EFFECT OF FIBER ORIENTATION IN UNI- DIRECTIONAL GLASS EPOXY LAMINATE

USING ACOUSTIC EMISSION MONITORING

V.Arumugam1, S. Barath Kumar

2, C. Santulli

3, A. Joseph Stanley

4

1 Assistant Professor, Department of Aerospace Engineering, Anna University, Chennai

2 PG student, Department of Aerospace Engineering, Anna University, Chennai

3Department of Chemical Engineering, Materials and Environment, Università La Sapienza, Rome, Italy

4 Professor & Dean, MIT Campus, Anna University, Chennai

ABSTARCT

Acoustic emission is one of the powerful techniques that can be used for in situ structural health monitoring

of composite laminates. One of the main issues of AE is to characterize the different damage mechanisms

from the detected AE signals. In this present work, pure resin and GFRP composites laminates with different

stacking sequences such as 0o, 90

o, angle ply[±45

o], crossply [0

o/90

o] are used to trigger different failure

mechanisms when subjected to tensile test with AE monitoring. The study of failure mechanisms is

facilitated by the choice of different oriented specimens in which one or two such mechanisms predominate.

Range of Peak frequencies in each orientation is investigated using FFT analysis. Fast Fourier Transform

(FFT) enabled calculating the frequency content of each damage mechanism. Randomly chosen hits from

each range of peak frequencies for the specimens with different orientations subjected to tensile test with

acoustic emission monitoring are analyzed using STFFT analysis. STFFT analysis is used to highlight the

possible failure mechanism associated with each signal. The predominance of failure modes in each

orientation is useful in the study of discrimination of failure modes in composite laminates from acoustic

emission data.

Keywords: Composite materials, Failure mechanisms, Acoustic Emission, FFT Analysis, STFFT Analysis.

1. INTRODUCTION:

Composite structures are widely used now a days

because of its high performance characteristics.

There has been a significant growth in the use of

fiber-reinforced materials to meet the increasing

demand for lightweight, high strength/stiffness and

corrosion-resistant materials in domestic appliances,

aircraft industries etc [1]. Hence the knowledge of

the damage behavior and the transition of damage

from a subcritical stage to a critical stage is of

considerable interest in material development and

application [2]. Many NDT techniques are used for

the characterization of the damage mechanisms in

composite structures. AE is one of the powerful

non-destructive technique for real-time structural

health monitoring of damage development in

materials and structures under quasi-static and

dynamic-cyclic loading [3-4]. This technology can

provide dynamic information of AE stress waves

when the structure is loaded. AE signal

characteristics such as energy, counts, rise time,

amplitude and spectral frequency distribution may

be related to the stress waves which are generated

by the source [5]. The types of damage that are

most frequently observed in composite laminates

are the matrix cracking, interlaminar failure,

fiber/matrix debonding and fiber failure. One of the

major goals for the researchers is to discriminate

these failure modes employing Acoustic emission

monitoring.

Many researchers has attempted to identify

the failure modes using Parametric based approach

and signal based approach [6]. For parametric based

approach, most of the studies have been performed

through AE parameters such as energy, counts,

amplitude, duration and rise time [7-9] while AE

waveforms and its FFT are used in Signal based

approach [11]. A number of investigators have

attempted to characterize the failure modes in

composite laminates using acoustic emission

parameters. Huguet and Godin et al [10-12] tracked

the critical waveforms of different failure modes

based on parametric based approach.i.e by using the

amplitude parameter. While Bussibaet al. used AE

technique to track the damage accumulation profile

in terms of AE parameter such as counts rate and

cumulative counts [13]. J.M Bertholet in his paper

[14] used AE parameter to discriminate the failure

modes using stacking sequences ( 0o, crossply,

0o/±45

o, 90

o/±45

o).

Pattern recognition is also one of the

technique used for the identification of AE signal of

the specific failure modes in composites[15].A

number of researchers have used UPR techniques

such as FCM clustering with PCA(principle

component Analysis), KSOM(Kohonen’s Self-

Organizing Map), K-Mean algorithm, Max-Min

algorithm etc. Godin et al. used the pattern

recognition technique to characterize the failure

modes of Glass/Polyester unidirectional specimens

when subjected to tensile loading [11]. A. Marec

used the Fuzzy C means with PCA to correlate each

clusters to the failure mechanisms [16] while

C.R.L.Murthy in his paper [17] used the KSOM for

the discrimination of the failure mechanisms and

the noises present in it.

FFT analysis is another promising technique

that is being used for the failure mode

characterization. Fast Fourier Transform (FFT)

analysis, points out the dominant frequencies, which

are directly related to the main failure

mechanism.The FFT power spectrum can be used as

the ‘fingerprint’ of each event and therefore may be

used as means of distinguishing them[18]. The

event primary frequency alone was enough to

characterize each acoustic event[18].Similar signals

having similar frequency content can indicate that

their source mechanism are also similar[6]. By this

technique, Giordano et al. captured the fiber

breakage using single fiber fragmentation tests in

carbon fiber polymer composites [19].

Some researchers have tracked the failure

modes using wavelet analysis.The Wavelet

Transform (WT) can decompose the AE signals in

time and wavelet scale domains and catch the

differences in these waves[20].Chun-Gon Kim in

his paper[20] has studied the different failure modes

using wavelet transform.L.H. Yam in his paper [21]

used the wavelet analysis to track the initial damage

occurring in composite structures.

Because FFT suffers with the loss of the

transient feature of signals. Therefore, it is

necessary to implement the time-frequencyanalysis

for damage characterization studies. The STFFT

can be a candidate for the time-frequency analysis

[12, 22]. STFFT can be used to obtain the time

frequency information of a small portion of the

signal.It calculates the local spectral density using

windowing techniques to analyze a dominant

section of the signal at a time. This gives a

perspective into how a waveform’s frequency

content is varying at any particular point or time of

the waveform. [23].

C.R Ramirez-Jimenez [18] used the primary

peak frequency to differentiate the failure modes by

means of a Fast Fourier Transformation using

different orientation, whereas Jürgen Bohse[24]

employed the AE energy parameter to characterize

the failure modes using specimen with different

orientations((90°) / (±45°) / (90° , angle ply and

crossply). Similarly Peter J. de Groot has employed

the peak frequency analysis to characterize the

failure modes using different orientations (0o, 10o

and 90o) [25].

One of the main complications in the study

of the mechanics of the composite material is the

multiphase failure behavior. The main problem

from an analytical and experimental point of view

in Acoustic emission technique are 1) how to

distinguish between the different failure modes and

2) how to access the individual and associated

failure modes.[26]. And at the same time, it is

difficult to characterize all the failure modes from a

single oriented specimen. In order to avoid these

complexities, specimens with different stacking

sequences may be used for the characterization of

failure modes in composite laminates. In this

present work, pure resin, GFRP composites

laminates with different stacking sequences such as

0o, 90

o, Angle ply[±45

o], Crossply [0

o/90

o] are used

to trigger different failure mechanisms when

subjected to tensile test with AE monitoring.The

range of peak frequency pertaining to the failure

mode in each orientation has been identified using

FFT analysis. Signals and its characteristics

representing different failure modes are identified

and validated using STFFT analysis and are

presented in this paper.

2. EXPERIMENTAL PROCEDURE

2.1 Specimen preparation

GFRP composite laminates with different

orientations such as (0o, 90

o, Cross ply[0

o/90

o],

Angle ply[±45o]) of size 300 x 300 mm are

fabricated using vacuum bagging technique. Four

layers of uni-directional glass rovings along with

LY556 epoxy matrix are used for the purpose of

fabrication of the laminates.For pure resin

specimen, epoxy LY556 alone was used. ASTM

D3039 Standard tensile specimens of size 280x18x

2.78 mm were cut from the fabricated laminates

using water-jet cutting to avoid machining defects

and to maintain good surface finish. Aluminum tabs

of size 60 x 18 x 3 mm are used to reduce the grip

noise.

2.2 Tensile testing procedure

ASTM D3039 tensile specimens that are cut from

the laminates are subjected to uni-axial tension

using an INSTRON 3367 universal testing machine

along with acoustic emission monitoring. Sixteen

specimens, four in each orientation (0o, 90

o, Cross

ply[0o/90

o], Angle ply[±45

o]) are tested. For all the

specimens the cross head speed was kept at 0.15

mm/min. Dominant AE parameters such as

amplitude, counts, rise time energy are recorded

during the tensile test with acoustic emission

monitoring.

2.3 Acoustic Emission Monitoring

An 8 channel AE system supplied by Physical

Acoustics Corporation (PAC) is used for this study.

The sampling rate and pre amplification are kept as

1 MSPS and 40 dB respectively. Preamplifiers

having a bandwidth of 10 kHz-2 MHz are used. The

ambient noise was filtered using a threshold of 45

dB. AE activities were sensed using wide band WD

piezoelectric sensor (the sensor that gives same

response over a wide frequency range). The

operating frequency range of the sensor is 100 –

900 KHz. High vacuum silicon grease was used as a

couplant. The amplitude distribution covers the

range 0-100 dB (0 dB corresponds to 1µv at the

transducer output). After mounting the transducers,

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 average wave

velocity in the material was found to be 3078 m/s.

The Pre-Trigger value and the Hits length value are

estimated as 26µsec and 4K.The timing parameter

in the hardware settings is calculated and are as

follows: Peak definition time (PDT) =32µs, Hit

definition time (HDT) =160µs, Hit lockout time

(HLT) = 300µs. The HDT is calculated from trial

and error method. Proper settings of the HDT

ensures that each signal from the structure is

reported as one and only signal.

3. Results and Discussion:

3.1 Frequency Analysis of AE signals

Frequency analysis is one of the promising

technique to discriminate the failure modes

occurring in composite structures.

Each failure mode generates an AE signal which is

related to the amount of strain energy released.

Therefore each AE waveform has a unique feature,

in the sense that its amplitude, duration and

frequency content are related to the damage

mechanisms [27] such as matrix cracking,

debonding, delamination and fiber failure. (a)

Matrix cracking has low to medium amplitude,

short to moderate duration with medium frequency

content. (b) Fiber debonding as well as

delamination generates AE hits which covers the

whole range of amplitudes and typically have long

duration and low frequency content.(c) Fiber

breakage generates medium to high amplitude and

short duration events with high frequency content

[14].

Acoustic emission characterization of failure modes

in composite materials is a complex phenomenon.

Therefore in this present work the different failure

mechanisms are identified using laminates with

different orientation such as 0o, 90

o, Cross

ply[0o/90

o], Angle ply[±45

o]. Matrix cracking signal

are characterized by testing pure resin specimens.

Fiber matrix debonding is tracked in Ninety degree

orientation specimens. Fiber failure and

Delamination signals are captured in zero degree

and angle ply specimens. The predominance of

failure modes in each orientation are used for the

acoustic emission characterization of damage

mechanisms.

After a thorough investigation of the AE data it is

found that four different ranges of characteristic

frequencies are involved in all the orientations. Fig

(2,5,8 and 12) shows the range of peak frequency

pertaining to the failure modes of composite

laminates obtained from different orientation during

the conduct of tensile test with acoustic emission

monitoring. The frequency range 90-110 kHz

corresponds to matrix cracking. The frequency

range of 130-200 kHz corresponds to delamination.

Debonding and Fiber breakage corresponds to a

frequency range of 230-250 kHz and 250-280 KHz

respectively. The sequence of failure events in AE

waveform are performed using STFFT.The STFFT

is performed for the primary or peak frequency

content in each range and the results are interpreted

with respect to the duration of the each failure

mechanism. This technique is used to analyse a

small portion of the signal at a time, which is also

called Windowing the signal.

3.2 Pure Resin Specimen:

For Matrix cracking studies, pure resin

specimens are fabricated using LY556 epoxy and

are subjected to tensile test with acoustic emission

monitoring. Fig (1) shows the AE waveform and

FFT magnitude pertaining to the matrix cracking

signal. From the FFT analysis, the frequency

content of matrix cracking was identified to be in

the range of 90 – 100 KHz which is evident from

[8]. Since Epoxy is a Viscoelasticity material in

nature, frequency range of signals for epoxy is

lower than fiber during the conduct of tensile

testwith AE monitoring [28-29].

Fig (1): Time domain & Frequency domain signals for Matrix Cracking failure mode

3.3 Transverse oriented Glass/epoxy specimen

(90o):

ASTM D3039 tensile specimens cut

perpendicular to the fiber direction (90o) are

subjected to tensile test with AE monitoring. The

failure mechanisms associated with glass fibers

oriented perpendicular to the loading direction are

matrix cracking and minor cases of debonding

[25]. Two ranges of peak frequencies are

identified during the conduct of tensile test for

transversely oriented specimen. The frequency

range 90 -110 KHZ corresponds to matrix

cracking which is evident from the frequency

content of pure resin specimen except that when

fibers are added to the resin there is a slight

increase in the peak frequency range [25]. Peak

frequency range 230-250 KHz indicates the

presence of a micro event and can be related to

fiber/matrix debonding. Fig (3a, 3b) shows the AE

waveforms and FFT magnitude obtained during

the tensile test, pertaining to matrix cracking and

fiber/matrix debonding. Short time FFT analysis is

performed on the waveforms to investigate the

characteristics of the failure modes in terms of AE

parameters such as amplitude and duration.

Waveform processing of AE signals using Fast

Fourier Transform is used to identify the

frequency content of various failure mechanisms

while STFFT analysis is used to convert time

series waveforms into frequency and time domain

components. Fig (3) shows STFFT analysis

performed for a portion of the AE waveforms

corresponding to the dominant frequency content

pertaining to the failure modes such as matrix

cracking and fiber/matrix debonding.. From the

figure it is found that matrix cracking is

characterized by low amplitude and low duration

whereas fiber matrix debonding is characterized

with moderate amplitude and duration[27].

Fig (2): Bar chart for the peak frequency ranges obtained in 90o orientation specimens

(a)

(b)

Fig (3) Time domain & Frequency domain of typical AE signals in 90o orientation specimen

a) Matrix cracking b) Debonding signal

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

90o Orientation

80-110 KHz

230-250 KHz

Peak Frequency Range

Matrix Cracking

101 KHz

Debonding

240 KHz

Fig (4): STFFT for the failure modes in 90o

orientation specimen

Fig (5): SEM micrograph of typical matrix

cracking

Fig (6): SEM micrograph of typical fiber /matrix

debonding in 90o orientation specimen

3.4 Angle Ply specimen [±45o]:

In the case of specimens cut from angle

ply (±45o) laminate subjected to tensile test with

AE monitoring, the dominant failure modes are

matrix cracking, debonding and interlaminar

failure[14]. Three ranges of peak frequency

content are identified in Angleply[±45o]

specimens. The frequency range 90-110 KHz and

230-250 KHz corresponds to matrix cracking and

debonding respectively which is evident from the

results obtained from transversely oriented (90o)

and pure resin specimens. The primary failure

mode that occurs in angle ply specimens subjected

to tensile test is interlaminar shear. The peak

frequency range (130-200 KHz) can be possibly

related to delamination. Fig (9a, 9b and 9c) shows

the AE waveforms and its corresponding FFT

magnitude obtained during the tensile test

pertaining to matrix cracking, delamination and

fiber/matrix debonding. STFFT analysis is further

performed for a small portion of the AE waveform

corresponding to the dominant frequency content

of the failure modes such as matrix cracking,

debonding and delamination. From the figure it is

found that compared to matrix cracking and

debonding, delamination is characterized by high

amplitude and high duration [27].

Fig (7): Bar chart for the peak frequency ranges

obtained in Angle ply [±45o] specimens

Fig (8): SEM Micrograph of typical Delamination

failure in Anlgeply [±45o] specimen.

(a)

(b)

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%84.80%

4.90% 9.50%

Angle Ply

80-110 KHz

130-200 KHz

230-250 KHz

Peak Frequency Range

Delamination

(c)

Fig (9): Time domain and frequency domain of typical AE signals in angle ply [±45o] specimen

a) Matrix cracking b) Delamination c) Fiber/Matrix Debonding.

Fig (10): STFFT Analysis for the failure modes in

angle ply[±45o] specimen

3.5 Longitudinal oriented glass/epoxy specimen

(0o):

For specimens loaded along the fiber

direction, the associated failure modes are matrix

cracking fiber/matrix debonding, delamination

and fiber failure [25]. During the conduct of

tensile test for 0o orientation specimens, four

range of peak frequency content are identified.

These peak frequencies are in the range of 90-110

KHz, 130-200 KHz and 230-250 KHz, which

Corresponds to matrix cracking, delamination and

fiber/matrix debonding respectively. Fig (12a,

12b, 12c, 12a, 12b and 12c) shows the AE

waveforms and its corresponding FFT magnitude

and STFFT for failure modes such as matrix

cracking, delamination, fiber/matrix debonding

and fiber failure. The Peak frequency content

pertaining to failure modes such as matrix,

debonding and delamination are validated from

the results of pure resin, 90o and angle ply [±45

o]

specimens. From the literature review, it is evident

that fiber failure always occurs at higher peak

frequency content with high amplitude and short

duration [13]. Hence the fourth range of peak

frequency content 250-300 KHz can possibly be

related to the mechanism of fiber failure. To

investigate the nature of the fiber failure

mechanism, STFFT analysis is performed for a

portion of AE waveform corresponding to the

dominant frequency content (250-280 KHz). From

Fig (14) compared to matrix cracking debonding

and delamination, fiber failure is characterized by

high amplitude and short duration [27].

Fig (11): Bar chart for the peak frequency ranges

obtained in 0o orientation specimen

(a)

(b)

(c)

(d)

Fig (12): Typical Time domain Signals obtained

in 0o orientation specimen a) Matrix cracking

signal b) Delamination signal c) Fiber/Matrix

Debonding d) Fiber Failure

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00% 81.00%

0.40%

8.00%

10.60%

0o Orientation

80-110 KHz

130-200 KHz

230-250 KHz

250-300 KHz

Peak Frequency

(a) (b)

(c) (d)

Fig (13): Frequency Domain signals a) Matrix cracking signal b) Delamination signal

c) Fiber/Matrix Debonding d) Fiber Failure

Fig (14): STFFT analysis for the failure modes in

0o orientation specimen

Fig (15): SEM Micrograph of typical fiber

breakage in 0o orientation specimen

Delamination170

KHz

Matrix

Cracking

101 KHz

Debonding

240 KHz Fiber Break

240 KHz

3.6 Crossply specimen [0o/90

o]:

In the case of crossply laminates [0o/90

o],

the dominant failure modes are matrix cracking,

debonding delamination, and fiber failure [14].

Four peak frequency ranges are identified during

the conduct of tensile test for cross ply specimen.

The frequency range 90-110 KHz and 230-250

KHz corresponds to matrix cracking and

debonding and 130-200 KHz and 250-280 KHz

corresponds to delamination and fiber failure

respectively.

The peak frequency content of the four

possible failure modes are further validated with

the results obtained from crossply laminates. Fig

(17 and 18) shows Time domain, Frequency

domain and STFFT for failure modes such as

matrix cracking, delamination, fiber/matrix

debonding and fiber failure.

Fig (16): Bar chart for the peak frequency ranges

obtained in cross ply [0o/90

o] specimen.

(a)

(b)

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

96.40%

1.56% 1.00% 1.00%

Cross ply

80-110 KHz

130-200 KHz

230-250 KHz

250-300 KHz

Peak Frequency Range

(c)

(d)

Fig (17): Time domain and frequency daomain signals a) Matrix cracking b) Delamination

c) Fiber/Matrix debonding d) Fiber faiilure.

Fig (18): STFFT analysis for the failure modes in

crossply specimen

Fiber orientation

Peak Frequency Range

90-110 KHz 130-200 KHz 230-250 KHz 250-280 KHZ

Pure Resin Matrix Cracking - - -

90o Matrix Cracking - Debonding -

Angle Ply[±45o] Matrix Cracking Delamination Debonding -

0o Matrix Cracking Delamination Debonding Fiber Failure

Crossply[0o/90

o] Matrix Cracking Delamination Debonding Fiber Failure

Table (I): Peak Frequency range for the different failure mechanisms in different orientations

4. CONCLUSION

In this study Glass/Epoxy laminates with

different fiber orientations are subjected to tensile

test with AE monitoring. Ranges of peak

frequency pertaining to all possible failure

mechanism are obtained from each orientation.

Each range of peak frequencies is possibly related

to specific micro failure within the material from

the knowledge of predominant failure in each

orientation.

Matrix cracking signal is tracked from the

peak frequency range 90- 100 KHz which is

obtained from pure resin specimens. Similar

ranges of peak frequency are identified from

specimens with different orientations and from

that it is concluded that Matrix cracking signal

ranges between 90-110 KHz. For transversely

oriented specimens two ranges of peak frequency

are identified which corresponds to the failure

modes such as matrix cracking and minor cases of

debonding. The second frequency range 230-250

KHz is related to debonding.

For angle ply specimens three ranges of

peak frequencies are identified which corresponds

to the failure modes such as matrix cracking,

debonding and delamination. Since the first two

ranges of peak frequencies corresponds to the

failure mechanisms such as Matrix cracking and

debonding which has been validated from the

results obtained from 90o and pure resin

specimens. Therefore the third peak frequency

range 130-200 KHz is related to delamination. To

capture the fiber failure signals, specimens with

fibers oriented along the loading direction is used,

where four ranges of peak frequency are obtained.

Since fibre failure always occurs at a higher peak

frequency content [13], the peak frequency range

250-280 KHz is related to the fiber failure.

The nature of the failure mechanisms in

GFRP laminates with different orientations has

also been validated using STFFT analysis.

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