testing temperature was -80 °C. For cryogenic
conditioning, out of these four compositions the maximum strength is as the fabricated
conditioned for GE composite with 0.1% CNT, which is 32.7% higher than GE composite.
Decrease in strength and modulus observed for short time span of liquid nitrogen conditioning
i.e. 0.25hr. Long cryogenic conditions resulted in increment in strength. To understand the
failure mechanisms, post failure fractography analysis was carried out using scanning electron
microscope. The design parameters are then calculated using Weibull distribution model.
Keywords : Fibre reinforced composite, Flexural properties, Low temperature, cryogenic
temperature and Multi walled carbon nanotubes.
x
Chapter 1
Introduction
1
1.1 Introduction to composite materials
The word composite means “made up of distinct parts”. Generally, composite materials defined
that which materials are made from two or more constituent materials with significantly differ in
physical or chemical properties, that when judiciously combined, produce material with superior
properties than individual constituents. The main idea of composite materials to make unusual
combination of properties for different potential applications. Composite materials are mainly
classified into three categories (i) Polymer Matrix Composites (ii) Metal Matrix Composites (iii)
Ceramic Matrix Composites. Many composite materials are made of two phases. One is named
as matrix, which is continuous phase, other one is reinforced phase.
Figure 1.1 Classification scheme for different composite types
Particle Reinforced composites sub classified into two categories, (i) Large particle and (ii)
Dispersion strengthened. Commonly used reinforced particles are Aluminium silica, Boron oxide
etc. The degree of reinforcement gives the high mechanical properties of the composite. It is
depends on strong bonding at the matrix–particle interface. Fibers are usually glass fibres, carbon
fibres and aramid fibres.
1.2 Polymer matrix composites
Polymer matrix composites made by combination of one matrix phase and one reinforced phase.
Matrix phase ductile in nature. Whereas reinforced in brittle nature.
2
1.3 The matrix phase
In polymer matrix composites the matrix phase is polymer. The purpose of polymer in PMCs to
get a ductile property to composite. Thus, it improves the fracture toughness. In Fibre reinforced
composites matrix phase assists several functions. The first function, Polymers binds the fibres
together and acts as the medium for stress transfer and distributed to fibres. The second function,
matrix act as shied around fibres protect from surface damage. It leads to improve mechanical
properties and protect the chemical reactions with environment.
Mainly polymers classified into two types:
(1) Thermo setting polymers
(2) Thermo plastic polymers
1.3.1 Thermo setting polymers
Thermo setting polymers are in state of viscous, during curing it changes irreversibly insoluble
polymer network. This process generally called as crosslinking. Thermo setting polymers have
high dimensional accuracy. Because of the cross linking, molecules of polymer difficult to
move/slide one on another results make it strong and rigid. General examples of thermosetting
polymers are epoxy, polyester, polyurethane and silicone.
1.3.2 Thermo plastic polymers
Thermo plastics polymers are gets soften/melt on heating. These polymers mainly suitable for
liquid flow forming. Many of thermoplastics polymers having high molecular weight.
Intermolecular force acting in between polymer chains, which weaken rapidly with increased the
temperature, leads to yielding a viscous liquid. Thermoplastics might be reformed by heating and
are typically used to produce different parts by several polymer processing methods such as
injection molding, compression molding, calendering, and extrusion. Some examples of thermo
plastic polymers are high and low density polyethylene, polystyrene and polymethyl
methacrylate (PMMA).
1.4 Fibre reinforced phase
Reinforcement phase is many forms such as flakes, whiskers, short fibre, continuous fibers or
sheets. The reinforcements are used in many composites have fibre form it results fibres are
stronger and stiffer than any other form. Some examples listed here.
3
1.4.1 Glass Fibres
Common glass fibres formed by the alumina-borosilicate glass having less than 1% w/w alkali
oxides. Many types of glass fibres are there like E-glass, S-glass and C-glass fibres. Here Esignificant good electrical insulator also having good strength. S-indicates this glass fibre having
high silica. By adding silica to the fibre withstand high temperatures than other glass fibres.
1.4.2 Carbon fibres
Carbon fibres have very light density. Depends on the arrangement of carbon atoms the structure
varies. When the carbon atoms arrange in form of three dimensional configuration the structure
indicates diamond. The raw material of the carbon fibre is the organic precursor fibres. The
carbon fibres having high Young’s modulus equal to about 1000 GPa.
Figure1.2 3K plain weave (a) glass fibre (b) carbon fibre
1.5 Nano reinforcement
Nano particles have nano scale size in geometry. Due to its size these particles have high specific
strength. Some of nano fillers listed below.
(i)
Alumina
4
(ii)
Silica
(iii)
Carbon nanotubes
1.5.1 Carbon nano tubes
A carbon nanotube is a tube-shaped material, made of carbon, having a diameter measuring on
the nano scale size. Carbon tubes are formed from the essentially the graphite sheet and the
graphite layer appears somewhat like a rolled-up continuous unbroken hexagonal mesh and
carbon molecules at the apexes of the hexagon. Depending on the process used for CNT
synthesis, CNTs can be classified into single-walled, double walled and multi walled carbon
nanotubes.[1] [2].
Figure 1.3: Classification carbon nanotubes
Carbon nanotubes have high specific strength and high specific stiffness due to these un
paralleled properties fibre reinforced nan composites have become attractive structural materials
not only in weight sensitivity aerospace industry but also some functional applications, marine,
armour, and automobile applications.[3] [4]. Addition of CNTs in epoxy improves the modulus,
stiffness as well as fracture toughness [5] [6].
1.6 Applications of FRP composites
FRP composites find application aerospace industry, marine vehicles, sports goods, structural
applications, cryogenic fuel tanks, hydrogen storage tanks, pressure vessels, satellite solar panels,
superconducting devices and thermal insulators [7] [8] [9].
5
Figure1.4: Applications of (a) Fibre reinforced composites (b) carbon nanotubes
1.7 Motivation of the current project
FRP composites have already been proven as trust worthy materials and being used extensively.
The main disadvantages in FRP composites are environmental degradation and low impact
resistance. This is might be due to poor matrix properties dominated properties. In present work
modify the matrix chemistry by adding CNTs to epoxy obtain a superior and reliable material for
low and cryogenic temperature applications.
1.8 Objective of the present work
Cryogenic tanks made by conventional materials like aluminium and steel. The main problem
finding in conventional metal tanks have continuous propagation of cracks in the primary (inner)
containment, leads to sudden dynamic liquid loads being applied to the secondary (outer)
containment. The conventional materials have high density, cost and less fatigue resistance than
polymer reinforced composites. The composites tanks will be enable the next generation of
rocket and spacecraft needed for space applications.
Conventional laminated composites have poor through thickness and poor interlaminar
properties. Further at low and cryogenic temperatures the tendency of micro crack generation is
higher. Addition of carbon nanotube to the polymer can significantly enhance their matrix
dominated properties. CNT is used to modify chemically with a vision to obtain a superior and
more reliable material with a less degree of dispersity.
6
Figure 1.5: Sectional view of compressed hydrogen tank.
References
[1] Allaoui, Aïssa, et al. "Mechanical and electrical properties of a MWNT/epoxy
composite." Composites Science and Technology 62.15 (2002): 1993-1998.
[2] Baughman, Ray H., Anvar A. Zakhidov, and Walt A. de Heer. "Carbon nanotubes--the route
toward applications." Science 297.5582 (2002): 787-792.
[3] Biercuk, M. J., et al. "Carbon nanotube composites for thermal management."Applied physics
letters 80.15 (2002): 2767-2769.
[4] Xie, Sishen, et al. "Mechanical and physical properties on carbon nanotube."Journal of
Physics and Chemistry of solids 61.7 (2000): 1153-1158.
[5] Xie, Xiao-Lin, Yiu-Wing Mai, and Xing-Ping Zhou. "Dispersion and alignment of carbon
nanotubes in polymer matrix: a review." Materials Science and Engineering: R: Reports 49.4
(2005): 89-112.
[6] Gojny, Florian H., et al. "Influence of different carbon nanotubes on the mechanical
properties of epoxy matrix composites–a comparative study."Composites Science and
Technology 65.15 (2005): 2300-2313.
[7] Sethi, Sanghamitra, Dinesh Kumar Rathore, and Bankim Chandra Ray. "Effects of
temperature and loading speed on interface-dominated strength in fibre/polymer composites: An
evaluation for in-situ environment." Materials & Design 65 (2015): 617-626.
[8] Ray, B. C. "Temperature effect during humid ageing on interfaces of glass and carbon fibers
reinforced epoxy composites." Journal of Colloid and Interface Science 298.1 (2006): 111-117.
[9] Ray, Bankim Chandra, and Dinesh Rathore. "Durability and integrity studies of
environmentally conditioned interfaces in fibrous polymeric composites: Critical concepts and
comments." Advances in colloid and interface science 209 (2014): 68-83.
7
Chapter 2
Literature survey
8
2.1 Literature survey
Literature survey is documentation of comprehensive analysis of published/unpublished work
from secondary bases information in the specific area of interest to researcher. One of the rich
storage source for secondary information and researchers is used to spend their time to read
several journals, newspapers and magazines. But now a days the computerized data base easily
available and even accessible for literature search. Reviewing the literature helps to researcher to
attention for further analysis more meaningful on specific aspects. So that the literature survey is
the important for collecting secondary bases information for the area of research which might be
helpful for research. The literature survey can be in field of area.
Some researchers have been attracted by the application of glass fibre /epoxy and carbon
fibre/epoxy composite materials in low temperature and cryogenic environment [1][2]. Gong et
al. [3] experimentally studied behaviour of the composite laminates at low temperature by 2
types of specimens 1) E-glass fibre and epoxy reinforced laminate. The volume fraction of fibre
is 0.4 and 2) carbon fibre and epoxy reinforced laminate, volume fraction of fibre is 0.55 and
made observations. Stress vs. strain curve shows linearity trend when sample exposed to low and
room temperature conditions and the curve extends up to failure of the sample. At low
temperatures brittleness is the failure characteristics found. The strength of the laminates at 77K
have showing higher strength than the strength at 296K for each tested samples. At 77K the
glass/epoxy and carbon/epoxy composites shows 15-20%, 30-40% than at 296K respectively.
Hence here observed a significant enhancement in strength after cryogenic conditioning. At
296K the specimens have a less damaged area at the tip of the notch as compared with exposed
at 77K. Because of micro rupture events might be damage area in composite. Some of the
examples for micro ruptures like matrix – cracking, fibre/matrix interface split and delamination
etc. Some of the energy is might be dissipated by each micro-rupture. Ray et al [4] used Araldite
LY-556 as matrix and HY-951 as a hardener with silane treated woven fabric E-glass fibre to
fabricate a laminated composite and evaluated its mechanical properties at low temperature
9
conditioning with different loading rates. The results reveled from the investigation the breaking
load for untreated samples showed less strength than cryogenically treated samples for all
loading rates. The reason might be when the sample expose to cryogenic temperatures the
material experienced thermal shock along with matrix phase gets hardened. During conditioning
polymer chains gets freeze leads to reduction in mobility of polymer. The scanning electron
microscope image of fractured sample of glass/epoxy laminate at cryogenic conditioned shown
in figure 2.1. The breaking load increases with increasing with cross head speed (mm/min) up to
50 mm/min, above the 50mm/min loading rate the breaking load decreases.
Figure 2.1: Schematic of Matrix cracking at cryogenic temperature i.e.77K. [4]
Sayer et al. [5] studied effect of the temperature on glass/carbon hybrid composite laminate
under the impact loading. The results reveled from the investigation, hybrid laminate have high
energy absorption compared to normal composite laminate at room temperature. Prasanth et al.
[6] have been investigated on the Mode-I fracture analysis of thermally aged of glass/epoxy and
glass-carbon/epoxy Hybrid Composites. There is a decrement in energy release rate of the glass
epoxy aged specimen about 10%-15% of the same material. But in case of pristine condition and
for glass/carbon hybrid specimens are showing decrement around 5%-10. The results carried out
by specimens test DCB (Double Cantilever Beam conditioned at -20oC for 500 hours. The results
10
shows that the decrement in energy release rate in glass/epoxy composite as compared to
glass/carbon hybrid [6]. N.K Naik [7] et al studied on two different hybrid (glass/carbon)
composites under quasi static loading. The results from the investigation revealed, for hybrid
composites, the ultimate tensile strain and strength is higher whenever the glass fibres placing
exterior and carbon fibre placing as compared to glass fibres placing interior and carbon fibres
placing exterior. Chensong Dong [8] et al have been investigated on tensile strength and flexural
strength of unidirectional hybrid composites. In laminated composites, the fibre phase plays a
key role in mechanical properties on in-plane direction. These laminated composites are weak in
z-direction (perpendicular to the plane of the laminate). The z-direction properties are generally
limited by the matrix. Scientifically engineered nano-fillers have been reported to be more
reliable choice to improve these properties, which have been acknowledged round the globe [9]
[10]. Along with this improvement, it is of great interest to investigate the effect of nano filler
incorporation on the environmental degradation of these potential materials. Qianqian Li et al
[11] studied on mechanical properties and microstructure analysis of 0.03 wt. % single walled
nanotubes (SWCNTs) as nano reinforcement, matrix as elastomeric epoxy with 0.3 wt. % block
copolymer. The results showed there is increment in young’s modulus, fracture stress and
fracture strain of SWCNT-epoxy composite with block copolymer 141%, 127% and 43%
respectively. Figure 2.2 represents stress vs. strain curves different composition of composites.
Figure 2.2: Tensile stress vs. strain curves of pure epoxy resin (ER), BCP-ER, SWCNT/ER, and
SWCNT/BCP/ER
11
David Hui [12] et al studied on mechanical, thermal and electrical properties of aligned carbon
nanotubes-polyimide composites. In the investigation they compared to properties of BPDAPDA/CNT and pure poly amide. The results reveled from the investigation the strength and
modulus of BPDA-PDA/CNT showed 2.3% and 12 times over the pure poly amide. Lu et al.
[13] reported enhanced the glass transition temperature and the mechanical strength on addition
of 0.25 wt. % of 3-glycidoxypropyltri-methoxysilane functionalized multi-walled carbon
nanotubes prepared by electron beam (EB) irradiation. Further, few studies suggested the
cryogenic property enhancement on adding CNTs to polymer resin. Figure 2.3 represents
Vickers’s hardness of pristine-MWCNT/epoxy and GPTMS-MWCNTS/epoxy.
Figure 2.3: Vickers’s hardness of pristine MWCNTs/epoxy and GPTMS-MWCNTs/epoxy
Chen et al. [14] studied on the cryogenic properties of CNT modified epoxy resin and noted
improvement in tensile strength as compared to that of neat matrix. By addition of 2 wt. % CNT
to epoxy the Young’s modulus increased 17.2%, 20% for Room temperature and 77K
respectively. The maximum strength showed 0.5wt. % CNT/epoxy as compared to other
composites observed from the figure 2.4.
12
Figure 2.4: Stress vs. strain curves epoxy and MWCNTs/epoxy nano composites at (a) room
temperature (b) 77K
Wei et al. [15] studied the flexural fatigue performance of CNT based polymer composites and
found that as the temperature was reduced from room temperature to 77 K, there was increment
in its fatigue resistance and fatigue performance was affected by the content of CNT used.
Takeda et al. [16] investigated cryogenic mechanical properties of CNT modified woven
glass/epoxy composites under tensile and fatigue loading. They observed that there was hardly
any improvement in young’s modulus and ultimate tensile strength on addition of CNT, but
noticed improvement in fatigue resistance. Thus they demonstrated that addition of CNT has a
potential to increase matrix dominated properties of composite materials at cryogenic
temperatures.
References
[1] Kasen, M. B. "Cryogenic properties of filamentary-reinforced composites: an
update." Cryogenics 21.6 (1981): 323-340.
[2] Hartwig, G., and S. Knaak. "Fibre-epoxy composites at low temperatures. "Cryogenics 24.11
(1984): 639-647.
[3] Gong, M., X. F. Wang, and J. H. Zhao. "Experimental study on mechanical behavior of
laminates at low temperature." Cryogenics 47.1 (2007): 1-7.
[4] Kumar, Surendra, and B. C. Ray. "Mechanical behaviour of FRP composites at low
temperature." (2007).
[5] Sayer, Metin, et al. "The effect of temperatures on hybrid composite laminates under impact
loading." Composites Part B: Engineering 43.5 (2012): 2152-2160.
[6] Pandya, Kedar S., Ch Veerraju, and N. K. Naik. "Hybrid composites made of carbon and
glass woven fabrics under quasi-static loading." Materials & Design32.7 (2011): 4094-4099.
13
[7] Dong, Chensong, and Ian J. Davies. "Flexural and tensile strengths of unidirectional hybrid
epoxy composites reinforced by S-2 glass and T700S carbon fibres." Materials & Design 54
(2014): 955-966.
[8] Zhou, Yuanxin, et al. "Fabrication and characterization of carbon/epoxy composites mixed
with multi-walled carbon nanotubes." Materials Science and Engineering: A 475.1 (2008): 157165.
[9] Hwang, Gan Lin, Y‐T. Shieh, and Kuo Chu Hwang. "Efficient Load Transfer to Polymer‐
Grafted Multiwalled Carbon Nanotubes in Polymer Composites."Advanced Functional
Materials 14.5 (2004): 487-491.
[10] Li, Qianqian, et al. "Mechanical properties and microstructure of single-wall carbon
nanotube/elastomeric epoxy composites with block copolymers."Materials Letters 125 (2014):
116-119.
[11] Jiang, Qian, et al. "Mechanical, electrical and thermal properties of aligned carbon
nanotube/polyimide composites." Composites Part B: Engineering 56 (2014): 408-412.
[12] Lu, Yanyan, et al. "Mechanical properties of 3-glycidoxypropyltrimethoxysilane
functionalized multi-walled carbon nanotubes/epoxy composites cured by electron beam
irradiation." Journal of Composite Materials 47.14 (2013): 1685-1694.
[13] Chen, Zhen-Kun, et al. "Reinforcement of epoxy resins with multi-walled carbon nanotubes
for enhancing cryogenic mechanical properties." Polymer 50.19 (2009): 4753-4759.
[14] Wei, Zhijuan, et al. "Flexural fatigue performance and electrical resistance response of
carbon nanotube-based polymer composites at cryogenic temperatures." Cryogenics 59 (2014):
44-48.
[15] Takeda, Tomo, et al. "Cryogenic mechanical properties of woven glass/epoxy composites
modified with multi-walled carbon nanotube and n-butyl glycidyl ether under tensile static and
cyclic loadings." Cryogenics 58 (2013): 33-37.
14
Chapter 3
Experimental procedure
15
3.1 Materials
For fabricating a laminate used epoxy resin as matrix it is Diglycidyl ether of Bisphenol A
(DGEBA) type epoxy. Triethylene tetra amine (TETA) used as hardener for curing. Both were
purchased from Atul Industries Ltd, India. The diameter of µm 3K plain weave glass fiber was
supplied by Saint Gobain, India. It served the need for reinforcement. Multi walled carbon tubes
having an outer diameter of 6 to 9 nm and 5 µm length were supplied by Sigma-Aldrich. Some
significant properties of the constituents of laminated composite are provided in Table-3.1.
Table 3.1: Properties of epoxy, glass fiber and MWCNTs
Properties
Density (g/cm3)
Epoxy
1.16
Glass fibre
2.58
MWCNTs
0.037
Tensile strength (GPa)
0.12
3.42
11-63
Tensile modulus (GPa)
4.11
72.31
250-970
Specifications of glass woven fabricWarp and weft density: 16 and 14 yarns/inch,
Fabric weight: 360 gm.
3.2 Fabrication process
3.2.1 Dispersion of MWCNT into epoxy resin
Fabricate the MWCNT reinforced glass/epoxy (CNT/GE) composite, the epoxy resin was
modified by incorporating MWCNTs in to it. The amount of CNT in CNT-GE composite was
0.3wt. % of epoxy for low temperature conditioning and 0.1 wt. %, 0.3 wt. % and 0.5 wt. % of
epoxy for cryogenic treatment. Pre-calculated CNT was slowly poured into 150 mL of acetone.
By using magnetic stirring the suspension was stirred at room temperature for 30 min at 1000
rpm. Followed by sonication for 30 min. Because of stirring and sonication, the CNTs gets
distributed throughout suspension. After sonication the suspension was mixed with precalculated epoxy. Magnetic stirring of epoxy/CNT/acetone mixture was done at 1000 rpm for 1
16
hr at 70 °C. Sonication was again carried out at 70 °C upto evaporate entire acetone. During
process might be air bubbles entrapped into the suspension. To remove these air bubbles,
suspension was vacuum degassed for 18 hrs. The figure shows the dispersion of CNTs in
composite and fabricate the fibre reinforced nanocomposite.
Fig 3.1: Dispersion of CNT in epoxy and further fabrication of laminated composite
3.2.2 Fabrication of Fiber reinforced nano-composites
After prepare the epoxy-CNT suspension mixed with required amount of hardener (10 wt. % of
epoxy. The volume percentage of matrix and fibres 50 and 50 respectively. By using hand –
layup method prepared the laminates. Followed by curing at 60°C and applied pressure 1 MPa in
hot compressed press for 20 min. Likewise, using the same parameters for prepare CNT/GE as
well as GE composites. For preparing GE composite, 14 layers glass fibre and required amount
of epoxy and hardener. Laminate was cut for flexural test (as per ASTM D7264) by using
diamond tipped cutter. The samples were then post-cured at 140 °C for 6 hr.
17
3.3 Material characterization
3.3.1 Mechanical characterization for low temperature conditioning
The samples were tested at different in-situ temperatures like -80 °C, -40 °C and RT (20 °C)
maintain holding time 10 min. and 1 mm/min loading rate. The entire test carried out by using 3point fixture environmental chamber of UTM-INSTRON 5967. Dynamical response of the
sample carried out by using DMTA –E242.
3.3.2 Mechanical characterization for cryogenic conditioning
All prepared samples were dipped in liquid nitrogen (77K) for different time durations of 0 hr.
(no conditioning), 0.25 hr., 1 hr., 4 hr. and 8 hr. After time duration samples drain out from the
liquid nitrogen and testes instantaneously at RT. The flexural test carried out by using universal
testing machine (UTM) –Instron 5967 as shown in figure 3.1. Here the sample dimensions as per
ASTM D7264 and loading rate 1mm/min.
Figure 3.2: Experimental set up of UTM-Instron 5967
18
3.3.3 Dynamic mechanical Thermal Analyser (DMTA)
The temperature range of dynamic mechanical thermal analyzer is from -100°C to 200°C at 10°C
heating rate. The samples were prepared by ASTM D7028. In DMTA, the specimens were
loaded in a 3-point bending mode using a frequency of 10 Hz.
Figure 3.2 Experimental set up for dynamic mechanical thermal analyzer (DMTA)
3.3.4 Fractographic analysis
To analyses the failure modes of composites, their fracture surfaces were observed under
Scanning Electron Microscope (SEM) with JEOL-JSM 6480 LVSEM operated at 20KV. The
dispersion of MWCNTs in epoxy in case of CNT-GE composite is observed under Field
Emission SEM. The surfaces of fractured samples were coated with a thin film of gold for
increasing its electrical conductivity.
19
Chapter 4
Low temperature
performance of CNT-GE
composites
20
4.1 Introduction
FRP composites are most promising materials in the world because of their superior properties
such as high strength to weight ratio, high corrosion resistance and low density etc. At low
temperatures matrix plays key role in strengthening the composite [1] [2]. Modify the matrix by
adding CNTs to get better mechanical and thermal properties at low temperatures [3] [4].
4.2 Results and discussion
4.2.1 Flexural behaviour at various testing temperatures
0.3%
CNT
0.3%
CNT
Figure 4.1: Flexural stress vs. strain curves for GE and CNT-GE composites for (a) -80⁰C (b) 20⁰C
The above figure 4.1 represents the stress vs. strain curves of CNT/Glass epoxy and glass epoxy
at low (-80⁰C) and RT (20⁰C). Figure 4 represents the flexural strength and flexural modulus vs.
temperature for CNT/glass epoxy and glass epoxy (GE).
21
Figure 4.2: Variation in (a) Flexural strength (b) Flexural modulus with testing temperatures for
GE and CNT-GE
From the figure 4.2 that the flexural modulus and strength both are dependent on temperature for
CNT/GE and GE. The below equations explains that strength and modulus dependency of
CNT/GE and GE on temperatures (for low temperatures (-80 °C, -40 °C and RT). For GE
composites:
For Glass/epoxy composites:
𝐸 (𝐺𝑃𝑎) = 24.53 − 0.03 𝑇 (°𝐶)
(1)
𝜎 (𝑀𝑃𝑎) = 370.56 − 1.1 𝑇(°𝐶)
(2)
For CNT/GE composites:
𝐸 (𝐺𝑃𝑎) = 28.84 − 0.05 𝑇 (°𝐶)
(3)
𝜎 (𝑀𝑃𝑎) = 435.45 − 1.36 𝑇(°𝐶)
(4)
The rate of decrement in modulus with increase in the temperature observed from the equations
(1) and (3). The decrement rate is 0.03 GPa/°C and 0.05GPa/°C for controlled GE and CNT/GE
respectively. From the equations (2) and (4) the rate of decrement in strength 1.1 MPa/°C and
1.36 MPa/°C for controlled GE and CNT/GE. The CNT/GE composite shows the higher strength
than glass/epoxy composite at room temperature (RT). Because CNT have high interfacial area
in CNT-epoxy matrix, the load transfer ability increases across the interface. At low
22
temperatures the strength and modulus improves than at room temperature. It might be attributed
by two reasons: (a) Hardening of matrix and (b) interfacial locking. Because of matrix hardening
the polymer molecules have less mobility results rigidity of matrix. Interfacial interlocking, as
temperature is decreasing, mechanical gripping established between CNT and epoxy due to
differential in thermal coefficients. The above two factors mainly effecting the strength and
modulus in case low temperature conditioning.
4.2.2 Dynamic Mechanical Thermal Analysis (DMTA)
Dynamical mechanical thermal analysis is devise to find the visco elastic response of the any
material. In this device applies a force (load) dynamically with wide range of temperature. Visco
elastic means the material show viscous property as well as elastic property. The storage/elastic
modulus (Eʹ) obtained from DMTA. The term (E’) represents the elastic property of the material.
Whereas the loss/viscous modulus (Eʺ) reflects the viscous property of material. The loss factor
denoted as tanδ. The damping tendency of the material. Damping means dissipating energy
under cyclic loads. It defined that ratio of Eʺ to Eʹ. The Eʹ, Eʺ and tanδ are determined using the
following equations [5] [6].
𝐸ʹ =
𝜎1
𝐸ʺ =
𝜎1
𝜀1
cos 𝛿
(5)
sin 𝛿
(6)
𝜀1
tan 𝛿 =
𝐸ʺ
(7)
𝐸ʹ
Where, 𝜎1 and 𝜀1 represent the stress at peak and strain at peak respectively and δ is the phase
difference between the dynamic stress to dynamic strain.
23
CNT
CNT
CNT
Figure 4.3: Variation in (a) Eʹ, (b) Eʺ and (c) tanδ with temperature for GE and CNT-GE
composites.
Figure 4.3 refers variation in storage modulus (Eʹ), loss modulus (Eʺ) and tanδ with temperature
for GE and CNT/GE composites. The reduction rate of Eʹ with temperature (upto Tg) is more in
CNT/GE composite compare to GE composite. The glass transition temperature (Tg) of the
material represents the change in slope of storage modulus and temperature curve. From figure
3.3 (a) it can be observed because of addition of 0.3 wt. % CNTs lowering in Tg from 136 °C to
125 °C. Due to entrapped CNTs into polymeric chains it results to reduce the formation of cross
links. Figure 4.3 (b) represents the variation in loss modulus (Eʺ) because of incorporation of
CNTs in composite. Figure 4.3 (c) show the tanδ value, it gives the damping property of the
material.
24
4.2.3 Constitutive flexural deformation model
In general, deformation/failure of the laminated composite is the final result of number of failure
micro mechanisms such as formation of local flexure, micro buckling, etc. The results in failure
modes like matrix cracking, fiber/matrix interfacial debonding. By using Weibull distribution
function, stress (σ)-strain (ε) relationship for a fiber reinforced composite can be modelled by the
given equations
𝐸𝜀 𝛽
𝜎 = 𝐸𝜀 exp [− (𝜎 ) ]
(8)
𝑜
Where E represents flexural modulus of the composite. Weibull scale ( 𝜎𝑜 ) and shape parameter
(β) respectively. The nominal strength of composite is denoted by𝜎𝑜 , randomness in strength was
measured by β. By taking double logarithm both sides of the equation 8 to evaluate 𝜎𝑜 and β
𝐸𝜀
ln [ln ( 𝜎 )] = 𝛽 ln(𝐸𝜀) − 𝛽 ln(𝜎𝑜 )
(9)
𝐸𝜀
Equation 9 represents a straight line between ln(𝐸𝜀) and ln [ln ( 𝜎 )] which can be seen from
figure 4.4. The slope of the straight line gives the value of β and from the value of intercept and
β the value of 𝜎𝑜 can be determined.
(a)
(b)
Figure 4.4: Weibull fitting of experimental for GE and CNT-GE at (a) -80⁰ C and (b) 20⁰ C
25
The parameters of the Weibull function for GE and CNT-GE composites obtained from figure
4.4 at various testing temperatures are reported in table 4.1
Table 4.1
Weibull scale (σo) and shape (β) parameters for GE and CNT-GE composites at various
temperatures
Temperature (oC)
-80
-40
20
Weibull scale parameter σo (MPa)
Weibull shape parameter (β)
GE
CNT-GE
GE
CNT-GE
1051.6±5.3
1572.5±292.6
2.01±0.13
1.73±0.23
852.4±45.3
1170.3±228.7
2.18±0.08
2.43±0.18
757.6±21.7
818.2±33.8
2.38±0.06
2.39±0.12
For both GE and CNT-GE composites, σo follows a similar trend that of similar to flexural
strength observed in figure. Alters the value of β by addition of CNT content to GE. Find the σo
and β values by using equation 9 and listed the values in table 3.1. The stress-strain curve was
drawn for GE and CNT-GE composites at -80 oC,-40⁰C and 20oC. Superimposed with the
experimental data as shown in figure 4.5. From figure 4.5 it can be evident that the experimental
and simulated stress vs. strain curves shows same kind of trend.
26
CNT
CNT
Figure 4.5: Comparison between experimental and simulated stress vs. strain for GE and CNTGE composites at (a) -80⁰C and (b) room temperature (20⁰C)
4.2.4 Fractography
The failure analysis of the GE and CNT (0.3%) -GE composites was carried out by using
scanning electron microscope (SEM). Fractured surfaces of GE composites at various testing
temperatures of SEM images observed from figure. Exposure to low temperature (-80 °C), due to
immobility of polymer molecules results the rigid of composite. Ultimately the material loss its
ductility and toughness. From figure3.6 (a) it can be observed that, at low temperatures the
dominating failure mode is brittle rupture of the matrix for GE composites. At low temperature,
the figure3.6 (b) indicates fractured surface in conjunction with debris and loose materials.
Because of shear loading the formation of debris failure modes attributed. The fractured surfaces
figure 3.6 (b) indicate fiber imprints and river line markings on the polymer in the interfilamentary region.
The major failure modes like fibre/matrix debonding observed at room temperature. At room
temperature from figure c. The fibre fragments and river line marking failure modes occur due to
low temperature conditions. From figure d it can be observed that scarp. These scarps type
failure mode observed at the end of the fibre due to applying load matrix gets deforms.
27
Figure 4.6: SEM images of fractured surfaces of GE composites of after flexural testing at (a, b)
-80⁰C and (c, d) room temperature (20⁰C)
The ribs was formed in case of CNT (0.3%)-GE composites, tested at low temperature (-80 °C),
was noticed as shown in figure 4.7(a). These ribs are formed might be high interlaminar matrix
thickness. In case of bulk polymer materials the ribs are arrest the cracks. Hence, it suggests ribs
may hold the enhancement in strength in case of CNT-GE composite. Figure 3.6 c represents the
delamination and fibre pull out failure modes in CNT/GE at room temperature. Figure 4.7 (b)
indicates the river line markings on the matrix surface, which is a signature of the brittle failure.
From figure 4.7(d) it can be observed
formation of scarps on the matrix, at the end to fibers.
Further observed under applied load, mirror, mist and hackle zones are present at the cross
section of fibre shown in figure 4.7 (c).
28
(b)
Figure 4.7: SEM images of the fractured surfaces of CNT – GE composites for after flexural
testing at (a, b) -80⁰C and (c, d) room temperature (20⁰C)
Figure 4.8 (a) represents the fractured CNT-GE composite at room temperature showing uniform
distribution and dispersion of the CNTs in polymeric matrix. The good dispersion of CNTs give
the better mechanical properties to composite. At room temperature the CNT-GE composite
shoes good interfacial bonding between CNT and epoxy. Figure 4.8 (b) shows the CNT pull out
phenomena, reported by various researchers which enhances the damage tolerance of the
composite.
29
Figure 4.8: SEM images of the fractured surfaces of CNT-GE composite after flexural testing at
room temperature (20⁰C) showing (a) distribution of MWCNTs in the epoxy (b) crack bridging
by MWCNTs.
4.3 Conclusion
The effect of in-service temperature on mechanical behaviour of CNT-GE and GE composites
was evaluated. At low in-service temperature environment, the CNT-GE composites exhibited
higher elastic modulus compared to GE composites, as confirmed from DMTA within the
studied range of temperature. Furthermore, addition of 0.3 wt. % MWCNT into GE composite
significantly lowered the Tg by 12 °C due to hindrance in crosslink formations. The
reinforcement efficiency (relative change in modulus) due to CNT incorporation in GE
composite is as high as 30%, when the testing temperature was -80 °C. It further reduces to 23%
when tested at room temperature (20 °C). The rate of degradation is significantly higher for
CNT-GE composites than GE composites (reducing the modulus of CNT-GE composite by 50%
than that of GE composite) due to presence of high interfacial area, providing more damage
nucleation sites, causing remarkable interfacial debonding. Within the temperature range of -80
°C to 20 °C, the magnitude of Weibull scale parameter (σo) was found to be enhanced by
incorporation of 0.3 wt. % MWCNT in GE composite. At -80 °C, the relative increment in σo
for CNT-GE composite was 50% with respect to controlled GE composite. The dependency of
30
in-service temperature on mechanical properties is more pivotal in case of MWCNT modified
GE composites than conventional GE composites.
References
[1] Lin, Y-S., C-H. Hu, and C-A. Hsiao. "Enhanced scratch resistance of flexible carbon fiberreinforced polymer composites by low temperature plasma-polymerized organosilicon
oxynitride: The effects of nitrogen addition."Composites Science and Technology 71.13 (2011):
1579-1586.
[2] Sethi, Sanghamitra, and Bankim Chandra Ray. "An assessment of mechanical behavior and
fractography study of glass/epoxy composites at different temperatures and loading
speeds." Materials & Design 64 (2014): 160-165.
[3] Jäckel, M. "Thermal properties of
temperatures." Cryogenics 35.11 (1995): 713-716.
polymer/particle
composites
at
low
[4] Dutta, Piyush K., and David Hui. "Low-temperature and freeze-thaw durability of thick
composites." Composites Part B: Engineering 27.3 (1996): 371-379.
[5] Guadagno, L., et al. "Effect of functionalization on the thermo-mechanical and electrical
behavior of multi-wall carbon nanotube/epoxy composites." Carbon49.6 (2011): 1919-1930.
[6] Prolongo, S. G., M. R. Gude, and A. Ureña. "Improving the flexural and thermomechanical
properties of amino-functionalized carbon nanotube/epoxy composites by using a pre-curing
treatment." Composites Science and Technology 71.5 (2011): 765-771.
31
Chapter 5
Cryogenic temperature performance
of CNT-GE composite
32
5.1 Introduction
FRP composites find application in aerospace industry, marine vehicles, structural applications,
cryogenic fuel tanks, hydrogen storage tanks, pressure vessels, thermal insulators, etc.[1][2] FRP
composites have been proved to be a better choice of material in replacing metallic materials in
various cryogenic applications mostly cryogenic fuel tanks for its efficient storage and
transportation [3][4]. This is due to their desirable properties like high resistance to corrosion,
high specific strength, specific stiffness, etc. Modifying the matrix using nano-fillers is a recent
trend observed in fabrication of FRP composites to enhance the matrix dominated properties [5]
[6] [7].
5.2 Results and discussion
5.2.1 Flexural performance after cryogenic treatment
The Figure 5.1 shows flexural stress vs. strain curves for GE and CNT-GE (all the compositions)
samples conditioned in liquid nitrogen for 0 hr, 0.25 hr, 1 hr and 4 hr. Figure 5.2 and 5.3
represents the flexural properties i.e. flexural modulus and flexural strength plotted against
conditioning time for various amount CNT content. From Figure 5.2 the flexural strength of
CNT (0.1%)-GE shows maximum strength as fabricated conditioned. The increment in strength
of CNT (0.1%)-GE around 32.7% than other CNT-GE and GE composites.
33
Figure 5.1: Flexural stress vs .strain curves for GE and CNT-GE composites conditioned in
liquid nitrogen for (a) 0hr (b) 0.25 hr (c) 1 hr and (d) 4hr.
This may be due to increased CNT/epoxy interfacial area due to high surface area of CNTs and
hence more stress/load transfer across the interface takes place.
Figure 5.2: flexural strength for GE and CNT-GE composites with conditioning time in liquid
nitrogen.
Hence, it results in more stress required to break the sample. At room temperature the strength of
CNT (0.3%)-GE and CNT (0.5%)-GE decreases compared to CNT (0.1%)-GE due to
34
agglomeration CNTs. Due to agglomerated CNTs will not able to show their nano scale
properties. It results their interfacial area decreases. Hence this lead to decrement in strength.
The flexural modulus also follow the same trend that of flexural strength for all composites
observed from figure.
Figure 5.3 Variation in flexural modulus for GE and CNT-GE composites with conditioning
time in liquid nitrogen.
From figure 5.3 it can be seen that after 0.25 hr conditioning, there is drop in modulus for all the
composites as compared to the modulus obtained for samples with no conditioning. It is
following the same fashion followed by variation in strength found after 0.25 hr conditioning.
After 1 hr conditioning, the modulus of all the composites except CNT (0.1%)-GE composite
decreased further.
0.25 hr thermal shock conditioning makes the polymeric chains gets frozen and results in drastic
matrix embrittlement. From figure 5.3 it can be observed that enhancement in mechanical
properties by the addition of CNTs into GE composites. The samples expose for 0.25 hr in liquid
nitrogen (-196⁰C), the strength and modulus drastically decreased. Whenever samples brought
from -196 oC
to room temperature the samples experienced thermal shock. In case of GE
composites, due to differential in thermal contractions between glass fibre and epoxy may have
generate localized thermal stress at interface of epoxy and fibre. Due to this thermal stresses
leads to formation of micro-cracks and micro voids at the interface. Interfacial debonding is
more in case of CNT-GE composite. Because in CNT-GE composite exist another interface that
is CNT/epoxy. Also, differential thermal contraction is more in case of epoxy (6.2 × 10 -5 K−1)
than CNT (0.73–1.49 × 10-5 K−1). The drop in strength by 36.2 %, 38.7 %, 60.2 % and 68.1 %
35
in case of GE composite, CNT (0.1%)-GE composite, CNT (0.3%)-GE composite and CNT (0.5
%)-GE composite respectively after 0.25 hr conditioning in liquid nitrogen. Further, exposed the
samples for 1 hr and test instantaneously. Because of formation of micro cracks, micro voids
and insufficient matrix hardening the strength and the modulus of the GE composite decreases as
compared to all CNT-GE composites. The CNT-GE (all the compositions) composite showed
relative increment in strength and modulus as compared to obtained after 0.25 hr conditioning.
This might be CNTs have obstruct the path propagation of cracks. Further exposing the samples
for 4 hr, recovery in flexural strength and modulus was observed for the GE composite and
CNT-GE composites (all compositions). Because of high amount of matrix gets hardening due to
available of enough time to polymer chains to freeze. Also, because of gripping between
CNT/epoxy improves the stiffness of the composite. The increment in strength by 45.2%,
55.82%, 73.9% and 126% of GE, CNT (0.1%)-GE, CNT (0.3%)-GE and CNT (0.5%)-GE
respectively as compared to flexural strength of 0.25hr conditioning. A very little increment in
strength observed after exposed samples for 8hrs compared to expose for 4hrs. This may
attributed the density of micro-cracks remained same and hence not much significant increase in
strength was observed.
Figure 5.4: Variation in failure strain for GE and CNT-GE composites with liquid conditioning
time.
From figure 5.4 it can be observed that the strain to failure is more in case of CNT (0.1%)-GE
than other all composites .It represents the toughness of the material. For short time (0.25hr.)
36
conditioning ductility is reduced due to thermal shock resulting frozen polymer chains. For long
term (4hr.) conditioning the residual stresses ahead of crack tip is released resulting enhancement
in ductility and toughness.
From the above discussion, that the mechanical properties of GE and CNT modified GE
composite is strongly impact on the duration of liquid nitrogen conditioning and content of CNT.
Structural integrity of GE composite could be efficiently improved with better reliable by
addition of CNT (0.1%).
5.2.2 Damage constitutive failure model
For durable and safe applications to it is need to design the critical parameters properly .With the
help of Weibull probability of functions modelling of stress strain relationship. The simulated
stress can be obtained with the help of equations from chapter 4.
Figure 5.5 represents the Weibull fitting plots of GE and CNT (0.1%) samples conditioned for 0
hr, 0.25 hr, 1 hr and 4 hr and the calculated Weibull scale parameter( 𝛽 )and nominal strength
(𝜎𝑜 ) values are plotted in figure 4.6 . From the Figure 4.6, it can be observed that the value of
nominal strength ( 𝜎𝑜 ) reduces
after conditioning GE and CNT-GE (all compositions)
composites after exposed to liquid nitrogen for 0.25 hr and further increases (except GE
composite) as the conditioning time increases to 1 hr. From the figure the trend in nominal
strength (𝜎𝑜 ) is quite similar to that of trend obtained for flexural strength. After short term
conditioning, the value of Weibull scale parameter ( 𝛽 ) was showed maximum for CNT (0.1%)GE composite showing minimum scatter. Further exposure of 1 hr and 4 hr resulted in decreased
variation in 𝛽. Overall results showed that the strength of all these composites obey the Weibull
distribution model.
37
Figure 5.5: Weibull fitting for experimental GE and CNT (0.1%)-GE composite conditioned in
liquid nitrogen (a) 0hr (b)0.25hr (c) 1 hr and (d) 4hr
38
Figure 5.6: Variation in Weibull shape parameter and scale parameter with varying of CNT
content and conditioning time.
The figure 5.7 represents the experimental and simulated stress vs. strain curves for different
conditioning time of 0 hr, 0.25 hr, 1 hr and 4 hr for different compositions. This shows that the
experimental stress vs. strain curves are in very close with the simulated stress vs. strain curves
for 0, 1hr and 4hrs whereas in case of short term exposure (0.25 hr) the experimental and
simulated stress-strain curves are not matching to that extent. Hence this implies that the
materials mechanical response become less uniform and less predictable under short term liquid
nitrogen exposure or thermal shock.
39
Figure 5.7: comparison between experimental and simulated flexural stress vs. strain curves for
GE and CNT-GE composites.
5.3 Fractography
Scanning Electron Microscopy (SEM) technique was adopted to identify the failure mechanisms
responsible for the overall failure of the GE and the CNT-GE composites. Figure 5.8 shows the
SEM images of fractured surfaces of GE composite and CNT (0.1%, 0.3%, 0.5%)-GE composite
after conditioning them in liquid nitrogen for 0.25 hr. It is evident from Figure 5.8 (a) that in case of
GE composite, the major failure modes are matrix cracking, due to differential thermal contractions
and formation of ribs.
40
Figure 5.8 SEM micrographs of the samples of (a) GE, (b) CNT (0.1%)-GE, (c) CNT (0.3%)-GE and (d)
CNT (0.5%)-GE composites conditioned in liquid nitrogen for 0.25 hr.
Figure 5.8 (b) shows the fractograph for CNT (0.1%)-GE composite and indicates that generation of
scarps due to multiple micro-cracks lowers the strength of this composite. in Figure 5.8 (c).
Debonding results due to large interfacial shear stress generated between fiber and matrix at the time
of loading. These micro-cracks are generated due to coalesce of many micro-voids formed in the
matrix. This can be seen from Figure 5.8 (d) of CNT (0.5%)-GE composite.
41
Figure 5.9: SEM micrographs of fractured samples of (a) GE, (b) CNT (0.1%)-GE, (c) CNT (0.3%)-GE and
(d) CNT (0.5%)-GE composites conditioned in liquid nitrogen for 4 hr.
Fibre fracture is the mechanism accountable for failure of GE composite after 4 hr. Also, as fiber
fracture is more energy absorbing failure mode, it can be observed from figure 5.9(a). Debries are
formed as shown in Figure 5.9 (b) due to abrasion between hardened matrix and fiber. The fracture
will proceed and the broken fibers will ultimately be pulled out of the matrix as shown in Figure 5.9
(c). Extensive river line markings and potholes, in addition to voids are observed in case of CNT
(0.5%)-GE composite as evident from Figure 5.9(d).
42
5.4 Conclusion
Evaluation of cryogenic treatment on the flexural performance of GE and CNT-GE composites
was done for various conditioning time. Present study suggested that liquid nitrogen conditioning
time has a strong impact on the mechanical behaviour of conventional composites as well as the
nano-filler engineered composite. In the as fabricated condition, CNT (0.1%)-GE composite
showed the highest strength among all the other composites fabricated i.e. the reinforcement
efficiency (relative change in modulus in comparison to GE composite) was 28 %. This further
dropped down to 25.1 % when cryogenically treated for short duration i.e. 0.25 hr. a longer
conditioning time enhances the strength of CNT-GE composite more effectively than GE
composite due to stiffening of CNTs and generation of clamping stress at CNT/epoxy interface.
Thus, this study suggests that the flexural behaviour of GE and CNT-GE composite is strongly
affected by liquid nitrogen conditioning time. This study also suggests that further high strength
can be expected by cryogenic conditioning of CNT-GE composite and GE composite when the
rate of cooling is significantly low to avoid thermal shock
References
[1] Zhou, Yuanxin, et al. "Fabrication and characterization of carbon/epoxy composites mixed
with multi-walled carbon nanotubes." Materials Science and Engineering: A 475.1 (2008): 157165.
[2] Park, Joung-Man, et al. "Effects of carbon nanotubes and carbon fiber reinforcements on
thermal conductivity and ablation properties of carbon/phenolic composites." Composites Part
B: Engineering 67 (2014): 22-29.
[3] Icten, Bulent Murat. "Low temperature effect on single and repeated impact behavior of
woven glass-epoxy composite plates." Journal of Composite Materials (2014):
0021998314531309.
[4] Huang, C. J., et al. "Cryogenic properties of SiO 2/epoxy nanocomposites. “Cryogenics 45.6
(2005): 450-454.
[5] Takeda, Tomo, et al. "Cryogenic mechanical properties of woven glass/epoxy composites
modified with multi-walled carbon nanotube and n-butyl glycidyl ether under tensile static and
cyclic loadings." Cryogenics 58 (2013): 33-37.
[6] Coleman, Jonathan N., et al. "Small but strong: a review of the mechanical properties of
carbon nanotube–polymer composites." Carbon 44.9 (2006): 1624-1652.
43
[7] Yu, Min-Feng, et al. "Strength and breaking mechanism of multi walled carbon nanotubes
under tensile load." Science 287.5453 (2000): 637-640.
1
List of Publications during M.Tech Project
[1] K. K. Mahato, M. J. Shukla, D. S. Kumar, and B. C. Ray, “In- service Performance of Fiber
Reinforced Polymer Composite in Different Environmental Conditions: A Review,” J. Adv. Res.
Manuf. Mater. Sci. Metall. Eng., vol. 1, no. 2, pp. 55–88, 2014.
[2] M. J. Shukla, D. S. Kumar, K. K. Mahato, D. K. Rathore, R. K. Prusty, and B. C. Ray, “A
comparative study of the mechanical performance of Glass and Glass/Carbon hybrid polymer
composites at different temperature environments,” IOP Conf. Ser. Mater. Sci. Eng., vol. 75, no.
1, p. 012002, Feb. 2015.
[3] D. S. Kumar, M. J. Shukla, K. K. Mahato, D. K. Rathore, R. K. Prusty, and B. C. Ray,
“Effect of post-curing on thermal and mechanical behavior of GFRP composites,” IOP Conf.
Ser. Mater. Sci. Eng., vol. 75, no. 1, p. 012012, 2015.
[4] M. J. Shukla, D. S. Kumar, D. K. Rathore, R. K. Prusty, "An assessment of flexural
performance of MWCNT embedded glass/epoxy composite after liquid nitrogen conditioning",
Journal of Composite Materials, Sagepub (under review)
[5] D.K Rathore, R.K Prusty, D.S. Kumar, “Evaluation of elevated temperature mechanical
properties of glass/epoxy laminated composites using multi-walled carbon nanotubes.”, Journal
of Composite Structures,(Under review)
2
Symposium/Conference attended
[1] Presented a poster on “Effect of MWCNT content on mechanical properties of liquid nitrogen
conditioned MWCNT embedded glass/epoxy composites”, at National symposium for Materials
Research MR-15, IIT Bombay during 21st -22nd May, 2015.
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