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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.