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Experimental and Numerical Investigation of Damping
Properties of Natural Fiber Reinforced Composites
Siddhesh Sawant1,Ashok Mache2
1Mechanical Engineering, Vishwakarma Institute of Information Technology, Kondhawa (Bk), Pune-
411048, India.
2Mechanical Engineering, Vishwakarma Institute of Information Technology, Kondhawa (Bk), Pune-
411048, India.
ABSTRACT
The importance of use of composite materials has been increasing consistently in various fields like mechanical,
civil and aerospace engineering etc. due to their good specific mechanical properties. Composites are known for
their high strength and stiffness with low weight. Most of the structural components are generally subjected to
dynamic loadings in their working life. Very often these components may have to perform in severe dynamic
environment where maximum damage results from resonant vibrations. Maximum amplitude of vibration must be in
the limited range for the safety of the structure. Hence, damping analysis has become very important to control the
vibration response and its amplitude. The present study involves experimental and numerical investigation of
damping properties of natural fiber reinforced composites and its hybrids i.e., jute/epoxy, hemp/epoxy, glass/epoxy,
glass/jute/epoxy, jute/glass/epoxy, aluminum/jute/epoxy and aluminum/glass/epoxy. Natural fibers are used to
fabricate fiber reinforced composites using hand lay-up method supported by compression molding machine.
Damping properties are determined by using half-power bandwidth method. Tensile strength and modulus of the
specimens are obtained experimentally. The experimental results of natural frequencies and damping ratio are
compared with numerical results obtained by using ANSYS R18.2.
Keywords: Composites, damping, FFT analyzer, frequency response function, half-power bandwidth
method, vibration.
.
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Volume 8, Issue IX, SEPTEMBER/2018
ISSN NO : 2249-7455
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1. INTRODUCTION
Natural fiber reinforced composites are widely used in the area of aerospace, automobile, sports equipments and
military sectors due to its multi-functional features like low density, biodegradability, high specific strength, fire
resistance, improved mechanical properties, resistance to corrosion and low cost when compared with artificial fiber
reinforced composites like glass, Kevlar, carbon fiber. The most commonly used plant fibers for reinforcement in
composite are sisal, jute, banana, Kenaf, Palmyra, hemp, coir, flax, ramie etc., which are best suited for low and
medium load engineering applications. It is important for an engineering material to have better energy dissipating
capability along with their stiffness and strength requirements. The composites are comparatively cheaper to
manufacture and have much higher surface finish. The use of composites has given more flexibility to design
engineers to modify the existing design or develop a new design. Composite material gives chance to designers and
engineers to increase material efficiency, therefore resulting in cost reduction and better utilization of resources.
Damping is the phenomenon by which mechanical energy is dissipated in dynamic system and it limits the resonant
amplitude of vibration. Damping is essential to study the dynamic characteristics of fiber reinforced composites.
Damping mechanisms in natural fiber composites differ entirely from those in conventional materials and energy
dissipation depends upon factors like viscoelastic nature of matrix and/or fiber, interphase, damage and viscoplastic
characteristics. Damping of a structure can be achieved by passive or active methods. A passive method uses the
inherent capacity of certain materials to absorb the vibration energy, thus providing passive energy dissipation. An
active method uses sensor and actuators to attain vibration sensing and activation to control the vibration in a real
time. However, fibers are the main factors controlling the properties of composites.
In particular, damping for materials with natural fiber is difficult to study due to their chemical constituents;
nevertheless it shows good damping characteristics due to their inherent porous nature. Many researchers have
investigated dynamic properties such as storage modulus, loss modulus and damping ratio of natural fiber rein-
forced composites. A. Mache et al. [1] studied the comparison of mechanical properties of jute-polyester composites
with hybrid jute-steel-polyester composites and summarized that hybridization is advantageous in terms of tensile
and flexural modulus, reducing the chances of brittle failure and arresting fall in tensile modulus due to moisture
absorption. Akash et al. [2] studied dynamic behavior of hybrid jute/sisal fiber reinforced polyester composites and
found that damping ratio of hybrid jute/sisal composites is higher than that of conventional composites and
monolithic materials i.e., 1.15 times jute laminate. C. Devalve and R. Pitchumani [3] contemplated damping
enhancement in fiber reinforced composite with addition of carbon nanotubes (CNT). Result shows that addition of
CNT increases damping by 133%. P. Sature and A. Mache [4] investigated mechanical properties and water
absorption study of jute fiber, hemp fiber, jute-hemp fiber, and jute-hemp-glass fiber reinforced composites and
found that hybridization of jute with hemp and jute-hemp with the glass fiber improves the mechanical properties
and can be considered as potential replacement to glass fiber. V. Allien et al. [5] presented investigation on free
vibration analysis of laminated chopped glass fiber reinforced polyester (CGRP) resin composite with two, four and
six layers. Result indicates that natural frequency of six layers CGRP composite material is more than two and four
layers CGRP composite material. A. Mache et al. [6] studied the effect of strain rate on mechanical response of jute-
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polyester composites and observed that there is significant effect of strain rate on stress-strain curves as tensile and
compressive failure strength increases with respect to increasing strain rate. M. Rajesh [7] studied influence of
surface pre-treatment with sodium hydroxide and hybridization effect of natural fiber on flexural test and free
vibration behavior. It is found that chemical treatment improves the mechanical and free vibration properties of
polymer composites due to the enhancement of interfacial bond between fiber and matrix as the result of chemical
treatment. J. Alexander and B. Augustine [8] studied damping characteristics of unidirectional [UD], woven fabrics
of basalt fiber and glass fiber at various end condition and observed that natural frequency of woven fabric
composite is higher than UD composite. Also, laminate with fixed-fixed end conditions are having high natural
frequency than cantilever and simply supported end condition. A. Erklig, M. Bulut [9] studied influence of borax
filler on vibration response of S-glass/epoxy composite laminates and found that the sample with 5 mass % of borax
filler has maximum damping ratio and loss modulus. K. Kumar et al. [10] analyzed sisal fiber (SFPC) and banana
fiber (BFPC) under the influence of fiber length and weight percentage. It is observed that an increase in fiber
content increases mechanical and damping properties. S. Chavan and M. Joshi [11] investigated vibration of
glass/epoxy composite in fix-free boundary condition and found that natural frequency increases with increase in
aspect ratio and no. of layers.
Many researchers have developed various solutions to analyze the dynamic performance of laminated
composites. Though experimental investigations on woven fabric composite laminated structures are not received
much attention. Therefore, the aim of this study is to compare damping properties of natural fiber reinforced
composites and its hybrid.
Nomenclature
FFT Fast Fourier Transform
FRF Frequency Response Function
2. EXPERIMENTAL DETAILS
2.1 Materials
Woven jute fiber, hemp fiber, glass fiber and aluminium sheet are used as reinforced materials and commercially
available epoxy 520 and PAM-hardener as a matrix material.
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TABLE 1 Physical properties of fibers
Physical properties
Jute
Hemp
Glass
Density (g/cm3)
1.4
0.86
2.55
Diameter(mm)
0.017
0.066
0.02
Tensile strength(MPa)
108
250
1950
Young’s modulus(GPa)
3.42
11
72
2.2 Fabrication of Laminates
The composite laminates are prepared by hand lay-up technique supported with compression moulding machine.
The laminate of size 300mm x 300mm x 3mm was made. Firstly, Epoxy resin-520 was mixed with PAM-hardener
in the ratio 10:1 by weight. Fibers are impregnated with resin and arranged one over the other with different stacking
sequence. Then, put whole assembly under rectangular mould plates and compressed the given structure to obtain
desire thickness. The laminate is then allowed to cure for next 24 hours at room temperature. Spacers are used
during the manufacturing to obtain uniform thickness. The manufacturing laminates are labelled as jute/epoxy
laminate (J/E), hemp/epoxy (H/E), glass/epoxy (G/E), glass/jute/epoxy (G/J/E), jute/glass/epoxy (J/G/E),
aluminium/jute/epoxy (Al/J/E) and aluminium/glass/epoxy (Al/G/E).
Fig. 1.Compression moulding machine
2.3 Tensile Testing
The composite materials fabricated are cut into required dimension using wire cutting machine. The tensile test
specimen is prepared according to the ASTM D3039 standard and tensile testing is carried out using the Universal
Testing Machine (UTM) (Model No-STS248) with an accuracy of ±1%. The UTM cross head speed is maintained at
1 mm/min. The capacity of the machine is 100kN. The testing process involves placing the test specimen in the
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testing machine and applying tension to it until it fractures. During the application of tension, the elongation of the
gauge section is recorded against the applied force. The experiments are repeated for several times and the average
values are used for discussion.
2.4 Modal Analysis
A cantilever rectangular plate of composite material is shown in Fig. 2. The specimen is clamped on test fixture at
one end and accelerometer is attached to the other end of specimen. The structure is excited by tapping free end of
specimen. A signal is recorded by unidirectional piezoelectric accelerometer. Four channelled FFT analyzer is used
to obtain relation of amplitude and frequency with time i.e. frequency response function (FRF). FRF is used to
calculate damping ratio by using half power bandwidth method.
Fig. 2. Modal analysis by FFT analyzer
1. Piezoelectric accelerometer, 2. Four-channel FFT analyzer, 3. Damping response displayed on screen,
4. Canti-lever beam support.
4
3
1
2
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2.5. Half-power Bandwidth Method
Fig. 3.Half-power bandwidth method
This method is based on curve of frequency response function (FRF) obtained from model analysis. Bandwidth is
defined as the width of the frequency response curve when magnitude is 1/√2 times the peak value i.e. represented
by point 1 in Fig. 3. If we draw a horizontal line at 1/√2 times the peak value, it will intersect the curve at two points
and gives corresponding values of frequencies i.e. ω1 and ω2. Then, damping ratio can be determined from
bandwidth using the expression;
(1)
Where, = Natural frequency corresponds to point 1.
2.6. Euler-Bernoulli Equation
The experimental results of natural frequency are compared with theoretical results using by Euler-Bernoulli beam
theory. The expression of natural frequency is given by equation;
(2)
where, E- Young’s modulus of beam (Pa), I- Moment of area (m2), ρ- Density of beam (m4), A-Cross sectional
area of beam (m2), L- Length of beam (m)
2.7 Logarithmic Decrement Method
One of the methods which is used to measure the damping ratio of an under damped system is the log decrement
method. In this system, vibration amplitude exponentially decays over time and natural log of amplitudes of any two
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successive peaks is called the logarithmic decrement. Logarithmic decrement can be calculated by equation (3);
(3)
Where, = Logarithmic decrement, A0= Maximum amplitude of first cycle, A =Maximum amplitude of nth cycle
Damping ratio can be obtained using logarithmic decrement by equation (4);
(4)
2.8 Etching Treatment for Aluminum
Sulphuric acid (H2SO4) A.R grade (Specific gravity=1.84) and ferric sulphate Fe2 (SO4)3 4H2O is used to make
solution. Solution is prepared by dissolving 122.5 g ferric sulphate Fe2 (SO4)3 4H2O and 0.185 liter of concentrated
sulphuric acid (H2SO4) in enough water to make a liter of Solution [12] and aluminum plates are dipped inside it.
The aluminum plates are immersed in this solution at 65 oC for 8 min. and then used to make laminates.
3. RESULT AND DISCUSSION
3.1 Tensile Testing
The different composite specimen samples are tested in the universal testing machine (UTM) and the samples are
left to break till the ultimate tensile strength occurs. Stress–strain curve is plotted for the determination of ultimate
tensile strength and elastic modulus.
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Fig. 4. Stress vs. Strain curve for different composites under consideration.
The results indicated that aluminum/glass/epoxy specimen gives better tensile strength then the other composites
under consideration. Aluminum/glass/epoxy has maximum tensile strength of 168.693MPa. The tensile strength of
glass/epoxy, jute/glass/epoxy, glass/jute/epoxy; hemp/epoxy, jute epoxy and jute/aluminum/epoxy composite
laminates are 158.44, 106.98, 61.46, 33.10, 48.27 and 38.54 respectively. The significant improvement in tensile
strength has been recorded due to hybridization. Hybridization of jute with glass fiber increases the tensile strength
of pure jute by 121%. As the composite materials are orthotropic in nature, the tensile testing of specimen was done
in longitudinal (L) and transverse (T) directions.
Fig. 5.Comparison of tensile strength for different composites.
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3.2 Vibration Analysis
Fig. 6. Frequency response curve obtained from modal analysis.
Vibration testing has been carried out on all composites under consideration. The composite specimen is clamped as
cantilever beam and modal analysis is carried out. The frequency response curve is obtained directly from the
software itself (RT Pro Photon 6.34.9104). Fig. 6 shows (Amplitude vs. Time) curve that represents exponential
decay of amplitude with time after disturbance and (Amplitude vs. Frequency) curve represents resonant frequency
of vibration. These results are validated numerically by ANSYS R18.2 shown in Fig. 7. Glass/jute/epoxy has
maximum natural frequency of 38.75Hz. Fig. 8 shows comparison of natural frequency for different composites.
Maximum damping ratio has observed for hemp/epoxy laminate. Also, hybrid jute/glass/epoxy laminate has higher
damping ratio than pure glass. The experimental results of damping ratio obtained from half-power bandwidth
method are compared with theoretical results obtained from logarithmic decrement method. Fig. 9 shows
comparison of damping ratio for different composites. The vibration testing of composites are tested for different
length i.e. 200mm, 230mm, 250mm. and it is observed that as length of the laminate increases, damping ratio
increases and natural frequency decreases.
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Fig. 7.Vibration analysis of aluminium/glass/epoxy composite laminate by ANSYR18.
Fig. 8.Comparison of natural frequency for different composites.
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Fig. 9. Comparison of damping ratio for different composites.
4. CONCLUSION
Tensile, damping and vibration characteristics of jute/epoxy laminate (J/E), hemp/epoxy (H/E), glass/epoxy (G/E),
glass/jute/epoxy (G/J/E), jute/glass/epoxy (J/G/E), aluminum/jute/epoxy (Al/J/E) and aluminum/glass/epoxy
(Al/G/E) composites are examined. Damping characteristics of the composite samples are determined by FFT
Analyzer. The main conclusions from this study can be summarized as follows;
It has been observed that the theoretical calculations of natural frequency and damping ratio matched
with experimental calculation very well for the cantilever structural beam with maximum errors of
27%.
Aluminium/glass/epoxy composite laminate has maximum tensile strength and can hold strength up to
168.693 MPa.
Tensile strength of composites is strongly affected by hybridization. Jute/glass/epoxy hybrid composite has
tensile strength of 106.98MPa which is higher than pure hemp and jute.
Hemp/epoxy laminate possesses highest damping capacity and has maximum damping ratio of 0.1025.
It has been observed that as length of the laminate increases, damping ratio increases and natural frequency
decreases.
This experimental damping analysis has explored to predict the dynamic behavior of composites in order to
design panels or other similar structure and to develop high performance materials for building and
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construction, railways, automobiles, aerospace, biomedical etc. Also, the applications of such high damping
materials may eliminate special energy absorbers.
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