Parametric Analysis on the Vibration Characteristics of
Sandwich composites Structure
Amith Kumar S J1, Sabeel Ahmed K2 & Santosh M B3
1, 2, 3
Department of Mechanical Engineering
Jawaharlal Nehru National College of Engineering, Shimoga-577 204, Karnataka, India
Visveswaraiah Technological University, Belgaum-Karnataka, India
joanesamith@gmail.com1, sabil_k@yahoo.com2, santhoshmb22@gmail.com3
response under static loading [1]-[3] and impact loading
[4]-[5]. Foam cored sandwich structures are increasingly
used in the aforementioned sectors for its low density
characteristics. Structural response of the sandwich
composite depends mainly on the strength properties of
the foam core material [6]. The purpose of the core is
limited to transmit shear stresses between the face skins
and to keep the skin distance approximately constant
during the deformation under transverse loading
condition [7]. Syntactic foam is one such core material
which yields higher strength and stiffness properties.
Syntactic foam based sandwich composites are presently
being used in wide variety of marine and naval
engineering applications [8]. Some of the researchers
investigated dynamic response of composite structures
using various techniques [9]-[10].
Abstract – Syntactic foam sandwich composites are
potential materials for aerospace and marine applications
because of their high specific properties and better energy
absorption characteristics. In the present work, syntactic
foam is prepared by uniform mixing of dry fly ash
cenosphere (solid waste material) and phenolic resin in
equal proportions. The effect of stiffening the syntactic
foam core by resin impregnated paper honeycomb (RIPH)
structure on vibration characteristics of sandwich
composite panels are experimentally investigated under
freely suspended configuration. Uni-axial Accelerometer, 4
Chanel real time analyzer and impulse hammer are used to
analyze vibration signals using half-power bandwidth
method. The results reveals that, frequency and damping
ratio of sandwich composites panels are significantly
influenced by the cell size of resin impregnated paper
honeycomb structure infused in syntactic foam core. The
effects of cell size on the core density of sandwich panels
and its influence vibration response are also discussed.
Index Terms — Frequency response, Half
Bandwidth, Sandwich Panels, vibration signals.
I.
A review of the literature reveals that, syntactic
foams of different materials have been successfully used
as structural materials. However, most of the studies
deals with the effect of change in volume fraction,
density, microsphere particle size (radius ratio) etc on
mechanical properties of syntactic foams. The benefit of
stiffening the syntactic foam by the incorporation of
resin impregnated paper honeycomb (RIPH) core
structure and their influence on vibration characteristic
is not found in the literature till date. The present work
explores this possibility by investigating the vibration
signal frequency response of sandwich composites with
core of syntactic foam integrated with RIPH structure.
power
INTRODUCTION
Composite materials offer high strength, higher
modulus and lower density. This is of utmost important
in many applications such as aerospace, marine and
automotive structures. Depending on whether the
structural design is strength-critical or stiffness-critical,
the material used should therefore have a high strengthto-weight ratio or a high stiffness-to-weight ratio.
Sandwich composites are produced by attaching two
thin but stiff skins to a lightweight thick core. Further,
the sandwich construction must exhibit good mechanical
II. MATERIALS AND METHODS
Materials used in the present investigations are, dry
fly ash cenosphere of density 450 kg/m3 supplied by
M/S Cenosphere India Pvt Ltd, Kolkota and phenolic
resin of density 1120 kg/m3 supplied by M/S Romit
Resins Pvt Ltd, Raigad- Maharastra are used for the
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International Journal on Mechanical Engineering and Robotics (IJMER)
fabrication of syntactic foam based Eco-core (blend of
dry fly ash cenosphere and phenolic resin).E-glass fabric
of 185 gsm and Epoxy resin LY 556 with Hardener HY
951 (in the ratio 10:1) supplied by M/S Insulation House
Bangalore are used for the fabrication of face sheets.
The configuration of developed sandwich
composites is shown in Fig. 3. Table 1 details the
developed sandwich composite panels.
Fig. 1 shows the warm press molding technique
used for the preparation of syntactic foam core. The
mold consists of two M.S plates of size (380 × 380 ×
18) mm. The spacing between the mold plates is
maintained at 12.5 mm using a metal frame. Mold
surfaces are coated with silicon grease and then wrapped
with aluminium foil to ensure easy removal of foam
core after curing. Syntactic foam is made by the blend of
dry fly ash cenosphere and phenolic resin uniformly
mixed in the ratio of 50:50 by weight. The mixture is
then thoroughly packed in the inner cavity of the metal
frame which is kept on the lower mold plate. After
packing the blend, the two mold plates are firmly
clamped and then heated. The core is cured at 140-150
0
C for 15 minutes in between the mild steel mold plates
followed by room temperature curing for at least 24
hours before its use in the fabrication of sandwich
composite. Provision is also made in the mold to allow
the hot gases to escape during the process of curing. The
developed composite out of this process called as
“stiffened-syntactic foam core”.
Mold plates are
clamped using
C-clamps
Heat
Syntactic foam + RIPH
Top Mold Plate
Metal Frame
Bottom Mold Plate
Heat
Fig.1 Warm Press Molding Process
For fabrication of sandwich composites, glass/epoxy
laminates with fiber mass fraction of about 0.5 is first prepared
by wet layup technique and then vacuum bonded to the core to
form the face sheets as shown in Fig. 2. The thickness of the
face sheets is maintained at 1.5 mm. The sandwich composites
are cured for 24 hours at room temperature and then post cured
at 100 ± 3 0C for one hour before being used for the
preparation of test panels.
Glass/epoxy face sheets + syntactic foam
Fig. 3 Configuration of sandwich composites panels
with core of (a) Syntactic foam (b) Syntactic foam with
kraft RIPH structure
Table 1 Details developed of sandwich composite panels
Vacuum Bag
Metal Frame with
gasket seal
To Vacuum pump
Mold Plate
Fig. 2 Vacuum bagging technique
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III. EXPERIMENTATION
The vibration testing instrument used in the
experiment is calibrated and adjusted. It mainly consists
of 4 channel real time analyzer, uni-axial accelerometer
and impulse hammer. The configuration of experimental
setup is shown in Fig .4.
Fig. 5 Half power band width
V. RESULTS AND DISCUSSION
The uni-axial accelerometer and impulse hammer
are interfaced with computer using the data acquisition
system (four channel real time analyzer) with NVgate
version 6.0 software. Frequency response function
(FRF) generated by the software is shown in Fig 6 a.
The degree of correlation of one signal with a second
signal is confirmed with coherence which is related to
the FRF. Coherence of the results varies from 0.94 to
0.99 shown in Fig. 6 b and is a function of frequency. In
modal analysis, this function shows the quality of a
measurement. A good impact produces a vibration
response that is perfectly correlated with the impact,
indicated by a coherence plot that is near one over the
entire frequency range. If there is some other source of
vibration, or noise, or the hammer is not exciting the
entire frequency range, the coherence plot will drop
below one in some regions. Natural frequency and
damping ratio of different sandwich panels at respective
modes are determined using half power band width
method. Table 2 details the summary of natural
frequency and damping ratio of the sandwich panels.
Fig.7 a and 7 b shows the mode1 and mode2 plots
respectively represented in terms of natural frequency
and damping ratio of different sandwich panels. High
damping and low natural frequency is encountered in
the case of SF panel. This may be due to the fact that,
low density and low stiffness of SF panel. However,
marginally increase in density and very high stiffness of
SFK5 panel results in low damping and high natural
frequency. Test panels with 10mm, 15mm and 20 mm
cell size of RIPH structure in syntactic foam (SFK10,
SFK15 and SFK20) core sandwich panels has shown
stable damping and natural frequency.
Fig. 4 Experimental setup
The sandwich panels to be tested are hanged from a
frame by thin strings. This simulates free boundary
conditions and therefore prevents the introduction of
unwanted stresses into the test panels that would need to
be accounted for in the calculations. The test panel is
tapped to induce the required vibration excitation using
impulse hammer, the frequency domain response
measured through accelerometer is displayed in terms of
acceleration v/s frequency. The frequency response
function (FRF) data is stored and processed to calculate
the natural frequency and damping ratio. The bandwidth
method is used to estimate damping ratio (𝜁) of the test
panels from frequency domain response. Bandwidth
(∆𝜔) is defined as the width of the frequency response
magnitude curve when the magnitude is 1⁄√2 times the
peak value. Equation (1) is used to determine the
damping ratio of the test panels. The variable parameter
considered in the present investigation is the cell size of
resin impregnated paper honeycomb structure which is
integrated with syntactic foam core. The effect of cell
size on natural frequency and damping ratio of syntactic
foam core sandwich composites is the aim of
investigation.
𝜁=
∆𝜔⁄2𝜔𝑛
(1)
Where, 𝜔𝑛 is the resonance frequency ∆𝜔 = 𝜔𝑅 −
𝜔𝐿 , 𝜔𝑅 and 𝜔𝐿 are the right and left frequency at 3 dB
below the resonance amplitude as shown in Fig.5.
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(a)
(a)
(b)
Fig. 7 plots of natural frequency and damping ratio of
different sandwich panels (a) mode I (b) mode II
(b)
Fig. 6 Frequency response function and its coherence
V. CONCLUSION
Table 2 Details summary of natural frequency and
damping ratio
Sandwich
panels
SF
SFK 20
Mode I
Natural
Damping
frequency
ratio
Hz
487.500
0.0064
519.750
0.0054
Stiffened syntactic foam core sandwich composites
are developed using RIPH structure in syntactic foam as
core and GFRP laminates as face sheets. The developed
composites are tested for vibration characteristics to
determine natural frequency and damping ratio.
Following important conclusions are drawn from this
investigation.
Mode II
Natural
Damping
frequency
ratio
Hz
1129.38
0.0091
1218.94
0.0078
SFK 15
SFK 10
524.947
536.562
0.0054
0.0053
1231.13
1239.77
0.0078
0.0066
SFK 5
589.375
0.0042
1365.63
0.0057
i.
Density of RIPH integrated syntactic foam core is
marginally increased with the decrease in the cell
size of RIPH structure.
ii.
Stiffness of the sandwich panel is increases with the
decrease in cell size of the RIPH structure in
syntactic foam core.
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iii.
Natural frequency of sandwich panels increases
with the decrease in cell size of RIPH structure in
syntactic foam core.
iv.
Damping ratio increases with the increase in cell
size of RIPH structure in syntactic foam core
sandwich composites. However, damping ratio is
very high in the case of sandwich composites with
core of syntactic foam without RIPH structure.
v.
The variation in cell size of RIPH structure in
syntactic foam significantly alters their vibration
response; this in turn promises its application in
stiffness critical structural applications.
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