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CHAPTER 3

EXPERIMENTAL INVESTIGATION

3.1 PREAMBLE

In this section, the preparation of an FRP Honeycomb core

sandwich panel with different cell sizes, and the experimental methods

adopted for assessing the mechanical characteristics, are explained. They are

broadly as under:

Manufacture of FRP Facings

Manufacture of FRP Honeycomb core

Manufacture of Sandwich panels

Testing of facing sheets

Static flexural and compression testing of Sandwich panels

Low velocity Impact testing of Sandwich panels

Dynamic testing of Sandwich panels

3.2 FLOW CHART OF THE RESEARCH

E-glass fabrics, epoxy thermosetting resin based honeycomb core

and facesheets were used to fabricate composite sandwich panels. The

reinforcement constituents of the composite facesheets and core are E-glass

0°/90° fabrics, and LY 556 epoxy resin with HY 951 hardener used as the

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3.0

Analysis of Static, Impact and vibration behaviour of Sandwich panels 4.0

Experimental Investigation- Manufacture of Sandwich panels with different cell sizes- Physical & mechanical characterization of Face sheet- Flexural testing of sandwich panels- Compression testing of sandwich panels- Low velocity Impact testing of sandwich panels- Experimental Modal analysis of sandwich panels under differentboundary conditions

Dissertation on “The Static, Impact and Vibration Behaviour of FRPHoneycomb Core Sandwich Composites”

Analysis ofImpact

Behaviour

Conclusions 5.0

Evolution of design criteria of sandwich panels

Introduction, Literature survey & Problem definition 1.0 &2.0

4.1Analysis of

StaticBehaviour

4.2

Correlation between cell size and coredensity, elastic constants,

compressive strength, shear strength,stiffness, of Sandwich panels

4.3Analysis ofvibrationBehaviour

Correlation between cellsize and low velocityimpact strength ofSandwich panels

Correlation between cell sizeand Modal parameters ofSandwich panels under

different boundary conditions

matrix material. The core material having regular hexagonal cell

configuration (with four different cell sizes of 8, 16, 20 and 25mm) was used

in the fabrication of the composite.

Figure 3.1 Flow chart of the research work

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The physical and mechanical properties of the facesheets have been

determined as per the appropriate ASTM standards. To assess the static

behaviour of the sandwich panels compression, Flexural 3-Point and 4-Point

tests have been carried out as per the guidelines of the ASTM standard to

determine the elastic constants of the core, such as the compression and shear

modulus, as well as on the compression and shear strength of the core. To

assess the low velocity impact behavior of the sandwich panels under falling

weight, impact tests have been carried out at different incident energy levels.

To assess the dynamic behaviour of the sandwich panels, experimental modal

analyses under different constraint conditions have been carried out. The

details of the tests are provided in the respective sub-sections. Figure 3.1

shows the flow chart of the research work carried out.

3.3 DEVELOPMENT OF THE SANDWICH COMPOSITE

The fabrication is carried out through the vacuum bag molding

technique, in which the refinement of the hand lay-up, uses the vacuum to

eliminate the entrapped air and excess resin. The process flow chart for the

manufacturing of FRP composites is shown in Figure 3.2. The Vacuum bag

hand lay-up process offers many benefits, when compared to the conventional

hand lay-up techniques. As it is a closed molding process, it virtually

eliminates potentially harmful volatile organic compound (VOC) emissions. It

also allows unlimited set-up time, because the resin system is not introduced

until all the reinforcements and core materials are in place. This method is of

particular benefit, when producing large facings, due to the weight savings

that can be gained, greater structural strength and efficiency gains against the

Hand Lay-up process. The vacuum system also facilitates good resin

distribution and consolidation of layers of the laminate. As a result, the

resulting mechanical properties of the Facings are likely to be markedly

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higher, than would be the case with hand laminating (Malin Akermo et al

1999).

Figure 3.2 Process flow chart for the manufacturing of sandwich

composites

3.3.1 Manufacture of face sheet

Bi-woven glass ‘E’ cloth, which is commercially available is used

for making the face sheet and is shown in Figure 3.3. The cloth ply was

trimmed to the correct size and impregnated in an adhesive made from a

mixture of LY556 epoxy resin and HY 951 hardener, mixed in the ratio of

100:10. The ply was stacked in 0 /90 orientation and was built to a thickness

of around 2.0 mm. The Vacuum hand lay – up technique was used to make

the facings and is shown in Figure 3.4. A Vacuum level of 450Hg/mm² is

maintained for 1 hour to avoid surface undulations and also to avoid air

pockets at the interface. The coupons were allowed to cure for about 24 hours

EpoxyE-glassfabric

Lamination of facesheet and core by wet

lay up

Vacuum Bagging

Curing

(1 day at roomtemperature)

Post Curing

(2 hours at 100oC)

Core (8, 16, 20 &25mm)

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at room temperature. The glass and resin content in the Facings was

respectively around 61.5 % and 38.5 %.

Figure 3.3 Bi-woven glass ‘E’ fabric

Figure 3.4 Manufacture of Face sheet using Vacuum hand lay-up technique

3.3.2 Manufacture of Honeycomb core

For the manufacturing of the honeycomb core the matrix used is

epoxy resin LY 556 mixed with a hardener HY 951 and the reinforcement is

glass ‘E’ fabric. The resin and hardener are mixed in the weight ratio of 10:1.

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To maintain the optimum strength, the resin glass ratio is found to be 35:65.

The molding tool used is hexagonally machined split molding tool made of

chromium plated mild steel, and is shown in Figure 3.5 (a). After ensuring

that the surface is clean and free from foreign particles the application of

release agents is done.

(a) Split Moulding Tool (b) Wet laminate under load

(c) Half honeycomb core (d) Joining of two halves of the core

Figure 3.5 Fabrication of the honeycomb core

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Figure 3.6 Honeycomb core

A coat of resin mixture is applied on the molding surface and the

plain weave glass ’E’ fabric is impregnated against the first half of the

molding tool surface, by ensuring a thorough wetting of the glass fabric. Then

the hexagonal mandrel is placed in the respective slots by pushing the glass

cloth downwards into the half hexagonal slot of the molding tool. The Load is

applied onto the wet laid-up laminate in order to improve its consolidation, as

shown in Figure 3.5 (b). This is achieved by sealing the wet laid up laminate

with a Peel-ply, perforated plastic film, and placing the breather over the

perforated plastic film. Above this, a bag with a Vacuum Valve (returnable) is

placed and sealed, which constitutes the vacuum bagging process. At one

corner of the bag a port for the vacuum is arranged and subjected to 450-500

Hg/mm2 of pressure for 120 minutes to consolidate and to increase the inter

laminar shear strength of the layers. After ambient curing, the laminate which

takes the shape of half-hexagon as shown in Figure 3.5 (c) is taken from the

molding tool, and a similar fabrication of the second half is done. The two

halves are joined together by placing the epoxy resin putty on the contact

surface, and allowing it to dry as shown in Figure 3.5 (d). Then it is cut to the

required thickness to form the hexagonal honeycomb core. Figure 3.6 shows

the honeycomb core.

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3.3.3 Manufacture of Sandwich panels

For the fabrication of the sandwich panel, the facings comprising of

the glass ’E’ fabric impregnated with the above said resin mixture, are

coupled with the open honeycomb structure using epoxy resin, compacted by

means of the vacuum bagging technique. After curing, the sandwich panel is

subjected to post curing in a hot-air oven at 100 oC up to 2 hours. The

vacuum method provides higher reinforcement concentrations, ‘better

adhesion between layers’, and more control over the resin/glass ratio

compared to the hand lay-up process. Figure 3.7 shows the honeycomb

sandwich panel under vaccum.

Figure 3.7 Fabrication of honeycomb sandwich panel

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Four types of sandwich panels of size 500 x 500mm are fabricated

with different cell sizes, i.e., 8, 12, 16 and 20 mm. The cell shape of the

honeycomb core is a regular hexagon. The membrane wall thickness of the

core is 0.2 mm and the height of the core is fixed at 8 mm. The thickness of

the top and bottom face sheets has been kept constant at 1 mm

The cell sizes were selected, based on the ease of manufacture and

testing requirements, stipulated by the relevant standards. A Cell size below 8

mm is extremely difficult, because the FRP sheets cannot be bent around the

mould. A Panel with a cell size higher than 25 mm poses severe restrictions

on flexural testing, as the width of the specimens become too unwieldy.

3.4 CHARACTERIZATION OF THE FACE SHEET

A thorough understanding of the physical and mechanical properties

of a material is paramount to determine whether the material is suitable for a

specific application. Hence, the face sheet must be characterized and

understood to determine how the selected material caters to the manufacturing

of the sandwich panels. The physical and mechanical characteristics of the

face sheet used for the fabrication of the sandwich panels are tested as per the

ASTM standards.

3.4.1 Tensile test

A Tensile test was conducted on the face sheet to determine the

tensile strength and modulus of the composite face sheets, as per the ASTM D

3039M-08. The Test specimens were sectioned from the composite panels

with the width of 25 mm, thickness of 2 mm and length of 220 mm. The Test

specimens were bonded with composite tabs of 4” in length at both the ends.

The tabs distribute the gripping stresses and prevent specimen failure caused

by grip jaws. As the face sheet exhibits a similar behaviour for 0° and 90°

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directions, only one direction is tested. The specimens were tested using the

universal test machine (Shimadzu) as shown in Figure 3.8. The test were

conducted at a cross head speed of 2 mm/min.

Figure 3.8 Tensile test setup for face sheet

The tensile strength ( uts) was calculated by using the Equation 3.1.

uts = F/A (3.1)

where F is the breaking load in N and A is the cross sectional area of the

specimen in mm2.

3.4.2 Compression test

The Compression test method, according to ASTM D3410M-03 was

used to measure the compressive strength and modulus values of the

composite face sheet panels. For this purpose, compression test specimens

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were cut from larger face sheet panels and tests along the ply-layup directions

were performed using the universal test machine (Shimadzu) at a crosshead

speed of 1.3 mm/min. The compressive stress was calculated by dividing the

load with the cross-sectional area of the specimens. The modulus was

estimated from the slope of the stress - strain curve.

3.4.3 Flexural test

The flexural test method according to the ASTM D790-10 was used

to determine the flexural strength and modulus of the composites. For this

purpose, test specimens 20 mm in width, 2 mm in height and 160 mm in

length, were sectioned from the face sheet panels using a diamond saw.

Specimens were tested in a 3-point bending apparatus under loading, both in

the longitudinal and tranverse directions of the fibers. The composite face

sheets were tested using the Schimadzu universal test machine at a crosshead

speed of 1.2 mm/min. During the test, the load versus central deflection was

recorded, and the slope P/ w and the elastic modulus were determined.

3.5 TESTING OF SANDWICH PANELS

While considering a sandwich construction for an application, care

must be given to the directional mechanical properties, and the design must

ensure that the best advantage of this attribute is incorporated. The biggest

advantage of a sandwich construction is that it possesses high stiffness at very

low weights. However, the stiffness behavior must be thoroughly understood,

so as to make a comprehensive computation of the deflection arising out of

the shear deflection because of low core densities as well as bending

deflection.

The panel can fail in any one of several different ways, depending

upon the geometric and fabrication characteristics of the panel, and how it is

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loaded. For example, a face sheet can fail in tension, compression, shear or

local buckling. Additionally, the core can fail in shear or by crushing. A face

sheet can separate from the core due to excessive shear or normal tensile

stress in the adhesive bond. Test methods have been developed to isolate and

simulate each of these specific failure modes.

3.5.1 Flexural test of Sandwich panels

Flexural tests were carried out as per the ASTM C393M-06, to

characterize the flexural properties of the sandwich composites. In accordance

with this standard, a series of 3-point bending, 4-point bending and short

beam shear strength (SBSS) tests are conducted. The three, four bending and

SBSS bending test setups are shown in Figures 3.9 a, b, c. This test method

was used to determine the flexural stiffness parameter (D), the core shear

modulus (G) and the core shear strength ( core ). The load-displacement

curves were plotted for all the test conditions. The SBSS test setup is similar

to the 3-point bend test except the use of specimen span length. The Test

parameters for the flexural testing of the sandwich composite specimens are

given in Table 3.1.

Table 3.1 Test parameters for the flexural testing of the sandwich

composite specimens

Type of bending test Span length

(mm)

Width

(mm)

Thickness

(mm)

Three-point 500 50 10

Four-point 500 50 10

SBSS 160 50 10

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Figure 3.9 Flexural test setup for (a) Three-point bending (b) Four-

point bending (c) SBSS test

By means of the flexure test, load can be applied to the specimen to

produce constant or variable bending moments, and constant or variable

shears. The concentrated load for all the specimens is applied by the movable

cross head, gradually at the cross head displacement rate of 2 mm/min. The

load is applied on the specimen till fracture and the maximum load at fracture

(a) (b)

(c)

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has been recorded for each case. Load - deflection curves are also recorded

using a deflectometer placed at the mid-span length of the specimen. The

slope of the load versus deflection curve is determined for each specimen.

The following equations are used from the ASTM standard C393M-06 to

compute ‘D’ and ‘G’.

For a point load at the mid-span of a simply supported beam, the

overall deflection including bending and shear is:

3 2 21 1 2 1

1 1 1 2 2 2 1

P L 1- 11L /8LD =

48 1- 2P L /P L (3.2)

2 21 1 1 2

2 3 31 1 1 2 2 2 1

P L c 8L /11L -1G =

b d+c 16PL /11P L -1 (3.3)

where,

P1 1 - slope of the load – deflection curve in three point

bending

P2 2 - slope of the load – deflection curve in four point bending

L1 - span length in three point bending test (160 mm)

L2 - span length in four point bending test (200 mm)

The shear strength of the core( core ) and the bending stress ( facing )

of the face sheet are computed using the equation (3.4).

core facingP PL= ; =

2bc 2t d+c b (3.4)

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3.5.2 Flatwise compressive test of Sandwich core

Flatwise compression tests of sandwich panels are conducted in

accordance with the ASTM C365M-11 standard to determine the f latwise

compressive strength and modulus. In order to prevent local crushing at the

edges of the honeycomb cores, the edges were stabilized with thin facings, so

that the load which causes failure in the core does not cause any damage to

the facings. The Setup used for compression tests is shown in Figure 3.10.

Figure 3.10 Flatwise compression test setup loaded with specimen

A constant crosshead movement rate is maintained at 0.5 mm/min,

as suggested by the ASTM standard. Specimens in this study have the

dimensions of 50×50×10 mm for length, width and height respectively, in

accordance with the ASTM C365M-11. The Load-displacement data is

obtained from the tests, and is used to carry out the calculations for the

modulus and compressive strength.

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coreP

Face sheet area (3.5)

p cEb L

(3.6)

Where, C is the core height in mm and b x L is the area of the face

sheet in mm2.

3.5.3 Low velocity impact test of the Sandwich panel

Low-velocity impact tests were carried out at different energy levels

on honeycomb sandwich panels, using an instrumented falling weight

apparatus to obtain information about the absorbed energy and maximum

impact force. Indigenously developed instrumented low velocity impact test

equipment was employed to perform the non-penetrating impact test. The

maximum impact energy is limited by suitably adjusting the falling height and

mass. The mass together with the height of the drop determines the energy of

the impact. With an increase in the mass and height of the fall, the potential

energy is converted to kinetic energy. The Instrumented falling weight impact

testing machine is shown in Figure 3.11 (a).

In accordance with the ASTM D3029-95 the impact test is

performed by sticking the specimen at the centre by a flat square dart. The

square dart is made of mild steel, and sized 25mm × 25mm. Figure 3.11 (b)

shows the specimen clamping apparatus having a fixture with a square slot of

100mm. This is specifically designed in order to assure the consistency of the

clamping force through the pre-loading of the four helical springs. The

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vertical guides of the impact are power lubricated frequently to minimize any

friction generated during the descent of the impactor.

This machine is capable of impacting samples at energies of up to

140J. For this test, samples of the size 150×150×10 mm were impacted. Table

3.2 lists the impact test parameters which give the range of the drop height

and impact mass to achieve the required impact energy. An inbuilt data

acquisition system along with an impact software was used to monitor the

position and acceleration of the impactor. The incident energy is calculated

based on the height history, while the dissipation of energy was derived from

both the acceleration and height histories of the impactor, assuming rigid

body motion. Tests were conducted under different impact energies, ranging

from 7 to 50 J.

Table 3.2 Impact Test Parameters

S.No Drop height mm Drop mass kg Impact energy J

1 500 2.576 12.64

2 1000 2.576 25.27

3 750 5.116 37.64

4 1000 5.166 50.19

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(a) Instrumented falling weight impact testing machine

(b) Specimen Clamping Fixture

Figure 3.11 Low velocity impact test setup

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3.5.4 Vibration testing of the Sandwich panel

The vibrational characteristics of the specimens are obtained by

studying their impulse response. Assessing the modal characteristics of

sandwich panels is very essential for design and manufacturing. The tests are

carried out for two boundary conditions C-F-F-F (One end Clamped) and C-

F-C-F (Two ends Clamped). In the cantilever (C-F-F-F) analysis, the

honeycomb sandwich panel is clamped at one end using a suitable fixture, and

the impact test is carried out. The accelerometer is placed at one of the corner

nodes, while the hammer is made to impact at multiple nodes. For the Fixed-

Fixed (C-F-C-F) analysis, the honeycomb sandwich panel is clamped at two

opposite ends using a suitable fixture, and the impact test is carried out. The

accelerometer is placed at one of the corner nodes, while the hammer is made

to impact at multiple nodes. The traditional “strike method” is used to

measure the vibration properties. The modal test setup is shown in

Figure 3.12.

Figure 3.12 Modal test setup

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The specimens are subjected to impulses through a hard tipped

hammer which is provided with a force transducer with a sensitivity of

2.25mV/N and the response is measured through the accelerometer with a

sensitivity of 10 mV/g. The hammer consists of an integral ICP quartz force

sensor mounted on the striking end of the hammer head. The striking end of

the hammer has a threaded hole for the installation of a variety of impact tips.

The tip functions to transfer the force of the impact to the sensor and protects

the sensor face from damage.

The impulse and the response are processed on a computer aided

fast fourier transformer (FFT) analyzer test system (LMS Inc.) in order to

extract the modal parameters with the help of built in software

(SMARTOFFICE). The types of specimens investigated in this study are in

the form of plates. The specimens are cut with a nominal length of 170 mm,

breadth of 150 mm and thickness of 10mm. In the total length of 170mm,

20mm were used for fixing the specimen to obtain a cantilever condition, thus

maintaining the effective test dimension as 150 x 150 mm, as shown in

Figure 3.13 a. Similarly, another specimen was prepared for the C-F-C-F

condition, and is shown in Figure 3.13 b. The sandwich specimen is subjected

to impulses at 25 station locations. The frequency response curves are

obtained for various grid points located on the specimen.

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(a) C-F-F-F condition

(b) C-F-C-F Condition

Figure 3.13 Specimens attached to the fixture for simulating the C-F-F-

F and C-F-C-F condition

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The FFT spectrum analyzer samples the time varying input signal,

computes the magnitude of its sine and cosine components, and displays the

spectrum of these measured frequency components. The FFT analyzer used

consisted of 8 channels for input and output which helps to plot the spectrum

with a linear, logarithmic, or dB amplitude scale and a linear or logarithmic

frequency scale. The FFT analyzer also helps in obtaining the frequency

response function (FRF), through which the dynamic behaviour of a structure

is determined. The Frequency Response Function (FRF) is a fundamental

measurement that isolates the inherent dynamic properties of a mechanical

structure. Experimental modal parameters (frequency, damping, and mode

shape) are also obtained from a set of FRF measurements. FRF is defined as

the ratio of the Fourier transform of an output response (X (w)) divided by the

Fourier transform of the input force (F (w)) that caused the output.