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CHAPTER 3
MATERIALS AND EXPERIMENTAL PROCEDURE
3.1 MATERIAL PROPERTIES
Two different categories of specimens have been made for this research work. The first is the fiber reinforced polymer (FRP) composite specimens, the other is the Polypropylene honeycomb sandwich structured specimens. The material properties of these two categories of specimens and their constituents are given below.
3.1.1 Material Properties of the FRP Laminates
The matrix material consists of low temperature curing epoxy resin with a specific gravity of 1.14 at 25°C, the solvent based high temperature curing hardener and the accelerator. The unidirectional glass fiber at a density of 2.50g/cm3 has been used as the reinforcement. The material properties ofthe composite specimen with respect to fiber direction were measured from the mechanical and dynamic tests. Two different diameters of fibers have been used for preparing two different categories of the FRP specimens. The various material properties such as tensile modulus, shear modulus and the Poisson ratio of the FRP laminates with the small and large diameter fibers were tested using the mechanical tests and given in Table 3.1.
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3.1.2 Material and Damping Properties of the Fiber and the Matrix
The various material properties like tensile modulus, shear modulus, longitudinal, transverse and longitudinal-transverse loss factors and the Poisson ratio of the fibers and the matrix materials with the small and large diameter fibers were tested using the mechanical tests and given in Table 3.2.
3.1.3 Material Specification of PP Honeycomb Core and PP Solid Materials (As per M/s Good fellow Cambridge Limited, London)
The Polypropylene honeycomb core is interleaved between the FRP specimens or skins. The PP honeycomb core, possessing high strength-to-weight ratio, high energy and sound absorption, and high corrosion resistance, was supplied by M/s Good Fellow Cambridge Limited of London. It is covered with polyester scrim, which facilitates a better bonding of the honeycomb with the surface panels, and also acts as a barrier to prevent the resin from leaking into the core cell. Material Specifications of PP Honeycomb Core and PP Solid Materials are given below.
Thickness - 10mm; Adhesive - None (cells are fused together); Cell wall - 0.25mm; Cell size (diameter) - 8mm; Core density - 0.08g.cm-3;Facing skin - Non-woven polyester veil; The Young’s and shear moduli of the PP solid materials are, E = 900 MPa and G = 320 MPa respectively. The shear moduli and compression modulus of the core in the transverse direction have been computed by the following equations of Meraghni et al. (1999).
( )( )
(1 2cos )2 sin (cos 1)hc
t ct c
wxz
t GG
k
( )( )
( 2 sin )2 (1 cos )( sin )hc
t ct c
w wyz
w
t G t kG
k t k
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( )( )
(1 2 cos )2 sin (cos 1)
t ct c
whc
t EE
k
where k is the hexagonal cell parameter and equal to ((d/2)/sin ), the cell
angle =2 /6=60°, tw the wall thickness, and d the diameter of the cell, as
shown in Figure 3.1.
Figure 3.1 Honeycomb cell structure and its parameters.
3.1.4 Material Properties of the FRP, PPHC, Fiber and Matrix
Materials
The various material properties like tensile modulus, shear modulus;
longitudinal, transverse and longitudinal-transverse loss factors, Poisson ratio
and density of the fibers, matrix, FRP and PPHC materials were tested using
the mechanical and dynamic tests. The findings are given in Table 3.3.
3.1.5 Mechanical Properties of the FRP, PPHC Materials under
Various Temperatures
The various material properties like tensile modulus, shear modulus;
longitudinal, transverse and longitudinal-transverse loss factors, Poisson ratio
of FRP and PPHC materials and the modulus values of the solid
polypropylene materials were tested under various temperatures from using
the mechanical and dynamic tests and given in Table 3.4.
k
tw
hc
Cell diameter d
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3.2 FABRICATION PROCESS
As already stated in the previous chapter, the two different
categories of specimens (the FRP laminated composite specimen and the
PPHC sandwich structured specimens) are needed for this research work. The
first type, i.e., FRP laminated composite specimens are prepared by simple or
single stage layup process, while the another type, i.e., the PPHC sandwich
structured specimens are prepared by two stage layup process.
3.2.1 Simple or Single Stage Layup Process
FRP laminated composites are prepared by simple hand layup
technique at room temperature. Glass fiber mats are cut into different angles
of pieces and placed in a platform. Epoxy resin is mixed with accelerator and
catalyst at appropriate proportions, stirred well. Then that resin system is
applied uniformly on the mats by using a brush. The air bubbles which are
entrapped inside the mat are released by using a steel roller. Composite
laminate plates are thus obtained by stacking the epoxy impregnated glass
fiber mats or layers.
The types of specimens prepared from single stage layup process are
given for the following studies.
For studying the effect of different fiber layups on damping :
The test specimens of the following 21 different layups with
the dimensions of 300 X 25 X 6.3 mm with a stacking of 8
layers were prepared. They are shown in Table 3.5.
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Table 3.5 List of different layups
Sl.No. Sample Nos. Lay ups 1 Lay- up No: 1 [60°]8
2 Lay- up No: 2 [±60°]2s
3 Lay- up No: 3 [90°]8
4 Lay- up No: 4 [±60°/±45°]s
5 Lay- up No: 5 [±60°/45°/30°]s
6 Lay- up No: 6 [±60°/45°/-30°]s
7 Lay- up No: 7 [±60°/90°/0°]s
8 Lay- up No: 8 [90°/60°/45°/30°]s
9 Lay- up No: 9 [45°]810 Lay- up No: 10 [±60°/±30°]s 11 Lay- up No: 11 [±45°]2s12 Lay- up No: 12 [±45/±30°]s 13 Lay- up No: 13 [±45°/90°/0°]s 14 Lay- up No: 14 [±45°/30°/0°]s 15 Lay- up No: 15 [90°/0°/±60°]s 16 Lay- up No: 16 [90°/0°]2s17 Lay- up No: 17 [90°/0°/±45°]s 18 Lay- up No: 18 [90°/0°/±30°]s 19 Lay- up No: 19 [±30°]2s20 Lay- up No: 20 [±30°/90°/0°]s 21 Lay- up No: 21 [0°]8
For studying the effect of different fiber diameters on
damping: The two different test specimens of the dimension
300 X 25 X 4 mm with a stacking of 12 layers for the smaller
and 4 layers for the larger diameter of the fiber were
prepared.
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For studying the different temperatures on damping: The
test specimens of the two different layups with the
dimensions of 300 X 25 X 6.3 mm with a stacking of 8 layers
were prepared.
3.2.2 Two Stage Layup Process
After reviewing various fabrication processes stated elsewhere
(Edwards 1998, Cabrera et al. 2008), the two stage layup process has been
introduced in this study. In order to prevent the epoxy resin from leaking into
the honeycomb core, and to maintain the uniform thicknesses of top and
bottom skins and obtain effective bonding between the skins and core, this
fabrication method was designed. In the first stage, the PPHC core mounting
on two stacked-up layers of glass fiber, impregnated with the epoxy resin was
installed on the bottom mould and allowed to cure under pressure after
mounting the top mould on the same. At the end of the first stage, the semi-
finished sandwich consisting of the PPHC core and the bottom FRP skin were
taken from the mould, then inverted upside down and mounted on the top
surface of another set of two stacked-up layers to fabricate the top skin. At the
end of this stage, a fully finished sandwich was obtained (Figure 3.2). Since it
is carried out in two different stages and periods, the resin flows evenly and
wets the skins and core uniformly and do not get cured while doing a lengthy
fabrication process. Figures 3.3.(a-c) show the PPHC core, semi-finished
PPHC sandwich and fully-finished PPHC sandwich.
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Figures 3.3 (a) The PPHC core, (b) Semi-finished sandwich and (c) fully-finished sandwich.
The types of specimens prepared from two stage layup process are
given for the following studies.
(a)
(b)
(c)
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For the effect of different orientations and thickness: FRP
of 6.3mm thickness and the three different sandwiches of
SW1 (‘ t ’ of FRP skins = 3.15 mm each, a core = 10 mm),
SW2 (‘ t ’ of skins = 1.575 mm each and a core = 5 mm),
SW3 (‘ t ’ of skins = 1.575 mm each and a core = 10 mm)
with different fiber orientations of 0° 30°, 45°, 60° and 90°
were prepared.
For the effect of different orientations and temperatures:
The sandwich with thicknesses of skins = 1.575 mm each and
a core = 5 mm) and the fiber orientations of 0° 30°, 45°, 60°,
90° were prepared.
For the effect of chopped fibers: The sandwich with the
thickness of top and bottom FRP skins = 3.15 mm each and a
thickness of core = 10 mm), and the following five groups of
the fibers were prepared:,
all continuous fibers,
alternate arrangement of continuous and two chopped
fibers,
the same with three chopped fibers, four chopped fibers,
and five chopped fibers.
Out of the above said five groups of chopped fiber systems, the
three chopped fiber systems, i.e., alternate arrangement of continuous and
three chopped fibers are shown below in Figure 3.4.
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Figure 3.4 The alternate arrangement of continuous and 3 chopped fibers of the FRP skins
3.3 EXPERIMENTAL SETUP
3.3.1 Impulse Technique on the FRP and the PPHC Sandwich Panels
Impulse technique has been performed to find the vibration
characteristics, i.e., the values of natural frequencies and loss factors of the
FRP and sandwich specimens for six different cases (or problems) of this
work. The schematic diagram of the impulse testing is shown in Figure 3.5 (a
& b). One end of the laminated FRP/Sandwich specimen is rigidly clamped in
a firm support; the other end, which is free to vibrate like a cantilever beam, is
properly positioned with an accelerometer. The input load is given by the
instrumented impact hammers and the output (response) is captured by the
accelerometer, and read by the National instrument Data Acquisition Card. It
is understood that the improper positioning of the accelerometer and the
clamping of the laminated specimen would influence the dynamic properties,
which may deviate from their corresponding theoretical values.
Choppedfibers
Continuousfibers
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Figure 3.5 The impulse testing setup with (a) FRP specimens and (b) Sandwich specimens
Using the half power bandwidth method, the natural frequencies and
the loss factors ( ) have been determined under different fiber orientations of
the first two modes as shown in Figure 3.6. The expression for the loss factor
) is given by the following Equation (3.1):
PC
Data acquisition card
Impact Hammer
FRP Specimen Accelerometer
Fixed end
(a)
PC
Impact Hammer
PP honeycomb Specimen Accelerometer
Data acquisition card
Fixed end
(b)
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1 2
2 n
f ff (3.1)
Figure 3.6 Half-Power width method
where, 1f and 2f = Bandwidth at the half-power points of resonant peak for
nth mode, nf = Natural frequency.
3.3.2 Impulse Technique on the PPHC Sandwich Panels under
Different Temperatures
The impulse technique was performed to determine the vibration
characteristics (the natural frequencies and loss factors) of the PPHC
sandwich under different temperatures ranging from room temperature to
80° C with an incremental value of 5° C. In order to heat and measure the
temperature of the sandwich specimens, a temperature regulated oven and a
data logger were used with the setup, as shown in Figure 3.7.
Frequency
Amplitude maximum, Amax
f1 f2fn
Amplitude factor = Amax 2
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Figure 3.7 Schematic representation of impulse testing with the oven
3.3.3 Impulse Technique on the Two Different Fiber Oriented (0° and
45°) FRP Specimens under Different Temperatures with
Dynamic Mechanical Analysis
The above setup (shown in Figure 3.7) was also used to calculate
the damping loss factors of two different fiber oriented (0° and 45°) FRP
specimens with the dimension of 300mm × 25mm × 6.3mm under different
temperatures. Dynamic mechanical analysis (DMA) was also performed in
the three point bending testing mode to check the effect of the visco-elastic
properties like the loss factor, the storage and loss modulus under the
influence of different temperatures.
The above said visco-elastic properties have been recorded from the
pure epoxy and two different fiber oriented (0° and 45°) FRP specimens with
PC Data Acquisition Card
Data Logger
Temperature Regulated Oven
Accelerometer
Fixedend
Impact Hammer
PPHC Specimen
Thermocouples
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dimension 35mm×12.5mm×3.3mm over the temperature spectrum of 20° C to
160° C under a constant excited frequency of 1 Hz at the heating rate of 5°
C/min and the strain amplitude of 1%. The temperatures at the maximum
values of the storage modulus, loss modulus and tangent delta are obtained
from their respective visco-elastic properties curves, and the glass-transition
temperature (Tg) is also noted for these two types of specimens.
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