CHAPTER 3 MATERIALS AND EXPERIMENTAL PROCEDURE …shodhganga.inflibnet.ac.in/bitstream/10603/16740/8/08_chapter 3.pdf · CHAPTER 3 MATERIALS AND EXPERIMENTAL PROCEDURE ... ASTM D3039

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CHAPTER 3

MATERIALS AND EXPERIMENTAL PROCEDURE

3.1 INTRODUCTION

Material selection is an important aspect of design and

manufacturing. Often the success of the manufacturing is critically dependent

on a material or materials performing as desired. The following may be

considered for selection of materials.

• Material characterization data base

• Flight/Ship/Automobile history

• Cost, availability, lead time.

The reinforcement material chosen is glass fiber. Fiber glass also

called glass fiber reinforced plastic or GFRP is a fiber reinforced polymer

made of a plastic matrix reinforced by fine fibers of glass. Fiberglass is a light

weight, extremely strong and robust material. Although strength properties

are somewhat lower than carbon fiber and it is less stiff, the material is

typically far less brittle, and the raw materials are much less expensive. Its

bulk strength and weight properties are also very favorable when compared to

metals and it can be easily formed using molding processes.

The materials used for preparation of laminates are procured and the

different type of specimens is prepared as per ASTM standard. ASTM D3039

standard is used for tensile test of composite laminates. Single lap joint tensile

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test specimen has been prepared according to ASTM D5868-01.Double lap

joint tensile test specimen has been prepared according to ASTM D 3528–96.

Laminated specimens have been prepared according to ASTM D5628-10 and

laminated lap joint specimen according to ASTM D7136.The ASTM E610-82

is the definition for Acoustic Emission, ASTM E976 STANDARD (1994

(AE)) is used for Pencil Lead Break Test.

3.2 MATERIAL PROPERTIES

The laminates were made from fiberglass roving (E-glass), LY 556/

Hardener HY 951 system for laminates with different orientation. The

properties are

Pot life : 30-60 minutes at 2000c

Curing time : 14-24 hours at 200 c

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The hardener HY 951 is thoroughly mixed with resin. If a large

quantity of a mixture is to be prepared and must be cooled, as otherwise

strong exothermic heat is developed and the mixture will gel in a short time.

3.3 METHODS OF LAMINATE PREPARATION

3.3.1 Hand Lay-Up Method

The simplest manufacturing technique adopted is laying down

unidirectional glass roving over a polished mould surface previously treated

with a releasing agent: after this, a liquid thermosetting resin is worked into

the reinforcement by hand with a brush or roller. The process is repeated a

number of times equal to the number of layers required for the final

composite. Epoxy resins are most commonly used with glass fibers because of

their good strength properties. Resin and curing agents are pre-mixed and

normally designed to cross-link and harden at room temperature. The major

advantage of this manufacturing process is its great flexibility, since it suits

most common mould sizes and complex shapes. Although tooling is normally

expensive, it can be re-used for several runs and the actual cost of the raw

materials make this process economically feasible.

3.3.2 Vacuum Bagging Method

Also known as vacuum moulding, it requires a pump that will make

use of atmospheric pressure to consolidate the material while curing by

applying vacuum to the mould cavity. Usually the fibers are placed on a

single mould surface and covered by a flexible membrane, sealed around the

edges of the mould. The space between the mould and the membrane is then

evacuated with a pump and the vacuum retained until the resin has cured.

Figure 3.1 shows an example of vacuum bagging as given by Kornmann et al

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(2005) where several layers of glass fabric were placed intercalated with

epoxy resin. When the stack reaches the desired amount of layers, it is

covered with a peel ply, a perforated film and a breather fabric and then

introduced into a vacuum bag. An inlet and an outlet are placed on the

breather fabric and then the bag is sealed with sealant tape. After that, vacuum

is applied and for this particular case hydraulic pressure is also applied.

Vacuum bagging processes normally deliver a better volume fraction ratio

since any excess resin is drawn out of the composite. Also, the presence of

voids is reduced due to the extra pressure applied. One disadvantage of this

technique is the difficulty in maintaining a good vacuum over very large

moulds from 10m. Also, particularly with epoxy resins, thick sections take

time to fabricate due to the exothermic nature of the cross linking reaction.

The reaction generates heat, which needs to be released for the resin to

consolidate.

Figure 3.1 Vacuum bagging technique

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3.4 SPECIMEN PREPARATION FOR DEFECT

CHARACTERIZATION STUDIES IN LAMINATES

3.4.1 Preparation of Laminates from Unidirectional Glass Fibers

GFRP composite laminates with different orientations such as

0o, 90o, cross ply (0o/90o), angle ply (+45o/-45o) of size 300 x 300 mm2 are

fabricated using vacuum bagging technique as shown in Figures 3.2 (a), (b)

and (c). Ten layers of unidirectional glass roving along with LY556 epoxy

matrix are used for the purpose of fabrication of the laminates.

(a) (b) (c)

Figure 3.2 (a) Zero degree orientation laminate (b) Angle ply laminate

(c) Cross ply laminate

3.4.2 Preparation of Tensile specimens from Unidirectional Glass

Fibers

ASTM D3039 Standard tensile specimens was cut using water jet

cutting (Figure3.3) from the fabricated laminate of size 280x18x3mm3 as

shown in Figure 3.4 to avoid machining defects and to maintain good surface

finish.

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Figure3.3 Specimens preparation using water jet cutting machine

Figure 3.4 ASTM D3039 standard tensile specimen

For pure resin specimen, epoxy LY556 alone is used. Aluminum

tabs of size 60 x 18 x 3 mm3 are used to reduce the grip noise and prevent

damaging the extremities of the laminate with possible unexpected failures

external to the grip length as shown in Figure 3.5.

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Figure 3.5 Specimens of epoxy matrix composite materials for tensile

test as per ASTM D3039

3.5 SPECIMEN PREPARATION FOR DEFECT

CHARACTERIZATION STUDIES IN LAMINATED JOINTS

WITH DIFFERENT ORIENTATION

3.5.1 Cutting of Specimens from the Composite Laminates

Specimens are cut from each GFRP laminate having dimensions

102x25.4x3mm3 for laminated joints, as show in Figure 3.6.

Figure 3.6 Specimens cut from water jet cutting machine

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3.5.2 Surface Preparation and Curing

Considering bonded joints the surface preparation takes an

important role in perfect bonding, hand abrasion technique was used to

introduce roughness in the surface. The entire mould with glass fabric lay-ups

has been kept in 24 hours for curing. The curing has been done properly at a

room temperature, to prevent the reduction in strength of the bond

drastically. The major defects found in bonding of two materials are

disbonding, porosity and voids are mostly due to improper curing. To prevent

dislocation of specimen in bond region proper load is applied.

3.5.3 Preparation of Aluminum Tabs

Aluminum Tabs of dimensions 25x25x3 mm3 has been prepared to

prevent the grip noise. Initially the surfaces of Aluminum tabs and specimen

are prepared by using emery sheet for good bonding. The specimen is then

allowed to cure for 24 hours under room temperature after bonding.

3.5.4 Specimen Specification for Joints

3.5.4.1 Single lap joint

Tensile test specimen has been prepared according to ASTM

D5868-01 from the fabricated laminate show in Figure 3.7. The dimension of

tensile specimen according to ASTM D5868-01are as follows:

Length - 179 mm

Width - 25 mm

Thickness - 3 mm

Gauge length - 129 mm

Bond length - 25mm

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Figure 3.7 Tensile test specimen of ASTM D 5868-01

3.5.4.2 Double lap joint

Tensile test specimen has been prepared according to ASTM D

3528–96 from the fabricated laminate shown in Figure 3.8. The dimensions of

tensile specimen according to ASTM D 3528–96 are as follows:

Figure 3.8 Tensile test specimen of ASTM D 3528–96

3.6 FABRICATION OF TEST SPECIMENS FOR ADHESIVE,

BOLTED, HYBRID AND LAP JOINT WITH ATTACHMENTS

3.6.1 Materials Used

The laminates were made following correct procedure as shown

earlier from fiberglass roving of E-glass (Figure3.9), LY 556/ Hardener HY

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951 system. The density, thickness of the fiber glass roving is 1.3334 x 10 -3

g/mm3, 0.2 mm respectively.

Figure 3.9 Fiber glass mat

3.6.2 Laminate Preparation Process

The required mould is placed on the table and a thin layer of resin is

applied on the surface of the lower mould and the first layer of glass fabric is

placed on the mould, rollers are used to squeeze the excess resin. The resin is

applied over the first layer and the second layer is placed over the first one.

The procedure is repeated with alternating layers of glass fiber and resin

mixture until all required thickness. The vacuum bagging and cured at a

pressure of 1 bar for 2 hours as shown in Figure 3.10.The GFRP laminate

obtained from the vacuum bagging will be having dimensions of

600x600x3.5mm3.

Figure 3.10 GFRP laminate under vacuum bagging

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3.6.3 Specimen Preparation for Joint Strength Analysis

The specimens are prepared using single lap joint technique. These

specimens are subjected to tensile testing with Acoustic Emission sensors

positioned at -40mm to +40mm from the centre of the specimen. Four to six

specimens are tested in each type. The failure strength, Load, Displacement,

and Event Location Data of the four different specimens are compared and

discussed.

As per ASTM D5868-01 standard tensile specimens of size

102x25x3.5mm3are cut from the fabricated laminates using water-jet cutting

to avoid machining defects and to maintain good surface finish as shown in

Figure 3.11.

Figure 3.11 Specimens cut from water jet cutting machine

Aluminum tabs of size 25x 25 x 3.5 mm3 are used to reduce the grip

noise. The specimens are attached by bolt, adhesive and hybrid (bolt and

adhesive). Specimens are prepared and attached using aluminium sheet for

attachment with a thickness of 0.8mm, 1.2 mm And 2.0 mm with varying the

angle of 300 and 450 as shown in Figure.3.12

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(a) (b)

Figure 3.12 Tensile test specimen prepared as per ASTM D5868-01 for

thickness of 0.8mm,1.2mm,2.0mm with varying angle for

attachment at 300and 450 (a) top view (b) side view

3.7 SPECIMEN PREPARATION FOR IMPACT TEST

3.7.1 Preparation of Normal Specimen

Impact test specimen has been prepared according to ASTM D5628-

10 from the fabricated laminate as shown in Figure3.13. The dimensions of

impact specimen according to ASTM D5628-10 are as follows;

Length - 58mm

Width - 58mm

Thickness - 3.5mm

Specification of the glass fiber lamina used:

Weave style - plain

Thickness - 0.23+0.02mm

Area weight - 320gsm

Thermal stability - continuous use up to 500 o c

Glass content - 97.5%

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Figure 3.13 Normal specimen

3.7.2 Preparation of Adhesive Lap Joint Specimen

Impact test specimen has been prepared according to ASTM D7136

from the fabricated laminate and attached with adhesive as shown in

Figure 3.14. The dimension of impact specimen according to ASTM D7136 is

as follows;

Length - 150mm

Width - 100mm

Lap length - 50mm

Thickness - 3.5mm

Specification of the glass fiber lamina used:

Weave style - plain

Thickness - 0.23+0.02mm

Area weight - 320gsm

Thermal stability - continuous use up to 500 o c

Glass content - 97.5%

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Figure 3.14 Single lap joint specimens for impact loading

3.7.3 Impact Testing on Composite Laminates

Laminates were subjected to drop impact test using a CEAST

Fractovis Drop impact tower (Figure 3.15). The normal impact specimens

with dimensions of 58mm x 58mm were clamped by using pneumatic fixtures

shown in Figure.3.16 (a). Adhesively bonded specimens with dimension

150mm x 100mm x 3.5mm were tested by using a suitable fixture as shown in

Figure 3.16 (b).

Figure 3.15 Fractovis plus impact testing machine

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Figure 3.16 (a) Normal specimen in fixture (b) Adhesively bonded

specimen in fixture

3.8 SPECIMEN PREPARATION FOR RESIDUAL STRENGTH

PREDICTION AND COMPARISION OF FAILURE MODES

3.8.1 Damage by Drop-weight Impact

Laminated specimens are prepared as per ASTM standard. The

specimens fabricated are divided into four categories. One group of

specimens are as-received, while the remaining specimens are subjected to

drop impact at three different heights using a CEAST Fractovis Drop impact

tower (Figure 3.17). The diameter of the hemispherical indenter is 12.7 mm

with a clamping.

Figure 3.17 Laminate with impact damage

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3.8.2 Determination of Depth using Ultrasonic A-SCAN

To determine the extent of damage due to impact Omni Scan MX

type Ultrasonic scan facility is used as shown in Figure 3.18.

Figure 3.18 Ultrasonic omni scan

3.8.3 Dressed Laminate

The impacted region is grinded by decreasing length from the top

surface (descending) as shown in Figure 3.19, maintaining the length of

impact region more than 20 times its depth for good strength of the bonding

patches. The thickness was maintained using vernier calipers correctly to

avoid deviation in the failure load values.

Figure 3.19 Dressed Laminate

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3.8.4 Repair Mechanism

The impacted or damaged specimens are repaired by scarf, single

lap and double lap methods. The repair mechanism involves the process

different technics. Glass fiber mat of different sizes (7mm x 18mm, 13mm x

18mm, 20mm x 18mm) are cut to apply patch in the grinded region. The

prepared layers are patched on the damaged region of the specimen as the

ascending order for scarf repair, using epoxy LY 556 & hardener HY 951.

The patched surfaces are prepared for the overlap; according to overlap length

the layers are prepared from glass fiber mat for the size 40mm, 50mm, 60mm

and three layers of lap for each specimen. In single lap, the three layers are

overlapped on one side of the patched region as per the length. In double lap,

the three layers are overlapped on both sides. All the overlapped specimens

are cured in room temperature for 24 hours and aluminum tabs of size 60mm

x 3mm x 18mm are bonded on the surface of prepared specimens using

epoxy-Araldite.

3.8.4.1 Scarf repaired specimen

Single lap of repair is done by layers of fiber cloth with resin

mixture is applied over the damage removed region in a pattern that the length

of each subsequent layers with increasing pattern of width, so that the inclined

region reduce the peel stress while applying load (Figure 3.20). The above

process is called as patching process. The wet layup technique is used here.

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Figure 3.20 Scarf repaired model specimen

3.8.4.2 Single lap repaired specimen

Single lap repair is done by layers of fiber cloth with resin mixture

is applied over the damage removed region in a pattern that the length of the

each subsequent layers to reduce the peel stress while applying load (Figure

3.21). The wet layup technique is used here. After the patching technique, the

overlapping the glass fiber layers as the number of 3 layers are placed on the

patching specimen.

Figure 3.21 Single lap repaired model specimen

3.8.4.3 Double lap repaired specimen

The same patching procedure is followed for the double lap repaired

specimen. After finishing the patching, the same patching procedure will be

followed. The overlapping is to be done on both sides of the specimen (Figure

3.22). The repair efficiency is determined by the changing overlapping length.

The repair efficiency is evaluated by the manufacturing of five specimens on

every overlap length. Compare the results by the tensile test and find the best

repair technique.

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Figure 3.22 Double lap repaired model specimen

The cured specimens are tested by the tensile test with AE and the

damage mechanism and the strength are compared to find the best repair

technique.

3.9 TENSILE TESTING SET-UP IN UNIVERSAL TESTING

MACHINE (UTM)

3.9.1 Tensile Testing of Glass/epoxy Composite Determination with

A.E. Sensors

The specimens prepared from the laminates are tested using an

INSTRON 3367 universal testing machine along with acoustic emission

monitoring as shown in Figure 3.23.

Figure 3.23 Tensile testing of glass/epoxy composite has been

determined with A.E. sensors

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3.9.2 Tensile Strength under AE Monitoring

The Specimens are subjected to uni-axial tension in 30kN

INSTRON 3367 Universal Testing Machine (UTM) under

acoustic emission monitoring using an 8-channel Acoustic

Emission setup supplied by Physical Acoustics Corporation.

The Specimens are mounted on the UTM machine and

dimension of the specimens are entered in the software.

The crosshead speed was maintained at a rate of 0.15mm/min

AE signals are recorded for each specimen.

3.10 ACOUSTIC EMISSION MONITORING

3.10.1 Equipment used in AE Monitoring

The process of AE monitoring is made possible using an array of

instruments. Each component has a unique role to play and is essential to

ensure proper monitoring. A brief description of each component is detailed

in this section.

3.10.1.1 Sensors

They are the key instruments that detect the mechanical transient

elastic waves generated from within a structure and convert them into

electrical signals. Usually piezoelectric resonant sensors are used for AE

testing. Figure 3.24 shows a plethora of various kinds of sensors available in

today’s market.

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Figure 3.24 Different types of Sensors

3.10.1.2 Couplants and holders

Sensors are placed on the surface of the material to be tested using

various couplants. These are mainly used to assist easy and complete

conduction of acoustic waves generated from the source. Commonly used

couplants are oil, glue, high vacuum grease, etc. Along with the use of

couplants, most field tests require additional holders (e.g. mechanical or

magnetic) to keep the sensors in place.

3.10.1.3 Pre-amplifiers

The main purpose of the pre-amplifier shown in Figure 3.25 is to

provide gain to boost and effectively and reject noise from areas outside the

sensor operating range.

Figure 3.25 Pre-amplifier

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3.10.1.4 Data acquisition system

Modern AE systems use computers and appropriate software

providing a menu- driven parameter input and system control. All the signals

received at the sensor end are acquired and stored in the acquisition system.

The new generation systems also enable extensive post-processing

possibilities. Acquisition systems have also been well adapted for continuous

monitoring of structures using wireless technology and web-based remote

monitoring. Figure 3.26 represents the schematic representation of acoustic

emission testing set up.

Figure 3.26 Acoustic emission monitoring Process

3.10.2 A Generic AE System

A schematic representation of an acoustic emission system and its

detection procedure is represented in Figure 3.27. The process chain basically

consists of the following stages, which take place in a very short time, so that

all stages can be perceived as simultaneous for testing purposes.

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Figure 3.27 Schematic representation of acoustic emission (AE)

3.10.2.1 Acoustic emission setup

An 8 channel AE system supplied by Physical Acoustics

Corporation (PAC) is used for the study. The sampling rate and pre-

amplification are kept as 1 to 3 MSPS and 40 dB respectively. Preamplifiers

operating in the frequency band 10 kHz-2 MHz are used. AE activities were

sensed using wide band WD and NANO sensors, filtering out frequencies

exceeding 900 kHz and using a threshold of 45 dB to eliminate the back

ground noise. High vacuum silicon grease used as a couplant. The amplitude

distribution covers the range 0-99 dB (0 dB corresponds to 1µv at the

transducer output). After mounting two transducers on the sample at a mutual

distance of 100 mm between them, so that they were both at the same distance

from the centre of the specimen length, a pencil lead break procedure was

used to generate repeatable AE signals for the calibration of each sensor.

Velocity and attenuation studies are performed on the laminates. The Pre-

Trigger values and the Hit length values are estimated as 26 µsec and 4K

respectively.

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3.10.2.2 Pencil lead break test

Among the characteristics of the AE instrumentation system,

sensitivity needs to be considered first. Of all the parameters and components

contributing to the sensitivity, the piezoelectric sensor is the one most subject

to variation. This variation can be a result of damage or aging, or there can be

variations between nominally identical sensors (ASTM Standard E 1781). To

detect such variations, it is desirable to have a method for measuring the

response of a sensor to an acoustic wave. Specific reasons for checking

sensors include: (1) checking the stability of its response with time; (2)

checking the sensor for possible damage after accident or abuse. A

recommended method to check the sensitivity of the sensor is the response of

the system to pencil lead break test (ASTM E976 1994). In this test, a

repeatable acoustic wave can be generated by carefully breaking a pencil lead

against the test panel. When the lead breaks, there is a sudden release of stress

on the surface of the panel where the lead is touching. This stress release

generates an acoustic wave. The Hsu pencil source uses a mechanical pencil

and the Nielsen source can be used to aid in breaking the lead consistently.

The pencil lead break test was performed to obtain the actual AE source for

this research. The test scheme is shown in Figure 3.28 (ASTM E976

STANDARD (1994).

Figure 3.28 Hsu-Nielsen source (NDT.net 2007)

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3.10.2.3 Hsu –Nielsen source

Hsu-Nielson device (named after developer of the technique) is an

aid to simulate an acoustic emission event using the fracture of a brittle

graphite lead in a suitable fitting. This test consists in breaking a 0.5 mm

diameter

(2-H) pencil lead approximately 3 mm (+/- 0.5 mm) from its tip by pressing it

against the surface of the piece. This generates an intense acoustic signal,

quite similar to a natural AE source that the sensors detect as a strong burst.

The purpose of this test is twofold. First, it ensures that the transducers are in

good acoustic contact with the part being monitored. Generally, the lead

breaks should register amplitudes of at least 80 dB for a reference voltage of 1

mV and a total system gain of 80 dB. Second, it checks the accuracy of the

source location setup. This last purpose involves indirectly determining the

actual value of the acoustic wave speed for the object being monitored.

3.11 INPUT PARAMETERS TO AE WIN SOFTWARE

3.11.1 Wave Velocity

The wave velocity is calculated by Hsu Nielsen pencil lead break

procedure method as shown in Figure 3.29. The pencil tip is broken at a

known distance from the sensors and the corresponding time difference at

which the signal is captured by the two sensors is noted down. Then the wave

velocity is calculated from the following relation.

Distance between the sensors dWaveVelocity

Time interval at which signal is detected

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Figure 3.29 Wave velocity calculations

3.11.2 Wave Velocity Study

The Acoustically emitted sound waves travel with different velocity

in different types of materials. The wave velocity must be found to determine

the other Acoustic Emission parameters. So the velocity is calculated using

the following steps.

Initially the sensors are fixed at two locations in the specimen

(Figure 3.30).

High vacuum grease is applied on the sensor and then fixed

using tape.

The distances between the two sensors are measured.

Velocity study is done using Hsu-Nielson source (pencil lead

break).

The test is performed at various locations within the sensors.

Then the velocity is calculated using the formula.

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47 A.E. Sensor Position 80 47

Sensor 1

2

Figure 3.30 A.E. Sensor position

3.11.3 Hit Definition Time (HDT)

The function of Hit Definition Time is to enable the system to

determine the end of the hit, close out the measurement processes and store

the measured attributes of the signal. Proper setting of HDT ensures that the

AE signal from the structure is reported as one and only signal. The

recommended range for the composites is 100-200µs (SAMOS AE user

manual, Rev:2).

3.11.4 Hit Lockout Time (HLT)

The function of Hit Lockout Time is to inhibit the measurement of

reflections and late arriving parts of the AE signal. With proper settings of the

HLT, spurious measurements during the signal decay are avoided and data

acquisition speed can be increased.

3.11.5 Peak Definition Time (PDT)

The function of peak definition time is to enable determination of

the time of true peak of the AE waveform. A proper setting of the PDT

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ensures correct identification of the signal peak from rise time and peak

amplitude measurements. The recommended range for the composites is

20-50µs (SAMOS AE user manual, Rev:2). PDT is calculated using the

following relation.

Distance between the sensors PDTWave Velocity

3.11.6 Data Acquisition Process

Monitoring of AE signals generated from uni-axial Tensile

Test is done by an acquisition system.

The signals were detected using two Nano 30 piezoelectric

transducers, which are attached to the specimen surface using

high vacuum grease as a couplant and are fastened by tape.

The signals from the transducer passed through PAC 2/4/6

G/A pre-amplifier before reaching the main unit.

Wave velocity test is performed on the specimen and wave

velocity is calculated.

Next the sensors are connected to the 8-channel AE data

acquisition system.

UTM is switched ON and the tensile load is applied.

Various AE parameters such as Amplitude, Counts, Energy,

Rise, and Duration are recorded during the test.

The data are processed.

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3.11.7 Sample Rate

This is the rate at which the data acquisition board samples the

waveforms on a per second basis. A sample rate of 1 MSPS (Mega sample per

second) means that one waveform sample is taken every µsec.

3.11.8 Pre-Trigger

This value tells the software how long to record (in µsec) before the

trigger point (the point at which the threshold is exceeded). The user may

enter a value in the pre-trigger edit box. The minimum allowable pre-trigger

value is zero. The maximum allowable pre-trigger value is calculated by

dividing the hit length by the sample rate in MHz If, for example the hit

length was 1k(1k = 1024) and the sample rate was 4 MHz, then the maximum

allowable pre-trigger value would be 1024/4 = 256 µsec.

3.11.9 Hit Length

This determines the size of a waveform message. The available hit

length is in the range of 1k- 4k. At a 4 MSPS sampling rate, a hit length of 1k

will allow up to 256 µsec of data, a hit length of 2k will allow 512 µsec of

data (2*256) and so on.

The timing parameters in the hardware settings are calculated for

different materials and are listed in Table 3.1. The HDT is calculated from

trial and error method. Proper setting of the HDT ensures that each signal

from the structure is reported as one signal only.

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Table 3.1 Timing parameters for glass fibre

HDT(µs)

PDT(µs)

HLT(µs)

Wave Velocity m/s

Glass fiber/epoxy ( Unidirectional)

160 32 300 3078

Glass fiber/epoxy ( Bidirectional)

300 30 600 3020

The description of the sensors used in the experiments is listed in

Table 3.2.The calibration certificates for both Nano30 and Wide Band sensors

are shown in Figures 3.31(a) and 3.31(b) respectively.

Table 3.2 Description of sensors

Sensors Model

Sensors Dimensions DIA X HT

(mm)

Operating Temperature

( 0C )

CaseMaterials

Operating Frequency

Range (kHz)

Nano 30 PAC

8 X 8 -65 to 177 Stainless

steel125 - 750

WD (Wide Band)

18 X 17 -65 to 177 Stainless

steel100 - 1000

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3.12 SCANNING ELECTRON MICROSCOPE

3.12.1 Scanning Electron Microscope Test on Specimens

The S-3400N is a powerful, yet user-friendly SEM through newly

developed electron optical and automated functions. The famous Hitachi

image quality convinces at high and low beam energies, and optimized

detector technology provides maximum information from your sample.

Variable chamber pressure allows charge up-free observation of any sample

without special preparation techniques such as coatings shown in Figure 3.31

(a). All samples must also be of an appropriate size to fit in the specimen

chamber and are generally mounted rigidly on a specimen holder called a

specimen stub as shown in Figure 3.31(b).

(a) (b)

Figure 3.31 (a) Scanning Electron Microscope (b) Specimens in SEM

setup for testing

3.13 SUMMARY OF EXPERIMENTAL PROCEDURE

In the present chapter, materials used and the different methods for

manufacturing fiber reinforced composite plates were explained; the different

fabrication techniques were described along with the production of laminates.

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Preparation of the specimens as per ASTM standards for different loading

conditions is also discussed in this chapter. The tensile test equipment was

described; one 30 kN load cell Instron 3367 Universal testing machine was

used for all the tests. Drop impact test was conducted using Fractovis drop

impact tower. Scanning Electron Microscope S-3400N was used to identify

the damages in the tested specimens.

All the necessary equipment required to conduct AE investigation

have been described along with the terminology of the standard data

processing associated with AE. The sensors used in this research, their

technical specifications are also presented. The recording apparatus was

described thoroughly including the response from the system and how the

signal is modified while travelling from the material to the processing

software. A common way of calibrating an AE test in order to ensure some

repeatability is described in detail. There are also several other parameters

that need to be defined prior experimentation in order to make the best out of

the recording system and obtain sensible data. The procedure involved in the

selection of these AE hardware parameters for different materials are

presented in this chapter. All the input parameters for the experimentation

purpose are also presented in this chapter. More details about the geometries

selected after damage observation are given in the following chapter.